Reliability of the Scanning Capacitance Microscopy and...

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No d’ordre: 2010-ISAL-0008 Année 2010

THESE

présentée devant

L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON

pour obtenir le grade de

DOCTEUR

ECOLE DOCTORALE : Electronique, Electrotechnique, Automatique

SPECIALITE : Dispositifs de l'Electronique Integrée

par

Ligor Octavian

Reliability of the Scanning Capacitance Microscopy and Spectroscopy for

the nanoscale characterization of semiconductors and dielectrics

Soutenue le 11 février 2010 devant la Commission d'Examen

Rapporteurs: Frédéric HOUZE Chargé de Recherche CNRS, HDR

François BERTIN Ingénieur CEA, HDR

Examinateurs: Daniel ALQUIER Professeur LMP

Christophe GIRARDEAUX Professeur IM2NP

George BREMOND Professeur INL/INSA de Lyon

Directeurs de these: Brice GAUTIER Professeur INL/INSA de Lyon

Invité: Jean-Claude DUPUY Professeur émérite INL/INSA de Lyon

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INSA Direction de la Recherche - Ecoles Doctorales

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CHIMIE

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Résumé du mémoire de thèse

Le développement des nanotechnologies, comme les progrès vers les limites de la micro-

électronique et de la nano-électronique, impose le développement des méthodes de

caractérisation capables de fournir des informations avec une résolution compatible avec la

dimension des objets à caractériser. Ceci est vrai pour la métrologie morphologique, mais

aussi pour autres propriétés des nano-objets comme les propriétés magnétiques ou les

propriétés électriques (charge, potentiel, champ électrique, résistance, capacité...). En

particulier, pour les nano-objets de la nano-électronique, les propriétés de transport, de la

rétention de charge comme des nanocristaux, ou la concentration des dopants sont des

paramétrés cruciaux qui contrôlent les propriétés finales des dispositifs basés sur ses objets.

Des nombreuses méthodes de caractérisation sont apparues, capables d'avoir une

résolution nanometrique ou même atomique, comme par exemple des techniques basées sur

la microscopie électronique ou la microscopie a sonde atomique. Même si certaines d'entre

elles fournissent une vraie résolution atomique (il est possible, par exemple, d'imager des

atomes individuels en utilisant la STM (Scanning Tunneling Microscopy) en ultra-vide, et il est

même possible de détecter la présence des atomes individuels de bore grâce a la modification

de la densité d'états des atomes voisins, ces méthodes de caractérisation nécessitent une

préparation complexe des échantillons ou une instrumentation complexe. Pourtant, pour des

applications industrielles, il faut prendre en compte non seulement la reproductibilité, la

quantitativite et la résolution de ces méthodes de caractérisation, mais aussi leur rapidité et

leur facilite d'utilisation.

C'est pour cette raison que les méthodes de caractérisation basées sur la microscopie a

force atomique (AFM) représentent des candidates très sérieuses, capables a fournir des

informations topographiques et électriques des surfaces, avec une résolution nanometrique.

La microscopie a force atomique, utilisée pour la caractérisation topographique des

surface, a évolué dans un grand nombre des différentes méthodes de caractérisation qui

profitent de la précision du positionnement de la sonde AFM dans les trois directions de

l'espace, et de l'opportunité d'utilisation des sondes AFM conductibles comme électrodes de

grille pour réaliser des mesures inspirées des méthodes de caractérisation macroscopiques.

C'est le cas de la KFM (Kelvin Force Microscopy) pour la mesure du potentiel du surface ou de

la SSRM (Scanning Spreading Resistance Microscopy) pour la caractérisation des profils des

dopants. Pour des mesures capacitives, l'analyseur d'impédance utilise pour des mesures

micrométrique ne peut pas être utilise a l'échelle nanometrique, a la cause d'un rapport

signal/bruit insuffisant et doit être adapte pour être capable a mesurer le signal capacitif

correspondant a la capacité de l'ordre de quelques attofarads de la structure M.O.S formée

par le système sonde-échantillon.

Cette dissertation a pour but de montrer les performances, les limites et le potentiel

d'une méthode de caractérisation appelée Scanning Capacitance Microscopy (SCM), utilisée


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pour la cartographie des dopants a nano-échelle et pour la mesure des propriétés des oxydes

minces.

En début du premier chapitre on présente des méthodes de caractérisation alternatives

pour la caractérisation des dopants, basées sur la microscopie a force atomique, la

microscopie électronique ou la sonde atomique tomographique. Des avantages et des

inconvénients de ces méthodes de caractérisation, en terme de sensibilité, résolution et

quantification sont mis en évidence. Le tableau suivant résume les conclusions de cette

comparaison :

SCM SIMS SSRM Holographie électronique

Sonde tomographique

ESEM

Détection porteurs atomes porteurs atomes atomes porteurs

Type de cartographie

2D 1D 2D 2D 3D 2D

Résolution 25 nm 1.5 nm pour le bore en Si

1-2 nm avec des pointes de diamant purs; 5-10 nm avec des pointes

commerciales

5-10 nm ~1 nm pas connu

Quantification possible

avec difficulté

oui, si des échantillons

étalons existent pour les atomes a étudier, dans

la même matrice

oui non oui non

Facilite d'utilisation

oui oui oui Non : nécessite la fabrication

d'une lamelle très mince

Non : nécessite une préparation très complexe de l'échantillon (une

pointe très aigüe)

non

La suite du premier chapitre expose en détails le fonctionnement de la SCM, d'abord en

en présentant le mode contact de l'AFM qui constitue le support pour les mesures capacitive.

Puis les méthodes de préparation des échantillons sont décrites : le clivage et le biseautage.

Les échantillons utilisés pour les investigations quant à la résolution, la localisation des

jonctions et la quantification sont présentés. Des mesures sur des échantillons de type paliers

de concentration (constitués de plusieurs couches de plusieurs centaines de nanomètre

d'épaisseur et de dopages différents) confirment l'utilité de la SCM pour des mesures

qualitatives (dites : "de défaillance") dans l'industrie de la nano-électronique. Les mesures

qualitatives avec la SCM peuvent confirmer très vite si l'implantation de l'échantillon a été

correctement effectue au bon endroit. Les zones de type n et de type p sont correctement

délimitées car la SCM est une technique capable de discriminer le type de dopants,

contrairement à d'autres techniques comme la SSRM. Des concentrations de dopage de l'ordre

de 1015 at/cm3 à 5.1019 at/cm3 de type n et p peuvent être imagées de manière qualitative

sans inversion de contraste, en choisissant correctement les tensions appliquées à

l'échantillon.

Des mesures évaluant la capacité de la SCM à localiser des jonctions, testant la

résolution géométrique (détection et séparation de puits quantiques), la résolution en termes

de dopage, et la quantitativite des profils des dopants sont aussi présentées dans le premier


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chapitre.

Des premiers problèmes rencontres pendant ces tests, lies a la reproductibilité des

mesures sont mis en évidence. Par exemple, pendant les essais pour la localisation des

jonctions, on peut observer que la forme du signal capacitif a l'endroit des jonctions en

fonction de la tension continue appliquée ne correspond pas du tout a la théorie. L'amplitude

du signal, qui est le paramètre principal pour la quantification des dopage varie de manière

significative d'une expérience a l'autre.

Ces évidences motivent le deuxième chapitre de la dissertation, où les paramètres

expérimentaux qui peuvent perturber le signal SCM, diminuer le rapport signal/bruit ou

conduire a une interprétation erronée seront étudiés. Les échantillons étalons (des oxydes

thermiques minces déposés sur un substrat faiblement dope, de concentration connue, 1015

cm-3) sont analysés en parallèle avec une méthode de caractérisation de référence dans

l'industrie de la microélectronique, l'analyseur d'impédance, afin d'obtenir des courbes

capacité-tension (C-V) dont la dérivée pourra être directement comparée avec le même type

de courbe obtenue par SCM en mode spectroscopique (Scanning Capacitance Spectroscopy,

SCS, dont le résultat théorique est la dérivée de la courbe C-V obtenue sur une structure

MOS).

L'une des sources connue dans la littérature pour son impact négatif sur les mesures

SCM est l'influence de la photo génération de porteurs due à la lumière dans la structure MOS,

laquelle est fournie par le laser du système de détection AFM.

En déplaçant le spot laser sur le levier AFM de quelques centaines de microns de la

zone ou les mesures sont faites, nous avons mis en évidence l'influence négative du laser sur

le rapport signal/bruit du signal capacitif (Figure 1), lequel chute de manière brutale lorsque le

laser pointe vers la zone analysée.

Figure 1 : Comparaison entre deux signaux SCM, dans la présence et dans l'absence du laser AFM

En outre, des échantillons avec des profils des dopants graduels ont été utilisés pour

mesurer le taux de génération des porteurs dans le semiconducteurs par le laser AFM qu'on

peut estimer aux alentours de 1017 cm-3.

Des mesures capacitives avec un analyseur d'impédance, sur des électrodes de grille

minces (quelques nanomètres d'épaisseur) qui ont un coefficient de transmission de la lumière


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significatif (laissant passer la lumière, contrairement à des électrodes plusieurs centaines de

nanomètres d'épaisseur), en présence de la lumière laser, ont été effectuées pour mieux

expliquer la diminution de l'amplitude du signal capacitif (Figure 2). En particulier, la remontée

en inversion due à la création de porteurs minoritaires, ainsi que la création de pics

supplémentaires correspondant à de nouveaux points d'inflexion créés par la lumière dans la

courbe C-V a été analysée, et ses conséquences sur le signal SCM explicitées.

Figure 2 : Des mesures capacitives avec un analyseur d'impédance, dans l'absence et dans la présence de la lumière laser, sur un oxyde

thermique de 5 nm déposé sur un substrat faiblement dope.

Nous avons montre que le laser peut avoir pas seulement des effets négatifs sur les

mesures capacitives, mais que des mesures capacitives a l'échelle nanometrique, en présence

de la lumière, d'une manière contrôlée, peuvent conduire a des nouvelles applications.

Nous avons également montre que des différentes sources de capacité parasite sont

présentes dans la configuration présente de l'AFM, et affectent le rapport signal/bruit. D'autres

sources de distorsion du signal capacitif ont été mises en évidence, parmi lesquelles la

topographie des échantillons, le contact en face-arrière et le contact entre la pointe et

l'échantillon. Parmi tous ces sources de distorsion du signal capacitif, la plus importante

semble être le contact pointe-échantillon.

Des mesures capacitives comparatives effectuées avec une pointe AFM sur la surface

d'un oxyde et avec une pointe AFM en contact avec un nano-électrode gravée a partir d'un

électrode micrométrique, utilisée pour des mesures C-V avec un analyseur d'impédance, ont

montre des différences claires du signal capacitif. Des phénomènes parasites sont présents

dans le signal SCM en comparaison des courbes C-V macroscopiques lorsque la pointe est en

contact direct avec la surface de l'oxyde (et lorsque les mesures sont faites à l'air ambiant), en

particulier :

Une hystérésis du signal qui se traduit par un décalage entre les courbes obtenues lors

de la montée en tension et lors de la descente en tension (trace et retrace sur la Figure

3).

Une remontée en inversion se traduisant par un pic dans la zone d'inversion de la SCS

lequel n'est pas présent dans les courbes C-V macroscopiques.


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Ces phénomènes disparaissent complètement lorsque la mesure est effectuée sur une

électrode propre, assurant un contact parfait entre la pointe et l'échantillon (voir Figure 3).

Parmi les causes de la distorsion du signal se dénombrent :

L'état de propreté de la pointe AFM

L'état de propreté de la surface

La couche d'eau présente sur la surface

La géométrie aiguë de la pointe qui amplifie le champ électrique.

Il faut noter par ailleurs que les mesures sur le nanoelectrode ont démontre que le

maximum du signal capacitif différentiel (SCS) est situe a la même tension (0.8 V) que le

point d'inflexion de la courbe C-V obtenue avec un analyseur d'impédance. Ceci constitue une

preuve que la SCM peut devenir, dans des conditions de travail correctes, une méthode de

mesure reproductible et fiables.

Figure 3 : Des mesures capacitives effectuées sur un nanoelectrode (gauche) et avec une pointe AFM (droit) sur un oxyde thermique de 5 nm déposé sur un substrat de type p dope 1015 cm-3

Les causes de l'instabilité du signal capacitif quand les mesures sont effectuées avec

une pointe directement sur la surface de l'échantillon sont approfondies dans le dernier

chapitre, qui concerne la caractérisation des oxydes, où nous mettons en évidence certains

des problèmes les plus difficiles du contact pointe AFM - échantillon:

Le chargement de l'oxyde pendant l'application des tensions nécessaires au

fonctionnement de la SCM.

La modification de topographie de l'échantillon pendant les mesures électriques par

AFM, qui se traduit par l'apparition sur la surface de bosses pouvant atteindre plusieurs

dizaines de nanomètres de hauteur et dont les conditions d'apparition dépendent de la

polarité de la tension appliquée et de son amplitude.

Figure 4 : Image SCM obtenue après chargement de l'oxyde pendant les rampes des tensions consécutives.


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Il y a quelques facteurs qui apportent leur contribution au phénomène du chargement

de l'oxyde. La minceur de l'oxyde favorise l'apparition des courants tunnel. Des mesures

réalisées sur des oxydes plus épais ont montre que le seuil de tension pour le chargement des

oxydes augmente. Sur des oxydes très épais (10 nm -15 nm) par rapport aux oxydes

normalement utilises avec la SCM (1 nm -2 nm), le phénomène du chargement n'apparait

plus.

Des oxydes de mauvaise qualité permettent plus facilement la capture des charges

dans des oxydes et le passage de courant tunnel assisté par les défauts (trap assisted

tunneling). Le chargement se passe plus facilement pour des tensions plus grandes ou pour

des pointes plus aigües. L'humidité de l'air qui génère une couche d'eau sur l'oxyde hydrophile

a également une contribution importante au chargement des oxydes.

Les oxydes représentent a la fois le diélectrique de la structure M.O.S utilisée pour

mesurer le taux de dopage, et un sujet séparé d'étude. Étant donne que les propriétés du

signal capacitif se modifient non seulement avec la concentration des dopants, mais aussi avec

les propriétés des oxydes, les profils des dopants ne peuvent pas être étudiés sans

caractériser de manière rigoureuse d'abord l'oxyde de la structure M.O.S.

En particulier, les états d'interface modifient la largeur à mi-hauteur du signal SCM.

L'épaisseur de l'oxyde, les charges fixes a l'interface oxyde - semiconducteur ou dans le

volume de l'oxyde déterminent le déplacement du maximum du signal SCM par rapport a sa

position idéale.

Des diélectriques différents ont été étudiés dans le troisième et dernier chapitre comme

des possibles candidats pour le diélectrique de grille nécessaire à la caractérisation des profils

des dopants:

Oxydes de silicium obtenu par oxydation plasma et oxyde de silicium natif ou obtenu

après irradiation sous rayonnement ultra-violet et atmosphère d'ozone (UV/Ozone)

Nitrures de silicium

Oxydes high-k, en particulier LaAlO3 (LAO) obtenu par épitaxie par jet moléculaire à

l'école Centrale de Lyon.

La tableau ci-dessous résume les conclusions obtenues :

Épaisseur Position du maximum du

signal SCS

Facile a croitre

Croissance sur la section des échantillons

Inversion de contraste

Oxyde natif

1.2 nm app. 0 V oui oui non

Oxyde UV-ozone

1.7 nm 3 V - 4 V oui oui non

Oxyde plasma

5 nm - 7 nm -1 V - -0.5 V non oui oui

Nitrure 7 nm - 9 nm app 1 V non oui oui

LaAlO3 5 nm 2 V - 5 V no oui non


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Il apparaît très difficile dans cette étude de trouver un compromis parfait entre les

contraintes inhérentes à la croissance de l'oxyde dans le contexte de la cartographie de

dopants (oxyde basse température pour ne pas redistribuer les dopants, de faible épaisseur

sans toutefois donner lieu à des courants tunnels, facile à faire croître dans le but de ne pas

alourdir les procédures de caractérisation...), et les contraintes inhérentes à la reproductibilité

et à la fiabilité de la SCM qui exigent un oxyde reproductible et de la meilleure qualité

possible. Dans l'état actuel de nos recherches, l'oxyde idéal n'a pas été déterminé.

En conclusion, dans ce mémoire de thèse, nous avons étudié les motifs expérimentaux

qui empêchent la SCM d'être une méthode de caractérisation quantitative. De nombreux

facteurs parasites qui provoquent des variations importantes des paramètres du signal SCM

ont été identifies et des solutions pour régler ces problèmes ont été proposes. Notre travail

s'inscrit donc comme une étape vers des mesures SCM plus fiables et plus reproductibles.

Contents

Contents

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CONTENTS ................................................................................................................................................................ 10

GENERAL INTRODUCTION ........................................................................................................................................ 15

SCANNING CAPACITANCE MICROSCOPY : PRINCIPLE AND OVERVIEW OF PERFORMANCES ..................................... 18

1.1 INTRODUCTION .................................................................................................................................................. 19

1.1.1. SECONDARY IONS MASS SPECTROMETRY (SIMS) .......................................................................................................... 19

1.1.2. ELECTRON HOLOGRAPHY ........................................................................................................................................... 19

1.1.3. SCANNING SPREADING RESISTANCE MICROSCOPY (SSRM) .............................................................................................. 20

1.1.4. TOMOGRAPHIC ATOM PROBE (TAP) ........................................................................................................................... 20

1.1.5. OTHER TECHNIQUES FOR DOPANT PROFILING................................................................................................................. 21

1.2. ATOMIC FORCE MICROSCOPY ............................................................................................................................ 21

1.3. THE PRINCIPLE OF SCANNING CAPACITANCE MICROSCOPY ............................................................................... 22

1.4. DESCRIPTION OF THE SAMPLES USED IN THIS STUDY ........................................................................................ 25

1.5. SURFACE PREPARATION .................................................................................................................................... 26

1.5.1 CLEAVING ............................................................................................................................................................... 26

1.5.2. POLISHING AND BEVELING ......................................................................................................................................... 27

1.6. QUALITATIVE CHARACTERIZATION OF DOPANT PROFILES ................................................................................. 29

1.7. JUNCTION LOCALIZATION .................................................................................................................................. 30

1.7.1 TERMINOLOGY ......................................................................................................................................................... 30

1.7.2. STATE OF THE ART .................................................................................................................................................... 33

1.7.3. COMMENTS ON JUNCTION CHARACTERIZATION .............................................................................................................. 39

1.8. RESOLUTION ...................................................................................................................................................... 44

1.9. DOPANT PROFILE QUANTIFICATION .................................................................................................................. 47

BIBLIOGRAPHY ......................................................................................................................................................... 49

CHAPTER 2 REPRODUCIBILITY PROBLEMS WITH THE SCM. OPTIMIZATION OF THE EXPERIMENTAL CONDITIONS FOR

SCM OPERATION ...................................................................................................................................................... 51

2.1. INTRODUCTION ................................................................................................................................................. 52

2.2. STATE OF THE ART ............................................................................................................................................. 54

2.2.1. LASER LIGHT ..................................................................................................................................................... 54

2.2.2. STRAY CAPACITANCE ................................................................................................................................................. 56

Contents

12

2.2.3. SURFACE RELATED PHENOMENA ................................................................................................................................. 56

2.2.4. TIP RELATED PHENOMENA ......................................................................................................................................... 57

2.3. C-V MEASUREMENTS .................................................................................................................................... 62

2.3.1. INTRODUCTION ....................................................................................................................................................... 62

2.3.2. THE PRINCIPLE OF THE C-V MEASUREMENTS WITH AN IMPEDANCE ANALYZER ...................................................................... 63

2.3.2.1. Introductive notions concerning impedance analyzer ................................................................................. 63

2.3.2.2. The series model .......................................................................................................................................... 64

2.3.2.3. The parallel model ....................................................................................................................................... 65

2.3.2.4. The choice of the model used with C-V measurements ............................................................................... 66

2.3.3. C-V MEASUREMENTS ON A TEST SAMPLE ...................................................................................................................... 69

2.3.3.1. The test sample ........................................................................................................................................... 69

2.3.3.2. Preliminary measurements .......................................................................................................................... 69

2.3.3.3. Oxide thickness ............................................................................................................................................ 70

2.3.3.4. Dopant concentration .................................................................................................................................. 71

2.3.3.5. Flatband capacitance, flatband voltage ...................................................................................................... 72

2.3.3.6. Interface states, surface potential ............................................................................................................... 73

2.3.3.7. A preliminary comparison between the C-V signal and the SCM signal ...................................................... 75

2.4. INFLUENCE OF THE LASER LIGHT ........................................................................................................................ 77

2.4.1. INTRODUCTION ....................................................................................................................................................... 77

2.4.2. EXPERIMENTAL EVIDENCE .......................................................................................................................................... 78

2.4.3. QUANTITATIVE ASPECTS ............................................................................................................................................ 79

2.4.4. COMPARISON WITH C-V MEASUREMENTS .................................................................................................................... 81

2.4.5. SOLUTIONS FOR THE ELIMINATION OF THE PARASITIC AFM LASER EFFECT ............................................................................ 87

2.4.6. CONCLUSIONS AND PERSPECTIVES ............................................................................................................................... 88

2.5. INFLUENCE OF THE PARASITIC CAPACITANCE OF THE GEOMETRY SETUP .......................................................... 89

2.5.1. INTRODUCTION ....................................................................................................................................................... 89

2.5.2. EXPERIMENTAL EVIDENCE .......................................................................................................................................... 90

2.5.3. COMPARISON WITH C-V MEASUREMENTS .................................................................................................................... 97

2.5.4. CONCLUSIONS AND PERSPECTIVES ............................................................................................................................... 98

2.6. ELECTRICAL CONTACTS ...................................................................................................................................... 99

2.6.1. INTRODUCTION ....................................................................................................................................................... 99

2.6.2. ELECTRICAL CONTACTS ............................................................................................................................................ 101

2.6.3. THE SAMPLE BACKFACE CONTACT ........................................................................................................................ 104

2.6.3.1. Experimental.............................................................................................................................................. 104

2.6.3.2. Comparison with C-V measurements ........................................................................................................ 105

2.6.3.3. Conclusions ................................................................................................................................................ 106

2.6.4. THE TIP-SAMPLE CONTACT ................................................................................................................................. 107

Contents

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2.6.5. CONCLUSIONS ....................................................................................................................................................... 113

2.7. INFLUENCE OF THE SAMPLE TOPOGRAPHY ON THE CAPACITANCE SIGNAL ..................................................... 113

2.7.1. INTRODUCTION ..................................................................................................................................................... 113

2.7.2. EXPERIMENTAL ...................................................................................................................................................... 115

2.7.3. DIRECTION OF APPROACH OF A TOPOGRAPHICAL FEATURE ....................................................................................... 117

2.7.4. SCAN DIRECTION ............................................................................................................................................. 118

2.7.5. DEFLECTION SETPOINT (DS) .............................................................................................................................. 119

2.7.6. SIZE OF THE TIP ............................................................................................................................................... 120

2.7.7. SOLUTIONS FOR DECREASING THE EFFECT OF STRAY CAPACITANCE ARISING FROM TOPOGRAPHY ............................................. 120

2.7.8. CONCLUSION .................................................................................................................................................. 122

2.8. TIP PROPERTIES ............................................................................................................................................... 122

2.8.1. INTRODUCTION ..................................................................................................................................................... 122

2.8.2. CHARACTERIZATION OF AFM TIPS WITH STANDARD AFM SAMPLE GRATINGS .................................................................... 124

2.8.3. CHARACTERIZATION OF AFM TIPS WITH SEM ............................................................................................................. 124

2.8.5. CONCLUSIONS ....................................................................................................................................................... 129

2.9. THE AFM PIEZOELECTRIC SCANNER ................................................................................................................ 129

2.9.1. INTRODUCTION ..................................................................................................................................................... 129

2.9.2. EXPERIMENTAL ...................................................................................................................................................... 132

2.9.3. CONCLUSIONS ....................................................................................................................................................... 138

2.10. CONCLUSIONS ............................................................................................................................................... 139

BIBLIOGRAPHY ....................................................................................................................................................... 140

CHAPTER 3. OXIDES CHARACTERIZATION WITH THE SCM ...................................................................................... 142

3.1 INTRODUCTION ................................................................................................................................................ 143

3.2. OXIDE DEFECTS AND THEIR INFLUENCE ON THE SCS SIGNAL ........................................................................... 143

3.2.1. MOBILE IONS ....................................................................................................................................................... 144

3.2.1.1. The nature of mobile ions .......................................................................................................................... 144

3.2.1.2 The influence of the mobile ions on the capacitive signal ......................................................................... 144

3.2.2. FIXED CHARGES ................................................................................................................................................... 145

3.2.2.1 The nature of fixed charges ........................................................................................................................ 145

3.2.2.2. The influence of the fixed charges on the capacitive signal ...................................................................... 146

3.2.3. INTERFACIAL TRAPS .............................................................................................................................................. 146

3.2.3.1. The nature of interfacial traps ................................................................................................................... 146

3.2.3.2. The influence of the interfacial traps on the capacitive signal .................................................................. 147

3.2.4. BULK OXIDE DEFECTS ............................................................................................................................................. 148

Contents

14

3.2.4.1. The nature of bulk oxide defects................................................................................................................ 148

3.2.4.2. The influence of the bulk defects on the capacitive signal ........................................................................ 148

3.3. STATE OF THE ART ........................................................................................................................................... 149

3.3.1. REQUIREMENTS FOR THE OXIDES USED WITH SCM MEASUREMENTS ................................................................................ 149

3.3.1.1 Low-temperature fabrication ..................................................................................................................... 150

3.3.1.2 Ease of formation ....................................................................................................................................... 150

3.3.1.3 Oxide reproducibility .................................................................................................................................. 150

3.3.1.4 Oxide thickness ........................................................................................................................................... 150

3.3.1.5 Oxide uniformity ......................................................................................................................................... 152

3.3.2 OXIDE PROPERTIES AND PROPERTIES OF THE SCS SIGNAL ................................................................................................ 153

3.3.2.1 The position of the flatband bias ................................................................................................................ 153

3.3.2.2 Full-width at half-maximum ....................................................................................................................... 155

3.3.2.3 Hysteresis of the signal ............................................................................................................................... 158

3.3.2.4 The amplitude of the SCS signal ................................................................................................................. 158

3.3.3 LOW-TEMPERATURE OXIDES ..................................................................................................................................... 159

3.4 OXIDES CHARACTERIZATION WITH THE SCM .................................................................................................... 161

3.4.1. OXIDE RELATED PARASITIC PHENOMENA AT THE TIP - OXIDE INTERFACE ............................................................................. 161

3.4.1.1. Oxide charging ........................................................................................................................................... 162

3.4.1.2. Anodic oxidation ........................................................................................................................................ 165

3.4.1.3. Oxide engravement ................................................................................................................................... 169

3.4.2. STUDY OF LOW-TEMPERATURE OXIDES FOR DOPANT PROFILING ...................................................................................... 170

3.4.2.1 Guidelines for a complete characterization of oxides with the SCM .......................................................... 170

3.4.2.2 Plasma oxide............................................................................................................................................... 174

3.4.2.3 Nitride ......................................................................................................................................................... 177

3.4.2.4 High-k dielectrics ........................................................................................................................................ 179

3.4.2.5 Native oxide ................................................................................................................................................ 180

3.4.2.6 UV/ozone oxide .......................................................................................................................................... 181

3.4.2.6 Summary of low-temperature oxides ......................................................................................................... 183

3.4.3 CONCLUSIONS AND PERSPECTIVES .............................................................................................................................. 183

BIBLIOGRAPHY ....................................................................................................................................................... 185

GENERAL CONCLUSION .......................................................................................................................................... 187

General introduction

General Introduction

16

The development of nanotechnologies, as well as the progress towards the limits of the

micro-electronics and nano-electronics, imposes the development of characterization methods

capable of providing reliable information with a resolution compatible with the size of the

objects to be probed. This is true for the morphological characterization (dimensional

metrology) but also for many other properties of nano-objects like the magnetic properties

(remnant magnetization) or the electrical properties (charge, potential, electric field,

resistance, capacitance...). In particular, for nano-objects involved in the nano-electronics, the

transport properties, the charge retention in e.g. nanocrystals, or the doping level are crucial

parameters which control the final properties of devices based on these objects.

Numerous characterization techniques have arisen which are indeed able to obtain sub-

micronic resolution or even atomic resolution, e.g. techniques based on the electronic

microscopy or scanning probe microscopies. Although some of them provide a true atomic

resolution (for example, it is possible to image individual silicon atoms using Scanning

Tunneling Microscopy – STM - under ultra-high vacuum, and even to check the presence of a

boron atom in its neighborhood thanks to the modification of the electronic density of states

due to its presence), they may also require complex sample preparation or heavy

instrumental setups. Yet, for a potential industrial application, the figures of merit of a

characterization technique are the reproducibility, the quantitativity, the resolution, but the

ease of use has also to be taken into account.

That's why techniques based on the atomic force microscopes (AFM) are very serious

candidates as nanoscale characterization techniques able to provide information on the

topography and on the electric properties of the surfaces with a nanometric resolution because

of their relative ease of use when implemented in air. Starting from the atomic force

microscope for the mapping of surface topography, they have evolved toward a great number

of different characterization techniques that take advantage of the precision of the positioning

of the AFM tip in all direction of space, and of the opportunity of using the conductive AFM tip

like a top electrode in order to perform techniques inspired from macroscopic setups. This is

the case for Kelvin Force Microscopy (KFM) for the measurement of surface potential or

Scanning Spreading Resistance Microscopy (SSRM) for doping profiling. For capacitance

measurements, the macroscopic setup can not be used because of signal to noise ratio

concerns but has to be adapted to reach the level of signal corresponding to the extremely

small surface of the capacitance formed from the tip/sample system.

This dissertation intends to show the performances, limits and potentialities of a

technique called Scanning Capacitance Microscopy (SCM) for the mapping of dopants at the

nanoscale and for the measurement of the properties of very thin dielectric oxides for

applications at the Metal-Oxide-Semiconductor (MOS) structure. After a short presentation of

alternative techniques based on scanning probe techniques, electronic microscopy or atom

probe tomography, some examples of dopant mapping in test samples will be presented with

the aim of underlining the satisfactory behavior of SCM to obtain qualitative images of dopants

of p-type or n-type in silicon. Samples containing quantum wells will demonstrate the

General Introduction

17

resolution of the technique and a discussion on the quantitativity and reproducibility will be

engaged, showing that the route toward fully reliable, reproducible and quantitative images is

not so easy, and that in the general case, only 'failure analysis' images can be obtained in

order e.g. to verify that the doping steps have been correctly implemented in a given region of

a sample or to distinguish between p-type and n-type regions of the sample.

This will motivate the second chapter of this dissertation, where all the experimental

parameters disturbing the SCM signal or lowering the signal to noise ratio, or leading to a

misinterpretation of the images will be reviewed and evidenced. Among them, the influence of

the laser light provided by the laser used by the AFM apparatus to measure the deflexion, the

parasitic capacitance of the sample – tip – lever – chip system and the influence of the

topography, the role of the tip-sample contact, of the measurement environment (humidity,

controlled atmosphere)... will all be addressed separately. To do so, the comparison of

Scanning Capacitance Spectroscopy (SCS) with macroscopic Capacitance-Voltage (C-V)

measurements will be a precious tool in order to better understand the role of each

parameter. The properties of the tip and the parasitic effects introduced by the piezotube used

in our experimental setup will also be taken into account. The aim of this part of the

dissertation is to provide guidelines which allow to improve the signal to noise ratio, the

reliability and the reproducibility of the measurements, and to point out the directions toward

which progress can be made to improve them significantly.

The third chapter will be an application of the previous chapters to the comparisons of

different dielectric oxides grown at low temperature in order to perform the doping mapping in

the semiconductor structures. Based on the understanding of the Scanning Capacitance

Spectroscopy developed in the second chapter, and given the constraints for doping mapping

exposed in the first, different kinds of silicon oxides or alternative oxides which could be

prepared at the laboratory will be compared and the suitability of each one for doping mapping

will be evaluated. Again, a trade off between all the constraints will have to be made and

directions will be indicated for improving the quality of oxides for such applications.

Finally, this work will be concluded by a summary of important experimental concerns

which should be solved for SCM to progress toward a better reliability, reproducibility and

quantitativity for both the mapping of dopants and the quantitative measurement of charges

in thin oxides.

Chapter 1. Scanning Capacitance Microscopy:

principle and overview of performances

Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances

19

1.1 Introduction

The measurement of the doping concentration is a key step in the fabrication process of

last generation micro and nanoelectronic devices. With the reduction of the size of the

structures, the performances of the characterization method must be enhanced to meet the

requirements of the International Technology Roadmap for Semiconductors (ITRS).

Several characterization methods, like Scanning Capacitance Microscopy (SCM), are able

to provide a 2D, or even a 3D dopant mapping in semiconductor structures. For now, none of

the specified characterization methods is able to fully achieve all of the ITRS goals in terms of

sensitivity, range, resolution, quantification and ease of characterization. Each of them has its

specific features with advantages and drawbacks.

Although the goal of this dissertation is not to provide details about each of them,

several techniques may be shortly described.

1.1.1. Secondary Ions Mass Spectrometry (SIMS)

This is a one dimensional method which is still a reference technique for doping

profiling because of its high sensitivity for dopants like boron in silicon, and its spatial

resolution which can reach 2 nm in optimal conditions. Two different techniques exist for SIMS

: so-called 'dynamic' SIMS and 'static SIMS', the former being the most often used for doping

profiling in silicon. It is based on the bombardment of the sample with an ion beam of Ar+, and

more often of O2+ or Cs+, with energies ranging from ~350 eV to 15 keV (the most often

encountered energies for an optimal depth resolution being in the 500 eV range for boron in

silicon for example). The ions emitted from the sample because of the bombardment

(secondary ions) are accelerated by an electric field, filtered with an electric and a magnetic

field and finally detected using a Faraday Cup or an electron multiplier. The intensity of a

given secondary ion (more precisely a given mass/charge ratio) is recorded as a function of

time as the surface is sputtered by the ionic bombardment. The depth scale of the resulting

depth profiles is retrieved by measuring the depth of the resulting crater with a mechanical

profilometer.

