I. FORSCHUNGSZENTRUM JUUCH GmbH

DIAMOND AND DIAMONDLIKECOATINGS

Preparation, properties and application

byS. Marinkovi6 and Z. Marinkovi6

with collaboration ofS. Krawczynski

~~~ ~~--~---

Scientific Series of the International Bureau Volume 2

Forschungszentrum JQlich GmbHScientific Series of the International Bureau

DIAMOND AND DIAMONDLIKE COATINGS

Preparation, properties and application

byS. Marinkovi6 and Z. Marinkovi6

with collaboration of

S. Krawczynski

German-Yugoslav-Cooperationin Scientific Research and Technological Development

Herausgeber Forschungszentrum JGlichGmbHund Vertrleb: ZENTRALBIBLIOTHEK

Postfach 1913. 0-5170 JGlichTelelon (02461) 61-5368 . Telelax (02461) 61-6103

Druck: Weka-Druck, L1nnich

Copyright: Forschungszentrum JGlich 1990

Scientilic Series 01 the International Bureau

ISSN 0938-7676

ISBN 3-89336-044-1

The work on "Diamond and Diamondlike Coatings" was done

in the framework of German-Yugoslav Bilateral Cooperation

on "High-Temperature Materials", Partner institutions:

The Boris Kidri6 Institute of Nuclear Sciences, Institute

for Materials, Vin~a/Belgrade, and the Central Division for

Fuel Element and Irradiation Technology of Forschungszentrum

JUlich GmbH.

The authors are indebted to the International Bureau of KFA

Jijlich for its support of these activities.

1.

2.

2.1

2.2

3.

3.1

3.1.1

3.1. 2

3.2

3.2.1

3.2.2

3.2.3

3.2.4

4.

5.

6.

i

CONTENTS

INTRODUCTION

PHYSICAL METHODS FOR PREPARATION OF DIAMOND AND

DIAMONDLIKE COATINGS

Sputtering

Ion beam

CHEMICAL METHODS FOR PREPARATION OF DIAMOND AND

DIAMONDLIKE COATINGS

Chemical vapour deposition

Conventional chemical vapour deposition

Hot filament chemical vapour deposition (CVD)

of diamond coatings

Plasma chemical vapour deposition

Direct current plasma chemical vapour deposition

Radio frequency (RF) plasma chemical vapour

deposition

Microwave plasma chemical vapour deposition

Thermal plasma chemical vapour deposition

CHARACTERISTICS (PROPERTIES) OF DIAMOND AND

DIAMONDLIKE COATINGS

APPLICATION OF DIAMOND AND DIAMONDLIKE COATINGS

CONCLUSION

REFERENCES

Page

1

9

9

10

27

27

27

34

41

41

45

74

90

94

102

114

118

1

1. INTRODUCTION

Diamond is the hardest and the best heat conductingmaterial known, transparent in the visible and infrared (IR),

except from 2.5 to about 6.5 ~m wavelength) with a highrefractive index (2.4), very high electrical resistivity

(> 10 20 ohm cm), exceptional chemical inertnes and high thermalstability.

Due to its unique properties, the diamond has attractedattention of men over centuries. For the same reason researchers

allover the world have tried to synthesize diamond.

One of the main obstacles in synthesizing diamond arisesfrom the fact that it is thermodynamically stable only at high

pressures (Fig. 1) [37], so that it exists under normalconditions only as a metastable phase. Although due to kinetic

reasons diamond remains unchanged at lower temperatures, at

about 1500 0C in an inert atmosphere begins its transformationinto the stable modification-graphite.

Since the first successful attempts to synthesize diamond

- those in Sweden [62,63] and USA [20], the laboratoryexperiments have been turned into an advanced industry. In theindustrial process a suitable carbon source (graphite) is

brought to the thermodynamically correct region in the presence

of a solvent/catalyst (one of the group VIII transition metals)whereby the diamond synthesis results.

In recent decades it has proved possible to produce

virtually pure carbon films with properties similar to those of

diamond by deposition from a gas phase at low pressures, usingmethods which provide activated, high energy species in the gas

phase. However, the films exhibited marked differences in their

optical, electrical, mechanical and other characteristics,depending on the method and conditions of preparation used.

This diversity of properties is a consequence of the ability

of carbon atom to form different strong carbon-carbon chemicalbonds.

The nature of the bond formed is determined by the type

of hybridization of valence electrons (spl, sp2, sp3) (Fig. 2),

2

the particular chemical bond being determined by the overlap

of the electronic orbitals. The sp3 hybridization leads to

formation of four strictly directed strong covalent bonds with

a resulting tetrahedral coordination. This well-known structure

of diamond (Fig. 3) leads, in turn, to its isotropic properties.

The sp2 hybridization, in which only three of the four

electrons of carbon atom are involved, leads to formation of

three coplanar, strictly directed, strong covalent bonds and to

the well-known hexagonal layer network of graphite. The fourth

(delocalized) electron is responsible for very weak forces

holding together the hexagonal layers (Fig. 4).