SIMS sensitivity and the depth resolution are not the same for all species. For example

both are excellent for boron in silicon, whereas a mass interference with the SiH+ ion impinges

seriously the results for Phosphorous.

In all this dissertation, test samples are characterized using SIMS which acts as a

reference technique for the concentrations and thicknesses.

1.1.2. Electron holography

This technique is based on a Transmission Electron Microscope setup. It uses a coherent

electron source divided into two different beams : one crosses the sample to be studied

(constituted of a very thin lamella), the other one serves as a reference. Both beams are


20

combined to form a hologram which can be interpreted after a Fourier Transform as a phase

and an amplitude image. The phase image is related to the type (p or n) of dopants. Very thin

samples needed for this method can be prepared using a Focused Ion Beam or by polishing.

1.1.3. Scanning Spreading Resistance Microscopy (SSRM)

SSRM is a technique very close to SCM. As the principle of Atomic Force Microscopy and

SCM will be exposed later, we will only describe the principle of SSRM.

As in SCM, the AFM tip is used as a top electrode. The tip is very stiff (40 N/m) and

coated with highly-doped poly-crystalline diamond. Pure diamond probes are also found in the

literature [1.1]. SSRM is a resistive method which measures the current flowing through the

sample when a continuous low voltage is applied. This current is proportional to the total

resistivity of the portion of sample located under the tip, and of the tip itself, including

therefore contributions from :

The tip

The contact resistance between the tip and the sample

The semiconductor under study which resistivity can be expressed as :

1

n p

ρ=q μ n+ μ p

The back-contact.

Where q is the elementary charge, n and p the mobility of the electrons and the holes

respectively and n, p the concentration of electrons and holes respectively. The resulting

signal is thus sensitive both to the concentration of and the mobility of the carriers.

For SSRM to provide a good signal, a very high pressure must be applied on the sample

surface (which justifies the use of very stiff cantilevers), which is truly scratched by the scan

of the surface. As a result, the number of scan is limited by the deterioration of the surface.

1.1.4. Tomographic Atom Probe (TAP)

One very powerful emerging method is the Tomographic Atom Probe (TAP) which is

based on a fabrication of a tip submitted to a strong electric field which allows the field

evaporation and ionization of the atoms located at the probe apex. The resulting ions are

sorted by time of fly and detected in a position sensitive detector. This allows a reconstruction

of the whole volume of the tip where the time-of-fly analysis has allowed to identify the nature

of all the atoms composing the original probe. This technique leads to very impressive results

when the samples are conductive. For semiconductors, as it is more difficult to obtain the field

evaporation, a laser beam is used to evaporate the atoms (Laser Assisted-TAP : LATAP). Test

samples similar to those used in this dissertation have been analyzed using this technique and

allow to identify strength and weaknesses at the present moment. Although the preparation of

the sample is very difficult and requires a skilled staff, delta-doped samples of boron in silicon

have been analyzed in three dimensions with a resolution comparable to SIMS [1.2]. However,


21

the level of noise is still rather high and concentrations lower than 1018 cm-3 can not be

detected yet.

1.1.5. Other techniques for dopant profiling

Some emerging techniques are based on the environmental scanning electron

microscope (ESEM). The detection of secondary electrons enables the technique to be

sensitive to the surface potential, but also to the surface band bending. Although a clear

contrast can be obtained with p-type semiconductors on test samples (one example will be

showed in the next sections), the result seems to be less convincing for n-type doping, and

the contamination of the surface seems to impinge the reproducibility of the technique.

It must also be mentioned that some techniques rely of the difference of the etch rate of

differently doped semiconductors to obtain a topographical image of the doped areas after a

wet etching using various acids.

The table below summarizes some of the features known from the literature for some of

the most often encountered techniques able to map the dopants in the semiconductors :

SCM SIMS SSRM Electron Holography

Tomographic Probe

ESEM

detection carriers atoms carriers carriers atoms carriers

Type of mapping 2D 1D 2D 2D 3D 2D

Resolution range 25 nm 1.5 nm for

boron in Si

1-2 nm with

pure

diamond

probes; 5-

10 nm with

commercial

probes

5-10 nm ~1 nm Not known

Quantification possible

Difficult Yes, if a

calibration

sample exists

of the atom

under study in

the same

matrix

yes no yes no

ease of use yes yes yes No : requires

the fabrication

of a very thin

lamella

No : requires a

very complex

sample

preparation (very

sharp tip)

no

1.2. Atomic Force Microscopy

The Atomic Force Microscope (AFM) belongs to the family of scanning probe

microscopes. It makes use of a very sharp probe to detect superficial properties of the

materials on nanometer and subnanometer scale.

The AFM probe is located at the free end of a cantilever that is attached to a piezo

scanner able to move in vertical direction, and in a horizontal plane with a sub-nanometer

precision. In some other configurations, the tip attached to the cantilever remains static and

the sample is attached to the piezotube controlling the direction in all the directions of space.

The AFM is commonly used for topography characterization in three operating modes:

non-contact, tapping, and contact AFM. As this dissertation deals only with SCM, which is

operated in contact mode, we will only describe this operating mode.


22

Changes in topography are detected in several manners. The most common way is the

use of a laser beam pointing at the reflecting cantilever rear, and being bounced off to a split

photo detector (Figure 1.2.1).

Figure 1.2.1. The Lennard Jones potential the governs the tip-surface interaction with the AFM (left). The AFM laser detection system (right)

In contact mode, the tip stays in contact with the surface at all time. The interaction

between the tip and the surface is repulsive. While scanning the tip on the surface any change

in the topography is accommodated by the probe via a feedback system. The deflexion

imposed by the topography is kept constant by the feed-back system so that the force applied

to the surface remains constant. The voltage applied on the piezotube in order to modify the

vertical position of the tip so that the deflexion remains constant represents the vertical

coordinate of the AFM image.

The AFM cantilevers used for the contact mode are less rigid (stiffness often in the

0.02-1 N/m range compared to the 40 N/m for classical high frequency tapping mode tips).

Considering that the tips are in permanent contact with the surface, they have to be more

resistant to wear than the AFM tips used with the other operation modes.

1.3. The principle of Scanning Capacitance Microscopy

SCM (Scanning Capacitance Microscopy) measures the differential capacitance dC/dV of

a metal - insulator - semiconductor structure formed by the contact between a conductive

probe and an oxidized semiconductor surface (Figure 1.3.1).


23

Figure 1.3.1. The MOS-like structure which describes SCM measurements. Here, the metal

is a representation for the AFM tip

The SCM operates in contact mode. With the SCM, the AFM is coupled to a capacitive

sensor, and, with the wear resistant, conductive tip, capacitive measurements can be

performed.

SCM operation is based on the Metal-Oxide-Semiconductor stack where the AFM tips

acts as the metal part. The Capacitance-Voltage (C-V) characteristic of this structure, as can

be measured by a macroscopic setup using large electrodes, can be found in Figure 1.3.2. But

because of the order of magnitude of the measured capacitance with the SCM because of the

very small size of the top electrode – 10-18 F, which is a million times smaller than the level of

noise (10-12 F), a high frequency resonant circuit coupled with a lock-in amplifier must be used

and only a differential capacitance can be measured.

The differential capacitance is measured as follows.

Figure 1.3.2. The operating principle of the SCM detector (Veeco Manual)

A Vdc point of operation must be chosen in the transition region of the C-V curve, where

the capacitance varies with the applied DC voltage . A Vac signal which determines a variation

of the system capacitance value is superimposed. The difference between the capacitance

values corresponding to Vpp is measured.

Ideally, for obtaining a real differential signal dC/dV, the applied Vac should be in a so-

called small signal range, with a maximal value of around 50 mV, as it is the case for


24

macroscopic measurements.

However, for practical reasons related to the noise/signal ratio, which are described in

chapter 2, much larger values have to be used in order to obtain a good SCM signal. Thus, the

measured signal is rather a ΔC/ΔVpp signal.

Further on, due to the specific design of the electronic circuit, the output signal

represents in fact a ΔC signal and it is not further divided by ΔV, which is a discrepancy with

the AFM's manual which states that a ΔC/ΔVpp is obtained as the output signal.

Figure 1.3.3. SCS curves for different Vac measured on a n-type substrate covered with a 2nm thermal oxide (left). The same SCS curves divided by the corresponding Vpp

This point can be verified as follows: first Scanning Capacitance Spectroscopy (SCS)

must be performed by applying a ramp of voltage and measuring the SCM signal for all the

values of the voltage ramp. If the applied Vac is sufficiently high, the change in capacitance ΔC

has to represent the difference between the accumulation and the depletion, that is, from a

certain Vac threshold, ΔC has to remain constant.

If ΔC reaches the difference between the capacitance in accumulation and the

capacitance in depletion, it will remain constant even though Vac increases. This means that for

higher values of Vac, the signal ΔC/ΔV should begin to decrease with the increase of the ΔV

(Vpp)

On the other hand, from the measurements in Figure 1.3.3, it can be seen that the

signal, no matter the magnitude of the Vac, monotonically increases until the saturation of the

detector. This is not a normal behavior if the SCM output signal would represent indeed

ΔC/ΔV.

This was confirmed by the Veeco company in a private communication : unlike the

name suggests for the SCM signal, ‘dC/dV’, these values do not get scaled by the ‘dV’ value,

but are the dC values observed when applying a given AC voltage: the lock-in outputs are not

divided by the dV signal. As a consequence, these signals will increase as the AC value is

increased (increased AC voltage mean that the C-V curve is sampled over a larger voltage

range), until at a certain point where the full depletion and accumulation voltage range is

reached, at which saturation will be obtained.


25

1.4. Description of the samples used in this study

During this thesis, several doping profile samples have been used for evidentiating the

SCM performances on doping profiles in terms of reproducibility, failure analysis images,

resolution, junction localization and quantification. Some of them have been grown at LETI,

Grenoble, others have been obtained from the company STMicroelectronics, Crolles.

Sample name Sample description Fabrication

Maya p p-type substrate 1015 cm-3, 6 p-type regions 250

nm wide with doping concentrations between 1017

cm-3 and 3.1019 cm-3 (Figure 1.3.5)

ST

Maya n p-type substrate 1015 cm-3, 5 n-type regions of

variable width with doping concentrations

between 2.1017 cm-3 and 2.1019 cm-3 (Figure

1.3.6)

ST

Diodes BP p-type substrate 1015 cm-3, 6 alternating p-type

and n-type regions with doping concentrations

between 1017 cm-3 and 1019 cm-3 (Figure 1.3.7)

ST

Maya 6 p-type substrate 1015 cm-3, 9 p-type regions 100

nm wide with doping concentrations between

5.1017 cm-3 and 5.1019 cm-3 (Figure 1.3.8)

LETI

Quantum wells 1 p-type substrate 1015 cm-3, 4 p-type regions of

500 nm, 50 nm, 10 nm and 3 nm wide with

doping concentrations between 1.1015 cm-3 and

3.1019 cm-3 (Figure 1.3.9)

LETI

Quantum wells 2 p-type substrate 1015 cm-3, 9 p-type regions of

480 nm, 240 nm, 120 nm, 60 nm, 30 nm, 15 nm,

and 7.5 nm wide with doping concentrations

between 1.1016 cm-3 and 3.1019 cm-3 (Figure

1.3.10)

LETI

The samples have been fabricated by RP-CVD.

The doping profile samples have been characterized by SIMS, used as reference for

comparison with the SCM doping profiles (Figures 1.4.1 - 1.4.6)

Figure 1.4.1. SIMS doping profile of Maya p sample (INL)

Figure 1.4.2. SIMS doping profile of Maya n sample (INL)


26

Figure 1.4.3. SIMS dopant profile of Diode sample (INL)

Figure 1.4.4. SIMS dopant profile of Maya 6 sample (LETI)

Figure 1.4.5. SIMS dopant profile of Quantum wells 1 (LETI)

Figure 1.4.6. SIMS dopant profile of Quantum wells 2 (LETI)

1.5. Surface preparation

1.5.1 Cleaving

The preparation of cross-sectioned silicon sample for SCM has been done by cleaving

with pliers.

A small scratch is performed with a diamond tip on the surface of the sample to be

cleaved and then the sample is cut with a pair of pliers. The first centimeters of the resulting

cross section (near the original scratch) will show a rough surface, but if the cut is done with

care, the crack will further orient itself along a crystalline plane and the rest of the cross-

section will show almost no roughness.

Several authors (as well as the user's manual from Veeco) use polishing as the next

step in order to retrieve a perfectly flat surface. Polishing allows also to stick two different

samples (including a reference sample) and to prepare them in a rigorously identical way. In

our study, considering that polishing introduces a lot of defects at the oxide-silicon interface,


27

we consider that this step is unnecessary to the cross-section sample preparation and is to be

avoided.

The cross-section sample preparation can be used for the SCM as well as for other

characterization methods such as the SSRM (Figure 1.5.1).

Figure 1.5.1. SSRM image obtained on the cross-section of the BP diode sample

For SCM qualitative characterization, the samples are put for several hours in a clean

environment such as a clean room in order that a thin native oxide grow on top on the cleaved

cross-section.

1.5.2. Polishing and beveling

We have used polishing for preparing beveled samples. On beveled samples, the doping

profiles are artificially magnified by geometrical means, which allows the characterization of

very small doping profiles.

For polishing, four different supports with different angles have been used:

Angle (degrees) amplification factor 1 / sin (θ)

5 44' 10

2 52' 20

1 9' 50

34' 100

Figure 1.5.2. Beveled surface after polishing

The most used support for the images presented in this dissertation was the one with

an angle of 5 44'. The other polishing supports usually lead to a beveled surface where the


28

doping profiles exceeded the AFM lateral range. For example, a doping profile lying on 4

microns becomes, after polishing with a support of 34' (x100), of 400 microns. The maximum

lateral AFM range is of 100 microns, if offsets are not performed.

For the polishing, a glass plate has been used with the Mecapol P300 polisher. On the

glass plate, colloidal silica (which is a colloidal suspension of a mixture of abrasive particles

dispersed throughout a chemically active liquid carrier) with a neutral pH is poured constantly

during the polishing.

The beveled sample must be oriented in the same direction as the direction of rotation

of the plate. The total time of polishing is around 2-3 minutes.

After the polishing, the sample must be immediately cleaned with a cotton swab under

deionized water (Figure 1.5.3).

Figure 1.5.3. Beveled samples cleaned only with deionized water after polishing (left) and cleaned with a cotton swab in deionized water (right)

As with the cross-sample preparation, the beveled samples can also be used by other

characterization methods, such as SSRM or SEM (Figure 1.5.4)

Figure 1.5.4. ESEM image of BP-diodes sample (Gilbert THOLLET - MATEIS). Beveled sample manufactured at INL

It must be specified that the two preparation methods presented above cannot be used

for samples containing localized microstructures such as transistors. In order to be able to

cleave or polish the samples exactly in the spot where the microstructure is located, a

dedicated industrial cleaving system should be used.


29

1.6. Qualitative characterization of dopant profiles

As presented in the introduction, failure analysis images constitute the primarily

application for the SCM. This means that the doping implantation of micronic and nanometric

structures can be rapidly verified by capacitance images with the SCM.

On the SCM images taken on the maya p, maya n and Diodes-BP samples, the

properties of the SCM measurements are evidentiated (Figures 1.6.1-1.6.3)

Figure 1.6.1 SCM image (left) and cross-section (right) of Staircase sample: maya n

Figure 1.6.2. SCM image (left) and cross-section (right) of Staircase sample: maya p

Figure 1.6.3. SCM image (left) and cross-section (right) of Diodes-BP

On the SCM images, a good contrast difference between the different doping regions is

obtained. The lightly doped regions lead to a higher capacitance signal than the highly doped

regions, according to the theory. The different doping regions are clearly defined and

correlated with the SIMS profiles in terms of geometry (width).

For the maya p sample, the SCM signal is positive and for maya n sample the SCM

signal is negative. SCM is able to make the difference between p-type and n-type regions due

to the phase of the SCM signal. This distinction is even clearer on the Diodes-BP sample,

where there is an alternance between the positive and the negative signals, in agreement with

the type of the doping regions.

Even if contrast reversal problems may appear because of the loss of the tip coating,

the wrong choice of Vdc point of operation or a high density of density of states at the oxide-


30

silicon interface, there will still be a difference in contrast between the different doped regions

and SCM qualitative images (failure analysis images) will still be able to evidentiate if the

geometry of the different doped regions is in agreement with the intended design of the

sample.

1.7. Junction localization

The control of the junction depth is a major issue in the processing of modern

semiconductor devices. The junction depths are predicted to be as low as 10 nm for future

deep sub-micron devices. The electrical characteristics of the transistor are affected by minor

changes of the junction position. Therefore, ITRS considers that an exact delineation of the

junction is critical.

1.7.1 Terminology

In semiconductor physics, the junction can be defined in several ways:

The metallurgical junction MJ can be defined as the position where the acceptor

concentration Na equals the donor concentration Nd. A plot of net doping concentration as a

function of position – Nd-Na = f(x) - is referred to as the doping profile.

The metallurgical junction is typically measured with SIMS, since this technique is only

sensitive to atoms.

The electrical junction EJ is defined as the position within the depletion region where

the active carrier concentrations of both types (holes and electrons) are equal (1010 cm-3).

The electrical junction can be measured with SRP, SSRM, and SCM which are

techniques sensitive to the carrier concentration. Further on, we will discuss how to

determine the location of electrical junctions with SCM.

There is no need to measure the MJ and the EJ simultaneously. One can calculate the

carrier distribution and implicitly the position of the electrical junction from the doping profile

and vice-versa.

Here is the algorithm of calculus for obtaining the doping profile when the carrier

distribution is known

At thermal equilibrium, there are no electron and no hole currents:

The density of electron current can be expressed as follows:

jn is equal to 0 at thermal equilibrium


31

Moreover:

Thus, it comes:

The charge density can be expressed as:

Moreover, it can also be defined as:

Thus:

Finally it comes:

The last equation allows us to find out analytically the doping profile from the carrier

distribution.

Here is the algorithm of calculus for obtaining the carrier distribution when the

doping profile is known.


32

Starting from a doping profile Nd – Na, the equation from above allows the calculus of

the potential and consequently, the carrier distribution n and p.

This equation doesn't generally have an analytical solution and has to be solved

numerically.

As an alternative to a home-made program which solves numerically the equation from

above, one may use a commercially TCAD program such as ISE (Dessis).

By introducing Na, Nd and the position of the metallurgical junction as input variables,

Dessis is able to calculate numerically the position of the electrical junction and the boundaries

of the depletion region, as in example from the Figure 1.7.1.

Figure 1.7.1. Simulation of the position of the electrical junction from the position of the metallurgical junction with TCAD-ISE

Although the concepts of metallurgical junction and electrical junction are very precise,

very often a junction is simply defined in terms of the depletion zone (the junction is within

the limits of the depletion zone).

The depletion region or the space charge region is the near-vicinity of the


33

metallurgical junction where there is a significant non-zero charge and the carrier

concentrations in the region are greatly reduced or depleted. It should also be mentioned that

the build-up charge and the associated electric field continues until the diffusion of carriers

across the junction is precisely balanced by the carrier drift. [1.3]

For a non-biased junction, the width of the depletion region can be calculated from the

formula:

where:

For a MOS structure:

1.7.2. State of the art

i) When imaging with the SCM across pn junctions, Williams [1.4] was the first to observe

that the SCM signal undergo a 180° phase shift at the passage between the n-type and p-type

regions. So, it has been concluded that the junction location may be estimated simply by

monitoring the phase of the SCM signal relative to the drive signal.

ii) One of the factors that may affect the measurements of the EJ position is the applied

bias.

Kopanski (NIST) [1.5] and Kleiman (Bell Laboratories) [1.6] were the first to observe

that the position of the pn junction shifted with the applied bias. At different biases, the SCM

signal passes through zero at different positions.

From the 2D SCM images on transistors, it can be seen the change in the position of

the junction with the applied bias.


34

Figure 1.7.2. image SCM of pn junction [1.3] (left). Image SCM of pn-junction in transistor [1.4]

This effect of the change of junction position with the applied bias has been confirmed

theoretically through simulations by Bell Laboratories [1.6].

For the simulations, Kleiman used a simplified 2D simulation model of a junction, with

the AFM tip as a plane electrode (Figure 1.7.3):

Figure 1.7.3. Simulation of the change of the position of the electrical junction with the applied bias [1.4]

The simulated pn-junction has the following parameters:

• 5.1017 at/cm-3 p-type (boron) substrate

• 3.1018 at/cm-3 n-type (phosphorus)

• a 2 nm oxide layer

• a 10 nm electrode (replacing the AFM tip)

• a bottom electrode

The high frequency capacitance data is calculated using steady-state small signal

analysis.

The top electrode is moved across the sample in 10nm steps and C-V and dC/dV -V

curves are generated for each electrode position across the sample and for different biases.

The graph dC/dV versus the electrode position across the junction and the applied bias

voltage Vb: dC/dV = f(x, Vdc) is plotted.

From the simulation, the following observations can be made:

• for Vdc = -1.5V, the p-type response is pushed roughly at the edge of the n-type depletion

boundary

• for Vdc = 0.5V, the n-type response has been pushed at the edge of the p-type depletion


35

boundary.

• the position of the sign changing gradually moves from one side of the physical junction to

the other, as a function of Vdc.

In the junction region, 0.99um < x < 1.05um, the sign of the dC/dV changes as Vb

increases from -2 V to +1 V.

iii) Further on, analyzing the data, it is O'Malley [1.8], [1.9] and Kopanski [1.10] who

proposed the use of the so called quiescent-point of operation.

The quiescent bias was defined as the bias where the apparent junction location would

coincide with the electrical junction. O'Malley [1.8] simulated pn-junction has the following

parameters:

• 1.1017 at/cm-3 p-type (boron) substrate

• 3.1017 at/cm-3 n-type (arsenic)

• a 2 nm oxide layer

• a 10 nm electrode (replacing the AFM tip)

• a bottom electrode

The 10 nm electrode was moved across the sample in 10 nm steps and C-V high

frequency curves and dC/dV-V have been generated for each electrode position across the

sample.

The Figure 1.7.4 shows the simulated data for an electrode location above the built-in

depletion region of the pn junction.

Figure 1.7.4. Junction simulation concerning the quiescent point of operation

The contour plot shows that within the depletion region there is an availability of both

carrier types. The applied bias Vdc alters the electric fields within the depletion region allowing

carriers to flow in from the adjacent n-type or p-type regions. The region under the tip will

appear to be p-type for a negative Vdc or n-type for a positive Vdc. This carrier movement

results in an extension of the p-type or n-type dC/dV-V response into the depletion region.

In conclusion, O'Malley has determined from simulations that the cause of the apparent

movement of the junction position with the applied bias is the flow of carriers from the

adjacent p-type and n-type regions under the influence of the applied bias.

The natural conclusion is that the applied bias must be chosen so that it doesn't attract


36

free carriers from neighbor regions.

For Vdc = -0.6V , it can be observed that the pn-junction appears unperturbed. Since in

the depletion region the semiconductor appears intrinsic (n=p=1010cm-3), a Vdc chosen at the

silicon mid-gap will cause no perturbations.

In practice, there are a series of supplementary factors which affect the choice of the

bias, such as the workfunction of the tip, the quality of the oxide. That is why the applied bias

can be chosen as an average between the n-type and the p-type peaks in dC/dV. This average

bias it is called the quiescent bias.

The obvious advantage of calculating the quiescent bias as the average between the n-type

and the p-type dC/dV peaks is that, this way:

• time-consuming simulations can be avoided. In addition

• the inaccuracies introduced by the simulations related to the tip modelization or the quality

of the oxide can be avoided.

Kopanski [1.12] confirms the accuracy of the formula only for symmetrical step

junctions. Only for symmetrical step junctions, the Fermi levels are equally distanced from the

intrinsic Fermi level and the bias voltage is midway between the voltages that produce the

peak SCM response on the p-type side and on the n-type side.

iv) The method of the most symmetrical C-V curve (Hall Edwards)

Edward [1.4] has constructed a model for explaining why the pn-junction position

moves with the applied voltage and how one can determine the real position of a pn-junction.

When the tip is above the depletion region, the applied bias Vdc attracts electrons or

holes in the depletion region from the p-region or n-region (carriers which overcome the

electrostatic field from the depletion region due to the net charge density from the positive

and negative P and B ions). These carriers attracted in the depletion region change the

contrast of the image, the carrier concentration under the tip being altered.

The only situation when the tip does not attract carriers from either side of the junction

is when the tip bias is equal to Vfl (flatband bias) for the depletion region.


37

Figure 1.7.5. Hal Edwards [1.4] simulations explaining the change of the junction's

position with the applied bias

Also, Edwards observed that the C-V characteristic at the EJ is a symmetrical curve. For

a tip bias higher or lower than Vfl, the system is in accumulation due to the carriers attracted

under the tip from the neighboring regions.

Based on this model, the location of the EJ can be determined by analyzing the

symmetry of the SCS curves in the vicinity of the junction.

The procedure Edwards proposes for finding the true position of the junction is as

follows:

• SCM images at different voltages must be obtained.

It is important to use an alternative sequence of bias voltages, in order not to charge or

deteriorate the oxide and obtain a false signal. Duhayon recommends (reference) that

between each applied bias to go back to Vdc = 0V and verify whether the signal corresponding

to this bias has changed or not. Only if the signal did not change, the previous measure has to

be taken into consideration.

• The SCM images at different biases are turned into dC/dV-V characteristics

•The dC/dV-V curves are numerically integrated to obtain C(V) curves. This results in a series

of C(V) curves, one for each pixel in the image.

•The pixel color is given by the voltage value of the minimum in the C(V) curve. For the p-

type Si, the minimum of C(V) is at the maximum voltage and the pixel value is white.

For the n-type Si, the minimum of C(V) is at the minimum voltage and the pixel value

is black.

The most symmetrical C-V curves should be located at the EJ.

v). The method of node accumulation (M.Stangoni)

Stangoni [1.10] proposes an alternative for the delineation of pn-junctions in which she

combines elements from the two previous methods: the SCS method for finding the most


38

symmetrical C-V curve, the dC/dV-x graph method to evidentiate the accumulation of the

nodes in the region of the pn-junction (a node being the location where the SCM signal crosses

the zero value)

The proposed procedure consists of two phases.

•1. The dC/dV-x signal is acquired across the junction at different tip biases.

•2. the position of the nodes P of the dC/dV-x curves are plotted as a function of the applied

bias Vdc.

As soon as the probe approaches the location where the CV curves exhibit the best

symmetry (which is at the electrical junction position in the ideal case), the PV curve (the

node curve versus the applied bias) shows a decrease in the slope (the P points accumulate).

Once the tip leaves the location of the best symmetry, the slope increases again.

Figure 1.7.6. dC/dV-x; P-V method of Stangoni for the determination of the junction's position

Finding the position of the EJ with this procedure means finding the interval in the P-V

space where the nodes accumulate. [1.19]

vi) Measurements on beveled samples

Stangoni [1.17] and Duhayon [1.18] suggested that one way to reduce the error in

junction delineation, is the use of samples beveled at very sharp aperture angles. The

principle behind this technique is that the uncertainty in the delineation of the electrical

junctions is the same in cross-sectioned and in beveled samples. Thus, when the distances

measured in beveled samples are scaled back to the cross-section case, the uncertainty in the

delineation of the electrical junction is divided by the geometrical magnification factor, leading

to an error of just some hundreds of angstroms.

However, this solution has also its drawbacks. Polishing will create more interface

states than usual that will distort the position of the electrical junction. Further more, beveling

of samples results in the carriers spilling effects (a different distribution of the carriers due to

different geometrical parameters), which produces the characteristic distortion of the junction

close to the surface, and thus to a systematic error in the location of the electrical junction.


39

1.7.3. Comments on junction characterization

The precision of the junction delineation depends of the type of pn-junction to be

analyzed:

• symmetrical or asymmetrical pn-junctions.

• abrupt or linearly-graded pn-junctions

• low-doped or high-doped junctions

as well as different combinations between these types.

The EJ as viewed by SCM will be always found in boundaries of the pn depletion region.

For abrupt junctions, the width of the depletion region varies as follows:

[1.1]

Figure 1.7.7. The width of the depletion region for different concentration of the p-type and n-type regions

For linear graded junctions, the width of the depletion region varies as follows:


40

Figure 1.7.8. The width of the depletion region for different doping concentrations

From the figure, it is clear that a better junction delineation will be obtained in the case

of highly doped abrupt junction. In this case, the depletion zone is much smaller and the

apparent position of the electrical junction cannot vary much.

Also, at higher doping concentration, the SCM signal is less sensitive to the

experimental conditions. The laser light, as well as oxide defaults have a smaller influence.

Each technique used for the location of a pn-junction has its advantages and

disadvantages. The latest techniques are more accurate but more complicated and some of

them time consuming. The earlier techniques are not so precise for most cases but are very

simple and quick. Each technique can be useful in certain cases, depending on the type of pn-

junction to be analyzed. In the case of high doped junctions, simpler techniques could be used

with a good precision. In the case of low-doped junctions, with a several microns depletion

region, more sophisticated techniques should be used.

When imaging with the SCM across pn junctions, the SCM signal was found to undergo

a 180° phase shift at the passage between the n-type and p-type regions. One way to

determine the position of the junction is by looking for the spot where the SCM signal passes

from positive to negative (Figure 1.7.9)

Even simpler, the junction location may be estimated simply by monitoring the phase

of the SCM signal relative to the drive signal. The phase signal contains only the information

related to the junction, without the unnecessary information (in this case) of the doping

concentration. The figure below shows line scans extracted from both the n-regions and the p-

regions and the change of sign in the SCM signal cant be clearly seen at the passage between

different regions.


41

Figure 1.7.9. SCM signal for the BP-diode sample (Vac = 500 mV, Vdc= 0 V).

For gradual junction, the SCM signal is presented in Figure 1.3.28. As expected from

the previous calculus (Figure 1.7.8), the junction localization is much clearer for abrupt, highly

doped junctions (Figure 1.7.10).

Figure 1.7.10. SCM signal for two different gradual doping profiles (very abrupt - black signal; less abrupt

- red signal)

Tests for the junction localization from the phase shift between the p-type and the n-

type regions have also been made on beveled samples (Figure 1.7.11).

Figure 1.7.11. SCM image (left) and cross-section (right) on beveled (x10) samples.


42

On the SCM images, especially on the image of the phase-1D (Figure 1.7.12), it can be

seen that the phase for the n-type dopants, although it is much smaller, it doesn't become

negative anymore, which can be seen as a consequence of the interface states created by

polishing.

The effect has also been encountered when recording SCS signals on M.O.S structures

with plasma oxides and nitrides as gate oxide (Chapter 3). It seems that a large density of

interface states affects the phase of signal, which can affect the accuracy of junction

localization.

Figure 1.7.12. SCM phase image (left) and cross-section (right) on beveled samples.

The other methods of junction delineation presented in the literature have also

been tested.

Here are, for example, attempts for junction delineation by the method of the most

symmetrical C-V curve (Hall Edwards) on the Diodes BP sample.

First, we have done simulations with DESSIS, that we present here, that confirm

Edward's theory regarding the shape of the C-V characteristic at the EJ (Figure 1.7.13)

Figure 1.7.13. Dessis simulation of the capacitive signal at the electrical junction

Further on, we have tried to obtain experimentally symmetrical SCM curves at a

junction's location


43

The work procedure was as follows:

• SCM images have been recorded at different biases: 0V, -0.2V, 0.2V, -0.4V, 0.4V.... -2.8V,

2.8V, -3.0V, 3.0V (Vac = 600 mV; Vdc=-3V;+3V with a 0.2V step)

• a SCS curve is constructed by taking into account all the measurements from a column of

the SCM image. The first SCS curve is formed by taking into consideration all the

measurements from the first column of the SCM image.

• 512 SCS curves have been obtained for each of 512 columns.

• the SCS curves have been analyzed in terms of symmetry. The most symmetrical SCS curve

should correspond to the position of the junction.

Figure 1.7.14. Diagram describing the work procedure for the junction delineation according with Hall Edwards procedure.

A representative SCS signal at the junction, obtained with the above procedure, is

presented in the figure 1.7.15:

Figure 1.7.15. Experimental SCS signal obtained for the junction delineation according with Hall Edwards procedure.

At the EJ, the curves should become more and more symmetrical. However, the

experimental signal is not at all similar to the theory. None of the SCS signal doesn't approach

the symmetry of the theoretical signal.

For understanding why are such important differences between the theoretical and the

experimental signals, we have verified the Hall Edward's method for the reconstruction of the

SCS signal on a substrate.


44

Figure 1.7.16. Comparison between the an SCS signal measured on a native oxide on top of a low-doped 1015 cm-3 substrate, Vac = 1 V (left) and an SCS signal reconstructed with the Hall Edward's method on the same sample, Vac

= 400 mV (right).

The reconstructed SCS signal doesn't match at all the recorded SCS signal. An

explanation for this mismatch is given in chapter 3, section 3.4.1.

After the beginning of the study of junction delineation par SCM, it soon become

apparent that the current experimental setup is not appropriate for such a high precision task.

The depletion zone is very sensitive to carrier photo-generation by the laser of the AFM

system.

High applied biases can attract carriers from the both sides of junction into the

depletion zone, changing the apparent position of the electrical junction. Also, high bias

measurements with a sharp tip, on a very thin, low-quality oxide, can lead to current

tunneling by hot electrons.

The polarization of the water layer always found on a hydrophilic oxide exposed to the

atmospheric conditions, under bias conditions, change the position of the flatband bias,

rending the measurements irreproducible from one measurement to another. Further more,

such humidity under bias conditions favorize the oxidation of the surface. Very close SCS

signals must be recorded in the junction region in order to be able to localize with precision

the junction's position, but the very first measurement could determine surface oxidation,

distorting the rest of the measurements.

Numerous oxide defaults of a low-quality oxide can also change the apparent position

of an electrical junction.

These observations underline the need for a better control of experimental parameters

that affect the SCS signal. This will be the goal of the next chapters.

1.8. Resolution

The spatial resolution with the SCM describes the ability of the SCM to resolve detail in


45

the doping profile that is being imaged.

In the literature [1.17], [1.21], the spatial resolution with the SCM is related to a series

of factors, among which the most important role is played by the size of the tip. The spatial

resolution can be artificially increased by imaging on beveled samples.

By comparison between the SIMS profiles and the SCM profiles, it can be observed

that, for maya p and maya n staircase samples, there is an noticeable difference between the

two. With the SCM profiles, the doping steps are rounded at the edges, which does not

correspond to the reality.

By comparing a maya p doping profile obtained on the cross-section of the sample with

a maya p profile obtained on a beveled sampled and reconstructed, it can be observed the

same difference (Figure 1.8.1).

Figure 1.8.1. Comparison between the maya p doping profile on the cross-section of the sample and on the reconstructed beveled sample

The SCM signal measured on the cross-section of the sample is rounded towards the

edges.

For doping profile samples with doping regions even narrower, the steps cannot be

seen at all (Figure1.8.2)

Figure 1.8.2. SCM signals on the cross-section of maya 6 sample with a diamond tip (left) and with a PtIr tip (right). The corresponding SIMS profile is presented in the Figure 1.3.8)

On the SCM profiles of MAYA 6 sample, which consists of steps of ~100 nm width , the

9 steps of different doping concentrations cannot be seen at all. If the SCM profile recorded

with a larger diamond tip is practically a Gaussian, on the SCM profile recorded with a smaller

PtIr tip, the steps can be barely guessed by small changes in the slope of the signal.


46

Comparing the two SCM signals, it can be concluded that the size of the tip influences the

resolution of the SCM.

The same conclusion results from the SCM measurements performed on quantum wells.

Figure 1.8.4. SCM signals on the cross-section of quantum wells sample with a diamond tip (left) and with a PtIr tip (right). The corresponding SIMS profile is presented in the Figure 1.3.10)

On the SCM profile recorded with the larger diamond tip, the seventh quantum well of

7.5 nm cannot be seen. However, this quantum well is clearly seen on the SCM image

recorded with the smaller PtIr tip.

Different solutions have been tested in order to achieve a better resolution: the use of

highly doped silicon tips (without coating) or the decrease of the tip size of diamond coated

tips by FIB engravement (Figure 1.8.5).

Figure 1.8.4. Diamond coated tip engraved by FIB at HELIOS Nanolab by ing. Armel Descamps

For now, such tests didn't give better results. The conductive highly doped silicon tips

are too fragile and they break or change their shape very easily with the SCM, in contact


47

mode.

The spiral engravement of the diamond coated tips resulted in a complete loss of

capacitive signal, probably because of a discontinuity of the conductive diamond coating.

We consider that achieving a better resolution with the SCM is related to the

manufacturing of smaller, conductive and resistant to wear AFM tips.

1.9. Dopant profile quantification

With the SCM, the capacitance detector output is in volts. The output value is

proportional to the doping concentration, but it only represents a voltage, not a capacitance.

The detector presents a series of operational amplifiers with unknown amplification constants

that render the output signal qualitative.

All the approaches towards the quantification of the doping profiles are based on a

semi-quantification method: recording the amplitude of the signal for a known doping

concentration (usually the substrate of the sample) and then determining the concentration for

the rest of the doping regions by comparing the amplitudes of the capacitance signal for these

regions with the amplitude of the signal for the reference region.

In the literature several attempts to established a working procedure for the

quantification of the doping profiles have been made.

Given that no home-made simulations have been made and the quantification attempts

did not lead to any results, we will not enter in the theory of the quantification of the doping

profiles.

Extensive work on this subject has been done by authors like J.J.Kopanski [1.19] and

L.Ciampolini [1.20]. Detailed explanations concerning the principles and the models used for

the simulations can be found in the manuals that accompany their software.

The attempts to quantify doping profiles have been done by using the software

developed by NIST - Fastc2d, in order to quantify the doping profiles for the maya p and the

maya n samples.

The working procedure has been as follows:

1. We have recorded an SCM image of the doping profile.

2. We have supposed known the substrate concentration and we have tried to obtain with

Fastc2d the doping concentration for the rest of the doping profile.

3. We have introduced as input values with Fastc2d the required parameters:

• the type of the doping profile;

• the amplitude of the output signal for all the doping regions;

• the biases that establish the point of operation (the ac bias voltage Vac, the dc bias voltage

Vdc and the sensor high-frequency voltage Vhf);

• the estimated oxide thickness (for the native oxides, the estimated thickness is around 1.2

nm - Chapter 3, Section 3.3.1.4 Oxide thickness);

• the estimated radius of the tip, base on the specifications given by the diamond tips


48

manufacturers.

4. We have compared the obtained concentrations with the doping concentrations of the SIMS

profiles.

The results have been off with almost a decade. Such a difference between the real

doping concentrations and the calculated doping concentrations may have several reasons:

Many of the input values of the required parameters for the simulation cannot be

known at this moment. Such parameters are:

• the tip radius. For the simulation has been used the value of the tip radius specified by the

manufacturers. However, ulterior SEM images of the tips (Chapter 2, Section 2.8.3

Characterization of AFM tips with SEM) proved that there may be important differences

between the tip radius specified by the manufacturers (approximatevely 50 nm for the

diamond tips) and the actually radius of the tips (up to 250 nm).

• the oxide thickness. The oxide thickness used with Fastc2d constitute only an

approximation based on previous work found in the literature (Chapter 3, Section 3.3.1.4

Oxide thickness). We had no means to measure the actual native oxide thickness. Attempt for

measuring the native oxide thickness have been made with C-V measurements. However,

these attempts have been unsuccessful. It is known that the native oxide is too thin and of too

low a quality so that interpretable C-V curves be obtained and the oxide thickness calculated

from the value of the capacitance in accumulation. The oxide thinness and its low quality lead

to strong tunneling currents which conduct to an unstable signal of the capacitance signal in

accumulation with C-V measurements. C-V and I-V measurements must be performed on the

native oxides and laborious simulations must be done in order to obtain the native oxide

thickness.

In addition, it must not be forget that the actual native oxide used with the doping

profiles is found on the cross-section of the doping profile samples, where it is not trivial to

deposit electrodes and perform C-V measurements.

• the value of the sensor high-frequency voltage Vhf is not known. The SCM electronics is a

black box and finding the value of the high-frequency voltage of the detector is not trivial.

• during the quantification attempts, a high instability of the SCM signal in terms of amplitude,

FWHM, position of the peak of the SCS signal and shape of the SCS signal has been observed

(Figure 1.9.1)


49

Figure 1.9.1 SCS signals on a 5 nm thermal oxide recorded with the same diamond coated tip, at Vac = 500 mV

Such variations are met on the same sample, using the same tip and the same

operation point. It became obvious that, if any quantification attempt is to be made, the

reproducibility of the SCM signal must be first insured.

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semiconductor probe tips', Appl. Phys.Lett, vol.74, no.24, 1999

[1.10] M. Stangoni, M. Ciappa, W. Fichtner, 'Accuracy of scanning capacitance

microscopy for the delineation of electrical junctions', J. Vac. Sci. Technol. B 22, no.1, Jan/Feb

2004

[1.11] J. J. Kopanski, J. F. Marchiando, D. W. Berning, R. Alvis, H. E. Smith, 'Scanning

capacitance microscopy measurement of two-dimensional dopant profiles across junctions', J.

Vac. Sci. Technol. B 16, no.1, 1998

[1.12] J. J. Kopanski, J. F. Marchiando, B. G. Rennex, 'Carrier concentration

dependence of the scanning capacitance microscopy signal in the vicinity of p–n junctions', J.

Vac. Sci. Technol. B 18, no.1, 2000

[1.13] C. J. Kang, C. K. Kim, J. D. Lera, Y. Kuka, K. M. Mang, J. G. Lee, K. S. Suh, C. C.

Williams, 'Depth dependent carrier density profile by scanning capacitance microscopy', Appl.

Phys. Lett. vo.71, no.11, 1997

[1.14] N. Duhayon, T. Clarysse, P. Eyben, W. Vandervorst, L. Hellemans, 'Detailed

study of scanning capacitance microscopy on cross-sectional and beveled junctions', J. Vac.


[1.15] T. Clarysse, P. Eyben, N. Duhayon, M. W. Xu, W. Vandervorst, 'Carrier spilling

revisited: On-bevel junction behavior of different electrical depth profiling techniques', J. Vac.


[1.16] Hal Edwards, Vladimir A. Ukraintsev, Richard San Martin, F. Scott Johnson,Philip

Menz, Shawn Walsh, Stan Ashburn, K. Scott Wills, Ken Harvey, Mi-Chang Chang, 'pn-junction

delineation in Si devices using scanning capacitance spectroscopy', Journal of Applied Physics,

vol.87, no.3, 2000

[1.17] N. Duhayon 'Experimental study and optimization of scanning capacitance

microscopy for two-dimensional carrier profiling of submicron semiconductor devices', thesis,

2006

[1.18] M. Stangoni, 'Scanning Probe Techniques for Dopant Profile Characterization',

thesis, 2005

[1.19] J. J. Kopanski, J. F. Marchiando, B. G. Rennex, 'FASTC2D Version 1: Software for

Extracting Two-Dimensional Carrier Profiles From Scanning Capacitance Microscope Images of

Doped Silicon'

[1.20] L. Ciampollini, 'Scanning Capacitance Microscope Imaging and Modelling', Series

in Microelectronics, vol.130, Hartung Gorre

[1.21] F. Giannazzo, D. Goghero, V. Raineri, S. Mirabella, F. Priolo, 'Scanning

capacitance microscopy on ultranarrow doping profiles in Si', Applied Physics Letters, vol.83,

no.13, 2003

Chapter 2. Reproducibility problems with the SCM.

Optimization of the experimental conditions for

SCM operation

Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation

52

2.1. Introduction

All the measurable parameters with the SCM have an appreciable variation from a

measurement to another (Figure 2.1.1).

The position of the maximum of the SCS signal varies within several hundreds of

milivolts from one measurement to another, on the same sample. Also, sometimes, there is an

important shift between the trace and the retrace of the SCS curves.

The amplitude of the SCS signal, used for the quantification of the doping

concentration, also shows variations between 10% of the signal until 1000%!

The shape of the SCS signal has a very strange behavior. The SCS signal presents very

rarely only one maximum. In most of the cases, two maxima, one positive and one negative,

are present. Sometimes, the SCS signal presents 3 or more maxima (see for example Figure

2.4.13).

Figure 2.1.1. Consecutive SCS signals recorded with the a same diamond coated tip, on the surface of a 5 nm thermal oxide, on

different regions of the sample

All these facts made us wonder whether the capacitance measurements performed with

the AFM are reproducible or not.

Different suppositions have been proposed to explain these measurements. Among

them, we enumerate:

The uniformity of the samples.

The SCM is a nanometric characterization method. It is quite possible that the oxide

properties vary from one spot to another and, in consequence, the parameters of the SCS

signal vary from one spot to another.

It is possible that the oxide doesn't present the same thickness all over the surface of

the sample. Even a variation of a few angstroms of the oxide thickness along the sample

would determine an important variation of the amplitude of the SCS signal. Also, it is possible

that the oxide defects not to be uniformly distributed, which would determine shifts of various

magnitudes of the SCS curve.


53

The quality and the thickness of the oxide.

Thin oxides often present lots of defects.

The thinness of the oxide means that tunneling currents are possible, which would have

as consequence an irregular shape of the SCS curve. The oxide defects could further favorize

the presence of tunneling currents of different magnitudes.

The variation of the size of the tip.

The AFM tip represents the gate electrode for the SCM measurements. It is known that

the tip dimensions are slightly different from tip to tip. Also, the tip can deteriorate during the

scans, changing shape and increasing its radius and/or losing its coating. Given that the

capacitance signal is proportional to the dimensions of the gate electrode, it seems reasonable

to consider that tips of different sizes, as well as the wear of the tip, may determine a

fluctuation of the amplitude of the SCS signal.

For verifying the above hypothesis, a 5 nm thermal oxide had been characterized with

an impedance analyzer (section 2.3) and from the C-V curve (Figure 2.1.2) its properties

have been calculated.

Figure 2.1.2 CV measurements performed on a 5nm thermal

oxide, at various frequencies.

From the C-V measurements it can be deduced the good quality of this oxide:

the measurements are reproducible. Measurements have been performed on dozens of

electrodes. Also, measurements have been performed with three different impedance

analyzers. Each and every time, the same results have been obtained. The

reproducibility of the measurements gives an indication about the uniformity of the

oxide properties.

the C-V curves don't present a hysteresis between the trace and the retrace at room

temperature, so there are very few mobile charges in the volume of the oxide.

the value of the capacitance signal in accumulation is in correlation with the oxide

thickness measured by TEM. This means there are no tunneling currents.

there is no increase of the C-V signal in inversion which indicates there are no sources

of minority carriers at the semiconductor-oxide interface.


54

the shape of the C-V curve varies very little with the frequency in depletion, which

indicates that there are few interface states at the semiconductor-oxide interface.

Although the initial hypothesis - concerning the quality of the oxides, the uniformity of

the oxides in terms of thickness and defects repartition, the different sizes of the tips, the

wear of the tips - maybe correct, it became obvious that there may exist other experimental

problems yet to be discovered.

It is also became obvious that a standard reproducible characterization method is

needed in order to understand the characterization challenges faced with the SCM. The C-V

measurements with an impedance analyzer, which provide very accurate and reproducible

results at micro scale and which represent the microscopic counterpart of the capacitive

measurements at nanoscale, is a good candidate.

This chapter contains the following main topics:

- a summary of the experimental conditions with the SCM that are discussed in the literature.

- a general presentation of C-V measurements with impedance analyzers, with the

evidentiation of the similarities and the differences between C-V measurements and SCM

measurements.

- a discussion concerning the parasitic factors that affect the reproducibility of SCM

measurements from the same perspective of comparison with the C-V measurements.

2.2. State of the art

In the literature, the study of the experimental setup concerning SCM is rather scarce.

We consider this to be the main reason for which SCM, after more than 15 years from its

development, to be still in the research and development stage.

2.2.1. Laser light

One of the main concerns regarding the SCM measurements, mentioned in the

literature is the AFM laser effect on the SCM signal.

The influence of the AFM laser on the SCM signal is studied or mentioned in several

publications.

As far as 1989, Williams [2.1] mentions the effects of the laser light, stating that when

a 1 mW laser beam is focused onto the tip and sample, both the amplitude of the capacitance

signal and the apparent location of the lateral depletion edge of a pn junction are significantly

modified by the light. He is also the first one to propose that the laser light may be seen not

only as a source of distortion of the capacitance signal, but it can be used to measure optical

related phenomena into semiconductors, such as carrier generation and recombination rates.

In [2.2], Kopanski et al. show that a significant percentage of the incident laser light on

the cantilever can spill over the cantilever edges. This would result in a decrease of the SCM

signal by comparison with the signal obtained under normal operating conditions (dark


55

conditions), which can lead to misinterpretations of the carrier concentration.

Kopanski also provides a solution for avoiding the laser light which, until today,

represents in our opinion, the only practical solution to avoid the influence of laser light upon

the SCM measurements. Kopanski proposes to back the AFM laser as far as possible away

from the tip, by placing it towards the middle of the cantilever. Thus, on a 200 microns

cantilever, the laser can be placed as far as 100 microns from the tip.

In [2.3], Buh et al. evidentiate the effect of laser light on the SCM signal in the case of

a pn junction sample and on the SCS signal (Figure 2.2.1).

Figure 2.2.1.a Line profiles of the SCM signal on

pn junctions with the laser switched on and the

laser switched off [2.3]

Figure 2.2.1.b. i) SCS signals on a MOS like

structure, with the laser on and the laser

switched off, and ii) the numerical integration

of these curves [2.3]

Besides the optical carrier generation, Buh mentions other effects of the laser light,

such as the decrease of the time constant of the minority-carrier generation in the inversion

layer and a decrease of the surface potential, with a corresponding increase of the capacitance

at strong inversion.

Buh is the first one to try and calculate the optically generated excess carriers into

silicon for the case of a commercial AFM detection system which, typically, has a power of 1

mW and a wavelength of 670 nm. According to him, the excess carrier density is estimated to

be around 1017 cm-3.

As a solution for removing the laser light from SCM and SCS measurements, Buh

proposes the modification of a commercial AFM by installing a switch which can turn off the

laser during SCM measurements.

Buh et al continues the study of the influence of the AFM laser in [2.4]. The light

transmission coefficient through the cantilever is studied. Also, an application of the laser light

with the SCM is proposed. Buh suggests that the laser can be used as an active component for

light pumping, for the SCM to measure optical properties of a semiconductor. The effective

carrier recombination lifetime is calculated from a transient capacitance signal measured as a

function of time, when the laser is switched off (Figure 2.2.2).


56

Figure 2.2.2 Transient capacitance signal measured

as a function of time. The dashed line represents the

laser power intensity Buh et al [2.4]

The parasitic effect of the laser light of the AFM detection system has been mentioned

in other publications, such as [2.5].

2.2.2. Stray capacitance

Another source of signal distortion mentioned in the literature is the stray

capacitance arisen from the interaction between the sample and the cantilever/cantilever

body assembly. Kopanski et al [2.2] are the first to observe this phenomenon. As a solution to

this problem, they propose such a geometrical arrangement that the chip that supports the

cantilever doesn't find itself above the sample, but in lateral of the sample (Figure 2.2.3). In

this way, the parasitic capacitance between the chip of the cantilever and the sample is

avoided.

Figure 2.2.3 SCM sample geometry that minimizes the

stray capacitance from the unintended coupling of the

cantilever and the supporting chip to the sample.

Kopanski et al [2.2]

2.2.3. Surface related phenomena

Stephenson et al [2.6] and Beyer et al. [2.7] mention the influence of surface


57

humidity in SCM measurements. They say that, under the influence of the strong electrical

fields between the tip and the sample, there is a possible decomposition of water from the

surface of the oxide and injection of protons H+ into the oxide. Such a charge injection into

the oxide will lead to signal distortion.

Stephenson et al [2.6] and Duhayon [2.8 - pg.118] also brought into discussion the

problem of surface moisture, but in conjunction with another problem in SCM measurements:

surface oxidation. Anodic oxidation is well known from other fields of the AFM, especially

nanolithography [2.9-2.11]. It is also common in STM (Scanning Tunneling Microscopy) [2.12]

and C-AFM (Conductive AFM) [2.13].

The oxidation depends of several factors: the humidity percentage, the magnitude of

the applied biases and the bias polarization, the tip size, the oxide thickness and the quality of

the oxide.

We have to mention that in the C-AFM field, the origins of the protrusions observed on

the surface of the sample still constitute a subject of dispute. There are three hypothesis

concerning the topographical modification after a C-AFM scan: anodic oxidation, electrostatic

repulsion of the AFM tip upon the charge trapping in the oxide and a mechanical deformation

of the silicon substrate. However, all the evidence suggest that the process responsible for the

apparition of the protrusions on the surface of the samples is the oxidation favorized by the

atmospheric conditions and the high biases used during the scans [2.13].

Another phenomenon related to the surface of the sample is the oxide engravement.

SCM measurements are done by scanning a tip on the surface of the oxide in contact mode. If

the interaction force between the tip and the surface is to great, oxide engravement can take

place [2.14]

Figure 2.2.4 The dielectric is scratched away by the tip (AFM profile – left image) which leads to a

thinner dielectric and an increase of the SCM signal (SCM profile – right image) Brezna et al [2.12]

2.2.4. Tip related phenomena

The tip depletion has been discussed in [2.8 - pg.121]. When scanning highly doped

regions, a stronger bias must be applied. This bias can lead to depletion in silicon based tips,

which can lead to signal distortion.


58

Another problem discussed in the literature, usually in correlation with the phenomenon

of contrast reversal is the tip wear [2.15]. The contact between the tip and the sample can

degrade not only the oxide, but also the tip. If the tip modifies its shape during scanning, a

higher SCM signal will be measured, not because of the modification of the substrate

concentration, but because of a greater 'gate' surface. In the case of the coated tips, tip wear

also means the removal of the coating, which can lead to the shift of the flatband bias and

thus, to contrast reversal.

One of the most discussed subjects in the literature is the contrast reversal. The SCM

signal should increase monotonically with the decrease of doping concentration. Many times at

the beginning of the SCM, cases have been observed when the SCM signal was increasing with

the increase of doping concentration, phenomenon called contrast reversal. Several

explanations have been proposed.

In theory, there is a monotonic dependence between the doping concentration and the

SCM signal: the higher the doping concentration, the lower the SCM signal (Figure 2.2.5).

Figure 2.2.5 The dependence between the doping concentration and the SCM

signal

The formula which describes this relation between capacitance and doping

concentration is:

The contrast reversal appears when this relationship of inverse proportionality is no

longer respected.

Several papers have been published on this subject and several opinions, that often

diverge, have been expressed. Before presenting them, we shall present a theoretical

background which can help create a clearer context for this matter.

We will consider, as a study subject, a MOS structure with: a p-type silicon substrate, a


59

5 nm ideal thermal oxide as dielectric and an aluminum metallic gate (workfunction 4.2 eV).

The bias is applied on the gate. The only variable parameter is the substrate concentration

which varies from 1015cm-3 to 1019cm-3.

As it is known from the MOS theory and C-V measurements on MOS structures, the

shape of the C-V curves modifies from one doping concentration to another of the silicon

substrate in the following way (Figure 2.2.6):

• the value of the capacitance in inversion increases with the increase of the doping

concentration

• the slope of the C-V curves diminishes with the increase of doping concentration

• the position of the inflexion point of the C-V curve shifts (towards the right in this case) with

the increase of doping concentration.

Figure 2.2.6 TCV simulation of C-V curves on a MOS structure with a 5nm

oxide thickness. The position of the inflexion points is indicated on the

figure.

As a result, the derivatives of these curves (the SCM signal) will have the following

shape. (Figure 2.2.7) (Tabulated CV v1.1 is a program developed at INSA de Lyon by the PhD.

student Christophe Busseret for capacitance simulations for M.O.S. structures).

The shape of the derivative curves modifies with the increase of doping concentration in

the following way:

C-V curves C-V derivatives (SCM signal)

the value of the capacitance in inversion

increases with the increase of the doping

concentration

• The amplitude decreases

the slope of the C-V curves diminishes

with the increase of doping concentration

• The width at half maximum

increases

the position of the inflexion point of the C-

V curve shifts (towards the right in this

case) with the increase of doping

concentration.

• The position of the maximum shifts

towards the right

The simulated curves correspond to an ideal case. Oxide defaults can further increase


60

the shift between the maximum of the signals.

Given the fact that SCM imaging is performed at a fixed Vdc and the maximum for

different doping concentrations are at different voltages, it can be clearly seen that this

characterization method is imperfect. Any single fixed bias cannot correspond to the maxima

of all the curves.

Figure 2.2.7 TCV simulation on dC/dV curves on a MOS structure

with a 5nm oxide thickness.

Several solutions have been found.

• First, SCM imaging is mainly used for qualitative failure analysis imaging. In such

measurements, it is not important to obtain the maximum signal for each doping

concentration, but to obtain only a contrast between the different doped regions. In order to

do this, the operating Vdc is usually chosen to the bias corresponding to the maximum or

around the maximum of the lightest doped region as in the figure below (Figure 2.2.8). In this

case, even if the signal is not corresponding with the maximum for the most doping regions,

the inverse proportionality relation between the doping concentration and the capacitance

signal is however respected.

Figure 2.2.8 The choice of Vdc operating bias with the SCM


61

• for doping quantification, simulations try and take into account the shifts between the

signals corresponding to various doping concentrations. However, instead of SCM imaging,

SCS measurements, first introduced by Hal Edwards et al [2.17], are preferred.

However, if the operating Vdc is not carefully chosen, the phenomenon called 'contrast

reversal' takes place, such as in the Figure 2.2.9. As it can be seen, the curve 3 will give a

higher signal than the curve 4 for the VDC0 operating point, although it corresponds to a higher

doping concentration.

Figure 2.2.9 An example where the chosen operating voltage

Vdc will determine the contrast reversal on the SCM image

Such an explanation has been given in the literature by Stephenson et al. [2.6] and

Smoliner et al [2.16]. They explained theoretically and experimentally the origins of the

contrast reversal as a function of the applied bias.

By using this theory, Smoliner has obtained intentionally contrast inversion on a

staircase sample, as a proof (Figure 2.2.10).

Figure 2.2.10 Doping profile of the epitaxial staircase structure determined by SIMS (left). Sections through SCM images taken at different operating voltages (right). Smoliner et al [2.16]


62

2.3. C-V measurements

2.3.1. Introduction

As it can be seen from the bibliography (paragraph 2.2) and from our own

measurements (Figure 2.1.1, Figure 2.1.3), SCM is a characterization method with many

problems concerning its reproducibility.

One area where SCM can be used for now with success is in obtaining qualitative failure

analysis images of doping profiles. These images only require a difference in contrast between

the different doped areas and they don't need a rigorous control of the reproducibility of the

signal.

That is why we considered necessary to use an alternative, calibrated method of

measurement of the properties of the MOS structures, in order to validate the data obtained

with the SCM.

Such a capacitance characterization method, closely related to the MOS structures and

transistors manufacturing, is the C-V measurements with an impedance analyzer.

This characterization method has been used in the microelectronics industry for more

than 50 years. A considerable know-how exists in this field. The measurements are

reproducible and quantifiable.

The C-V measurements with an impedance analyzer are not only reproducible and

quantifiable, but also versatile. They have a multitude of applications, illustrate in the table

below.

Parameter or phenomenon studied Technique used

Substrate Doping concentration (uniform) Values of Chf for accumulation and inversion

Doping profile near the Si-SiO2 interface

Chf(V) curve in depletion

Minority carrier lifetime Deep depletion transient capacitance

Oxide Oxide thickness Values of Chf or Clf for accumulation

Oxide leakage current (when not too important)

Quasi-static method

Image charge ΔVFB between experimentally measured Chf(V) and computed ideally Chf(V) curves.

Density of mobile ions ΔVFB from the hysteresis of the Chf(V) curves

Hot carrier trapping ΔVFB from Chf(V) curves

Oxide breakdown (dielectric strength, wear-out)

Voltage ramping, or time before failure measurements

Interface Energy distribution of the interface

states in the silicon gap

Quasi-static, DLTS, conductance methods

Relaxation time of interface states Conductance method

Standard deviation of surface potential Conductance method

Recombination velocity at the interface Deep depletion transient capacitance

Chart after Barbottin [2.18 - pg.261]


63

In the next paragraphs, we are going to describe the basic principles of C-V

measurements. We will demonstrate how to obtain data concerning the properties of a MOS

structure from a C-V signal, on the test sample that we also used for SCM measurements.

2.3.2. The principle of the C-V measurements with an impedance analyzer

2.3.2.1. Introductive notions concerning impedance analyzer

The impedance analyzer used for the measurements is an AGILENT 4284A.

The measurements of the capacitance of a MOS structure are done by adding the

sample in an AC circuit.

The frequency of the AC circuit can be chosen by the user in the interval (100Hz;

1MHz).

In order to keep the harmonics of the signal frequency from giving rise to conductance

and capacitance values with no physical signification, only AC voltages of small amplitude can

be applied. The small signal range is defined as the range of signal amplitude in which the

measured capacitance is independent of the AC gate voltage amplitude [2.19 - pg.585].

Commonly, the maximum AC voltage used with the impedance analyzer is 50 mV.

The DC voltage can be varied in the interval (-10V; +10V).

A C-V voltage ramp is obtained by choosing an AC gate voltage amplitude, a voltage

frequency and by slowly varying the DC gate voltage in a chosen interval, with a

predetermined voltage step. For each DC voltage, the capacitance and the admittance of the

MOS structure are measured.

The MOS capacitor under tests can be modeled as an ideal capacitor which presents a

parallel and a series resistance (Figure 2.3.1).

Figure 2.3.1 Simplified modelisation of the MOS

structure under tests

The parallel resistance takes into consideration the tunneling currents through the gate

oxide. The series resistance takes into consideration the substrate resistivity, the resistance of

the contacts, the resistivity of the coaxial cables and other elements of the instrument's

electronic circuit.

However, the impedance analyzer is not able to measure directly these parameters

(C,Rs,Rp).


64

For the measurement of the MOS capacitance, the impedance analyzer is able to use

two equivalent models, a series model and a parallel model (Figure 2.3.2).

Figure 2.3.2 The equivalent series model (left) and the parallel model (right) used by the impedance analyzer for a MOS capacitor. Cms series measured capacitance;

Rms series measured resistance; Cmp parallel measured capacitance; Rmp parallel measured resistance. All the electric components from this model are considered

ideal.

The impedance analyzer measures the real and the imaginary components of the total

impedance of either of these models by the use of an admittance bridge. Other impedance

analyzers may use lock-in amplifiers. The output measured variables are (Cms;Rms) for the

series model and (Cmp;Rmp) for the parallel model.

In order to obtain the values of the parameters (C,Rp,Rs) of the M.O.S. structure, the

relations between (Cms;Rms) and (C,Rp,Rs) or between (Cmp;Rmp) and (C,Rp,Rs) must be found

and calculations must be performed.

2.3.2.2. The series model

In the series model, the real capacitance of the M.O.S. structure is approximated with

an ideal capacitance in series with an ideal resistor (Figure 2.3.3).

Figure 2.3.3 The equivalence between the simplified capacitance model and the parallel

capacitance model

The equivalent impedance Z*RpC of the capacitor C in parallel with the resistance Rp is:


65

The total equivalent impedance of the (Rs,Rp,C) model is:

The equivalent impedance of the series capacitance model is:

By identifying the real part and the imaginary part of the two equivalent impedances,

we obtain:

(1)

(2)

As it was obvious from the beginning from the comparison of the two diagrams, the

measured capacity Cms is not equal to the capacity C of the M.O.S. structure. However, this

equality may become true in certain conditions. Further details will be given in the next

section, when it will be discussed what model should be chosen for the measurements, series

or parallel.

2.3.2.3. The parallel model

In the parallel model, the capacitance of the MOS structure is approximated with an

ideal capacitance in parallel with an ideal resistor (Figure 2.3.4).

Figure 2.3.4 The equivalence between the simplified capacitance model and the parallel capacitance model

The impedance of the MOS capacitor is the same as in the previous case. However, for

calculus reasons, it will be written under a different form.


66

The equivalent impedance of the parallel capacitance model is:

The equality between the two impedances gives:

After a calculus similar to the one presented for the series model, which involves the

identification of the real terms and the imaginary terms, we obtain:

(3)

(4)

2.3.2.4. The choice of the model used with C-V measurements

On an unknown sample, measurements must be performed by using both the series

model and the capacitance model, on several range of frequencies.

Following these measurements, several situations can be met:

1. Cmp stays constant with the variation of the frequency.

From the Cmp formula (4), it can be seen that Cmp will remain constant with the

variation of the frequency when Rs is negligible. When Rs->0, Cmp->C and Rmp->Rp (Figure

2.3.5)


67

Figure 2.3.5 The equivalence between the simplified capacitance model when Rs is negligible and the parallel

capacitance model

In this situation, the measured parameter Cmp represents the capacitance C of the MOS

structure.

2. Cmp varies with the frequency while Cms stays constant.

When Cmp varies with the frequency, it means that there is a non-negligible series

resistance (4). If Cms stays constant, it means that, this time, Rp is infinite, Cms->C (formula 2)

and the capacitance of the MOS structure can be extracted from the series model (Figure

2.3.6)

Figure 2.3.6 The equivalence between the simplified capacitance model when Rp is infinite and the

series capacitance model.

3. Cmp and Cms vary with the frequency

In this case, both Rp and Rs cannot be ignored. The formulas (1-4) cannot be simplified

anymore.

Yang et al [2.20] propose the extraction of the unknown parameters C, Rs and Rp by

performing measurements within the parallel model (Figure 2.3.5), at two different

frequencies. As a result, four experimental values will be obtained, Cmp1, Cmp2, Rmp1, Rmp2. The

unknown parameters will be written as functions of these four experimental values.

The equivalent impedance of the parallel model is:


68

This relation can be written using the expression of the quality factor Q. For a complex

impedance, the Q factor is the ratio of the reactance to the resistance. In the case of a

capacitor:

A high quality factor capacitor is always best than a low quality capacitor. The reason is

that is always easy to add supplementary resistances into a circuit. An ideal capacitor has a

quality factor Q->∞, with zero effective resistance.

The total impedance of the parallel model becomes:

The total impedance of the MOS structure is calculated in a similar way:

By equalizing the imaginary parts and the real parts of the two impedances and writing

the obtained expressions for two different experimental frequencies, we will finally obtain:

and

Thus, in the situation when the Cmp and Cms vary both with the frequency, the

impedance of the MOS structure can be calculated from the experimental values Cmp1, Cmp2,

Rmp1, Rmp2 measured with the parallel model, for two different frequencies.


69

2.3.3. C-V measurements on a test sample

2.3.3.1. The test sample

For comparisons between C-V and SCM measurements, we have used a sample which

consists in a p-type silicon substrate, 1015 cm-3, covered with a 5 nm thick thermal oxide.

Metallic electrodes (Nickel - Gold) which serve as the gate for the MOS structure have

been fabricated by evaporation with an electron gun (Figure 2.3.7)

Figure 2.3.7 Ni-Au electrodes deposited on a p-type substrate covered with a 5 nm thick thermal oxide.

The size of the electrodes has been measured with an optical microscope. There are

electrodes of six different sizes, 100 μm x 100 μm, 150 μm x 150 μm, 200 μm x 200 μm, 300

μm x 300 μm, 400 μm x 400 μm and 600 μm x 600 μm.

2.3.3.2. Preliminary measurements

As we have presented in 2.3.3.4., preliminary measurements must be performed on

the sample, at different frequencies, using both models, the parallel model and the series

model. From the observation of the behavior of Cms and Cmp with the variation of the

frequency, we can establish which model must be used for the following measurements and

whether any corrections are necessary to the measured capacitance in order to find out the

capacitance C of the M.O.S. structure.

For the C-V curves presented in Figure 2.3.8, we have used the following parameters:

AC voltage Vac=50 mV, DC voltage step 20 mV. The C-V curves have been obtained for each

of the frequencies: 100 Hz, 1 kHz, 10 kHz. The measurements in Figure 2.3.8 have been done

on 600 μm x 600 μm electrodes.