In consequence, graphite is, in contrast to diamond,

soft, black and electrical conductor. In addition, and again

in contrast to diamond, it is characterized by an exceptional

anisotropy of properties.

Under the conditions of deposition of carbon from a vapour

phase, simultaneous formation and coexistence of different

carbon/carbon bonds is virtually inevitable. Thus in

synthesizing diamond by this method, simultaneous formation of

sp2 bonds takes place. Therefore the product of deposition was

never "pure" diamond coating, but rather a "diamondlike" layer

containing, in addition to the sp3 bonds, greater or lesser

amount of graphitic material, i.e. the sp2-bonded carbon atoms.

After early successful attempts to grow diamond epitaxially,

i.e. with very low deposition rate, it is only in recent years

that it has become possible to deposit diamond layers at

acceptable and even high growth rates and yet without any

detectable amount of sp2 bonds and, therefore, with properties

close to those of natural diamond.

The aim of this study has been to present an up-to-date

condensed review of the rapidly progressing knowledge concerning

formation of diamond and diamondlike coatings by deposition of

carbon from a vapour phase. The study should help newcomers to

the field to rapidly orient themselves; it might also help those

working in the field to have an overall picture of the progress

made during the recent years.

It should be mentioned that, in contrast to the situation

with BN coatings [66] where the relative scarcity of literature

3

TIK '"eocoVopour

-toccLiquid

Uqwld.000

\.000

S9lid

\ "00 , 10 '00 ceo\ P/ba,

3000 \\\\

2000 \ .\ Diamond

.......... Merollic\ "- ,\

, ,\

-, ,\

100 200 300 .00 '00 600 700

Plkoo(

Fig. 1: The carbon phase diagram - the part aroundthe three-phase equillibrium graphite-liquid-vapouris in the top right hand corner in semilogarithmicrepresentation [37].

sp

Fig. 2: Schematic diagram of the hybridizations of sandp atomic orbitals permitted in carbon. The 0 orbitals areshown un-shaded and the IT orbitals are shown shaded .

4

(al

Fig. 3: The diamond structure seen from two points of view:(a) the conventional cubic unit cell; (b) a view showinghow layers are stacked - these layers run perpendicularto the body diagonals of the cube.

(b)

Fig. 4:

5

The structure of the hexagonal form of graphite.

6

has prevented the authors to preSent a complete picture of the

BN synthesis by certain methods, the literature on the diamond

growth is so abundant that the problem was how to make a proper

selection of the papers to be included into the present review.

Concerning the systematization of the review, the point

that was difficult to decide consisted in whether to arrange the

material according to methods used to produce coatings, or

according to the properties of the coatings. The systematization

according to the coating preparation methods seems to be more

appropriate, and therefore was adopted here.

Consequently, the chapter on physical methods used for

diamondlike coatings (DLC) and diamond coatings (DC) preparation

(sputtering and ion beam) is the first one, and chemical methods

are dealt with in the following chapter. The chapter on chemical

methods is divided in two parts - chemical vapour deposition

(CVD), containing conventional CVD and hot-filament-assisted

CVD, and plasma assisted CVD, containing direct current plasma

CVD, radiofrequency plasma CVD, microwave plasma CVO and thermal

plasma CVD.

Within each of these sections, a description of the method

is presented, followed by a review of the published literature

on the subject. To help the reader to get a more complete

picture of the diversity of methods, apparatus and techniques

used, as well as of properties of the materials obtained, these

data are presented in the condensed form of tables, the

additional text discussing and summarizing the results and

presenting the authors' conclusions.

It was considered helpful to present an overall summary

of the properties of OLC and DC obtained, as well as of their

possible applications, the following two chapters being devoted

to these subjects. The last chapter is conclusion.

The already mentioned abundance of the literature on OLC

and DC, very rapidly increasing in the last years, as well as

the authors' intention to present an up-to-date review, i.e. to

prepare (and publish) the review in a relatively short time,

have necessarily resulted in an incomplete survey of the

published literature. The authors hope nevertheless, that the

important results have not been omitted. A great help were the

7

recently published review articles covering some of the subjectstreated in the present review.

A problem of terminology has been a rather difficult one.

Because of the fact that structure and properties of coatings

obtained using the deposition methods depend on the method and

conditions of preparation used, these vary in a broad range.

The properties which have been reported for the coatings

prepared range from graphitelike (electrically conducting and

optically opaque) or polymerlike (electrically insulating, soft

and optically transparent) to diamondlike (electrically

insulating, very hard and optically transparent). Different

names which have been used to designate the prepared coatings

are diarnondlike carbon, i-carbon, ale, a-C:H, hard carbon, dense

carbonaceous. Different names have been often used to designate

coatings with similar properties.

The difficulty in defining criteria in order to

distinguish between polymerlike, diamondlike and diamond

coatings stems also from the fact that different researchers

have used different methods to characterize the coatings

prepared.

Therefore, the criteria adopted in this work and the

definitions of diamondlike and diamond coatings used are

somewhat vague.