70

Figure 2.3.8 C-V measurements on 600 μ x 600 μ electrodes, in both series and parallel models, in the frequency interval 100 Hz - 10 kHz, at Vac = 50

mV

Cmp remains constant with the variation of the frequency. This means that the series

resistance is negligible. The measured capacitance in the parallel model Cmp represents the

capacitance of the MOS structure, presented in Figure 2.3.1.

2.3.3.3. Oxide thickness

From the value of the capacitance in accumulation Cox, oxide thickness tox can

be calculated.

In accumulation, the majority carriers are attracted towards the interface. In our case,

the majority carriers are the holes and they are attracted towards the Si-SiO2 interface for

negative voltages applied to the gate. The capacitance of the MOS structure is given by the

capacitance Cox of the dielectric (Figure 2.3.9).

where:

Cox is the capacitance in accumulation Cox=2.26 10-9 F

εo is the permittivity of vacuum εo=8.85 10-12 F/m

εSiO2 is the relative permittivity of SiO2 εSiO2=3.9

S is the surface of the gate S=600μm2

tox is the dielectric thickness -


71

Figure 2.3.9 The MOS structure in accumulation for a p-type substrate

From this relation, the oxide thickness can be calculated.

For our test sample, we obtain tox = 5.5 nm

2.3.3.4. Doping concentration

From the values of the capacitance in accumulation Cox and in inversion Cmin,

the doping concentration can be calculated.

In inversion, the majority carriers are repelled away from the interface. In our case,

the majority carriers are the holes and they are repelled away from the Si-SiO2 interface for

positive voltages applied to the gate. The capacitance of the MOS structure is given by the

capacitance Cox of the dielectric in series with the capacitance of the depletion layer (Figure

2.3.10).

The total capacitance in inversion is:

This equation can be solved iteratively and the doping concentration can be found. A

program with an iterative algorithm can be written to facilitate the process.


72

Figure 2.3.10 The MOS structure in inversion for a p-type substrate

In our case, we can simply verify the substrate doping concentration of 1015cm-3,

measured with a four-point probe, by replacing this value into the equation and seeing

whether the equality is verified.

Cox the capacitance in accumulation Cox=2.26e-9 F

Cmin the capacitance in inversion Cmin = 2.55e-11 F

εo the permittivity of vacuum εo=8.85e-12 F/m

εSi the relative permittivity of silicon εsi=11.7

k Boltzmann constant k = 1.38e-23 J/K

T temperature T=300 K

q the elementary charge q=1.6e-19 C

ni intrinsic concentration 1.10cm-3

Na acceptor doping concentration -

2.3.3.5. Flatband capacitance, flatband voltage

By knowing the doping concentration, the flatband capacitance and the

flatband voltage can be calculated.

At the flat-band condition, variations in the gate potential give rise to the addition /

subtraction of incremental charge in the substrate, at a depth LD. We remind that the Debye

length LD is the distance at which the electric field generated by a perturbing charge falls off

by a factor 1/e.

The capacitance of the MOS structure at the flatband condition is:

From calculus, we obtain CFB=2.6 10-10 F

Knowing CFB, we can find the flatband voltage from the C-V curve. The voltage VFB

corresponding to CFB is VFB=-0.56 V (Figure 2.3.11)


73

Figure 2.3.11 CFB and VFB calculus from the C-V curves

For substrates with low doping concentrations, the flatband voltage will always be close

to the inversion region of the C-V curve, while for substrates with high doping concentrations,

the flatband voltage will be very close to the accumulation region of the C-V curve.

2.3.3.6. Interface states, surface potential

From C-V measurements performed at two different frequencies, the interface

state density can be calculated.

In the proximity of Si/SiO2 interface, carriers generated by different sources (intrinsic

energy levels, doping impurities, interface states etc) are characterized by a cut-off frequency

beyond which they cannot follow anymore the variations of the surface potential imposed by

the gate bias.

The C-V measurements can be considered as a filtration procedure, where the

frequency chosen for the measurements represents the cut-off frequency. All the carriers that

have a lower cut-off frequency than the measurement frequency will not bring their

contribution to the capacitance signal.

This allows us to calculate the interface state density for a given MOS structure. The

method used here to calculate the interface state density has been used for the first time by

[2.21].

From the C-V measurements performed with the parallel model, we will consider for our

calculus the two measurements performed at the two extremes of the frequency range: 100

Hz and 10 kHz (Figure 2.3.12). In the figure, it can be clearly seen that the capacitance signal

given by the interface states at the lower frequency is higher than the capacitance signal at

higher frequency, which means that, at higher frequency, less carriers are able to respond to

the change in the gate voltage.


74

Figure 2.3.12 Calculus of the interface state density from the measured C-V curves at two frequencies

The capacitance of the MOS structure for the high frequency can be written:

HF

1 1 1

C ox SemiconductorHFC C= +

and the capacitance of the MOS structure for the lower frequency:

LF

1 1 1

C ox SemiconductorLFC C= +

where Cit is the capacitance given by the interface states.

From these two equations, we obtain:

The interface state density is:

The above formula is a function of the gate bias. For finding out the the energy

distribution of the interface states, we have to represent the interface state density as a

function of the surface potential:


75

Figure 2.3.13 The interface state distribution as a function of the surface potential

2.3.3.7. A preliminary comparison between the C-V signal and the SCM signal

From the C-V curves that we presented, it is obvious that the capacitance

measurements with an impedance analyzer are highly reproducible. The properties of the C-V

curve don't change from a measurement to the next. The measurements done on different

electrodes give exactly the same results. Measurements performed in different days on the

same sample confirm the reproducibility of the technique.

The value of the capacitance in accumulation is always 2.26.10-9 F on the 600μ x 600μ

electrodes. The value of the capacitance in inversion remains the same: 2.55.10-11 F. The

inflexion point of the C-V curve (which represents the maximum of the SCS signal) is always

at -0.8 V. The C-V curves have always exactly the same shape.

On the other hand, the SCM signal is instable and sometimes presents significant

differences even between consecutive measurements. As it can be seen from the example

from Figure 2.1.1, the parameters of the SCS signal vary significatively: the amplitude of the

signal, the position of the maximum, the shape of the signal. Sometimes, the SCS signal

presents a single maximum, other times two or more local maxima.

A direct comparison between the C-V and SCS measurements done on our test sample,

several discrepancies can be observed (Figure 2.3.14).


76

Figure 2.3.14 C-V measurements (left) and SCS measurements (right) performed on a 5 nm thermal oxide deposited on a 1015 cm-3 p-type silicon substrate

The SCS signal presents hysteresis between the trace and the retrace. With the C-V

signal, the hysteresis is given by the presence of mobile charges in the oxide. However, the C-

V curve doesn't present any hysteresis at all. Even if the structure is stressed (a high DC bias

is applied to the MOS structure for several minutes), the trace and the retrace in the case of

the C-V signal will still coincide. In addition, the mobile charges don't play any role at room

temperature. The sample must be heated in order that the mobile charge be able to migrate

through the oxide under the influence of an electric field.

The SCS signal presents a minimum in the inversion region which usually corresponds

to the presence of minority carriers, which doesn't appear on the C-V curve.

From the integral of the SCS signal, it can also be observed that the capacitance level

in the inversion region is higher than the capacitance level in accumulation (Figure 2.3.15).

Physically, this is not possible. The capacitance in accumulation represents the capacitance of

the dielectric which is the maximum capacitance of the MOS structure. Even if minority

carriers reach the Si/SiO2 interface in inversion, the capacitance signal cannot surpass the

accumulation capacitance value.

Figure 2.3.15 The integral curve of the SCS signal. the bias is applied on the substrate. The curve has been inversed and normalized.

In the next paragraphs, we will investigate the source of these problems, based on the

problems already signaled in the literature and on our experimental observations.


77

2.4. Influence of the laser light

2.4.1. Introduction

The movements of the tip on the surface of the sample and the deflexion of the

cantilever are detected with most commercial AFMs by a laser detection system (Figure 2.4.1)

Figure 2.4.1 Diagram of the laser detection system (left). CCD camera image of the laser positioning at

the end of the cantilever (right)

From the theory of semiconductors, it is known that when a semiconductor is perturbed

from the equilibrium state, an excess or deficit in the carrier concentrations relative to their

equilibrium values is created inside the semiconductor. Recombination/Generation (R-G)

phenomena become dominant inside the semiconductor.

One of the carrier generation mechanisms inside a semiconductor, besides direct

thermal generation, thermally assisted generation by R-G centers and impact ionization, is

photogeneration. If the incident photon energy is greater than the band gap energy of the

semiconductor, then the light will be absorbed and electron-hole pairs will be created as the

light passes through the semiconductor. The photogeneration process always creates an equal

number of electrons and holes (equal number of majority and minority carriers).

In this context, considering the wavelength of the laser and the positioning of the laser

on the cantilever, it seems pertinent to question whether the laser light of the AFM detection

system perturbs or not the capacitance measurements, which are sensitive to the carrier

concentration.

The laser used on the Veeco system has a wavelength in the red domain (670 nm).

Thus, the photon energy is greater than the band gap energy of the silicon.

Band gap of the silicon 1.12 eV

Photon energy for a wavelength of 670 nm 1.85 eV

The laser beam is positioned at the end of the cantilever, exactly above the spot where

the electrical measurements are performed (Figure 2.4.1)


78

As discussed in the section (2.2.1), many research groups have already performed

studies regarding the impact of the laser light upon the SCM measurements [2.1-2.5].

In the next section, we present our measurements which show how the SCM

measurements are distorted by the laser light of the AFM detection system.

2.4.2. Experimental evidence

In order to evidentiate whether the laser has a significant influence on the capacitance

measurements on the Veeco system or not, SCS measurements have been performed on

different oxides deposited on low-concentration substrates, in the presence and in the absence

of the laser. The samples used for the analysis consist of various oxides (5 nm thick thermal

oxides, native oxides, plasma oxides) deposited on p-type substrates with a doping

concentration of 1015cm-3 corresponding to a 8 .cm resistivity

Considering the fact there are no ways to shut down the laser by means of the software

controlling the apparatus, we have chosen the method proposed by Kopanski et al [2.2] : we

tried to minimize the laser influence by displacing the spot along the axis of the cantilever so

that the laser is located far (several tens of microns) from the point where the tip touches the

surface (Figure 2.4.2).

For a 1015 at.cm-3 substrate, the diffusion length of carriers in silicon is in the 5 μm

range [2.22]. The cantilevers used for our measurements are approximatively 200 m in

length. If the laser spot is placed towards the middle of the cantilever, the pairs generated by

the laser should recombinate before reaching the point where the SCS is performed.

Figure 2.4.2 CCD camera image of the laser positionement at the end of the cantilever (left) and

towards the base of the cantilever (right)

The positioning of the laser represents a compromise between the quality of the

topography measurements, which need the laser to be reasonably at the end of the cantilever,

and the SCS measurements. Several comparisons have been made between SCS

measurements recorded with the laser spot placed roughly at the middle of the cantilever and

measurements taken by temporary obstructing the laser beam : no difference between these

measurements has been observed.

However, in any cases, remnant light may still influence the measurements.

In Fig.2.4.3 SCS is operated on low-doped p-type substrate covered by a thin native

oxide with and without the presence of the AFM laser light. The difference between the

resulting SCS measured in both conditions emphasizes the decrease of the signal to noise ratio


79

due to the electron-hole generation originating from the impact of the laser light on the

surface.

Figure 2.4.3 SCS on a native oxide grown on a low doped p-

type substrate (1015 at.cm-3 ) with and without laser

This is the first clear evidence why in most cases, no signal at all was previously

obtained with the SCS measurements on native oxides. In the image, it can be seen that the

signal decreases more than 6 times in the presence of the laser. The amplitude of the peak

decreases from more than 1.2 V in the absence of the laser to approximatively 200 mV in the

presence of the laser, close to the level noise of 50 mV on the Veeco system

It must also be pointed out that the generation of electron/holes pairs by the laser light

has also created an additional peak in the negative voltage region corresponding to the

inversion zone.

Figure 2.4.4 also illustrate the influence of the laser on a 5 nm thermal oxide.

The strong decrease of the signal to noise ratio due to the laser light is evident. The

thermal oxide under study has been grown on a low doped substrate (1015 at/cm3), in order

that the SCS signal is as strong as possible which explains this strong influence of the

photogeneration of carriers.

Figure 2.4.4 SCSon a 5 nm thick thermal oxide

grown on 1015 at/cm3 p-type substrate

2.4.3. Quantitative aspects

We have tried and quantify the intensity of the light on the surface of the samples and


80

the excess of carriers generated by the laser.

On an intrinsic substrate, a gradual doping profile has been created by thermal

diffusion of phosphorous. The sample was cleaved and the SCM measurements have been

performed on the cross-section. The cross-section of the samples is covered by a thin native

oxide.

SIMS measurements have been used as a reference characterization method, in order

to know the value of the concentration doping in correlation with the distance from the surface

of the sample (Figure 2.4.5)

Figure.2.4.5 SIMS profile of Phosphorous diffused in

a silicon intrinsic substrate.

Figure 2.4.6 SCM profile of Phosphorous diffused

in a silicon intrinsic substrate.The dotted green

line indicates the depth from which the SIMS

profile is too noisy too provide the real

concentration of phosphorous.

The profile measured by SCM (Figure 2.4.6) in the presence of light is strongly altered

from 200 nm and deeper (where the carriers concentration falls under 1017 cm-3 according to

SIMS) and does not correspond to the SIMS profile any more. The absolute value of the SCM

signal decreases strongly and even becomes positive at 600 nm depth. The strong influence of

the applied DC voltage on the measured SCM signal indicates that this result is due to the

presence of minority carriers generated by the laser and attracted by the tip. When the laser

light is placed away from the scanning zone, the influence of the laser in the zone probed by

the tip is removed and a SCM profile corresponding to the SIMS profile can be retrieved.

The doping concentration value at 200 nm, where the SCM signal in the presence of

light becomes different from the SCM signal in the absence of light, is between 1017 cm-3 and

1018 cm-3. In consequence it can be estimated that the Veeco AFM laser generates a

concentration of electron-hole pairs between 1017 cm-3 and 1018 cm-3. This value is in

accordance with the value calculated in the literature for the same system [2.4].

However, it must be pointed out that this is only an estimation. Several causes have

prevented it us to make an accurate measurement.

First of all, not all the impurity phosphorous atoms are activated because of the doping


81

method. Given the fact that SIMS measures the concentration of phosphorous atoms,

while SCM is sensitive to carriers, there should be a slight difference between both

profiles.

Another reason is that the doping profile starts immediately at the edge of the sample.

Given the drift of the piezoelectric tube in x-y direction and the danger that the tip falls

off the surface, it is possible that the SCM measurements have not taken into

consideration the first tenths of nanometers of the doping profile. In consequence, a

slight shift in the depth direction may exist between the SIMS profile and the SCM

profile. In order to be sure that the entire doping profile has been measured, a capping

layer should have been deposited on the surface of the sample.

Since one of the SCM strengths is to provide a high signal for low concentrations, future

experimental setups should take care of minimizing if not suppressing the impact of the laser

light on the sample's surface or choose another technique to measure the deflexion. This is an

important condition for SCM to be able to provide reliable information e.g. on a junction

located under a shallow trench isolation (STI) where both the n-side and the p-side are low

doped.

From the Figure 2.4.6, it can be also pointed out that SCM without the laser light is

able to provide information about the carrier distribution at concentrations lower than SIMS,

which is limited by the noise in this particular configuration (low erosion speed, severe

interference between the masses of SiH and Phosphorous, which imposes to use a high mass

resolution and leads to a lower signal).

2.4.4. Comparison with C-V measurements

In the literature, there are two main effects of light that influences the capacitance

measurements [2.3].

One effect is a decrease in the time constant of the minority carrier generation in the

inversion layer resulting in a low frequency like characteristic, even at high frequencies.

Another effect is a decrease of the surface potential, resulting in a reduction of the

width of the depletion layer.

However, we considered that neither of these two phenomena cannot fully explain the

drastic decrease of the SCM signal in the presence of light (Figure 2.4.3)

Comparisons with macroscopic curves have been performed in order to better

understand the role of illumination on SCS measurements. The same thermal oxide as in

Figure 2.4.4 has been investigated using 600 x 600 m NiAu electrodes illuminated by a 20

mV laser, (wavelength 632.8 nm), leading to the macroscopic C-V of Figure 2.4.7. The

electrode consists in a 3 nm thick layer of Ni in contact with the oxide and a 300 nm thick Au

top layer. The creation of minority carriers which modify the capacity in depletion and

inversion is evident in this figure and corresponds to the observations from reference [2.3].


82

Figure 2.4.7 Effect of a laser on the macroscopic C-V curve for a 5

nm thick thermal oxide and thick NiAu electrodes

Figure 2.4.7 also shows the influence of white light focused on the surface of the

sample on the C-V curve The increase of the signal in the inversion region is clearly detectable

and will also lead to a drop in the SCS signal. This shows that, since in AFM measurements the

sample surface is not protected by anything else than the cantilever, even the ambient light

can have a rather detectable influence on the results provided by SCS.

However, this still doesn't seem to explain the decrease of the SCM signal in the

presence of light. The calculated derivatives of the C-V curves in Figure 2.4.8 show that the

influence of the light on the amplitude of the peak from the depletion region is minimum.

Figure 2.4.8 Numerical calculated derivatives of the C-V curves from

the Figure 2.38

The amplitude of the peak in the presence of light stays unchanged and the amplitude

of the peak in the presence of the laser decreases very little. In consequence, the decrease of

the surface potential in the presence of light, resulting in a reduction of the width of the

depletion layer, doesn't seem to explain the drastic decrease of the amplitude of the SCS peak

under the influence of the AFM laser.


83

We are proposing the following two explanations :

The consequences in terms of SCS signal could be understood by the following

amplitude of the SCM oscillator Aosc(t) is extracted from the stray capacitance (1e-12 –

1e-13 F) with a lock-in amplifier. When the C-V curve is monotonous and Vac as low as

possible, the variation of Aosc(t) will be reasonably sinusoidal at the same frequency as

Vac(t), leading to a SCM signal equal to A1 in Fig.2.4.9. But if capacitance increases

again before the period Vac(t) is over, then the corresponding Aosc(t) is distorted. The

higher minimum of the C-V curve due to the increase of the signal in inversion reduces

the maximum variation of capacitance and thus the SCM output drops down to A2 in

Figure 2.40. The spectral component of Aosc(t) at the same frequency as Vac(t) is also

reduced, reducing the SCM output further.

Thus, the decrease of the surface potential in the presence of light, resulting in a

reduction of the width of the depletion layer, could have much more influence on the

SCS signal than on the derivatives of the C-V curves

Figure 2.4.9 Schematics of the SCM oscillator amplitude Aosc

with time when the corresponding C-V curve is monotonous

(dotted line) and when it increases in inversion (plain line).

Another explanation could be that, in the case of the C-V measurements with an

impedance analyzer, the surface is mostly protected by the influence of the light by 300

nm thick electrodes that prevent the light from reaching the semiconductor. The

minority carriers that distort the C-V curves are pumped under the electrodes from the

exposed edges of the electrodes. On the other hand, in the case of the SCS, the laser

beam is obstructed only by a metal layer 3-5 nm thick, that represents the back

coating of the cantilever.

In order to test this hypothesis, we have tried to reproduce with the C-V measurements

the same experimental conditions as with the SCS.


84

Figure 2.4.10 shows the influence of illumination on the same thermal oxide as

Fig.2.4.7, but covered with 5 nm thick aluminum electrodes that present a transmission

coefficient to light in comparison with previously used 300 nm NiAu electrodes that don't let

the light illuminate directly the area under them.

Figure 2.4.10 Effect of light on the macroscopic C-V curve for a 5 nm thick

thermal oxide and 5 nm thick Al electrodes. The arrow in the figure

indicates the change of slope of the C-V curve measured under illumination

Before discussing the results, it must be pointed out that using Al electrodes, the bad

electrical properties of the interface between the electrode and the oxide leads to the

existence of charges at the metal-oxide interface which contribute to the increase of the

capacitance in depletion, even at high frequencies. This effect is well known by people

involved in the macroscopic C-V characterization and imposes the choice of the electrodes

(like NiAu instead of Al). A post metallization annealing is sometimes used to enhance the

electrical properties of the interface between the Al electrode and the oxide.

Figure 2.4.11 Numerical calculated derivatives of the C-V curves from the

Figure 2.4.10


85

Besides the increase of the capacitance in inversion arising from the choice of the metal

for the gate electrodes, several other important changes can be observed by comparison with

the C-V curves from the Figure 2.4.7

The first difference is that the minimum capacitance in the depletion region shows even

higher values, which leads to the modification of the position of the inflexion point of

the curves. In Figure 2.4.11, it can be observed that the shift of the inflexion point

translates into the shift of the peak from the depletion region. This effect can explain

why the maxima of the SCS signals measured in the presence and in the absence of

the laser beam, in Figure 2.4.3 (on the native oxide) and in Figure 2.4.4 (on the

thermal oxide) are shifted.

A second difference arises from the fact that the use of the laser leads to a change in

the slope of the C-V curve in the negative voltages (indicated by the arrow in Figure

2.4.10). This means that in this case, even if a very small amplitude for Vac were used,

the change of the slope due to illumination would lead to a bad interpretation of the

SCS in terms of doping concentration (in the case of doping mapping) or oxide

thickness (in the case of oxide characterization). In any case, this change of slope leads

to a decrease of the SCS signal.

The modification of the slope also explains why the peak of the blue curve (laser) shifts

less than the peak of the red curve (white light), although the increase of the minimum of the

C-V curve in depletion is higher. The explanation is that while the increase of the minimum of

the C-V curve in depletion shifts the peak towards left, the modification of the slope shifts the

peak towards the right.

Another surprising observation is the behavior of the interface states in the presence of

light. In the dark, the presence of the interface states can be observed on a C-V curve, at the

frontier between the depletion region and the inversion region (Figure 2.4.12). The signal

provided by the interface states lowers with the increase of the frequency of the Vac voltage

because the interface states have less time to react.


86

Figure 2.4.12 The variation of the interface states with the variation of

the frequency

Thus, it can be concluded that the presence of the laser is able to create additional

peaks in the SCS, each corresponding to a different inflexion point of the C-V curve in Figure

2.4.10. This can explain why, sometimes, on the SCS images, multiple peaks can be observed

(Figure 2.4.13).

In summary, the decrease of the signal to noise ratio with illumination can now be

explained by

The change of slope of the C-V curve

The increase of the minimum of the C-V curves in depletion which reduces the dynamic

range of SCS and changes the position of the maximum of the SCS peak

The increase of C-V curves in inversion which may distort the SCS when high values of

Vac are used

Figure 2.4.13 SCS on a 2 nm thermal oxide grown on a low

doped n-type substrate (1015 at.cm-3 ) with and without laser


87

2.4.5. Solutions for the elimination of the parasitic AFM laser effect

The only practical method at our disposal in order to eliminate the parasitic laser light

was to displace the laser spot towards the base of the cantilever. This method has been first

proposed by Kopanski et al [2.2] and it is fairly efficient.

The laser-beam-reflexion detection method is the most common method in commercial

AFMs to measure cantilever deflection. However, there are many other detection methods for

measuring the cantilever deflection [2.23]: the tunneling detection system, the heterodyne

detection system, the homodyne detection system, the polarization detection system.

However, this approach is improbable. From the all types of characterization methods

with the AFM, SCM is the only one strongly affected by the parasitic light of the laser beam. It

is unlikely that the detection system will be changed only for the SCM on commercial systems

although it can be envisaged for a dedicated instrument.

A practical modality to eliminate the laser effect, proposed in the literature is by simply

turning off the laser. Buh et al [2.4] proposed a home made solution which consists in an

installation of a switch that can be used to turn the laser on and off. This solution is valid only

for the SCS. SCM imaging cannot be performed with the laser off. However, for the

quantification of the doping profiles, the critical information is obtained with the SCS

A similar solution, by turning off the laser temporarily, called dark lift, has been

recently perfected by Veeco for its systems. With the dark lift, each line is scanned twice as for

the lift mode used for e.g. Electric Force Microscopy (EFM) measurements or KFM

measurements. The goal of the first passage, done with the laser on, is to sense the

topographical details. At the second passage on the same line, the laser is turned off. The

AFM, unable to sense the topographical features anymore, will use the topographical data

recorded at the first passage. The second passage is used to do the capacitance

measurements in the dark. This mode can be used only for SCM imaging, but not for the SCS.

One of the critical practical problems with solutions that turn on/off the laser is the

resulting drastic decrease of the lifetime of the photodiode.

A more viable alternative would be the installation of a hatch in front of the photodiode.

The hatch would block the laser beam and prevent it from reaching the surface of the sample

when closed off.

The most practical solution to prevent the laser beam of reaching the surface would be

to modify the parameters of the cantilevers used with the SCM. Larger cantilevers would block

more effectively the light. A thicker backside metallic layer in combination with thicker silicon

cantilevers would completely reflect/absorb the incident laser beam. Buh et al [2.4] discussed

in detail the silicon and the coating metal layer in function of their thicknesses.

Another range of solutions could concentrate on the properties of the laser beam : a

theoretical possibility would be the use of photodiodes that generate a laser beam with a

wavelength corresponding to a smaller photon energy than the silicon band gap energy (or


88

any other semiconductor that might be studied with the SCM). If we take into account the

case of the silicon, it would mean that the photon energy should be smaller than Eg=1.12 eV,

which will mean the use of photodiodes/photodetectors starting with the mid infrared

spectrum.

Other solutions may concern the decrease of the intensity of the incident light on the

surface of the sample. This can be done either by using lasers less powerful or more

divergent.

2.4.6. Conclusions and Perspectives

We have shown that the AFM laser strongly affects the signal/noise ratio of the SCS

signal. Considering that the amplitude of the SCS signal is the main parameter used in the

attempts to quantify the doping profiles, it is vital that this parasitic effect be completely

removed.

In this work, the effect of the laser upon the capacitance signal has been removed by

displacing the laser spot towards the base of the cantilever. Gradual doping profiles have been

used to estimate the carrier generation. Comparisons between SCS and C-V measurements in

the presence of light have been made and the phenomenon of the decrease of the SCS signal

has been better explained. C-V measurements have been performed on usual micrometric

electrodes and on thin electrodes for better reproducing the experimental conditions with the

AFM

We have shown that, in the future, gradual doping profiles as the one in Figure 2.4.6

can be used as reference samples for measuring the intensity of the laser light. Such gradual

profiles can be further ameliorated in the following way:

Profiles with all the impurity ions activated should be fabricated.

Such samples should present a capping for an accurate positioning by reference to the

edge of the sample where the doping profile starts.

The variation of the doping concentration should be monotonous.

The doping profile should be as large as possible, extending on several microns. Thus,

the error of the doping positioning would be decreased..

As Buh showed [2.3], studies of the carrier recombination time from the transient

response curve are possible, given that the appropriate setup is put in place. While C-t ramps

can be easily performed with Nanoscope or by using an external software like LabView (Figure

2.4.14), these signals must be correlated with the precise time when the laser beam doesn't

hit the surface any more. A switch controlling the laser or an electromagnetic switch

controlling an obstructing hatch, commanded by a Labview station could be used.


89

Figure 2.4.14 Transient C-t ramp recorded with Nanoscope (left). Zoom in of the transient C-t ramp in

the transition region (right)

We have shown that the presence of light can lead to the presence of multiple peaks on

the SCS signal, some of them caused by the interface states. Interface states can be studied

in this way, in correlation with C-V measurements performed on thin electrodes.

Quantification attempts with an external high intensity laser are possible, in correlation

with simulations.

2.5. Influence of the parasitic capacitance of the geometry setup

2.5.1. Introduction

SCM characterization of doping profiles is done in most cases on the cross-section of

the samples (Figure 2.5.1).

On the other hand, oxides are usually deposited on the surface of the wafers and SCM

measurements are performed on the surface of the samples (Figure 2.5.2).

Figure 2.5.1 Geometry setup for the

characterization of doping profiles

with the SCM

Figure 2.5.2 Geometry setup for the

characterization of dielectrics with the SCM

During our measurements, a strong decrease of the signal/noise ratio has been


90

observed on the SCS measured on the surface by comparison with the SCS signal measured

on the cross-section of the samples, as it can be seen in Figure 2.5.3 : the signal measured

on the plane surface is practically at the SCM noise level, which is 50 mV. This observation

was valid even in the case when the same tip was used, during the same session of

experiments for both measurements (plane surface and cross-section). The problem remained

unsolved even after removing the effects of the parasitic AFM laser light.

In the literature, a clue about this problem may be found in the publication of Kopanski

et al [2.2]. As we have presented in the bibliography section, it is considered that the tip-

cantilever/body assembly and the sample form a capacitance that could affect the SCM signal.

This hypothesis is indeed reasonable.

The cantilever and the sample are a part of the same electrical circuit. Both the

cantilever and the substrate of the sample are conductive. The cantilever is connected to the

mass of the circuit, while a bias is applied on the XY stage which is in contact with the sample.

The cantilever and the substrate are separated by dielectric layers – the air between the

cantilever and the sample and the dielectric layer of the sample. In theory, a capacitance

effect should occur.

Figure 2.5.3 SCS signal on two native oxides, on low-doped 1.5.1015cm-3 p-type substrates, in the cross-section setup (black

curve) and on the plane surface (blue curve)

In addition, it is straightforward to say that the main difference between the two

experimental setups from the Figure 2.5.1 and the Figure 2.5.2 is the different positioning of

the cantilever with respect to the sample.

As a result, we decided to study the effect of the cantilever positioning.

2.5.2. Experimental evidence

In the comparison between the two SCS signals from Figure 2.5.3, several factors that

can influence the amplitude of the SCS signals may have overlapped.

The measurements are made on different samples. Although both substrates have a

low-doped concentration, the doping concentration is probably not exactly the same.

The substrates don't have the same crystalline orientation which can mean different


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concentration of interface states (see Chapter 3 : Oxides). As a result, the oxide

thickness may not be the same.

In order to isolate the possible effect of the cantilever positioning from other factors,

we have decided that the measurements should be done on the very same sample, on its

surface. The measurements on the cross section of the sample would be reproduced by

positioning the tip at the very edge of the sample.

In this way, all the measurements are done on the same substrate covered by the

same oxide. Of course, it is possible that the density of oxide defects be different from one

spot to another, but we considered that such variations of the density of oxide defects cannot

determine a drastic variation of the signal amplitude.

The measurements have been done on a sample consisting in a good quality 2 nm thick

thermal oxide deposited on a low-doped 1015 cm-3, n-type substrate.

In both cases that have been studied, the SCS signal and the output of the SCM sensor

have been recorded.

The SCM sensor includes a high frequency oscillator driven with a frequency that can be

varied in the interval 0.88 GHz – 1.05 GHz, which output amplitude varies because of the shift

of its resonance frequency due to C. The demodulated amplitude variation of the oscillator at

the frequency of VAC as a response to C constitutes the SCM signal. The measurement of

this curve, and particularly its slope which governs the SCM output, is a direct indication of the

sensitivity of the SCM during the measurement.

Figure 2.5.4 shows the output of this sensor as a function of frequency for different

positions of the sample far over or on the sample surface.

Figure 2.5.4 Output of the SCM sensor as a function of the frequency for different positions of the sample over or on the surface.

Figure 2.5.5 presents the SCS signals for the tip positioned at the edge of the sample

and towards the middle of the sample. The high value of 1 V for the VAC value was chosen in

order to be able to have a signal for the SCS curve taken towards the middle of the sample.


92

For the SCS curve at the edge of the sample we have used the same VAC = 1 V in order to

compare the two curves.

From Figure 2.5.4 and Figure 2.5.5, it can be clearly seen the correlation between the

sensitivity of the SCM output sensor and the SCS signal. The maximum value of the sensor

output drops from 5 V when the tip is positioned at the edge of the sample to 200 mV when

the tip is positioned towards the middle of the sample (25 times weaker). The peak of the SCS

signal drops from -1000 mV when the tip is positioned at the edge of the sample to -50 mV

when the tip is positioned towards the middle of the sample (20 times weaker). It must be

reminded that the SCS signals show negative values because of the nature of the substrate

(n-type semiconductor).

Figure 2.5.5 The SCS signals on the same sample which consists

in a 2 nm thermal oxide deposited on a low-doped, 1.5 1015cm-3

n-type substrate. The SCS signals are measured for the tip

positioned at the edge of the sample (the black curve) and

towards the middle of the sample (the blue curve).

The important change in the slope of the resulting curve demonstrates that when the

AFM tip is positioned directly in the middle of the sample, with the cantilever and chip just

over the sample surface, the sensitivity and the dynamic range of the measurement drop

dramatically. The stray capacitance is lower when the measurement is conducted at the edge

of the sample, i.e. when the cantilever is not over the sample any more. This means that the

cantilever position, and thus any topographic feature, influences the quantitative

interpretation of SCS because of the change of sensitivity due to the parasitic capacitance, and

that any attempt to reduce the stray capacitance would be beneficial for the reproducibility

and quantitativity of the measurements.

In particular, when SCM is performed in the middle of a plane sample, the sensitivity is

reduced dramatically in comparison with the case when the experiments are conducted at the

edge of the sample. This is an important cause of nonreproducibility of the SCM signal when

comparisons are attempted between experiments performed e.g. on a cross section and

experiments performed on a plane or beveled sample, which leads to a completely different

situation from the stray capacitance point of view, and thus to completely different

sensitivities and applied voltages during SCM operation. From Figure 2.5.5 it can be seen that


93

the sensitivity drops of a factor 25 from one configuration to another.

Another question which arises is what is effectively responsible for the huge drop in the

sensitivity of the sensor and hence, in the amplitude of the SCM signal.

We have noticed that the sensitivity of the sensor doesn't drop too much as the

cantilever advances above the sample. In order to verify how the SCS signal and the

sensitivity of the sensor modify with the advancement of the cantilever above the surface of

the sample, we have made the following experiment.

We have positioned the cantilever exactly at the edge of the sample and we have

recorded the signal of the sensor. Then, we made several shifts with various steps, from 50

m steps to 500 m steps towards the end. For each step the sensor output has been

recorded. The measurements are presented in Figure 2.5.6.

Figure 2.5.6 The signal of the output sensor for different shifts of the cantilever above the sample, from the edge towards the middle of the sample.

From Figure 2.5.6, It can be seen that the sensitivity of the output sensor doesn't drop

suddenly with the advancement of the cantilever above the sample. There is a monotonous

decrease in the sensitivity of the sensor, decrease that continues even after the shift of 200

m – 250 m. Given the fact that the cantilever length specified by the manufacturer is of 225

m ± 10 m, it means that the cantilever is not the sole responsible for the decrease of the

sensor output.

Until the 200 m – 250 m shift, the sensitivity of the detector doesn't decrease to

much. Based on the results, we can conclude that, in fact, the chip of the cantilever is the

main responsible for the drop of the capacitance signal when a measurement is

performed towards the middle of a plane sample.

The parasitic capacitance between the cantilever and the sample, although noticeable,

is not the main factor which determines the drop of the sensitivity of the output sensor.

Besides the two observed parasitic capacitances, chip-sample and cantilever-sample,

there are another few observations to be made.

Even if the measurements are done on the edge of a plane sample, but which is placed


94

on a metallic holder, the signal will still drop, because of a parasitic capacitance given by the

cantilever/ cantilever chip - sample holder assembly. If the sample is pasted on a metallic

holder in order to insure a good electrical contact, the sample edge where the measurements

are to be done must be positioned at the edge of the holder.

For the SCM, it is not critical to insure an ohmic contact between the sample and the XY

stage of the AFM, as it will be showed in the paragraph 2.6. However, with other

characterization methods, like C-V measurements with an impedance analyzer, SSRM, C-AFM,

an ohmic contact between the sample and the stage is crucial and the samples will be pasted

with silver dye or GaIn on metallic holders. If any comparison is intended to be made

between measurements with any of these characterization methods and SCM measurements,

we have to make sure from the beginning that the sample is placed on the metallic holder in a

correct position for the SCM measurements.

Even if the cantilever/ cantilever chip – sample/ sample holder capacitance is

eliminated, a parasitic capacitance can still exist between the XY stage of the AFM and the

cantilever/ cantilever chip (Figure 2.5.7).

Figure 2.5.7 Output of the SCM sensor as a function of the frequency in relation with the position of the cantilever chip by reference to the

XY stage

Although less important, it still affects the sensitivity of the SCM sensor. In order to

avoid this parasitic capacitance, it is better to position the sample at the very edge of the XY

stage.

Another parasitic capacitance is formed between the XY stage of the AFM and the

SCM electronic module (Figure 2.5.8). When the SCM electronic module is just above the

stage, not only the maximum value, but also the slope which controls the sensitivity of the

SCM sensor signal, drop. In order to avoid this parasitic capacitance, it is better to position the

sample in such a way that the SCM electronic module is not above the XY stage (dotted


95

curve).

Figure 2.5.8 Output of the SCM sensor as a function of the frequency in relation with the position of the SCM electronic module by reference to

the XY stage

Following is a summary of parasitic capacitances that can appear and distort the SCM

signal:

Parasitic capacitance Influence on the SCM signal

Cantilever -sample/ sample holder/ XY stage Small

Cantilever chip – sample/sample holder/ XY stage Great

SCM electronic module – XY stage Small

The values of all these parasitic capacitances can vary from one experiment to another.

A different geometrical setup can lead to the increase or the decrease of the distance between

the respective metallic surfaces and implicitly, to an increase or a decrease of the parasitic

capacitances. For example, the distance between the SCM electronic module and the XY stage

will be greater when the measurements are done on the cross section of a sample placed on a

vertical holder than in the case where the measurements are performed on a plane sample.

Regarding the parasitic capacitance between the sample and cantilever, an important

problem can appear if the distance between the sample and the cantilever changes during the

same measurement, because of the topography of the sample (Figure 2.5.9).

In this case, the topography of the sample, which changes the overall geometry and

thus stray capacitance of the system, will always play a role in the final SCM signal, and has to

be taken into account.


96

Figure 2.5.9 Illustration of the parasitic capacitance modification between the cantilever and the sample, because of the topographical modifications

The topographical features would increase the distance between the cantilever and the

surface of the sample as a function of their height. This is why only perfectly flat samples are

studied in this report (RMS topography < 0.5 nm)

Further, we will try and give an explanation of why the sensor sensitivity drops when a

very high parasitic capacitance by comparison with the tip-sample capacitance, is introduced

in the circuit.

In Figure 2.5.10 we present a general diagram of the circuit. The parasitic capacitances

that might appear have been taken into account (the red capacitor in the Figure 2.5.10)

Figure 2.5.10 Diagram of the SCM sensor [2.28]

As it can been observed, the parasitic capacitance is connected in parallel with the tip-

dielectric-substrate capacitance. As it is well known, in a system of parallel capacitors, the

weight of the resultant capacitance is given by the capacitance with the highest value.

The parasitic capacitances have far greater values than the measured capacitance

between the tip and the sample which is of the order of 10-17 – 10-18 F.

As an example, we have calculated the capacitance between the cantilever and the

sample. The cantilevers that we used for the experiments have the dimensions: length=200

m; width=30 m; tip height (distance cantilever sample) = 10 m. The capacitance is of the

order of 10-15 F, which is a thousand times greater than the capacitance between the tip and

the sample.


97

2.5.3. Comparison with C-V measurements

With an impedance analyzer, the sample is glued on a metallic holder with silver dye in

order to assure a good electrical contact (Figure 2.5.11). The assembly is placed on the XY

stage of the apparatus, from which it is isolated with an insulating material, usually mica.

Two probes make the contact with the microelectrodes from the surface of the sample

and with the substrate, respectively.

Figure 2.5.11 A typical setup for C-V measurements. 1. XY stage; 2. isolating layer, usually mica; 3. metallic holder on which the sample is glued using silver dye; 4.

substrate. 5. dielectric layer; 6. micrometric electrode (gate); 7. Metallic contact to the substrate (back contact); 8. Metallic contact to the gate.

From the Figure 2.5.11, it can be seen that the only capacitance in this setup is the

capacitance to be measured, between the microelectrode and the sample.

On the other hand, in the case of the SCM, besides the capacitance between the

sample and the tip, there are several other capacitances that appear, which distort the SCM

signal (Figure 2.5.12). The origin of these parasitic capacitances is given by the very design of

the apparatus.

Figure 2.5.12 A typical setup for SCM measurements on the surface of a sample. 1. cantilever-sample (or sample holder, or XY stage) parasitic capacitance; 2.

cantilever chip – sample (or sample holder, or XY stage) parasitic capacitance; 3. electrical module – XY stage

From the geometry of the AFM (Figure 2.5.13), it must be also noticed that the piezo-


98

electric tube, which has a metallic shell, is located right above the cantilever holder. It is very

possible for this part of the AFM head to represent another source of parasitic capacitance.

However, given its position, we had no possibility to test its influence upon the SCM sensor or

the SCS signal.

Figure 2.5.13 Image of the AFM head with the attached SCM module (Veeco manual)

2.5.4. Conclusions and perspectives

In this paragraph, we have shown that the geometry of the AFM setup generates

several parasitic capacitances that influence the SCM measurements.

Some of them (cantilever chip – sample/ sample holder/ XY stage) are very important

and determine a huge drop of the signal/noise ratio. Other sources (cantilever-sample/ sample

holder/ XY stage, SCM electronic module – XY stage) are less important, determining a small

drop of the sensitivity of the signal/noise ratio.

It must be noticed that a small modification of the detector output by parasitic

capacitances doesn't really affect the measurements. For now, SCM is not an absolute

characterization method. The electronic circuit has unknown amplification factors. The

amplitude of the SCM signal has a real significance only relative to other measurements

performed in exactly experimental conditions. The parasitic capacitances have a critical impact

on the SCM measurements only when they determine a dramatic drop of the sensitivity of the

detector.

In this light, it can be said that the measurements performed on the cross

section of the samples are very little affected.

The main problem which remains is the measurements that have to be performed on a

planar sample. For now, such measurements can be done only on the edge of the sample,

which is unrealistic. Such measurements can also be performed by applying very high Vac

voltages to compensate for the drop in sensitivity of the signal, which is also unrealistic.


99

In the end, we have to make the following remarks : Considering the absence of this

subject – the parasitic capacitance introduced by the geometry of the experimental setup on

the capacitance measurements – from the instrument manual, we cannot know whether the

manufacturer is aware of this problem or whether this problem has been solved for the new

generations of SCM.

Our results are valid for our instrumental setup (Dimension 3100). SCM performed on

other systems may not be sensitive to the geometry of the experimental setup and such

capacitive couplings may not appear. Feedback from other users may be appreciated.

2.6. Electrical contacts

2.6.1. Introduction

In the previous paragraph, we have seen that several parasitic capacitances, in parallel

with the tip-dielectric-sample capacitance, affect the sensitivity of the detector output. In this

paragraph we continue to investigate whether there aren't other parasitic factors of the SCM

electronic circuit that may influence the output signal.

From the diagram of the electronic circuit (Figure 2.6.1), it can be seen that a part of

the circuit is exposed to a series of factors that may change from one experiment to another

and may alter the results.

Figure 2.6.1 Diagram of the SCM electronic circuit (Veeco Manual)

On the one hand, we have the main electronic - the Nanoscope controller, the

Dimension electronics, the capacitance measurement electronics, the resonant capacitance

sensor module (Figure 2.6.2).


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Figure 2.6.2 Nanoscope electronics (Veeco manual) Figure 2.6.3 Sample setup (Veeco manual)

On the other hand, the portion of the circuit from the electronic module to the XY stage

(Figure 2.6.3) is susceptible to changes that may take place from one measurement to

another.

An UHF transmission line connects the resonant capacitance sensor with the cantilever

holder. The contact between the cantilever holder and the cantilever is insured by a metallic

spring clip. The cantilever tip is put in contact with the surface of the sample. The backside of

the sample is put in contact with the XY stage or with a metallic sample holder.

The UHF transmission line may introduce parasitic elements into the circuit.

It is known that the electrical components are not ideal. Each component, resistor,

capacitor, inductor, even wires, present parasitic resistance, capacitance or inductive effects.

The contact between the cantilever holder and the cantilever may be affected by

the presence of contaminants on the cantilever chip or on the spring clip.

The manufacturer, in the instruction manual, advices to insure an ohmic contact

between the semiconductor sample and XY stage by pasting the sample on a metallic

holder with silver dye or InAs paste. Only by pasting the sample on a metallic holder with InGa

eutectic and with silver dye, an ohmic contact can be assured. If not, the contact will be a

Shottky contact and additional series resistance will be present in the circuit.

The total resistance between the metal and the semiconductor can further increase

because of the native oxide from the back face of the sample and possible contaminants.

A small contact resistance between the metal and the semiconductor is obtained for low

values of the barrier height between the metal and the semiconductor and for high

concentrations of the semiconductor. [2.24 - Ch.5 Metal-Semiconductor Contacts, pg.304].

Also, the contact resistance vary with the type of substrate, p-type or n-type, because of the

differences in the barrier heights.

If the contact resistance is important relative to the bulk resistance of the

semiconductor, the RC time constant associated with the contact resistance can affect the

frequency response of the circuit.


101

Problems may appear at the contact between the tip and the surface of the oxide.

Some samples present important topographical features that may affect the tip-sample

contact. With the measurements performed in atmospheric conditions, a water layer is usually

present on top of the hydrophilic oxide. Contaminants may be found on the tip and on the

sample. The conical geometry of the tip may further induce capacitance parasitic effects.

2.6.2. Electrical contacts

The UHF transmission line (Figure 2.6.4) insures the electrical contact between the

resonant capacitance sensor and the cantilever holder. The UHF transmission line is glued on

the holder and it continues with a metallic wire that makes the contact, at the end of the

spring clip, with the back face of the cantilever.

Figure 2.6.4 Image of the chain which insures the electrical contact between the SCM detector and the cantilever (Veeco Manual)

The geometrical parameters of this group of components, as well as the connexion

between them, are important and can influence the outcome of the sensor output and of the

measurements.

We start to exemplify this affirmation by showing the output the SCM sensor for the

following cases: the transmission line is not connected to the detector, the transmission line is

connected but there is no cantilever on the holder, the transmission line is connected to the

detector and there is a cantilever in place, ready for measurements (Figure 2.6.5).


102

Figure 2.6.5 The detector output for different configurations described in the figure

It can be observed that the detector output has no meaning except for the standard

setup: the transmission line connected to the detector and the cantilever in place. The

properties of the transmission line and the cantilever affect the detector output and bring the

detector signal at maximum in the frequency range of the oscillator (0.88 GHz – 1.05 GHz).

The detector sensibility varies with the geometrical properties of these components. For

example, a variation of the length of the transmission line determines a variation of the output

detector and implicitly, of the SCM signal (Figure 2.6.6).

Figure 2.6.6 The detector output for two transmission lines of different lengths

The two signals from the figure have been recorded by using two Veeco holders with


103

transmission lines of different lengths. For the shorter transmission line, the detector output is

smaller. The difference of sensitivity is significant and can have a negative impact upon the

capacitance measurements.

A loosened spring clip may also determine a loss of signal during the scans (Figure

2.6.7).

This problem, in appearance a simply mechanical problem easily fixed, can create a lot

of difficulties. The user may easily and incorrectly assume that the bad tip is at the origin of

the poor SCS signal. There are no contact problems between the cantilever and a loosened

spring clip when the tip is not engaged on the surface and the detector output doesn't signal

any anomaly.

Only when the tip is engaged on the surface, the strain exerted upon the cantilever

may determine intermittent lose of contact between the cantilever and the spring clip.

Figure 2.6.7 SCS curves with the spring clip loosened and in correct configuration

This problem may be detected during the measurements by observing small, unusual

variations of the position of the laser spot on the photodiode.

The choice of the spot where the cantilever makes the electrical contact with the spring

clip may also slightly affect the output of the detector (Figure 2.6.8).


104

Figure 2.6.8 Output of the SCM sensor as a function of the contact

cantilever chip -holder

Although the difference in the detector output is not significant, it is obvious that the

position of the cantilever on the holder can bring modifications of the SCM amplitude signal for

measurements performed on the same sample, every time we reposition a cantilever on the

holder.

2.6.3. The sample backface contact

2.6.3.1. Experimental

In order to see what is the influence of the back face contact of the sample upon the

SCM signal, the following experiment has been done.

For the experiment, it has been used the test sample consisting in a n-type substrate

doped 1015 cm-3, covered with a 2 nm thermal oxide.

The sample has been cut in two pieces. One piece has been pasted on a metallic holder

with InAs eutectic and silver dye. SCS measurements have been performed on the two

samples, one pasted on a metallic holder, the other one simply put in contact with the XY

stage. The results are showed in Figure 2.6.9.

Figure 2.6.9 SCS measurements on two pieces cut from the same sample.


105

A significant series resistance would diminish the value of the capacitance in

accumulation, which would lead to the modification of the slope of the C-V curve. The

amplitude of the SCS signal should therefore diminish. As it can be observed, there is a slight

difference between amplitude of the two measured signals. Also, the shift between the trace

and the retrace for the SCS signal performed on sample with a metallic back contact (not

shown) seems to be slightly smaller.

However, the difference in amplitude between the two signals is very small, in the

range of noise level and normal variation of amplitude of the SCS signal from one

measurement to another for the characterizations done in ambient conditions. Apparently,

although the measurements are done at a frequency of 1 GHz, the evidence shows that the

series resistance given by the back-contact of the sample doesn’t have a major impact on the

capacitance signal.

Because the contact resistance depends on the type of the substrate, the same

experiment has been performed on a sample with a p-type substrate 1015cm-3, covered with a

5 nm thermal oxide.

The results are similar to the previous ones. There is very little difference between the

SCS curve on the sample pasted on a metallic holder and the SCS curve placed directly on the

XY stage.

2.6.3.2. Comparison with C-V measurements

With the C-V measurements, it is known that if the sample is simply put on the metallic

holder, there is no ohmic contact between the back face of the sample and the holder. The

contact will be a Schottky contact.

Even if the sample is pasted with InGa and silver dye on a metallic holder, the series

resistance can still become to great at high frequencies (Figure 2.6.10)

Figure 2.6.10 C-V measurements which shows the influence of the series resistance upon the measured capacitance

On the other hand, we have seen from the Figure 2.6.9 that the amplitude of the SCS


106

signal changes very little when there is no ohmic contact between the back face of the sample

and the holder.

The nature of the back contact influences the C-V measurements, but doesn't seem to

influence the SCS measurements in the same amount. In order to understand why, we

calculated the capacitance and the total impedance for an ideal capacitor in series with an

ideal resistor, for different frequencies.

*j

Z RCw

= -

We have taken into consideration that the series resistance of a Shottky contact is

approximatively 1kΩ.

In the case of C-V measurements, the value of the capacitance in accumulation is of

the order of nano-farads. Here is the table with the calculated values of the total impedance

for different frequencies:

Frequency Capacitive impedance ZC Series resistance R Total impedance Z

1 kHz 159 kΩ 1 kΩ 160 kΩ

1 MHz 159 Ω 1 kΩ 1.159 kΩ

1 GHz 159 mΩ 1 kΩ 1 kΩ

As it is well known, the capacitive impedance varies inversely proportional with the

frequency. For small frequencies, the capacitive impedance is high and has the dominant

contribution to the total impedance. For high frequencies, starting with 1 MHz, the series

resistance has a dominant role and influences significatively the results.

With the AFM, the measured capacitance is of the order of femto-farads and the

measurements take place at a frequency of around 1 GHz.

Frequency Capacitive impedance ZC Series resistance R Total impedance Z

1 GHz 159 kΩ 1 kΩ 159 kΩ

Although the SCS measurements are performed at much higher frequencies than the C-

V measurements, the capacitance of the MOS structure is also much smaller because of the

small size of the tip. Thus, the capacitive impedance will be much greater than the series

resistance, which shouldn’t play any role at all.

2.6.3.3. Conclusions

The back face contact slightly influences the SCS measurements. Although the

difference in the amplitude of the signal is very little, from our experience, it seems to always

be slightly higher in the case of the samples with a metallic back-contact.

There are arguments in the favor and against the use of an ohmic contact for the back

face of the sample.

In the favor of the procedure, the main argument is that it is best to have a series


107

resistance as small as possible. To insure an ohmic back-contact represents the standard

procedure with all the electric measurements. The amplitude of the signal is slightly higher

and the difference between the trace and the retrace seems to be slightly smaller.

On the other hand, such a procedure is useless with the AFM if only qualitative

measurements are envisaged. Compared with the capacitive impedance, the series resistance

is negligible. In addition, the procedure has several drawbacks.

The creation of an ohmic back contact is time consuming. It may add an unnecessary

step to the overall procedure of the characterization of a MOS structure.

As we have seen in the previous section 2.5, a parasitic capacitance can appear

between the metallic holder and the AFM cantilever/chip, which will distort the signal.

After the use of silver dye and InGa eutectic for gluing the sample on a metallic holder,

it is unlikely to be able to submit that particular sample to further technological procedures

(annealings, HF etching of the gate oxide, cleaning procedures etc).

2.6.4. The tip-sample contact

One of the most common parasitic features with the SCS is the presence of a secondary

peak.

In the literature, some authors consider that this effect may appear as a consequence

of tip depletion [2.8]. Many tips used with the SCM are not metallic tips, nor they have a

metallic coating, such as the diamond coated tips that we have used for the measurements

performed during this thesis or the highly doped semiconductor tips.

In order to verify this hypothesis, we have performed simulations of the tip-oxide-

substrate with DESSIS and we have compared the results with the capacitive signals obtained

with the SCM. (DESSIS - Device Simulation for Smart Integrated Systems - from ISE AG, is a

multidimensional, electrothermal, mixed-mode device and circuit simulator for one, two, and

three-dimensional semiconductor devices. It incorporates physical models and numerical

methods

for the simulation of most types of semiconductor devices. A real semiconductor device, such

as a

transistor, is represented in the simulator as a virtual device whose physical properties are

discretized onto a non-uniform grid of nodes).

Figure 2.6.11 ISE-TCAD simulation of a M.O.S structure with a semiconductor tip of the same type of

doping concentration as the substrate. A secondary peak appears on the dC/dV signal as a result of tip

depletion (left - simulated C-V curve; middle - derivative of the C-V curve; right - modulus of the

derivative of the C-V curve)


108

A M.O.S structure has been simulated (Figure 2.6.11): a 2 nm thermal oxide on top of

a n-type semiconductor of different doping concentrations. We represented the tip as a highly

doped n-type semiconductor square electrode. The shape of the AFM tip has not been

introduced into the equation. We considered that the 3D shape of the tip would have not been

relevant for the goal of this simulation and it would have introduced supplementary and

unnecessary complications to the simulation.

If the substrate has the same type of doping concentration as the tip, a secondary peak

appears on the dC/dV signal. The explanation is that when the tip is in accumulation, the

substrate will be in inversion and vice-versa.

In the case when the substrate is of a different type of doping concentration (p-type)

than the tip, a secondary peak doesn't appear anymore (Figure 2.6.12)

Figure 2.6.12 ISE-TCAD simulation of a M.O.S structure

with a semiconductor tip of the same doping

concentration as the substrate. The effect of tip

depletion superimposes with the substrate depletion

In this case, both semiconductors, the substrate and the tip, will be in accumulation or

in inversion regime in the same time and the two effects will superimpose.

However, the results obtained with the simulations don't match the experimental

evidence.

Figure 2.6.13 SCS signal obtained with a full-metal

(tungsten) tip on a 5 nm thermal oxide (tips fabricated

by K. Akiyama at Tohoku University - Japan and

provided by Prof. George Bremond)

On one hand, a secondary peak appears even when the tip and the substrate are doped


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with different type of dopants. On the other hand, a secondary peak appears on the SCS

signals when full-metal tips are used (Figure 2.6.13).

Another explanation has been found for the presence of a secondary peak on many of

the samples we have characterized, based on a comparison between the C-V and SCS

experimental setups.

Although SCM is a derivate technique from macroscopic C-V measurements with

impedance analyzers, there are several important differences between the 2 techniques. One

of the differences is that, in the case of C-V measurements with an impedance analyzer, the

measurements are done on deposited metallic electrodes.

The use of electrodes has several advantages.

The surface of the electrodes can be measured with an optical microscope. As we have

seen in the paragraph 2.3.4, the properties of the gate oxide and the properties of the

substrate can be calculated from the experimental C-V curve (the thickness of the oxide, the

doping concentration of the substrate, the surface potential, the flatband bias, the density of

the interface states, the defects density in the volume of the oxide etc) if we know the

dimension of the gate electrode.

The electrodes protect the MOS structure from most of the parasitic light, as we have

shown in the paragraph 2.4.4. The electrodes have a thickness of several hundred

nanometers and the light doesn't transmit across them. Minority carriers can be generated

only at the edges of the electrodes and migrate under the electrodes.

The electrodes also protect the MOS structure from contaminants. The surface of the

oxide is cleaned before the electrode deposition. The electrodes are deposited with an electron

gun in ultra-vacuum. In consequence, there are no contaminants between the surface of the

oxide and the gate electrodes.

The oxides are hydrophilic. A layer of water forms on the surface of the oxides from the

humidity in the air. The water molecules can be easily polarized under the influence of an

electric field. Further on, they can dissociate and generate mobile charges on the surface of

the oxide. These ions can be attracted to the tip/ repulsed away from the tip under the

influence of an alternative voltage. Under the influence of strong electric fields, oxide charging

or even anodic oxidation may occur. The metallic electrodes prevent the generation of a layer

of water on the surface of the oxide.

The C-V measurements on the thermal oxide don't present any sign of hysteresis or

increase of the signal in inversion (Figure 2.6.14).


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Figure 2.6.14 C-V curves operated on 100 μm× 100 μm NiAu electrodes

on a thermal 5 nm thick silicon oxide. The measurements show that

there is no hysteresis and no an increase of the signal in inversion

To test the role of the tip-sample contact in the interpretation of SCS, a comparison

has been made between C-V curves operated on NiAu electrodes and the same measurement

operated using a large, home-made tin tip directly on the oxide.

The size of the tip has been chosen so that the level of the signal is sufficient to record

C-V curves using the same impedance analyzer as used for the C-V curves recorded on the

electrodes.

2.6.15 Example of C-V curves obtained with a macroscopic tin tip directly on the oxide surface (without electrodes). The hysteresis obtained while ramping from negative to positive and then from

positive to negative voltages indicates the presence of charging effects

The results can be found in the Figure 2.6.15 and 2.6.16 : an hysteretic behavior

and/or an increase of the signal in the inversion region is recorded, significative of the bad

electrical contact between the probe and the oxide.


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2.6.16 Example of C-V curves obtained with a macroscopic tin tip directly on the oxide surface (without electrodes). The increase of the signal when

positive voltages are applied indicates the presence of minority carriers.

However, these experimental results cannot constitute more than a preliminary proof

that something may be wrong with the tip-sample contact with the AFM. Although the C-V

measurements with a home-made tin tip directly on the surface of the oxide respect in

principle the AFM concept, there are important differences.

The dominant forces between an AFM tip and the surface of a sample are the Van der

Waals forces. The forces between the macroscopic tin probe and the sample are rather

mechanical: the probe is forced pushed against the surface of the sample.

The tin probe presents numerous microscopic fissures which render the probe-sample

contact flawed. It is very likely that between the apex of the tin probe and the surface of the

sample remain lots of air gaps, which is not the case between the apex of the AFM tip and the

surface of the sample.

Therefore, in order to confirm this hypothesis, we recorded SCS curves on the same

sample (thermal silicon oxide, 5 nm thick) and on the same electrodes but with a reduced

size, so that it becomes possible to obtain a signal compatible with the range of the SCM. In

order to be sure that we respect exactly the same experimental conditions as with the C-V

measurements, a nanometric electrode has been cut directly in a micrometric electrode using

the tip of the AFM. Very hard diamond coated tips (stiffness in the 40 N/M range, typically

used for the Scanning Spreading Resistance - SSRM - mode of the AFM) have been used to

engrave the electrode. The deepness of the engraved region is higher than the thickness of

the electrode, so that the region delimited by the grooves is electrically independent of the

rest of the electrode. The resulting electrode is ≈ 700 nm × 700 nm large. SCS recorded on

this submicronic electrode can be found in Figure 2.6.17.


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Figure 2.6.17 Topographical image and SCS measurement on a

700nm x 700nm electrode, on a 5nm thermal oxide. SCS doesn't

present a secondary peak, nor hysteresis.

A direct comparison can be made with the measurements from Figure 2.6.18, where

the AFM tip has been placed directly on the thermal oxide, as usual with the AFM.

Figure 2.6.18 SCS operated with the tip directly on the oxide:

hysteresis and minority carriers are present in the signal. Voltage

is applied to the sample.

From the comparison between the macroscopic C-V curves form Figure 2.6.14 and from

the SCS spectroscopy shown in Figure 2.6.17 and Figure 2.6.18, it is clear that the quality of

the tip-sample contact is a major parameter which influences dramatically the interpretation of

SCS.

No double peak is obtained in SCS when the measurement is made on a correct

electrode, with a clean interface between the electrode and the oxide, whereas a hysteresis

and a peak in inversion (which traduces an increase in the signal in the inversion part of the

C-V curve) is recorded directly on the oxide surface, similar to C-V curves obtained from a

metallic tip directly on the oxide surface in Figure 2.6.15 and Figure 2.6.16.


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All comparisons put together accreditate the role of the tip-sample contact in the

interpretation of SCS and emphasize the need to enhance it if SCS aims at becoming a

metrological tool at the nanoscale.

It must be noted that the maximum of the SCS curves from Figure 2.6.17 is situated at

+0.8V (the bias applied on the substrate), the same value that the inflexion point of the C-V

curves from Figure 2.6.15. (-0.8 V, the voltage applied on the gate).

Although this observation may be considered trivial, given that the both measurements

have been done on the same sample (the same gate metal with the same workfunction, the

same oxide with the same density of defects and the same thickness, the same substrate), it

represents however the first perfect match that we have observed between the C-V and the

AFM measurements.

This correlation is, in our opinion, a significant and promising sign that SCM can

become a great metrological tool, once the numerous experimental variables are understood

and controlled.

2.6.5. Conclusions

The components (the UHF transmission line, the holder, the cantilever) and the

electrical contact between the holder and the cantilever have a minor impact upon the

sensitivity of the detector and may vary from one measurement to another. The problems may

deepen if the standard components are not used and taken care.

The sample backcontact seems to influence very little the capacitance measurements.

Even at the high frequencies used with the AFM, the series resistance is insignificant by

comparison with the capacitive impedance. The explanation is that the very small values of

the measured capacitance conduct to a very high capacitive impedance, several orders higher

than the series resistance.

The face contact however, between the tip and the sample, is negatively influenced by

a multitude of factors: contaminants on the tip and on the surface, surface humidity,

topography etc.

Each of these factors will be analyzed and their influence evaluated in the next

paragraphs. The oxide charging and the anodic oxidation will be discussed in the Chapter 3:

Oxides.

2.7. Influence of the sample topography on the capacitance signal

2.7.1. Introduction

Some samples studied with the SCM present topographical features, either because

unavoidable defects during the sample preparation, or because of the intrinsic structure of the


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sample to be analyzed.

Many of the SCM measurements must be performed on a cross-section of a sample,

given that the doping profiles constitute multi-layers of different doping concentrations.

Cleaving the samples is not an easy procedure and can result in topographical artifacts.

The topographical artifacts that can result from cleaving can lead to misinterpretations

of the doping profile (Figure 2.7.1).

Figure 2.7.1 Comparison between a SIMS profile and an SCM image. A drop of the SCM signal

can be observed on the left of the cross-section, caused by a topographical artifact.

The other method of preparation of a sample for SCM analysis is by polishing. We

present an example on beveled samples obtained by polishing. Because the

topographical pattern doesn't match the doping pattern, the effect of the surface roughness on

the SCM image is obvious (Figure 2.7.2).

Figure 2.7.2. Beveled surface of staircase like sample. Topographical image

(left). SCM image (right)

Sometimes, the nanodevices that are to be characterized have an inherent topography.

It is the case, for example, of the standard sample from Veeco which represents a RAM

memory (Figure 2.7.3).


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Figure 2.7.3. Veeco standard sample of a RAM memory.

Topographical image (left). SCM image (right) (Veeco

Manual)

As it can be seen from the Veeco images, the topography presents features of up to

400 nm high and the topographical features match perfectly the different doping domains that

appear in the SCM image. The question that automatically arises is how can someone know

how much of the SCM signal is due to the doping concentration and how much is due to the

changes in the topography of the sample.

One possible application with the SCM is the study of charge storage effect of individual

nanodots and the amount of charge stored in individual nanodots for the long-term memory

retention.

In this case the nanodots themselves introduce topographical features that cannot be

avoided (Figure 2.7.4). But how much of the SCM signal is due to the doping concentration

and charge retention and how much to the topography variations?

Figure 2.7.4. InAs nanodots. Topographical image (left). SCM image (right) after charging

a 500 nm x 500 nm area (center of the image, dark contrast)

2.7.2. Experimental

In order to be able to study separately the supposed stray capacitance given by the

topography of the sample, this signal should be isolated from all the other sources of

capacitance signal.

We believe that this stray capacitance is given by small hops of the tip which determine

the interposition of an air layer between the tip and the sample for short periods of time,


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which leads to the creation of a capacitance-like structure (Figure 2.7.5).

Figure 2.7.5. Possible parasitic capacitance that may

appear between the tip and the sample because of

topographical features of the sample

In order to verify this hypothesis, metallic samples have been designed. On doped

substrates, different metallic layers have been deposed by evaporation with an electron gun.

On some metallic layers (aluminum, nickel) metallic steps of different heights have

been fabricated by lithography or by AFM engravement. These metallic steps are intended to

simulate the topography of the samples from the previous examples - the topography of the

Veeco standard samples, beveled samples or cross-section samples (Figure 2.7.6).

Figure 2.7.6a Nickel metallic surface obtained

by AFM engravement of a micrometric

electrode used for CV measurements

Figure 2.7.6b SCM signal on the nickel surface.

Vdc = 0 V. Vac = 100 mV

Other metallic layers (titan) present in addition a granular topography, similar to the

case of nanodots or topography features met on thick high-k dielectric layers (Figure 2.7.7).

Figure 2.7.7a. Titan metallic surface obtained by Figure 2.7.7b SCM signal on the titan surface, given


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electron canon deposition and lithography by the step and surface rugosity. Vdc=0V. Vac=100mV

A conductive tip in contact with a metallic sample should not give any capacitance

signal at all. But if the conductive tip leaves the surface for a short period of time, a

capacitance formed by the tip, air and the metallic surface will appear and a capacitance signal

will be observed.

The study was made by analyzing various parameters that could affect the SCM signal

(different topographical features of the samples, size of the tip, deflection setpoint – the

interaction force between the surface and the tip – gains, scan direction).

2.7.3. Direction of approach of a topographical feature

The first measurements were done on a nickel metallic step obtained by AFM

engravement of a micrometric electrode used for C-V measurements.

By scanning this metallic surface, there shouldn’t be any capacitance signal at all,

because there is an ohmic contact between the surface and the conductive tip. However, at

the position of the step, a capacitance signal can be clearly seen (Figure 2.7.8 and Figure

2.7.9).

Figure 2.7.8. SCM signal at the position of the metallic step. Perpendicular scan on the step

A large diamond coated tip, specific to SSRM applications, utilized to previously engrave

the metallic step, has been used. Scans were performed in the directions perpendicular and


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parallel to the step (Figure 2.7.8 - 2.7.9) .

Figure 2.7.9. SCM signal at the position of the metallic step. Parallel scan on the step

From the two SCM images, it can be observed that the capacitance signal is higher

when the scans are performed perpendicular to the metallic step.

Figure 2.7.10. Superimposed average topographical profile and average SCM signal. Left

image - Perpendicular scan. Right image – parallel scan

2.7.4. Scan direction

The possible scan directions using an AFM cantilever are represented in Figure 2.7.11.

Figure 2.7.11. Scanning directions with an AFM tip

By analyzing the capacitance signal between the trace (backward movement) and the

retrace (forward movement), it can be seen that the capacitance signal almost vanishes in the

first case (Figure 2.7.13). From these experiments, we conclude that the best scan direction

for diminuating the stray capacitance is a backward movement of the cantilever.


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Figure 2.7.12. Topography of the titan 10nm step surface

Figure 2.7.13 Difference between trace (left) and retrace (right) of the capacitance signal.

DS=0.3V. Vac=100mV

2.7.5. Deflection setpoint (DS)

By variating the deflection setpoint, it can be seen that for a lower DS (Figure 2.7.14),

the stray capacitance disappears for almost all the topographical features, although the

deflection error signal doesn't become more important.


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Figure 2.7.14a. DS=0.3V+0.2V. Vac=100mV Figure 2.7.14b. DS=0.3V-0.2V. Vac=100mV

2.7.6. Size of the tip

The parasitic capacitance signal diminishes in the case of smaller tips (Figure 2.7.15).

However, we cannot be sure if this happens because smaller tips follow better the

topographical features or because smaller tips give a smaller capacitance signal.

Figure 2.7.15. SCM images performed with tips of different sizes. Diamond coated tip (left),

PtIr coated tip (right)

2.7.7. Solutions for decreasing the effect of stray capacitance arising from

topography

The topographical features due to the preparation of the sample are random and it is

very hard to avoid stray capacitance, even when optimizing the different scan parameters.


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However, the advantage in the cases is that the topographical pattern doesn't match the

doping pattern and the stray capacitance can be identified by comparing the SCM image

with the topographical image . On the other hand, when the topography is an intrinsic part

of the sample, the topographical pattern match the doping pattern and it is hard to decide

whether or not the topography influence the capacitance signal. Even if one can reasonably

deduce that the stray capacitance will be present only where topographical transitions exist,

the situation is even more complicated when analyzing very small structures, several

nanometers in size (like nanodots).

We must notice that, although we evidentiated experimentally the existence of the

stray capacitance given by topography, a explanation has yet to be found for this

phenomenon. SCM measures a difference of capacitance, not an absolute capacitance. When

the tip shortly leaves the surface at the metallic steps, even if a temporary capacitance

appears (tip-air-metallic surface), this capacitance is not variable. Thus, an SCM signal

shouldn't appear.

One probable cause can be the water layer on the surface of the sample. It is possible

that the H2O dipoles follow the frequency of the AC bias applied to the sample and generate a

variation of capacitance. For verifying this hypothesis, SCM measurements in a controlled

environment should be performed.

In the case of the SCM measurements performed in atmospheric conditions, we

propose several solutions for eliminating/diminishing the stray capacitance given by

topography

One solution for diminuating the role of the stray signals from the SCM images and

increasing the signal/noise ratio is to use high Vac. Vac biases of hundreds of milivolts and

even several volts are being used. However, we considered that, although high Vac do not

affect the correctitude of the results in the case of failure analysis images, such biases are

unrealistic. SCM is a differential measurement method. The SCM signal should reflect as

accurately as possible the slope of a C-V curve for the quantification to be possible. Other

issues concerning the depletion under the tip makes us consider that high Vac biases should be

avoided as much as possible.

Using sharper tips when there are topographical issues. We must accentuate that the

choice of the tip varies from application to application. Using sharper tips is better for the case

when dealing with samples with significant topography, as with the any topographical

measurement with the AFM. It is also for the best in the cases when resolution is an issue, like

on samples with very small nanodevices. However, sharper tips may not be the best choice for

other samples. Sharper tips means a smaller signal/noise ratio.


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Optimization of the deflection setpoint. As it has been demonstrated, the factor that

influences the most the stray capacitance given by the topography is the deflection setpoint.

In these cases, it is best to try to optimize the deflection setpoint, preferably with prior test

measurements on standard sample as the ones we proposed.

The best solution is, in our opinion, using SCS measurements instead of SCM scans.

Stray capacitance generated by topography is an issue only when imaging. It is best to use

SCM scans only for qualitative failure analysis. We consider that the best capacitance method

with the AFM, for obtaining accurate information about the doping profiles and oxide

properties, is by performing SCS measurements. In addition, SCM images present a major

disadvantage in the fact that the scans are done at a fixed dc bias.

2.7.8. Conclusion

With the metallic samples we proposed, we have isolated the stray capacitance

generated by the surface topography from other various factors. We have shown how this

stray capacitance varies with different parameters, such as the size of the tip, the deflection

setpoint, scan direction.

We have proposed several solutions to counter this stray capacitance, the best of

which, in our opinion, is to perform topography independent SCS measurements at fix point

instead of SCM scans.

2.8. Tip properties

2.8.1. Introduction

We continue the analysis of the tip-sample contact by taking a closer look to some of

the properties of the tip.

In order to see what influence might have the tip on the capacitive measurements, we

have recorded the signal given by 19 different diamond tips, from two different

manufacturers, on a well-known sample. The sample consists in a lightly doped n-type

substrate, 1015cm-3, covered with 2 nm thermal oxides. The results are shown in Figure 2.10.1

and 2.10.2.


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Figure 2.10.1 SCS signals recorded with 9 different new tips from Nanosensors, on a 2 nm thermal oxide

The measurements have been performed in the absence of parasitic laser light (Section

2.2) and parasitic capacitances (Section 2.2). The sample doesn't present any topography at

all (Section 2.6). All the electrical contacts have been verified (Section 2.5).

Figure 2.10.2 SCS signals recorded with 10 different new tips from Veeco, on a 2 nm thermal oxide

However, it can be seen that not much has change by comparison with the Figure 2.1.1

from the beginning of this chapter when recording SCS signals with different tips and in

different experimental conditions (unknowingly).

Although now we have a good signal/noise ratio for most of the tips that we use, the

same problems persist. There is a high variation in the amplitude of the signal. The maxima of

the SCS curves are located at different voltages for different tips, although the sample

remains the same and the tips are covered with the same p-doped diamond material.

In the following sections of this paragraph, we will characterize the AFM tips by

different means in order to find out the source of the non-reproducibility of the signal. We will

also try and find out the radius of the tips, required for the calculation of the doping

concentration. We will also discuss some of the properties of the tips required for the

capacitance measurements with the AFM.


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2.8.2. Characterization of AFM tips with standard AFM sample gratings

The tip represents one of the 'plates' of the MOS capacitor. Thus, the dimension of the

tip affects the amplitude of the signal. The higher the tip radius, the higher the measured

capacitance. It is important to know the radius of the tips in order to explain the differences

from case to case in the amplitude of the capacitance signal. Also, changes in the amplitude

of the signal during a scan could be explain and by the change of the tip radius during the

scan (tip wear).

Standard calibration samples can be used for learning the tip radius (Figure 2.10.3).

2.10.3 GZT1 standard sample NTMDT for measuring the tip radius. Images from NTMDT company

The calibration sample consists of a topography with very sharp features.

The final AFM image in these places will be a convoluted signal between the

topographical features and the AFM tip. After the AFM image is obtained, a specialized

software, like SPIP from NTMDT, is needed for the deconvolution of the tip.

2.10.4 AFM image on the GZT1 grating sample with a diamond coated tip (left) and the

deconvoluted image of the tip - SPIP software (right). Armel Descamps, Ingenieur INSA

Such a procedure has several important applications for the SCM.

The radius of the tips can be obtained. Also, the tip wear can be evaluated by

comparing the tip size before and after the SCM measurements.

However, this procedure gives us no information about the coating of the tips or about

possible contaminants on the surface of the tips.

2.8.3. Characterization of AFM tips with SEM

The AFM tips can be visualized with a Scanning Electron Microscope. By direct

visualization, it is possible to obtain the radius of the tips. It is also possible to observe

eventual problems concerning the properties of the tips (shape, coating etc)


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The description of the principle of a Scanning Electron Microscope exceeds the

framework of this thesis, so we will move on directly to the characterization of the tips

performed with SEM.

The diamond coated tips, as seen with the SEM, have a pyramidal shape and a granular

structure, given by the diamond coating (Figure 2.10.5)

Figure 2.10.5. SEM images of a diamond coated tip

SEM is a very powerful characterization tool, capable to measure the tip radius. With a

sufficiently high magnification, the end of the tip can be observed and recorded (Figure

2.10.6).

Figure 2.10.6 SEM images of a diamond coated tip at 50.000x magnification (left) and 100.000x magnification

(right)

A comparison between different tips can also be made. (Figure 2.10.6-2.10.7). While

the diamond tips exceed easily 200 nm in diameter, the PtIr tips have a diameter smaller than

50 nm.


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Figure 2.10.7 SEM images of a PtIr5 coated tip at 5.000x magnification (left) and 80.000x magnification (right)

The images presented previously have been made with an old JEOL JSM-840. The

images taken with modern SEMs, like ESEM FEI-XL30 are even clearer and allow us to have

better images of the shape of the tip.

Figure 2.10.8 SEM images of a diamond coated tip (left) and a PtIr5 coated tip at 65.000x magnification

The most important results with the MEB by comparing with the previous

characterization method with the AFM, concern the state of the coating of the tips and the

contaminants.

In Figures 2.10.9-2.1011 different problems with the coating can be observed.


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Figure 2.10.9 SEM image of a diamond coated tip at 20.000x magnification

Figure 2.10.10 SEM image of a diamond coated tip at 20.000x magnification

The state of coating has an important impact upon the SCS signal.

The tip wear determines a change in the amplitude of the SCS signal over time. The

complete loss of coating at the end of the tip means a shift of the peak of the SCS signal, due

to the change of the gate workfunction, from the one corresponding to the coating to the one

corresponding to the silicium. On an SCM image, the loss of coating translates into contrast

reversal.

Problems with the tip coating, observed with SEM can constitute one of the reasons of

the non-reproducibility of the SCS signal.

Figure 2.10.11 SEM image of a diamond coated tip at 20.000x magnification (left) and 80.000x magnification (right)

Figures 2.10.12-2.10.15 represent a few examples of SEM images of contaminated tips.

More than 50% of the tips examined with SEM presented some form of contamination.


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Figure 2.10.12 SEM image of a contaminated diamond coated tip (18.000x)

Figure 2.10.13 SEM image of a contaminated diamond coated tip (20.000x)

The contaminants on the SCM tips can have an unpredictable effect on the capacitance

signal, ranging from the total loss of signal, shifts of the SCS peak, modification of the shape

of the SCS signal etc.

Figure 2.10.14 SEM image of a contaminated (40.000x)

Figure 2.10.15 SEM image of a contaminated (40.000x)

With SEM, the shape and the size of the tip, the state of the coating, possible contaminants,

can be observed.

SEM has also several drawbacks related to the characterization of AFM tips.

SEM is destructive with regard to the cantilever. The cantilever must be pasted on a metallic

holder in order to be able to perform the SEM characterization. We have tried to find a solution

around this problem by manufacturing a cantilever holder which fixes the cantilever by

mechanical means. This method was a partial success. The cantilevers were not destroyed

anymore in the process, but it appeared the risk that the cantilevers fall off the holder and

break during the characterization.

SEM characterization is time consuming, especially if we take into account the high number of

characterizations that must be performed (for each tip). The SEM measurements are done in


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ultra-vacuum, which takes time to put in place. Localizing the tip with SEM and obtaining a

high resolution image is no less time consuming. Also, this method assumes the simultaneous

access to a SEM and to an AFM, which is a requirement hard to meet for any laboratory.

2.8.5. Conclusions

The AFM tip and its properties have a great impact upon the capacitive measurements.

The tip represents the gate of the MOS structure. Its dimensions (its radius) must be

known if any quantification of the doping profiles is to be attempted.

The slightest problem concerning the wear of the tip over time, the loss of coating,

contamination, render the capacitive measurements false.

The tip geometry has an impact upon the intensity of the electrical field. For the same applied

bias, a sharper tips will generate a stronger electrical field which will favorize unwanted

tunneling currents or even the dielectric breakdown.

In the semiconductor industry, there is an entire field of research concerned with finding the

best materials for the gate of transistors. Metal electrodes are replacing polysilicon electrodes

in order to eliminate gate depletion and to be able to adjust the threshold voltage through the

control of the gate workfunction. With the SCM, there exists a similar interest in the

engineering of the workfunction of the tip. In order to minimize the electrical fields across the

thin gate oxide, biases as small as possible should be applied to the M.O.S structure. In order

to be able to do this, the tip workfunction (the choice of the metal coating) should be adjusted

in such a way that the peak of the SCS signal be located as close of possible of zero volts.

The tip workfunction can be measured with Kelvin Force Microscopy. We have done extended

measurements with the KFM, trying to find out the workfunction of various tips: diamond

coated tips, PtIr tips, highly doped silicon tips. Layers of different metals (gold, nickel,

platinum, titan, aluminum, copper) have been deposited on semiconductors or mica and

served as samples for measuring the workfunction of the tips. However, the same non-

reproducibility of the signal observed with the SCM and C-AFM, has also been observed with

the KFM. Variations of the tip workfunction up to 500 mV have been noticed.

Rigorous methodologies for the control of the properties of the tips must be put in place.

2.9. The AFM Piezoelectric Scanner

2.9.1. Introduction

With the AFM, the positioning of the tip relative to a sample with nanometric resolution

is accomplished with piezoelectric transducers which expand and contract proportionally to an

applied voltage [2.25 - pg.420-pg.423].

The commercial piezoelectric transducers installed on most commercial AFMs are

cylindrical tubes (Figure 2.9.1). These tubes are most often fabricated from a lead zirconium

titanate material, ranging in size from 3-20 mm in diameter, and 10-50 mm in length, with a


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wall thickness of 0.5-1 mm.

There are two main electrodes, one on the exterior of the tube and the other on the

interior of the tube. The electrode on the exterior of the tube is further divided into 4

electrodes.

Figure 2.9.1 The structure of a piezoelectric scanner

The application of a voltage between the electrodes on the inner and outer surfaces of

the tube causes the tube to bend or to increase or decrease its length.

A voltage applied between the inner electrode and one of the outer electrodes causes

the tube to bend. The application of complementary biases to two electrodes on opposite sides

of the tube doubles the magnitude of the bending. Biases applied on the other two electrodes

induces bending orthogonal to the previous mode. These two bending modes are used for

lateral displacements of the probe relative to the sample (x-y displacements).

Vertical displacements are imposed by changing the bias of the inner electrode relative

to all four of the outer electrodes.

Ideally, the relative length of the piezoelectric scanner varies linearly with the applied

bias. However, the piezoelectric scanners are very sensitive elements, susceptible to non-

linearities caused mainly by ageing or by working in an improper environment, for example in

excessive temperatures (over 150o C) or excessive humidity.

Among the non-idealities that can occur with an AFM piezoelectric scanner and that can

have a pronounced negative impact upon the electric measurements, we mention: hysteresis,

creep, cross-coupling.

It is said that the piezoelectric transducer manifests a hysteretic behavior when the

paths of extension (trace) and contraction (retrace) of the PZT scanner are different due to a

hysteretic relationship between the applied voltage and the change in its dimensions (Figure

2.9.2)


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Figure 2.9.2 Example of a hysteresys curve [2.26]

As a consequence, the surface of a sample seems to be different in the two AFM images

scanned in the forward direction (trace) and the backward direction (retrace). The

discrepancies are more accentuated when the images are scanned with the piezoelectric

transducer very contracted or very extended.

The differences between the trace and the retrace become even more evident when the

scan is performed on large images, of tens of microns.

The scanner creep is a phenomenon that occurs especially when an abrupt voltage is

applied to the scanner. Such a voltage is usually applied in the common situation when the

user wants to move the tip from one spot to another (to make an offset).

When such a voltage is applied, the scanner doesn't extend or contract all at once.

Instead, the change occurs in two steps. In the first step, the main change in the shape of the

scanner occurs as a response to the applied bias. In the second step, called creep, the scanner

slowly continues to extend or contract, although there isn't any modification in the applied bias

(Figure 2.9.3).

Figure 2.9.3. Example of the creep effect [2.26]

The cross-coupling refers to the fact that the lateral bending of the scanner, meant to

generate a movement of the tip in the x–y plane, can also generate an undesired vertical

movement of the tip.

Because of the fact that the cantilever is in feedback, the scanner is commanded to

retract the tip in order to avoid the approach to the surface. As a consequence, it retracts in


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the z direction and the AFM will register an artificial concave surface (Figure 2.9.4)

Figure 2.9.4. Example of the creep effect [2.25]

Further on, we will give a few examples of how these non-linearities may impact the

electrical measurements with the AFM.

2.9.2. Experimental

1. In the case of the doping profile characterization on the cross-section of the samples,

the doping layers are to be found, in most of the cases, on the very edge of the sample

(Figure 2.9.5)

Figure 2.9.5. Doping profile on the cross-section of a sample. 1-initial engagement of the tip; 2-offset of

the tip to the doping profile.

Engaging the tip directly on the doping layer is a very dangerous and delicate

operation. Such a procedure would suppose to be able to engage the tip only a few hundreds

or tens of nanometers off the edge of the sample. The tip could easily slip off the surface and

break.

Such an operation is also hindered by the fact that the position of the tip on the

backface of the cantilever is not known with precision. Different cantilevers have the tips

located in different regions of the cantilever.

A safe way to bring the tip on the doping profile is to engage the tip a few microns off

the edge of the sample (Figure 2.9.5 – position 1) and to slowly approach the doping profile

by consecutive offsets. The downside of this method is that the consecutive abrupt voltages

applied to the piezoelectric scanner accentuate the creep effect. As a result, the SCM images

may present a drift, which is exemplified in Figure 11.6. Besides the intrinsic problems related


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with analysis of such an image, the creep further endangers the tip of falling off the surface.

Figure 2.9.6. Example of creep with doping profile measurements on the cross-section of a

sample

As a parenthesis, it can be said that the problem is even greater with SSRM

measurements. In the case of the resistive measurements, the force between the tip and the

sample is much greater than in the case of the SCM measurements. If the SSRM tip falls off

the surface, not only the shape of the tip or its coating are endangered, but there is also the

possibility that the tip or even the whole cantilever break.

One other problem met with the SSRM measurements is that no good electric signal is

obtained in the first minutes of scanning, because of the creep effect (Figure 2.9.7). With the

SSRM, a very good electrical contact between the tip and the sample must be achieved in

order to obtain an electrical signal. The creep effect, probably combined with a cross-coupling

effect which determines a scanner movement in the z direction, doesn't let the tip to achieve

the necessary electrical contact with the surface of the sample.

Figure 2.9.7. SSRM signal (left) in the first minutes after the offset on the doping profile. The SIMS profile of the dopant profile (right)

2. The creep effect also hinders the SCS measurements on the oxides, on the surface of

the samples.

With the SCS measurements, the tip must stay still in a spot, while the capacitance-

voltage ramps are performed. When the AFM passes from SCM mode in SCS mode, the tip is

brought from a corner of the scanned area towards the middle of the image, where the SCS

measurements are performed by default. During this operation, an abrupt voltage must be


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applied on the scanner, which triggers the creep effect. During the SCS measurements, the tip

will not stay still in the same spot (Figure 2.9.8)

Figure 2.9.8. Topographical image (left) and SCM image (right) on a high-k dielectric. On both images, it can be observed the elongated shape of the spot where the SCS measurements have been

performed. This shape is an indication of the drift of the tip during the SCS measurement.

A solution to prevent the drift during the SCS measurements is to wait several minutes

after passing from image mode in the ramp mode. During this time, the creep effect will

slowly vanish and the measurements will be more stable.

3. An interesting application with the electric measurements is the study of the charging

and the anodic oxidation of the samples during SCS measurements (paragraph 3.4.1) or C-

AFM measurements .

Such a study requires several stages. In a first stage, a high voltage (10 volts for

example) is applied for several minutes in one spot of the sample and the anodic oxidation

takes place.

This operation affects not only the surface of the oxide, but also the tip, so the tip must

be changed and current measurements are performed with a new tip.

In a third stage, the electronic module and the tip are changed in order to perform

capacitance measurements.

During all these operations, exactly the same spot of nanometric size, where the anodic

oxidation took place, must be localized. This can be done by the use of nano-marks [2.27].

Additional oxidations may be performed in order to be able to find the same spot when

measurements are performed with a new tip. (Figure 2.9.9).


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Figure 2.9.9 Nanomarks created on the surface of a sample by anodic oxidation which helps find the spot of interest (the central spot) for consecutive electrical measurements. Image

performed by Wael Hourani, phd. Student, INSA de Lyon

The problems due to creep appear when the tip must be placed again exactly over the

central spot. Consecutive AFM images show that the tip drifts from one image to another

because of consecutive offsets and zoom in operations (Figure 2.9.10).

Even when the AFM image is finally centered, we cannot be sure that, when switching

from the image mode to the ramp mode, the tip will be placed on the oxidized surface and it

will not drift. In order to avoid this possibility, it is best to zoom in on the region of interest as

much as possible before switching to the ramp mode and to leave the piezoelectric tube for

several minutes to stabilize.

Figure 2.9.10. Effect of creep as a result of consecutive offsets, from one scan to the next

4. A very useful and powerful tool with the AFM is 'point&shoot'. After scanning the zone

of interest, several spots can be selected where SCS ramps are to be performed.

The point&shoot is dangerous to use with samples that present important topographical

features, because the feedback loop is deactivated during this operation. However,

point&shoot is a very useful tool for electrical measurements on doping profiles and oxides,

because such samples don't have any important topographical features.

Thus, consecutive automated SCS measurements can be made on all the regions of a

doping profile or in several regions of an oxide, in order to study its homogeneity.

However, because of the consecutive offsets during the point&shoot (consecutive


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abrupt voltages applied to the scanner), there is the possibility that the tip may not perform

the measurements in the chosen spots (Figure 2.9.11)

Figure 2.9.11. Topographical image (left) which shows the distance between the places where measurements are to be performed (261 nm). SCM image (right) which shows the actual distance between the places where the measurements have been done (143 nm)

This is especially problematic in the case of point&shoot measurements on doping

profiles. There is the danger that, because of the creep, SCS measurements are not done on

all the doping regions. Instead, several consecutive measurements may be done on the same

doping region and none on the other regions.

5. A very serious problem with the AFM is the oxide charging and the anodic oxidation for

the measurements done in air (section 2.6.8). The air humidity, combined with the small size

of the tip and the conical shape which generates strong electric fields, and with high applied

biases, can lead to the oxide charging or even to anodic oxidation. Further, contaminants on

the tip and on the sample can distort the measurements.

One solution in the case of oxide characterization that can solve all these problems

consists in the fabrication of nanoelectrodes. Nanoelectrodes protect the surface from

contaminants and humidity, insure a good electrical contact with the surface, as in the case of

macroscopic C-V measurements, eliminate the stray capacitance problems generated by the

conical shape of the tip. In addition, the size of such nanoelectrodes can be measured with the

AFM.

Nanoelectrodes can also constitute a good solution for C-AFM measurements. The main

impediment in the reproducibility of I-V measurements with the AFM is the anodic oxidation

because of the environment humidity, correlated with strong applied biases used with this

characterization method. The unknown surface of the tip prevents a quantitative

characterization of the oxides. Performing I-V measurements on nanoelectrodes would solve

all these problems.

The procedure that we use to fabricate such nanoelectrodes is the following. A thin

metallic layer (5 nm) is deposited on the top of the oxide to be analyzed, by evaporation with

an electron gun. Nanoelectrodes are engraved in the metal with the AFM, by using diamond


137

coated tips, specific for SSRM measurements.

By using a nanolithography program or even the point&shoot tool, arrays of such

nanoelectrodes can be fabricated.

Figure 2.9.12. Fabricated nanoelectrode by engraving a nickel layer on top of a 5 nm

thermal oxide

However, with a faulty piezoelectric scanner, because of the creep effect, instead of

creating arrays of nanoelectrodes, all the surface of interest will be randomly engraved,

without being able to fabricate nanoelectrodes.

For now, we have been able to fabricate only isolated nanoelectrodes, one at a time

(Figure 2.9.12)

6. As we have presented in chapter 1, one way to prepare the doping profile samples is by

bevel polishing. In this way, the sample area can be magnified up to a few hundred times and

the geometrical resolution can be improved (Figure 2.9.13).

Figure 2.9.13 Setup for measurements on a beveled sample

Performing measurements on beveled samples instead of the cross-section of the

samples, has several implications. First of all, the scanned area is much larger. If a doping

profile extended over 4 microns for example, as a result of a magnification by a factor 10, the

doping profile on a bevel sample will be 40 microns large. Second, the height difference

between the start and the end of a scan line will be of several hundreds of nanometers.


138

Figure 2.9.14. The AFM scan line on a beveled sample. For an amplification coefficient x10 of the

bevel, the height difference between the start and the end of a scan line is 1.000nm for a 100 x 100 μ

image.

Scanning large surfaces with a faulty piezoelectric scanner, on samples that present

such height differences, can lead to the total loss of the electric signal. Because of the cross-

coupling effect, the tip will not stay in contact with the surface and electrical measurements

cannot be performed.

A way to solve this problem is to use a special holder in such a manner that the

beveled sample to be placed in a horizontal position.

Figure 2.9.15. Metallic holder for beveled samples

2.9.3. Conclusions

The examples above show how the non-linearities of the piezoelectric scanner can have

a negative impact on the electrical measurements with the AFM. These problems are

magnified mainly because numerous offsets are necessary to be made with these type of

measurements, which amplify the effect of creep.

The best way to counter such problems is to make sure that the piezoelectric scanner is

perfectly functional and well calibrated at all times. If it is not possible, several other

precautions can be taken.

● When engaged on the surface of the samples, special care must be taken that the

scanner is not in a retracted or contracted position.

● Immediately after engagement, the system should be left for at least a few minutes to

stabilize before starting any measurement. The same thing is valid after each offset that must

be performed during the characterization of the sample.

● In the case of the need of an offset in order to change the location where the


139

measurements are performed, it is better to avoid this operation whenever possible.

Sometimes, offsets can be avoided, especially in the case of oxide characterization on plane

surfaces, by disengaging the tip from the sample, making the offset by moving the sample

with the XY stage positioning system and re-engaging the tip on the sample.

● In the case of the measurements performed on the cross-section of the samples,

special care must be taken in order to avoid the tip falling off the sample. The best way to

insure this is by deposing a capping layer on the surface of the sample.

Finally, it must be noted the difference between all the types of problems presented in

the previous sections of this chapter (parasitic light, parasitic capacitance) and the scanner

problems presented in this paragraph. Parasitic light and parasitic capacitance are problems

that appear because of the intrinsic design of the AFM. Piezoelectric issues may sometimes

appear because of maintenance problems of the AFM.

2.10. Conclusions

SCM is a very sensitive technique, prone to numerous sources of stray signal. The

quality of the topography, of the oxide and of the tip, stray laser and stray capacitance, make

SCM a difficult technique to work with.

We have shown that the AFM laser strongly affects the signal/noise ratio of the SCS

signal. Also, the AFM laser can lead to the creation of the additional peaks on the SCS signal.

Considering that the amplitude of the SCS signal is the main parameter used in the attempts

to quantify the doping profiles, it is vital that this parasitic effect be completely removed.

The geometry of the AFM setup generates several parasitic capacitances that influence

the SCM measurements.

Some of them (cantilever chip – sample/ sample holder/ XY stage) are very important

and determine a huge drop of the signal/noise ratio. Other sources (cantilever-sample/ sample

holder/ XY stage, SCM electronic module – XY stage) are less important, determining a small

drop of the sensitivity of the signal/noise ratio.

The electrical components such as the UHF transmission line, the holder, the cantilever,

the electrical contact between the holder and the cantilever, the back contact of the sample,

have a minor impact upon the sensitivity of the detector and may vary from one measurement

to another if necessary precautions are not taken.

The face contact however, between the tip and the sample, is negatively influenced by

a multitude of factors: the properties of the tip, contaminants on the tip and on the surface,

topography.

Other problems related to the quality of the oxide and to the phenomena affecting the


140

oxide will be presented in the next chapter.

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Chapter 3. Oxides characterization with the SCM

Chapter 3 Oxides characterization with the SCM

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3.1 Introduction

The oxide layer represents the dielectric of the MOS structure substrate - oxide - tip

characterized with the SCM.

The properties of the oxide layer between the tip and the substrate affect the SCS

curve, making sometimes hard to discern which features of the SCS signal are due to the

doping profile and which ones to the oxide layer. A good quality oxide is required in order to

be able to study the properties of the doping profile in terms of resolution, quantification or

localization of a p-n junction. Further on, good quality thermal oxides cannot be grown on top

of a doping profile. The high temperatures used for the growth of such oxides (1000o-1200oC)

would determine the diffusion of the impurity atoms in the silicon, destroying the very doping

distribution that is to be characterized.

The oxides can constitute a separate subject of study. Given a known substrate, of

known uniform doping concentration , the properties of the SCS signal would be determined

by the oxide properties. It is better to use a low doping concentration, 1014 cm-3 – 1016 cm-3,

because the SCS signal is higher on such substrates. Thus, the oxide defaults (fixed charges,

mobile charges, interface states) can all be characterized at nanoscale with the SCM.

3.2. Oxide defects and their influence on the SCS signal

The oxides present defects that can be grouped in the following categories (Figure

3.2.1):

- mobile charges (mobile ions)

- fixed charges

- interfaced trapped charges (interface states)

- oxide trapped charges (bulk oxide charge)

Figure 3.2.1 Oxide defects - diagram after Pierret [3.1] pg.651.

Each type of defect has a specific influence on the capacitive signal.


144

3.2.1. Mobile Ions

3.2.1.1. The nature of mobile ions

The mobile charges in the oxides are alkali (positive) ions, mainly ions of Na.

The mobile charges in the oxides have been discovered when trying to explain a

phenomenon encountered by measuring the C-V characteristics on thermal oxides. It has been

noticed that the C-V characteristics were:

- always shifted in the direction opposite to the applied polarity on the gate

- the C-V curves were always situated to the negative side of the theoretical curves

The alkali ions have been identified as the cause of these abnormalities, because of

previous knowledge concerning these ions.

It was already known that alkali ions, especially sodium ions, are abundant on the

hands, in the glass apparatus, chemical products used in laboratories.

In addition, even from the 19th century, has been demonstrated that alkali ions can

move through quartz or crystalline silicon at low temperatures.

The hypothesis has been confirmed by two experiments.

In one experiment, called neutron activation technique, the oxides have been

bombarded with neutrons, trying to render the sodium, if it existed, radioactive. The analysis

showed the presence of the sodium in the oxides.

In another experiment, semiconductor structures have been intentionally contaminated

before metallization by rinsing the oxidized silicon wafers in a dilute solution of NaCl. These

intentionally contaminated devices showed severe instabilities under bias-temperature

stressing.

3.2.1.2 The influence of the mobile ions on the capacitive signal

As has been already mentioned, the C-V characteristics are always shifted in the

direction opposite to the applied gate polarity. Also, C-V curves are situated to the negative

side of the theoretical curves

The effect of the mobile charges on the SCS signal is similar to the effect on the C-V

characteristics (Figure 3.2.2).

There is a hysteresis and the SCS curves should be situated to the right of the

theoretical signal (the bias being applied in this case on the substrate).

The current methods for detecting the presence of mobile charges in the oxide are:

1. 'bias temperature stressing'.

2. in extreme cases, the instability can be observed by simply biasing the device at

room temperature.

- trace a C-V characteristic;


145

- apply a certain bias, positive or negative, for a period of time;

- redo C-V characteristic.

If there are mobile ions, the curve should be shifted in the directions opposite to the

applied bias.

Figure 3.2.2 The influence of mobile charges on the SCS signal. Green

curve - SCS curve in the absence of mobile charges. Red curve - SCS

curve for an oxide stressed to a negative bias applied to the substrate

and the alkali ions shifted towards the Si-SiO2 interface. Black curve -

SCS curve for an oxide stressed to a positive bias applied to the

substrate and the alkali ions shifted towards the gate-SiO2 interface.

The current methods for reducing the amount of mobile charges are:

1. prevention of the alkali-ion contamination throughout the MOS fabrication process

(which is extremely difficult)

2. Technological procedures developed to minimize the effects of residual alkali-ion

contamination

- phosphorous stabilization (PSG);

- chlorine neutralization.

3.2.2. Fixed Charges

3.2.2.1 The nature of fixed charges

From considerations enumerated below, it has been postulated that the fixed oxide

charge is due to excess ionic silicon that has broken away from the silicon crystal and is

waiting to react in the vicinity of Si-SiO2 interface when the oxidation process is abruptly

terminated.

- the fixed charge is independent of the oxide thickness, the semiconductor doping

concentration and the semiconductor doping type (n or p);

- the fixed charge varies as a function of the Si surface orientation.

QF (the amount of fixed charges) is the largest on {111} surfaces, smallest on {100}

surfaces, and the ratio of the fixed charges on the two surfaces is approximately 3:1.


146

3.2.2.2. The influence of the fixed charges on the capacitive signal

The C-V characteristic is translated toward negative biases relative to the theoretical

curve.

In the SCS case, the only difference is that the signal is shifted toward positive biases

relative to the theoretical curve, given the bias applied not to the gate, but to the substrate

(Figure 3.2.3).

Figure 3.2.3 The influence of fixed charges on the SCS signal

The presence of the fixed charges can be determined by calculating the flatband bias of

the structure and compare it with the experimental value. However, it must be taken into

account that the shift caused by the fixed charges can superimpose with the shift given by

mobile charges and bulk charges.

Regardless of the oxidation conditions, the fixed charge can always be reduced to a

minimum by annealing in an inert atmosphere (Ar, N2).

3.2.3. Interfacial Traps

3.2.3.1. The nature of interfacial traps

Interfacial traps – also referred to as surface states or interface states – are allowed

energy states in which electrons are localized in the vicinity of a material's surface.

The interfacial traps introduce energy levels in the forbidden gap at the Si-SiO2

interface. Actually, the interface states can, and normally do introduce levels distributed

throughout the band gap, but such levels are usually obscured by the much larger density of

conduction or valence band states.

The physical origin of the traps has not been totally clarified [3.1].

The weight of experimental evidence, however, supports the view that the interfacial

traps primarily arise from unsatisfied chemical bonds or so called 'dangling bonds' at the

surface of the semiconductor.

When the silicon lattice is abruptly terminated along a given plane to form a surface,


147

one of the four surface-atom bonds is left dangling. The thermal formation of the SiO2 layer

ties up some but not all of the Si-surface bonds. It is the remaining dangling bonds that

become the interfacial traps.

The interfacial trap density, like the fixed oxide charge, is the greatest on {111} Si

surfaces, the smallest on {100} surfaces, and the ratio of midgap states on the two surfaces

is approximately 3:1.

3.2.3.2. The influence of the interfacial traps on the capacitive signal

The manifestation of a significant interfacial trap concentration within a MOS structure

is the distorted or spread-out nature of the C-V characteristics.

- in a semiconductor, to a first-order approximation, all energy levels above EF are empty and

all the energy levels below EF are filled.

- the interfacial traps charge and discharge as a function of bias.

- in inversion, Ev/Ec passes above/below the fermi level.

The SCS signal on a sample which presents interfacial traps has a larger value of the

width at half maximum compared to the theoretical curve (Figure 3.2.4)

Figure 3.2.4 The influence of interface states on the SCS signal

The presence of the interfacial traps can be revealed by comparing C-V characteristics

before and after a hydrogen annealing. After the annealing, the transition region of a C-V

curve should be more abrupt. In consequence, the SCS signal should be narrower.

The interfacial traps can be reduced by annealing in the presence of hydrogen at

relatively low temperatures (≤500C), from 1011, 1012 states/(cm2 eV) to values smaller than

1010 states/(cm2 eV). There are two well-known procedures in the semiconductor industry:

• postmetallization annealing

• hydrogen annealing

Burte et al [3.2] conducted a study of the density of interface states with the annealing

temperature. The oxides have been annealed in forming gas at temperatures ranging from

250°C to 620°C. A minimum of the density of interface states has been observed for the

samples annealed at 450° C.


148

3.2.4. Bulk oxide defects

3.2.4.1. The nature of bulk oxide defects

The SiO2 network often contains imperfections and impurities [3.3] which can be

electrically active if they introduce energy levels in the SiO2 band gap.

The defects in the bulk of the oxide can come from various sources related to the oxide

growth or to various processes which follow the oxide growth.

During the oxide growth, foreign atoms can be introduced by the oxidizing ambient -

H2O, H2, Na, Cl - or by the substrate - B, P, As. Other types of oxide growth, like plasma

assisted oxidation, or wet-oxidation introduce various defects in the bulk of the oxide.

Metal deposition on top of the oxide is mainly responsible for metallic contamination.

The metal atoms can penetrate the oxide as deeply as 2 nm.

During lithography (e-beam, ion beam, X-rays) the radiation can cause bond rupture

and displacement damage.

Post-oxidation annealings let the hydrogen enter the oxide.

A particular case is constitute by the Na ions. Although they may be considered as

mobile charges, as long as the oxide is not stressed under temperature conditions, the sodium

will not move through the oxide and it will act as a volume defect. This explains why

characterization methods using a physical analysis as SIMS indicate a greater sodium

concentration than the electrical methods. SIMS detects the entire amount of sodium present

in the SiO2, while the electrical characterization methods will measure either the quantity of

mobile sodium, or the electrically active sodium trapping centers.

These defects may act as recombination centers or as trapping centers, donor or

acceptor. A donor trap is neutral when filled with an electron and positively charged when

devoid of the electron. An acceptor trap is negatively charged when filled with an electron and

neutral when devoid of the electron.

The trapping mechanism is related to the phenomenon of charge injection into the

oxide by various mechanisms (electrical, optical, thermal). With the SCM, a particular interest

is represented by the electrical charge injection into the oxide, given the strong electrical fields

generated by the geometry of the tip combined with the water layer present on top of the

hydrophilic oxide.

3.2.4.2. The influence of the bulk defects on the capacitive signal

The introduction of trapped charges in the bulk of the oxide will determine a shift of the

C-V curve along the voltage axis. The sign of the shift of the C-V curve will be opposite to the

sign of the charges injected into the oxide (Figure 3.2.5).


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Figure 3.2.5 The influence of volume charge on the SCS signal. The continuous line represent the SCS signal in the absence of

bulk charges. The dotted signals represent the SCS signals in the presence of dotted charges. The shift with the SCS (the bias

applied on the substrate) has the same sign as the sign of the charges.

In the case of volume charges, the shift of the C-V curve depends not only of the

density of volume charges, but also of their distribution in the volume of the oxide. The

volume charges located near the gate will have less impact upon the C-V curves.

The defects distribution into the oxide can be evidentiated by successively etching the

oxide and recording the shifts of the C-V curve for different oxide thicknesses. The drawback

of this method is that metal depositions must be performed after each etching in order to

create a new gate. These depositions may also modify the remaining SiO2 because of the

metal atoms which penetrate through the oxide creating new trap centers and modifying the

trap distribution.

With the SCM, this problem doesn't exist, given that no face contacts or back contacts

are needed (chapter 2, section 6).

3.3. State of the Art

The bibliography related to different types of oxides, in terms of growth and

characterization, is extensive. In this thesis, we are going to limit our approach to the oxides

used with the SCM and characterized by the SCM.

3.3.1. Requirements for the oxides used with SCM measurements

An oxide used as a dielectric layer with the SCM for the measurements of the doping

profiles, besides the number of defaults as low as possible, has other several basic

requirements that must meet.

- low-temperature fabrication

- ease of formation

- reproducibility of the oxide quality

- reproducibility of the oxide thickness

- uniformity of the oxide layer in terms of thickness and defects.


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3.3.1.1 Low-temperature fabrication

One of the most important and limiting conditions for an oxide that can be used in SCM

measurements is that the oxide must be grown at temperatures smaller than the diffusion

temperature of the semiconductor dopants. This condition eliminates from the start the best

known oxide, the thermal oxide, which is grown at temperatures passing 1000oC.

However, thermal oxides can be used in the analysis of the properties of the oxides as

the standard for the other oxides. Thermal oxides can be used with uniform substrates, thus

eliminating any danger of doping diffusion. The capacitance signal measured on thermal

oxides offers a standard to which we can refer when measurements are done on other oxides.

Any parameter that influences the capacitance signal is needed to be understood and

calibrated with a reference sample. We consider that thermal oxides of the highest quality

constitutes the best reference sample in the study of oxide properties and their influence on

the capacitance signal.

Given the fact the thermal oxides cannot be used for the analysis of the doping

concentration, different other oxides, grown at low temperature, have been proposed in the

literature and their quality analyzed.

- native oxides;

- dry oxidation under UV/ozone;

- dry oxidation;

- wet chemical oxidation.

3.3.1.2 Ease of formation

The ease of formation of the oxide is a criteria necessary for practical reasons. For

mass measurements it is important that the sample preparation be quick.

The easiest low-temperature oxide to grow is the native oxide. All that must be done is

to leave the sample in a proper environment for a few hours, after the previously native oxide

has been removed or after the sample has been cleaved.

3.3.1.3 Oxide reproducibility

The oxide reproducibility is also important for mass measurements.

In order to make comparisons between results on different samples, it is important that

the technological processes insures each time the same quality of the oxide on all the

samples.

3.3.1.4 Oxide thickness

The oxide thickness is an important and sensitive criteria. If the oxide is too thin,

tunneling currents will make impossible any measurement attempt. If the oxide is to thick, the


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sensitivity will drop until, eventually, we will not have any signal at all. The oxide thickness

should be correlated with the oxide dielectric constant. Oxides with higher dielectric constants

will allow the use of thicker oxides.

The thickness of the oxides for the SCM is measured in the literature by different

means, mostly often by TEM and XPS.

Goghero et al [3.4] measured thin wet-chemical oxides and low-temperature thermal

oxides by TEM and they obtained a thickness of 2-3 nm for the chemical oxide and 3-6 nm for

the thermal oxide

Yang et al [3.5] measured the thicknesses of a thermal oxides to be around 3 nm by

ellipsometry.

Duhayon [3.6] measured the thicknesses of different oxides with the XPS.

For some types of oxides, like the UV/ozone and the native oxides, the oxidation

reaches saturation after a few nanometers and their thickness can be estimated.

Duhayon [3.6] presents in her thesis a plot of the UV/ozone oxide thickness with

respect to the oxidation time. Based on her measurements, but also on the data of other

authors, Duhayon showed that the oxidation process stops after the UV/ozone oxide reaches a

thickness of around 1.7 nm-1.8 nm.

Figure 3.3.1 The oxide thickness, as determined with XPS, plotted in function of the oxidation time.

Duhayon [3.6], pg.78

Oxide's thicknesses monitoring study done at Standford shows that the native oxide

reaches a maximum thicknesses of approximately 1.2 nm after several days (Figure 3.3.2).

In conclusion, it may be said that the UV/ozone has a maximum thickness of 1.8 nm

and the native oxide a maximum thicknesses of 1.2 nm.


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Figure 3.3.2 The thickness of native oxide as a function of time. U.Thumser, P.Beck,

D.Stewart, Standford University

3.3.1.5 Oxide uniformity

Another important property of the oxides is the oxide uniformity, in terms of

thickness and defaults.

Thickness variations across a sample and different default densities from one spot to

another will surely influence the capacitive signal (FWHM, flatband voltage and hysteresis)

which will lead to misinterpretations of the doping concentration and to contrast inversion.

In the literature, one way to verify the oxide uniformity at nanoscale is by plotting the

Vdc max versus the number of measurements [3.7]

Figure 3.3.3 Analysis of oxide uniformity O. Bowallius et at [3.7]

We consider that this method is not fully efficient if the measurements are done in air,

on surfaces that present contaminants and surface moisture. The contaminants and the

humidity of the surface may be responsible for the differences in flatband voltage from one

spot to another. This method could be rather used to calculate an average of the flatband bias

for the oxide in question.


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Finally, it must be restated that these conditions must be met only by the oxides used

for measurements aiming at the doping concentration.

If the measurements aim at the analysis of the very oxide, the only condition which

stays valid is the oxide thickness. It must not be forgotten that the SCM is able to perform

measurements on thin oxides only.

Measurements on thicker oxides could be performed with the SCM by using

nanoelectrodes. On thicker oxides, bigger nanoelectrodes could be used for compensating for

the loss of signal because of the thickness of the oxide. Thus, we consider that SCM can be

used to perform measurements even on oxides hundreds of nanometers thick with electrodes

of nanometric scale.

3.3.2 Oxide properties and properties of the SCS signal

As we have seen, the oxide defaults and, implicitly, the oxide quality can be determined

by evaluating the parameters of a SCS curve:

- the position of the flatband bias (shift of the flatband bias);

- the FWHM of the curve;

- the hysteresis of the signal – the difference between the trace and the retrace.

However, other factors may influence these parameters. Here is a list of the factors

which may influence the parameters of a SCS curve

3.3.2.1 The position of the flatband bias

With the SCM, the bias for which the maximum amplitude of the capacitance signal is

obtained, is taken as an accurate indicator of the flatband bias, which is very near.

In the ideal case, the flatband bias is the difference between the workfunctions of the

substrate and the tip. Further on, given the fact that the tips (doped silicon, doped diamond,

PtIr5, chromium) have very close workfunction values of those of the silicon substrate, the

flatband bias should be situated around 0 V.

The near-zero flatband voltage is very important in SCM measurements. The

nanometric dimension of the tip and its sharp shape, combined with strong applied biases, will

lead to tunneling currents and injection of charge into the oxide during the experiments. A

near-zero flatband voltage would allow the use of small biases.

In an ideal case (no defaults in the oxide) the flatband voltage should be around 0 V.

The workfunction of the tips used with the SCM (metal coated tips, heavily doped silicon tips,

doped diamond tips) is very close of the workfunction of silicon. In almost all the cases, the

difference between the two workfunctions is not greater than 1 V.

In order to meet the condition of near-zero flatband voltage in practice, the

concentration of defaults, which cause a shift of the flatband towards higher values, should be

kept as low as possible.

However, in current SCS measurements, the flatband bias is shifted towards higher


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values. The causes that may determine the shift of the flatband bias are:

• fixed, mobile charges, oxide trapped charges

• charging and discharging of the interface states

• the state of the oxide surface (contaminants, surface moisture etc)

• tip degradation

We have already shown that the fixed and mobile charges, which are positive in

most of the cases, cause a shift of the flatband voltage to the right of the ideal flatband

voltage.

Beyer et al. [3.8] have shown that different initial voltages in the case of a dc-bias

ramp can alter the position of the flatband bias.

Figure 3.3.4 Shift of the dC/dV peak with the variation of the onset of the dc-bias sweep. Beyer et al [3.8]

The measurements have been done on a n-type low-doped substrate. The sweep has

been done starting from positive biases towards negative biases. The bias was applied to

the tip.

A shift of 1.6 V in the peak position, to the left, has been observed, when the starting

bias changed from +2V to +8V. The authors estimated that 1.6 V corresponds to a quantity of

trapped charges of 5.1012 cm-2.

The increase of positive charge in the dielectric layer has been explained by two

possibilities:

A field assisted emission of weakly bound electrons, especially from trap states, under

the influence of the high electrical field at the starting point

A proton H+ incorporation in the oxide layer due to the decomposition of water layer

from the surface of the hydrophilic oxide. This proves once again, besides surface oxidation,

the negative effect on the SCM measurements of the water layer present on the surface.

Several authors, like Giannazo et al [3.9] have shown that a tip degradation, especially

in the case of metal-coated tips, will determine a shift in the flatband voltage. Once the metal

coating is washed off during scanning letting the doped silicon at the surface of the tip, the

workfunction of the tip will change.


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3.3.2.2 Full-width at half-maximum

FWHM can vary as a function of the following parameters:

◦ - interface states given by the quality of the oxide

◦ - interface states which resulted from the roughness of the surface (higher for

higher roughness)

◦ - different response time of the interface states (the sweep rate of the dC/dV-V

characteristic)

◦

We have already seen that, according to the theory, the SCS signal on a sample which

presents interfacial traps should have a larger value of the width at half maximum compared

to a theoretical, ideal curve.

Giannazzo et al [3.9] have demonstrated this fact by comparing SCS signals between

theoretical curves that do not take into consideration interface states and SCS experimental

curves.

The theoretical characteristics have been

calculated for a MOS structure formed by a

parabolic shaped tip with an apex of 30 nm

radius, and with the corresponding

experimental parameters: substrate doping

concentration varying from 1015 cm-3 to 1019

cm-3, a 2.6 nm oxide thickness, Vpp = 300 mV.

The author states that a shift in the voltage

axis has been applied to the experimental

results, considering that the theoretical model

didn't take into consideration the fixed charges.

It can be seen that the experimental curves

are broader than the theoretical ones, which

can be explained by the presence of the

interface states. Also, it can be noticed that the

effect of the interface states is more

accentuated for lower concentrations of the

substrate.

Figure 3.3.5 Comparison between

experimental and calculated SCS signals

for p-type and n-type silicon Giannazzo et

al [3.9]

Also, Yang et al [3.5] have shown the difference in the FWHM of the SCS signal by

experimental means.


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Figure 3.3.6 Experimental dC/dV-Vdc data on

uniformly doped p-type silicon for two different

oxides. Yang et al [3.5]

The nitride gate oxide is known from the industry to present no interface traps. The

SiO2 oxide has a high interface states density. The measurements have been done on the

same type of substrate and the oxide had similar thicknesses.

From the two SCS signals, it can be seen that the curve measured on the thermal oxide

is much broader than the curve on the nitride oxide.

Bowallius et al. [3.7] showed that on a cross-section of a sample, as a

consequence of higher roughness than on a planar surface, because of cleaving, the

FWHM is also greater than on the surface of the same sample.

Here are his results on the surface of a sample and on a cross-section for native oxide

and for a wet-chemical oxide:

Geometry Oxide type FWHM

Planar (100) Native 0.9 V

Planar (100) Wet-chemical 0.9 V

Cleaved Native 1.5 V-2 V

Cleaved Wet-chemical 1.6 V

However, we must observe that the generation of interface states due to cleaving has

not been isolated from other effects, such as the crystalline orientation. The surface of the

sample has a (100) orientation, while the cross-section of the same wafer cannot still have a

(100) orientation. Thus, we consider that the enlargement of the SCS signal has, in this case,

multiple causes.

Goghero et al [3.4] showed that the FWHM increases with the roughness of the

surface caused by cleaning the native oxide with higher concentrations of HF.

Yang et et [3.10] also showed that an increased roughness of the surface due to

polishing determines higher values for FWHM.


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Figure 3.3.7 dC/dV-Vtip data on samples

polished using 0.02 um colloidal silica and 0.25

um diamond suspension. Yang et al [3.9]

The sample polished with a suspension containing larger grains determines a higher

roughness on the surface and higher densities of interface states. The difference of densities

states for the two samples is reflected by different FWHM of the two signals

Beyer et al [3.8] observed that the FWHM of the SCS signal increases with the sweep

rate.

Figure 3.3.8 Impact of the dc-bias sweep rate on the dC/dV FWHM on a a) native oxide b) thermal

oxide. Beyer et al [3.7]

Beyer considers that the interface states have different response time constants to the

changes introduced by the electrical fields and that the interface states with longer response

times will remain into a non-equilibrium state during fast sweeps.

Based on the observation that fast sweeps broaden more the peaks in the case of the

thermal oxide, the author concludes the state of equilibrium is achieved harder on the thermal

oxide because of tighter bound carriers, in the case of emission of charges, and more effective

barriers, in the case of capture of charges.

According to these observations, the best choice seems to be performing the sweeps as

slow as possible. Slow scans will allow the system achieve equilibrium and the signal will not

be distorted.


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3.3.2.3 Hysteresis of the signal

Hysteresis can appear as a consequence of several factors:

- the presence of mobile charges in the oxide

- the limits of the sweep interval

- experimental conditions (the state of cleanliness of the surface etc)

As we have seen, the mobile charges which consist, in principal, of alkali ions, can

move through the oxide, from one interface to another, under bias. The reverse of the

direction of the electrical field from trace to the retrace of the SCS curve, determine the

reverse in the movement of these positive ions. When a positive bias has been applied on the

gate, it is very likely that the positive ions be situated to the oxide-silicon interface. When a

negative bias has been applied to the gate, it is likely that most of the positive ions be

localized at the air-oxide interface. Thus, a hysteresis, or a difference between the trace and

the retrace of the SCS signal appears. However, it must be emphasized that, with the SCM,

the hysteresis present in almost any SCS measurement is not caused by mobile charges

(section 3.4.1).

The limits of the sweep interval can influence the dimension of the hysteresis. The

amplitude of the electrical field which determines the movement of mobile ions through the

oxide, depends of the values of the sweep interval. If the limits of the sweep interval are

small, one might have no hysteresis at all. If the limits of the sweep interval are to high, the

movement of the mobile ions may be amplificated by the injection of additional charges into

the oxide.

In chapter 2 has been shown that measurements made on thermal oxide presented no

hysteresis with CV measurements and SCS measurements on a nanoelectrode. However, CV

measurements made with a probe directly on the surface of the oxide and SCS measurements

made with a tip also directly on the surface of the oxide, in air, showed the presence of a

hysteresis. Our conclusion is that hysteresis can appear as a result of an unproper surface,

humidity and contact problems between the tip and the sample.

The position of the flatband bias or more precise, the position of the maximum of

the SCS signal the FWHM and the hysteresis are the parameters of the SCS signal that

can bring information about the oxide defaults. Also, the sweep rate and the limits of the

sweep interval can be related with the defaults in the oxide and can distort the above

mentioned parameters.

3.3.2.4 The amplitude of the SCS signal

With the C-V measurements, the difference between accumulation and inversion is not


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dependent of the density of defaults of the oxide. However, with the SCS, one doesn't

measure the difference between the accumulation and inversion, but the difference between

two values of capacitance corresponding of the extremity values of Vac.

If the slope of the C-V curve is higher

because of the interfacial traps, then the

difference between Vmax and Vmin (the

amplitude of the SCS signal) should be

smaller for the same Vac. In conclusion, the

amplitude of the SCS signal should depend

of the density of state interfaces.

However, Yang et al [3.5] have shown

that this is not the case. They showed that,

for each Vdc, the interface states don't

respond to the ac signal, because of its high

frequency. For each Vdc, the dC/dV signal

can be expected to have the same value as

in the case of a trap-free sample.

Thus, the resulting SCS signal will have a

FWHM corresponding to the respective

amount of interface states from the oxide,

but the amplitude of a trap-free sample.

Figure 3.3.9 a)Effect of interface traps on C-V

characteristics; b) Expected SCS signal when

interface traps don’t respond to the Vac

signal; Yang et al [3.5]

Their experimental results confirmed this hypothesis and have been shown in the

Figure 3.3.6. The amplitude of the two signals is practically the same, although one of the

oxides present interface states.

In conclusion, the amplitude of the SCS signal doesn't depend at all of any

type of oxide defaults.

3.3.3 Low-temperature oxides

The low temperature oxides proposed as a dielectric for SCM measurements in the

literature are:

- native oxides

- dry oxidation under UV/ozone

- dry oxidation

- wet chemical oxidation

Bowallius et al. [3.7] have compared a native oxide and a wet-chemical oxide.

The native oxide was obtained simply by exposure to air and the wet-chemical oxide

was obtained in a solution H2SO4+H2O2.

The quality of the two oxides was evaluated by the authors by comparing the flatband


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bias difference between two different doping concentrations, 1014cm-3 and 1017cm-3.

Figure 3.3.10 SCS curves for a. native oxide and b. wet-oxide. Bowallius et al [3.7]

The author's conclusion is that the difference of almost 2 volts between the

workfunctions of the 1014cm-3 substrate and 1017cm-3 p-type substrate is unrealistic and is

caused by large numbers of defaults.

Based on these measurements, the wet-chemical oxide has a better quality and should

be preferred as a top oxide for SCM measurements, while the native oxide is not the best

choice as a top oxide, especially if quantitative measurements are taken into consideration.

The experiments made by Bowallius represent an example of how the position of the

maximum of the SCS signal can be used for a qualitative general estimation of the oxide

quality.

Goghero et al. [3.4] compared the quality of a wet-chemical oxide and a low-

temperature thermal oxide.

The wet-chemical oxide was obtained by immersing the sample in a H2O2 solution for

20 min, followed by heating the sample in air to a temperature between 150oC and 200oC.

The low-temperature thermal oxide was grown at 200oC for 1h in an oxygen gas flow.

In order to evaluate the relative quality of the two

oxides, Goghero evaluated the hysteresis of the SCS

signal.

He found that the hysteresis obtained in the case of

the wet-oxide is almost two times higher than for the

thermal oxide, for two different concentrations.

He also found that the values of the hysteresis in the

two cases remained unchanged from day to day

measurements (the hysteresis measurements are

reproducible).

On the other hand, the flatband bias were not constant

from day to day.

As a consequence of these observations, Goghero

Figure 3.3.11 SCS curves in forward and

reverse scans for a) wet-oxide b)thermal

oxide. Goghero et al [3.4]

considers that the hysteresis criterion is more reliable than the peak position criterion in

determining the oxide quality. In this particular case, the low-temperature thermal oxide

seems to be a better choice than the wet-oxide, as a top oxide for the SCM.


161

However, it should be noticed that the preparation method for the wet-oxide doesn't

coincide with Bowallius preparation method.

Goghero et al [3.11] have also compared a UV/ozone oxide with a wet-chemical

oxide.

The UV/ozone has been grown by exposing the surface sample to UV/ozone illumination

for 10 min.

The wet-chemical oxide was obtained by the immersion of the sample into a 30% H2O2

solution for 20 min.

The properties of the two oxides have not been determined by SCS measurements, but

by assessing the SCM signal. The conclusion was that the UV/ozone oxide has a better quality,

measurements with this oxide showing a better stability and smaller variations of the signal.

In her thesis, Duhayon [3.6] thoroughly analyzed several types of oxides: native oxide,

wet-chemical oxide with H2O2, UV/ozone oxide, UV oxides obtained with a UV-gun and

simultaneously baked at 150oC. Duhayon's conclusion, similar to the conclusion of Stangoni

[3.12] was that the best oxide for the SCM is the UV/ozone oxide.

3.4 Oxides characterization with the SCM

In the present chapter we will definitivate our presentation upon the problems at the

tip - sample interface. We will present how these contact problems affect the properties of the

SCS signal and how the parameters of the voltage ramp should be optimized in order to obtain

results with a physical signification in the context of numerous current setup problems.

We will continue with the study of the properties of the SCS curves and their

interpretation.

In the end, we will present a few trails made on different dielectrics to be used as gate

oxides for the study of doping profiles.

3.4.1. Oxide related parasitic phenomena at the tip - oxide interface

During SCS voltage ramps, the properties of the SCS curves often change without

apparent justification.

In chapter 2, we have seen that for almost all the measurements, there is a shift

between the trace and the retrace of the SCS signal, phenomenon that doesn't happen when

the measurements are performed on nanoelectrodes (chapter 2 Figure 2.6.17).

Very often, there is a difference in amplitude between the trace and the retrace of the

SCS curve.

The position of the peak of the SCS signal may change when several consecutive

measurements are performed in the same spot.

Such phenomena have been further investigated. In order to understand what is


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happening at the tip-oxide interface, AFM and SCM scans have been performed before and

after SCS measurements. The AFM images recorded before the SCS measurements constitute

a proof that the aimed regions have nothing special by comparison with the rest of the

sample. The AFM images recorded after the SCS measurements are meant to verify if the

topography or the electrical properties have changed in the spots where the voltage ramps

have been performed.

3.4.1.1. Oxide charging

The first evidence of the oxide charging during SCS measurements has been observed

on the substrate staircase samples covered with a native oxide.

The change of capacitance contrast in the spots where SCS measurements have been

performed has been observed on the SCM image following the SCS curves (Figure 3.4.1).

Figure 3.4.1. SCM image of a p-type staircase sample covered by native oxide following

several voltage ramps [-2V; 2V]. The image shows modifications of the oxide properties

in the spots where the SCS measurements took place

In order to be sure that the surface of the sample didn't present any contrast before

performing the SCS measurements, an SCM image of the area has been recorded before the

SCS curves, showing an uniform capacitance image (Figure 3.4.2 a).

Figure 3.4.2. SCM images on the same surface before (a) and after (b) SCS

measurements of a type p 1015 cm-3 substrate covered by a native oxide. The ramps

have been taken between Vdc [-3V; 3V]. After the SCS measurements, 2 darker spots

can be noticed in the places where the measurements took place.

The SCM image recorded after SCS measurements have been performed, shows two

darker spots in the spots where the measurements took place (Figure 3.4.2 b).

We didn't find in the literature evidence of capacitance contrast modifications following

SCS measurements. However, Beyer et al [3.7] scanned a surface by modifying the dc bias in

steps of 0.4 V from 0 to 2 V. A subsequent scan of the surface at Vdc = 0 V showed that the

stripes scanned at different voltages present a different contrast which suggests a charge

injection into the oxide.

We believe that the two processes are similar. In our case, the oxide charging takes


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place following a voltage ramp in one spot. In Beyer's case, the oxide charging takes place

following the surface scan at a fixed voltage.

Beyer et al [3.7] reported that reiterated scans of same area with the dc bias

maintained at zero reveal the relaxation of the excess charge. On the other hand, in our case,

multiple scans at Vdc = 0 V performed after the voltage ramps didn't show any relaxation of

the excess charge on the native oxide. However, on 2 nm and 5 nm thermal oxides, the

relaxation of the excess charge takes place. We conclude that the relaxation of the excess

charge doesn't take place on the native oxide probably because of its thinness and low-quality

which cause a permanent breakdown of the oxide.

There are several factors which contribute to the oxide charging.

Thin oxides favorize the existence of tunneling currents. Measurements performed on

thicker oxides showed that the oxide is charging only for higher values of the applied bias or

not at all.

Low quality oxides allow more easily the phenomenon of charge trapping due to the

high number of defects in the volume of the oxide (bulk defects). The oxide charging seems

to occur for higher voltages on thermal oxides than on plasma oxides or nitride dielectrics.

This observation is in correlation with Goghero et al (Figure 3.3.11) who considers that the

value of the hysteresis represents an indicator of the quality of the oxide. Further

investigations, under vacuum conditions, which will remove the water layer from the surface

of the oxide, must be performed.

High biases applied to the MOS structure generate higher electrical fields which allow

the injection of carriers into the oxide. The geometry of the tip further amplifies the intensity

of the electrical field.

The most important factor of all seems to be the water layer present on the surface of

the hydrophilic oxide. Test measurements performed in a nitrogen atmosphere with a home

made enclosure have shown that the oxide charging occurs for higher applied voltages.

The lack of appropriate test oxides and the limited access for installing a home made

setups with the AFM in order to create a stable, humidity free atmosphere, don't allow us to

support the above statements with experimental evidence. Further investigations with the

appropriate equipment and the appropriate test samples should be performed.

We have seen that the oxide charging can be detected by scanning the surface after

performing a voltage ramp. By correlating the information given by the SCS measurements

with the information given by the SCM images, we conclude that the oxide charging can be

detected directly with the voltage ramps (Figure 3.4.3).


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Figure 3.4.3. SCS measurement at a sweep rate of 3 V/sec(left) and SCM signal recorded at Vdc = 0 V

(right) on a type n substrate 1015 covered by a 2 nm thick thermal oxide

The value of the signal on the SCM image recorded at 0 V is of approximatively -300

mV. On the SCS curves, the capacitance signal at 0 volts corresponds to -300 mV on the trace

and to -200 mV on the retrace. From these values, we conclude that the trace is the signal

which give the right values. The retrace represents the characterization of the oxide after the

oxide charging. This conclusion corresponds to the intuitive answer. It is intuitive to consider

that the trace of the signal corresponds to the properties of the oxide before the charging

while, during the retrace, the oxide is already charged.

On the other hand, if the SCS scans are done at a much slower voltage sweep, we can

observe that not even the trace of the signal at 0 V doesn't correspond anymore to the value

of the SCM signal recorded at 0 V.

Figure 3.4.4. SCS measurements performed on a type n substrate 1015

covered by a 2 nm thick thermal oxide, at 0.5 V/sec

In the Figure 3.4.4, the value of the capacitive signal at 0 V for the trace is -400 mV

and the value of the capacitive signal at 0 V for the retrace is - 200 mV. We have seen that,

on the SCM image (Figure 3.4.3 b), the value of the signal on the SCM image recorded at 0 V

is of -300 mV.

We consider that, in order to obtain right values of the SCS curves, it is best to perform

the voltage ramps at a faster rate in order minimize the danger of oxide charging. We

emphasize that this conclusion is valid only for ambient measurements and thin oxides which

allow the presence of tunneling currents, in order to avoid the oxide charging because of the

surface water layer. The sweep rate may influence in other aspects the response time of the

interface states or the recombination centers in the volume of the oxide.


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The limits of the voltage ramp also influence the SCS signal (Figure 3.4.5)

Figure 3.4.5. SCS measurements performed on a type n substrate

1015 covered by a 2 nm thick thermal oxide, between -5V;5V

In this case, we observe that for the retrace of the signal, not only that the value of the

capacitive signal at 0 V is even more distant from the real value of - 300 mV, but also the

amplitude of the peak is not the same as the peak of the trace.

The SCM image recorded immediately after the SCS measurement shows a change not

only on the SCM image, but also a change on the topographical image (Figure 3.4.6), in the

spot where the SCS measurement took place.

Figure 3.4.6. Topographical image (left) and cross section of the topographical image (right)

following a voltage ramp between -5V;5V (Figure 3.4.5), on a type n substrate 1015 covered by

a 2 nm thick thermal oxide.

This phenomenon is addressed in the next paragraph.

3.4.1.2. Anodic oxidation

In the previous paragraph, we have seen that applied biases to the M.O.S structure

lead to the oxide charging.

We have noticed that, when higher voltages are applied, even the topographical image

presents changes after SCS measurements (Figure 3.4.6).

The magnitude of the surface oxidation depends of the value of the applied voltage

(Figure 3.4.7).


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Figure 3.4.7. Topographical image (left) and cross section of the topographical image (right) following

an applied bias of -10 V in the center of the image, for 20 seconds, on a type n substrate 1015 covered

by a 2 nm thick thermal oxide. The air humidity percentage: 33.4%

In the Figure 3.4.8 it can be observed the proportionality between the applied bias and

the height of the oxidized surface for negative biases applied on the substrate.

Figure 3.4.8. Variation of the height of the oxidized features with the

applied bias, for negative biases. The voltage stress has been applied

for 20sec at an air humidity percentage of 33.4%

For positive biases, the measured dependence is not linear and the oxidation threshold

increases (Figure 3.4.9)


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Figure 3.4.9. Variation of the height of the oxidized features with

the applied bias, for positive biases. The voltage stress has been

applied for 20sec at an air humidity percentage of 33.4%

If, for negative biases applied on the substrate, the voltage threshold for the anodic

oxidation was -5 V, for positive biases we didn't observe any modification of the surface

topography until + 8 V, on a 2 nm thermal oxide. Even after this threshold, for higher biases,

the topography changes very little, around 2 nm in height.

These observations stay unchanged for a p-type substrate. The tests made on the 5 nm

thermal oxide grown on top of a 1015 cm-3 p-type substrate showed an important topography

change for negative biases applied on the substrate (Figure 3.4.10) and a very small change

for positive biases applied on the substrate (Figure 3.4.11).

Figure 3.4.10 Surface oxidation. Applied bias: -7V on the substrate, air humidity: 48%


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Figure 3.4.11 Surface oxidation Applied bias:+7V, air humidity: 48%

This data is in correlation with the bibliography from the nanolithography field and the

experimental observations made with TUNA [3.13] and nanolithography.

It must be noticed that, in the literature, the phenomena are clearly explained only for

positive biases applied on the substrate. In this case, OH- and O- species are attracted towards

the oxide - silicon interface and the anodic oxidation takes place.

In the case of negative biases applied on the substrate, there are several possible

explanations for the topographical modifications observed with the AFM. Besides the

hypothesis of the anodic oxidation, one other hypothesis is that the topographical

modifications are given by a substrate deformation under the intense electrical field. Another

possible explanation is that the topographical modifications seen on the AFM images are given

by an electrostatic repulsion of the AFM tip, because of the charge accumulation in the

dielectric [3.13]. However the most probable explanation according to us remains for now the

anodic oxidation.

As in the case of oxide charging, this phenomenon is related to the oxide thickness, the

quality of the oxide, the applied bias (the limits of the ramp interval), the stress time, the

polarity of the applied bias, the air humidity percentage. The tip sharpness and geometry may

further amplify the intensity of the electrical field for the same applied voltages.

For a complete study of anodic oxidation of the surface each of the above mentioned

parameters should be thoroughly studied.

In order to avoid humidity, experiments in vacuum or in a controlled atmosphere

(nitrogen) should be performed. Voltage ramps should be performed at different air humidity

percentages.

Sharp tips (PtIr) and cylindrical blunt tips could be used.

Good quality thermal oxides of different thicknesses as well as low quality oxides of

different thicknesses should be used for each of the above mentioned experimental conditions.

However, there are several conclusions that can be deduced based on the


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measurements and the results shown above.

It is imperative to limit the values of the applied biases. The boundaries of the voltage

ramp must be as low as possible. This will not be possible unless the peak of the SCS curve

will be situated as close of 0 volts as possible. The position of the peak of the SCS curve can

be controlled in several ways: by engineering the workfunction of the tip and by choosing the

right oxide thickness. Also, the voltage ramps should be performed as fast as possible in order

to minimize the stress time.

The oxides should be grown on p-type substrates. The topographical modifications are

far less important for positive biases. For p-type substrates the position of the SCS peak is

located towards positive biases applied on the substrate.

In order to minimize the strength of the electrical field (and given that resolution is not

the primarily concern in the case of the study of the properties of the oxides), less sharp tips

should be used.

The electrical measurements should be performed, if possible, into a humidity-free

environment.

3.4.1.3. Oxide engravement

Another problem that has been observed is the engravement of the oxide by the tip.

In contact AFM, the friction between the surface and the tip determines a deterioration

of the tip. However, because the doped diamond coating is very tough, it seems that the

surface also deteriorates (Figure 3.4.12). The problem is further worsen by the method

chosen for avoiding the laser light with the SCM measurements: shifting the laser position

towards the middle of the cantilever, which diminuates the sensitivity of the AFM detection

system.

Figure 3.4.12 Sample: Thermal oxide, 2 nm. Engravement effect after several scans (10 scans)

The choice of the tip being made (diamond coated tips), the only parameter that could

lower the deterioration of the surface is the elastic constant of the cantilever. Cantilevers with

lower elastic constant determine a softer interaction tip-surface, thus a smaller impact on the

deterioration of the surface.


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3.4.2. Study of low-temperature oxides for doping profiling

3.4.2.1 Guidelines for a complete characterization of oxides with the SCM

Currently, the oxide characterization with the SCM means trying to characterize oxides

of unknown properties with an irreproducible and uncalibrated characterization method.

First, a minimum reproducibility must be insured. In the previous chapter, we have

shown a number of causes that distort the SCS signal and impinge the interpretation. In the

present chapter, it has been shown that, with the ambient measurements, oxide charging and

topographical modifications are commonly met. Before any characterization attempt be made

in terms of oxide or doping profile characterization, all these instrumentation problems should

be solved.

Second, SCM should be first calibrated for oxide characterization with test samples.

Thermal oxides of good quality with low density of defects should be used as test samples.

Thermal thin oxides grown by RTO have also proven to be a very good alternative

(Figure 3.4.13).

Figure 3.4.13 SCS signal on a thermal oxide grown by RTO in an oxygen atmosphere, at 940oC, during 4 minutes

(left). C-V curves performed with an impedance analyzer, in the parallel model, at Vac = 50mV

The C-V curves measured with an impedance analyzer, in the parallel model, at

different frequencies, show no variation of the capacitance level in accumulation. There are no

indication of tunneling currents. The curve modifies with the frequency only in the region

concerning the interface states. The calculated oxide thickness, from the level of the

capacitance in accumulation is of 8.4 nm.

The oxide thickness correlated with the absence of tunneling currents on the C-V curve

can explain why the SCS curves don't present any indication of oxide charging or anodic

oxidation.

The test samples should be intentionally contaminated with a controlled density of each

type of defects (interface states, fixed defects at the silicon-SiO2 interface, bulk defects,

mobile defects).

Test samples characterization should be performed with SCM and with other

characterization methods such as ellipsometry, FTIR, DLTS, SIMS etc. From the properties of


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the SCS signal (its shape, the position of the peak, the FWHM, the amplitude, the hysteresis),

the detection capability and the sensitivity of the SCM for oxide characterization should be

determined.

Determination of oxide thickness with the SCM

Thermal oxides of different thicknesses (between 1 nm and 10 nm), grown in the

exactly the same experimental conditions, on the same low-doped substrate should be

available in order to be able to determine the SCM capability and precision of measuring the

oxide thickness.

Figure 3.4.14 TCV simulation: C-V curves (left) and their derivatives (right) for different oxide

thicknesses (1 nm, 5 nm and 10 nm) on a 1015 cm-3 thermal (ideal) oxide

Other methods of measuring the oxide thickness, such as ellipsometry should be

available. Measurements with the SCM, with the same tip, should be performed on all the test

oxides, of different thicknesses. The oxide thickness is a function of the shift of the SCS peak,

the amplitude of the SCS peak as well as the FWHM of the SCS signal. Comparisons between

the SCS experimental data and simulations can be performed (Figure 3.4.14).

Tests measurements on thermal oxides of two different thicknesses, grown in the same

technological conditions and on the same n-type substrate 1015 cm-3, showed differences of

the SCS signals in terms of position of the SCS peak, FWHM and amplitude (Figure 3.4.15).

The peak of the SCS signal is shifted towards right for the thicker oxide. FWHM of the

SCS signal for the 2 nm oxide is greater than the FWHM of the SCS signal for the 1.5 nm

oxide. The amplitude of the SCS signal is greater for the 1.5 nm oxide. All these modifications,

of the position of the peak, of the FWHM and of the amplitude of the SCS signal with the oxide

thickness are in correlation with the theory.


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Figure 3.4.15 Variation of the position of the peak, the FWHM and the

amplitude of the SCS signal with the oxide thickness

Determination of the presence and the density of mobile charges with the SCM

The presence of mobile charges into an oxide is given by the hysteresis of the C-V

curve, when the sample is heated and the oxide is electrically stressed.

We have seen that, with the SCM, oxide charging or/and anodic oxidation occurs when

the oxide is stressed leading to the modification of the SCS parameters and hysteresis. Also,

in our laboratory, there are no means to heat the sample during the measurements.

In order to be able to detect and quantify the mobile charges with the SCM, the

following experience should be put in place.

A thermal oxide of good quality (without defects), grown on a low-doped 1015 cm-3

substrate should be available. The oxide should intentionally be contaminated with alkali ions,

in different concentrations. The samples should be available for characterization with other

methods of characterization, such as C-V measurements, SIMS etc, in order to determine the

amount of alkali ions in the oxide.

SCM measurements should be performed in a controlled environment (ultra-vacuum).

The sample should be heated around 100oC during the SCM measurements (technological

means are available with the AFMs, such as Peltier cells).

Determination of the presence and the density of bulk defects with the SCM

The presence of bulk defects into an oxide is given by the shift of the SCS curve.

It must be kept in mind that the shift of the peak of the SCS signal has a number of

other causes: variations of the oxide's thickness, fixed charges at the silicon - oxide interface.

The experiences should be conceived in such a manner that the shift of the SCS signal be

given only by the defects in the volume of the oxide.

From the literature, it is known that the alkali ions act, at room temperature, as defects


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in the volume of the oxide and not as mobile charges. In consequence, a similar experience as

described above, for the mobile charges, could be put in place in order to evidentiate the bulk

defects with the SCM.

A thermal oxide of good quality (without defects), grown on a low-doped 1015 cm-3

substrate should be available. The oxide should intentionally be contaminated with alkali ions,

in different concentrations. The samples should be available for characterization with other

methods of characterization, such as C-V measurements, SIMS etc, in order to determine the

amount of alkali ions in the oxide.

SCM measurements should be performed in a controlled environment (ultra-vacuum).

Determination of the presence and the density of fixed charges at the silicon -

oxide interface with the SCM

Fixed charges, at the silicon - oxide interface can be evidentiated by the shift of the

SCS peak.

One modality of evidentiating the fixed charges with the SCM is by taking advantage of

the different density of fixed charges on the substrates with different crystalline orientations

In the literature, it is known that there are 3 times more fixed charges on a (111)

surface than on a (100) surface. As a result, the SCS signal measured on a (111) surface

should be shifted more to the right of the theoretical value than a SCS signal measured on a

(100) surface.

Two samples with different crystalline orientation should be used for this experiment.

Both samples should have the same low-doped substrate, covered by a thermal oxide grown

in the same time. The surface of one sample should have the crystalline orientation (100), the

other one (111).

Another way of evidentiating the fixed charges with the SCM is by using the fact that

the amount of fixed charges can be reduced by annealing in a nitrogen atmosphere.

Two samples should be used for this experience. Both samples should have a p-type

substrate, 1015 cm-3, (111), covered by a thermal oxide.

One of the samples should be annealed in a nitrogen atmosphere. Measurements on

both samples should be performed. The comparison between the SCS signals should

evidentiate the detection capability with the SCM of fixed charges at the silicon - oxide

interface.

Determination of the presence and the density of the interface states with the

SCM

HF oxide cleaning, cleaving, polishing, the crystalline orientation can all introduce

interface states.

In order to evidentiate the interface states with the SCM, the following experience could


174

be performed.

The surface of a silicon sample, 1015 cm-3, could be altered by polishing or by etching

with a solution of HF or KOH. On this sample and on an unaltered silicon sample, a thermal

oxide should be grown.

Alternative characterization methods, such as DLTS, C-V, should be used for the

characterization of the density of interface states on these two samples.

SCS measurements performed on the two samples should give a different FWHM of the

SCS signals.

The investigation of the interface states could be further continued by performing

annealings on the two samples in a hydrogen atmosphere

During this thesis, we didn't have access to the fabrication of thermal oxides. It has

also not been possible to perform modifications to the existing AFM according to the solutions

we proposed for the amelioration of the capacitive signal. Given this situation, we evaluated

different low temperature oxides by the proximity of the SCS peak to the value of zero volts.

3.4.2.2 Plasma oxide

The plasma oxide has been obtained with a RIE Nextral NE 1110, made by ALCATEL.

Several tests have been made for finding the optimal growth parameters, the

apparatus and its standard operating conditions being mainly used for the etch of oxides.

The parameters that can be varied are: the pression of the oxygen, the flux of oxygen,

the power of the electrical field in which the oxygen plasma is created and the oxidation time.

The first tests showed with the AFM a variation of the topography of the surface due to

a selective etch of the silicon with respect to the doping concentration (Figure 3.4.16 and

Figure 3.4.17).

Figure 3.4.16 Doping SIMS profile of the maya p sample (left). Topography of the sample

put in the chamber in vertical position (right)


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Figure 3.4.17 Topography of the sample put in horizontal position, the polished face upward

(left). Topography of the sample put in horizontal position, the polished face downward

The highly doped regions have been less etched while the low-doped regions have been

more affected. The etched has been more or less aggressive, depending on the position of the

cross-section.

1. native oxide cleaning: BOE 30sec

2. O2 debit = 10 sccm, power =75 W, pressure = 50mtorr, t=5 min

3. oxide cleaning: BOE 10min + rinsing

4. O2 = 10 sccm, P =75 W, p=50 mtorr, t =10 min

The increase of the oxygen pressure led to a substantial decrease of the plasma attack

of the surface (Figure 3.4.18).

0. Surface cleaning with acetone

1. native oxide cleaning: BOE 30sec + rinsing with deionized water

2. O2 = 10sccm, P = 75 W, p = 100mtorr, t = 5 min

3. Nettoyage BOE 10min + rincage

4. O2 = 10 sccm, P = 75 W, p =100 mtorr, t =10 min

Figure 3.4.18 Topography of the sample put in horizontal position, the polished face downward,

for 100mTorr


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In the final step, optimal values for the oxygen flux, the power and the oxidation time

have been searched in order to obtain a thinner oxide. The oxide thickness has been

measured by ellipsometry.

p=100mtorr

t O2 = 10sccm, P=75W O2 = 10sccm, P=60W O2 = 20sccm, P=75W

30 sec 5.6 5.2 nm 5.6 nm

2 min 4.8 - 6.3 6.2 nm 6.4 nm

5 min 5.6 – 7.2 7.0 nm 7.3 nm

8 min 6.0 – 7.5 7.4 nm

10 min 6.4 – 8.0

From the SCM images, no contrast inversion can be seen (Figure 3.4.19).

Figure 3.4.19. SCM image of the BP-diodes (left). SCM profile of the BP-diodes (right). Vac=400mV; Vdc=0V

However, from the SCS sweep on the lowest-doped region – the substrate, it can be

seen that the phase signal is negative (Figure 3.4.20).

Figure 3.4.20. SCS – amplitude (left). SCS phase (right) of the substrate of the diode sample

The C-V measurements performed with the impedance analyzer (Figure 3.4.21)

suggest strong leakage currents (from the shape of the curves in accumulation) and a great

density of bulk charges (from the shift of the C-V curves).


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Figure 3.4.21. C-V curves performed with an impedance

analyzer, in the parallel model, at 1kHz, Vac = 50mV. The

curves shifts towards left for greater applied biases

We conclude that the assisted plasma oxides are not of the best quality and don't

represent the ideal dielectric for SCM measurements on doping profiles.

3.4.2.3 Nitride

The main reason for trying to use this dielectric for SCM measurements is its dielectric

constant, almost 3 times higher than the dielectric constant of an oxide. This means that

thicker layers could be used without signal loss. Also, with a thicker dielectric, the danger of

tunneling currents should diminish.

For the nitride growth, we took advantage of the know-how that other research teams

had and we used directly the optimal parameters possible for obtaining a dielectric with

minimum defaults and the optimal thickness.

The silicon nitride films have been deposited by direct PECVD with a SEMCO furnace.

The parameters used for the deposition are:

frequency 440 kHz

temperature 370oC

pressure 1500 mtorr

power 1000 W

total debit 800 sccm

The SCS signals on three samples of different thicknesses showed promising results.

The peak of the SCS signals (Figure 3.4.22) are close to zero volts.


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Figure 3.4.22. SCS signals obtained on a low-doped substrate

covered with 7 nm, 8 nm and 9 nm, as measured by ellipsometry

The nitride layer used as a dielectric for doping profile characterization showed that it is

possible to obtain a good capacitive signal (Figure 3.4.23)

Figure 3.4.23. SCM image of a doping profile (left) and cross-section of the doping profile,

with the nitride layer as gate dielectric

However, as in the case of oxide plasma assisted deposition, there are problems with

the phase of the signal. From Figure 3.4.23, it can be seen that the substrate (left) has the

lowest signal. As the SCS signal taken on the substrate of the signal prove (Figure 3.4.24) the

small capacitive signal given by the substrate is not caused by the wrong choice of the applied

bias, but by a phase inversion.

An instrumentation problem made that the phase couldn't be recorded for the most

part of the thesis in the ramp mode, but only in the scanning mode. This is the reason why

many SCS signal along this thesis are presented only in absolute value. The SCS curves

recorded in Figure 3.4.22 represent only the amplitude of the signal, without the possibility of

recording the phase of the signal.


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Figure 3.4.24. SCS signal recorded on the low-

doped substrate of the BP doping profile (Vac =

500 mV)

Further investigations confirmed that, on low doped substrates, the phase of the

capacitive signal is inverted, as in the Figure 3.4.24, as if the substrate would be a n-type

substrate.

We observe that this is the same phenomenon occurred with the oxide deposited by

plasma assisted oxidation (section 3.4.2.20).

At the present time, we do not have a consisting explanation about the change of

phase with the dielectrics deposited by plasma assisted oxidation.

From the data we have - the phase inversion happens only on low-doped substrates,

the nitrides are known for the great density of bulk defects, plasma can affect greatly the

surface of the sample leading to an important topography and interface defects - we speculate

that a great density of defects in the bulk of the dielectric or at the silicon - dielectric interface

can lead to a charge accumulation which could be detected by a phase inversion of the

capacitive signal.

3.4.2.4 High-k dielectrics

In the industry of microelectronics, the high-k dielectrics represent an alternate

solution for the gate dielectrics. The permittivity of high-k dielectrics allows to reduce the

thickness of the gate dielectrics or to increase the dielectric thickness without diminishing the

capacitance of the M.O.S structure.

The LaAlO3 layer studied with the SCM has been deposited by Molecular Beam Epitaxy

(MBE) in a O2 atmosphere, at a pressure of 10-5 torr, on a low-doped p-type substrate 1015

cm-3. The estimated thickness of the dielectric layer, following the deposition, is approximately

4-5 nm.

The peak of the SCS signal is shifted towards high biases (5.5 V) - the black curve in

the Figure 3.4.25. The shift reflects a high density of defects.


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Figure 3.4.25. SCS signals of a high-k LaAlO3 dielectric deposited by

MBE on a p-type low-doped substrate 1015 cm-3, before (black curve)

and after annealing in a N2 atmosphere, at 400oC, by RTA.

After annealing by RTA in a nitrogen atmosphere, at 400oC, the SCS curve shifted

more than 3 V, which shows a great amelioration of the dielectric properties. Such an

improvement following a nitrogen annealing reflects the density of fixed charges at the

dielectric-silicon interface.

In spite of the amelioration of the dielectric's quality brought by the thermal annealing,

the peak of the SCS signal continues to be shifted towards biases that are too high for doping

profile characterization.

However, it must be taken into account that the fabrication of good quality high-k

dielectrics is still at the beginning. High-k dielectrics may represent, in the future, a solution

that should be taken into consideration as gate dielectrics for doping profile characterization.

3.4.2.5 Native oxide

Native oxide is currently used as gate oxide for SCM measurements. It has several

important advantages.

It doesn't require a special preparation. It is enough to leave the samples, after

cleaving, exposed to air, in a proper environment, for a few hours.

It's thickness is known. The maximum thickness that the native oxide can reach is

approximately 1.2 nm (Figure 3.3.2).

The failure analysis images obtained with the native oxide as a gate oxide are of good

quality (chapter 1).

The maximum of the SCS signals obtained on native oxides are located around the

value of zero volts (Figure 3.4.26)


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Figure 3.4.26. SCS signal on a native oxide on top of a low-doped

p-type substrate. The diminished amplitude of the retrace (red

curve) suggests strong tunneling currents

The native oxide has a major drawback. The native oxide is a very thin oxide of poor

quality. Tunneling currents are prone to appear during capacitive measurements. In Figure

3.4.26 for example, the diminished amplitude of the retrace suggests the presence of strong

tunneling currents. Further more, in most of the cases, tunneling currents transforms the

M.O.S structure into a short circuit and no SCS signal is recorded on native oxides. This

phenomenon renders impossible any rigorous attempt of doping profile quantification.

However, it must be emphasized that the tunneling currents appear as a consequence

of the limits of the voltage sweep with the SCS. On SCM images recorded with a continuous

applied bias of approximately zero volts, no contrast inversion or signal loss has been noticed.

3.4.2.6 UV/ozone oxide

In the literature, the UV/ozone oxide represents the best solution for SCM

measurements [3.6], [3.12]. Nakanishi et al [3.14] showed that, during UV/ozone oxidation,

the Si-H and Si-OH bonds are replaced with Si-O bonds and a good SiO2 network is expected.

An UV gun or an UV cleaner is used to grow an UV/ozone thin oxide or to ameliorate

the existing native oxide, under a flux of oxygen. In most cases, during the UV/ozone

oxidation, the samples are heated at around 150oC.

For our experience, we have used a UV/ozone cleaner from Jelight Company (42-220).

The cleaner has a timer which allows us to control with precision the growth time.

We have chosen four samples cut from the same p-type low-doped 1015 cm-3 substrate.

We have cleaned the native oxide in a 1% HF solution during 10 seconds and we have rinsed

the samples with deionized water.


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Figure 3.4.27 UV/ozone oxide growth time, after Duhayon [3.6]

Given the UV/ozone growth time (Figure 3.4.27), we have chosen four oxidation times

for each sample: 10 minutes, 30 minutes, 90 minutes and 270 minutes. We have taken into

account that our UV/cleaner doesn't have the possibility to heat the sample during the

oxidation which may slow down the process.

The SCS signals recorded on all four samples are presented in Figure 3.4.28.

Figure 3.4.28. SCS signals on UV/ozone oxides . Vac = 300 mV.

The signals present a number of irregularities. For the oxides grown for 90 minutes and

270 minutes the amplitude is greater and the FWHM smaller than those corresponding to the

oxides grown for 10 minutes and 30 minutes. The shift of the SCS signals, from one oxide to

another, does not seem to have a physical justification other than the improper experimental

conditions.

However, from this data, a conclusion is clear. While the position of the peak of the

SCS signal for the native oxide is located around zero volts, the position of the peak of the

SCS signal for the UV/ozone oxides shifted towards the value of 3 V - 4 V. The difference of

thickness between the two oxides cannot result in such a magnitude of the shift of the SCS

signal. We conclude that the quality of the UV/ozone oxides is poorer than the quality of the

native oxide.


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This result coincides with the work of the Dr. Didier Goghero done in our laboratory in

2005, on UV/ozone oxides. He concluded that an oxygen line and a mean to heat the samples

must be added in order to reach the results found in the literature.

3.4.2.6 Summary of low-temperature oxides

Thickness Position of the peak

of the SCS signal

Ease of growth Growth on the

cross-section

Contrast

reversal

Native 1.2 nm app. 0 V yes yes no

UV-ozone 1.7 nm 3 V - 4 V yes yes no

plasma 5 nm - 7 nm -1 V - -0.5 V no yes yes

nitride 7 nm - 9 nm app 1 V no yes yes

LaAlO3 5 nm 2 V - 5 V no yes no

3.4.3 Conclusions and perspectives

In our opinion, the best way to characterize oxides with the SCM is as follows:

1. Any information on the studied oxide, acquired by other characterization methods, is

welcome.

Previous knowledge about the oxide thickness and the density of various types of

defaults, can help correlating the SCS experimental data with defaults concentration.

Also, such knowledge could help to the theoretical calculation of the SCS signal, which

will further improve the interpretation of the SCS signal.

2. The first option would be to acquire the SCS data on nanoelectrodes previously

deposited on the surface of the studied oxide. Such nanoelectrodes would assure the

protection of the surface from contaminants and humidity and a good electrical contact. Also,

the known geometrical dimensions of the electrodes would help in quantifying the oxide

properties.

In the ideal case, the same sample, or similar samples, should also have electrodes for

CV measurements. Comparisons between C-V and SCS could be made.

3. In the study of an oxide, multiple samples of the oxide in question, on substrates of

different doping concentrations and of different thicknesses would also help in quantifying the

oxide properties.

4. The SCS measurements should aim a complete evaluation of the oxide by taking into

account all the properties that can be deduced from a SCS curve. From our knowledge, no

author from the literature didn't complete a full study on an oxide.


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5. Before any measurement, one should verify the quality of the tip and the state of the

equipment. In order to do this, a standard sample, preferably a thermal oxide of good quality

deposited on a low-concentration substrate, should be used.

A test SCS should be performed on this standard sample, in standard experimental and

operating conditions. If all is normal, the measured SCS signal should correspond, in terms of

amplitude, FWHM, hysteresis and flatband bias to the previous established values.

6. The SCS measurements should be immediately followed by a topographical scan of the

spot where the measurement was done. In air, during a SCS sweep, there is always the

danger of oxidation of the surface. A topographical scan would verify if the topography has

changed.

The topographical scan should be accompanied by a SCM scan at Vdc=0V and a small

Vac. Thus, it can be seen if a injection of charges took place during the SCS.

From all the dielectrics we have taken into account, for now, there is no ideal oxide that

could play the role of gate oxide for the doping profile characterization.

The native oxide continues to represent the best compromise due to the proximity to

zero volts of the peak of the SCS curve, in spite of its thinness and its low quality.

UV/ozone oxide remains a strong candidate, given the possibility that the necessary

equipment for its growth (heating of the substrate, oxygen line) be available.

The high-k dielectrics remain a strong candidate, especially due to the high permittivity

constant, which would allow thicker dielectric layers to be used. Future developments

concerning the growth technology could impose the high-k dielectrics as the best choice for

SCM measurements.

Another choice that we have taken into account is the growth of thermal oxides at

temperatures below the doping diffusion temperature. It is known that the density of defects

of an oxide increases exponentially with the decrease of growth temperature. We think

possible that, by choosing the growth temperature right below the doping diffusion

temperature, the thermal oxide to be good enough for SCM measurements.

A few attempts have already be made, with promising results (Figure 3.4.29).


185

Figure 3.4.29. Thermal oxide grown by dry oxidation at 700oC

The numerous problems with the oxidation oven - especially the humidity proofness of

the oven - made that the dry oxidation be in fact a humid oxidation which lowered the quality

of the oxide and rendered the oxide growth uncontrollable.

However, we consider oxides growth by thermal dry oxidation or by RTO, by using a

growth temperature right below the doping diffusion temperature, to be a very good lead in

obtaining the needed gate oxide for SCM measurements.

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General conclusion

General Conclusion

188

This work was devoted to the experimental study of the scanning capacitance microscopy

(SCM) and spectroscopy (SCS) for the mapping of the dopants in the semiconductor structures

and for the characterization of thin oxides. SCM has appeared to be a very powerful technique

for doping mapping as long as qualitative images are needed, for example in order to check

whether fabrication steps like implantations have been correctly operated during the

fabrication of devices (presence or absence of doping of a given type in a region where it

should be present). When quantitativity is needed, the only way of performing a calibration of

SCM images for doping mapping seems to grow exactly the same oxide on two different

samples, one being a calibration sample from which a semi-calibration curve associating

doping levels and SCM signal levels will be measured and applied to the unknown sample

(semi-calibration). This is a very constraining method, which imposes to stick two samples

together and polish them in exactly the same way so that the resulting oxide (possibly of poor

quality) is the same for both.

In this work, we have chosen to study the experimental reasons which prevent SCM and

SCS from being a fully quantitative technique.

In the first chapter, we have shown the capabilities of SCM for doping mapping using a

series a experimental situations and test samples covering almost all frequently encountered

structures in the industry of silicon microelectronics : doping staircases of p-type and n-type

structures, quantum wells and p-n junctions. Bevels have been realized in order to artificially

enhance the resolution in a single direction. Qualitative images have been obtained for a wide

range of doping levels between 2.1015 at.cm-3 to 5.1019 at.cm-3. SCM is able to detect

quantum wells of ~ 7 nm width. By adjusting the applied voltages (especially DC voltage), it is

possible to avoid the well-known contrast inversion, and recover a SCM profile corresponding

to e.g. a SIMS analysis. Also, it is obvious that SCM is a wonderful characterization method for

the detection of dopants of different type (p-type or n-type) especially compared to Scanning

Spreading Resistance Microscopy (SSRM) which can not distinguish between both types. All

these results confirm the usefulness of SCM as a qualitative imaging technique.

Problems start to arise when the doping profile is not precisely known by means of

another technique like SIMS. On the one hand, contrast inversion can be avoided by adjusting

the Vdc voltage but in the absence of any other information it is difficult to decide whether

contrast inversion takes place or not. On the other hand, even theoretically, the 'ideal' Vdc is

not the same for p-type and n-type regions, which further complicates the quest for ideal

experimental conditions. Moreover, a large number of experimental parameters may further

distort the SCM image and modify the measured signal levels and thus the interpretation.

In the second chapter of this work, we have studied all the parameters playing a role

in the interpretation and reproducibility of SCM signal. To do so, we have chosen to work with

test samples of thermal silicon oxide of known thickness and quality, and make a direct

comparison between Capacitance-Voltage (C-V) curves obtained with a macroscopic setup and

General Conclusion

189

results obtained on the same test samples with SCM and Scanning Capacitance Spectroscopy

(SCS). This approach has allowed to clearly point out the influence of the ambient or laser

light on the SCS signal. The comparison of SCS with or without the light provided by the laser

measuring the deflexion of the AFM evidentiate the drop of the signal to noise ratio due to the

creation of electron-holes pairs in the sample under study. This effect has been explained

using the macroscopic C-V curves on test samples covered with metallic electrodes of different

thicknesses and exposed to laser or ambient light. Experimental examples of the influence of

the laser light on the interpretation of doping profiles have been provided. Solutions have been

proposed to avoid or minimize this effect.

The influence of the parasitic capacitance has also been addressed in a context where

the preparation of the samples for doping mapping may require a polishing process leading to

possible important topographical features. It appears that SCM measurements in the middle

of a planar sample suffer from a drop of the sensitivity of the system compared with

measurements on the sample edge or even on a cross section. Using samples covered with a

metallic layer, which should not lead to any signal at all by SCM measurement, we have shown

that a parasitic signal was very still often present. The influence of experimental conditions

has been studied in order to try and find the optimal conditions for the minimization of this

parasitic capacitance. Although the origin of a parasitic signal at the same frequency as the

applied alternating voltage Vac remains unclear, the presence of a water layer on the surface

along with abrupt variations of capacitance whenever a topographical feature is encountered

could play a role in its presence. We have also thoroughly studied the influence of all the parts

of the experimental setup (cantilever, chip, cantilever holder, electronic modules...) on the

SCM signal and sensitivity.

Finally, we have studied the role of the tip sample contact, which appears also to be of

major importance in the quality of the SCM/SCS signal. By a direct comparison of SCS

obtained with a macroscopic C-V setup and SCS obtained on the very same electrode cut from

the electrode used for macroscopic measurements, we have emphasized that SCS was

indeed able to perform an exact measurement of the position of the inflexion point

of the C-V curve (related to the flatband voltage of the MOS structure) when the

measurement was performed on a clean, metallic, water (or any other kind of contamination)-

free electrode. When the tip is positioned directly on the surface of the oxide, parasitic

features appear on the SCS like a hysteresis (difference between the trace and the retrace), or

a peak in the inversion region of the C-V curve. Again, comparisons with macroscopic C-V

curves have allowed to better understand the role of the tip-sample contact in SCS

measurements.

Contaminations possibly present on the AFM tips, reproducibility of the quality of the tips

and artifacts due to the behavior of the piezotube have also been taken into account as

possible sources of errors on the SCM signal.

In the third and last chapter, the use of the SCS in order to assess the quality of

General Conclusion

190

different top oxides for doping imaging has been addressed. A precise description of what can

be expected from SCS curves has been provided and the problem of anodic oxidation has been

described, leading to the conclusion that voltages applied to the MOS structure during

characterization should be limited in order to limit anodic oxidation, growth of 'hillocks' on the

surface and leakage currents through the oxide (leading to a modification (degradation) of its

properties). With the aim of choosing an oxide for the MOS structure used for doping mapping,

we have compared between different candidates : native SiO2, plasma oxide, UV/Ozone SiO2

oxide, silicon nitride... Also, as an illustration of what SCM can bring to the oxide growers, the

use of SCS in order to assess the role of an annealing process on the quality (especially the

position of the flatband voltage) of a LaAlO3 oxide grown by Molecular Beam epitaxy has also

been shown.

These experiments have shown that the compromise between the ease of growth,

thickness (trade off between leakage currents and signal to noise ratio), position of the SCS

maximum in order to limit the value of the applied voltages and limitation of leakage currents

is not easy to find. For qualitative imaging, native oxide grown on clean conditions (clean

room for example) remains a good compromise. Although it is very thin (1.2 nm) and leads to

important leakage currents, it allows to get a good contrast with no contrast reversal with 0 V

applied on the sample. Oxides grown at higher temperatures, like the RTO oxide used in this

work, represent a promising alternative. Advances on the growth and the control of the

properties of alternative oxides (high-k oxides) could be a solution for the future, as these

kind of oxides are often obtained at low temperature.

All these results put together emphasize the importance of the measurement

environment on the quality of capacitance measurements. SCM under vacuum and controlled

environment (dry air, nitrogen), along with a system eliminating the laser light during SCM

measurements (like the dark lift recently operated by Veeco on its systems), a better quality

of the tips (coating adhesion, absence of contamination, reproducibility, known shape, better

knowledge on the work function of the tip...) would allow to improve significantly the reliability

of SCM technique.

FOLIO ADMINISTRATIF

THESE SOUTENUE DEVANT L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON

NOM : LIGOR DATE de SOUTENANCE :11/02/2010

Prénoms : Octavian TITRE : Reliability of the Scanning Capacitance Microscopy and Spectroscopy for the nanoscale characterization

of semiconductors and dielectrics NATURE : Doctorat Numéro d'ordre : 2010-ISAL-0008

Ecole doctorale : Electronique, Electrotechnique, Automatisme

Spécialité : Dispositifs de l'Electronique Intégrée Cote B.I.U. - Lyon : T 50/210/19 / et bis CLASSE : RESUME : This work was devoted to the experimental study of the scanning capacitance microscopy (SCM) and

spectroscopy (SCS) for the mapping of the dopants in the semiconductor structures and for the characterization of thin oxides. SCM has appeared to be a very powerful technique for doping mapping as long as qualitative images are needed, for example in order to check whether fabrication steps like implantations have been correctly operated during the fabrication of devices (presence or absence of doping of a given type in a region where it should be present). When quantitativity is needed, the only way of performing a calibration of SCM images for doping mapping seems to grow exactly the same oxide on two different samples, one being a calibration sample from which a semi-calibration curve

associating doping levels and SCM signal levels will be measured and applied to the unknown sample

(semi-calibration). We have shown the capabilities of SCM for doping mapping using a series of experimental situations and test samples covering almost all frequently encountered structures in the industry of silicon microelectronics : doping staircases of p-type and n-type structures, quantum wells and p-n junctions. Qualitative images have been obtained for a wide range of doping levels between 2.1015 at.cm-3 to 5.1019 at.cm-3. SCM is able to detect quantum wells of ~ 7 nm width. SCM is also able to differentiate between

dopants of different type (p-type or n-type) especially compared to Scanning Spreading Resistance Microscopy (SSRM) which can not distinguish between both types. All these results confirm the usefulness of SCM as a qualitative imaging technique.

We have studied all the parameters playing a role in the interpretation and reproducibility of

SCM signal: stray light, stray capacitance, the tip-sample contact, the influence of strong

electrical fields, the sample’s topography, the quality and the properties of the top oxide. We

have proposed solutions for eliminating all these parasitic factors and to render the SCM

measurements reproducible and quantitative.

MOTS-CLES : SCM, SCS, doping profiles, oxide characterization, AFM, resolution, junction localization, quantification, interface states, fixed charges, volume charges, mobile charges, stray light, stray capacitance, C-V measurements, impedance analyzer, M.O.S structure, nano-electrodes.

Laboratoire (s) de recherche : Institut des Nanotechnologies de Lyon Directeur de thèse: Brice Gautier Président de jury : Composition du jury : Frédéric HOUZE, François BERTIN, Daniel ALQUIER, Christophe GIRARDEAUX,

George BREMOND, Jean-Claude DUPUY

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