DLC are carbon coatings for which the combination of the

methods of characterization used permits to conclude that, along

with presence of hexagonal graphite arrangement, they contain a

considerable proportion of carbon-carbon bonds with tetrahedral

coordination, making the coatings similar in their properties to

diamond.

Diamond coatings are nearly pure carbon coatings for which

a combination of methods used for characterization permits to

conclude that the carbon atoms of which they consist are in

tetrahedral coordination, with undetectable or insignificant

proportion of carbon atoms in the hexagonal graphite

arrangement, making properties of the coatings close to those

of diamond.

In addition to the well-known cubic diamond, four other

metastable carbon phases have been identified (lonsdaleite -

8

hexagonal diamond [37], chaoite - another hexagonal phase and

two other cubic phases [64,24]. All these phases are stable at

the proper high-pressure, high temperature conditions. However,

although the crystal structure of these phases was identified,

very little is known about their properties. In some papers

reporting diamondlike coatings preparation, one or more of these

phases have been formed. In the present review under the term

diamond we shall mean, unless otherwise stated, the well-known

cubic diamond.

9

2. PHYSICAL METHODS FOR PREPARATION OF DIAMOND AND

DIAMONDLIKE COATINGS

In physical methods of diamond and diamondlike coatingspreparation, no chemical changes are supposed to take place.

The methods which are included under this heading are sputtering

from carbon target and a group of methods which use ion beam

(IB) techniques for coating preparation (direct IB, mass­selected IB, IB sputtering and dual IB).

This classification is, however, only partly correct,

because in IB methods not only beams of carbon are used, butalso beams obtained by ionization of various hydrocarbonmolecules, which undergo different chemical changes in the

process of coating formation.

2.1 Sputtering

Sputtering is the emission of material particles fromsolid surfaces (target) which are bombarded with high energy

particles, usually Ar gas ions.

Diamondlike carbon films have been produced using directcurrent (de) planar magnetron sputtering of a graphite target

in ultrapure argon gas at a pressure of 10-2 mbar and at asubstrate temperature close to ambient temperature [98].

The target power was varied in the range 5-500 W (the targeteffective area was about 20 cm2).

The films prepared at low sputtering power are very dense(2.1-2.2 g/cm3), with electrical resistivity of 10 4 ohm cm and

hardness of 12-24 GPa. The films possess a very low optical gapas compared to the films obtained by other methods. The range

is 0.40-0.74 eV, meaning that the film is transparent in the

IR region. Analysis of their optical properties shows that the

films possessing the best DL properties consist principally oftetrahedrally sp3 bonded atoms with the sp3/sp2 coordination

of carbon atoms ratio equal to 3.1. As the sputtering power

increases, there is a sp3 to sp2 transition in the amorphous

10

carbon matrix, so that the films prepared at higher power

consist of approximately equal mixtures of sp3 and sp2

coordinated atoms.The authors conclude that the presence of hydrogen is not

strictly neccesary for obtaining stable tetrahedral bonding in

the amorphous carbon matrix, but that hydrogen does improve DL

properties by reducing the refractive index and increasing the

infrared optical transparency.

It is interesting to note that no paper reporting

preparation of DC and DLC by the well known cathodic sputtering

technique could be found. The amorphous carbon films prepared

using a commercial RF sputtering system [97,67] do not show any

diamondlike properties.

2.2 ION BEAM

Since Aisenberg and Chabot's paper describing preparation

of DLC by ion beam beam deposition [2], a number of researchers

in the world used this and related methods.

This rapidly growing field has resulted in many

publications, only some of which have been included in the

Table 1 (without the pretense that the best selection has been

made) .Several excellent review articles have been recently

published on the subject [4,64,77]. Other recent papers [50,79,

83,3] may help to get additional informations about ion beam

deposited carbon layers, and [113,116,40,115] are dealing with

the method and application in general.

Compared to other plasma sources, ion beams permit

a greater control of deposition parameters. They can have

a narrow energy distribution and specified direction, the

beam energy and ion current density being controlled almost

independently over a wide range of process conditions.

The advantage of this method in comparison with most plasma

techniques lies also in the fact that in the latter bombardment

conditions are controlled by a variety of parameters including

plasma power, gas pressure, gas composition, flow rate and

11

system geometry. Furthermore, the separation of the ion

generating plasma volume from the substrate minimizes

interaction between high energy plasma electrons and

the substrate, so that high energy particle interaction is

produced exclusively by the well-defined beam ions.

It follows from the above said that, in order to take full

advantage of the control offered by the ion beam deposition

techniques, it is important to maintain the ion beam energy,

current and chemical identity of the species while transporting

the beam to the substrate or target. In this regard, it is most

important to minimize the pressure in the region of beam

transport in order to prevent energy and spatial distribution of

the beam ions from extensive broadening due to their interaction

with background gas.

The pressure is usually 10-4 - 10-6 mbar, because in this

pressure range mean free path is sufficiently long and most of

the ions will maintain their energy (e.g. for a pressure of

about 1x10-4 mbar, 90% of the Ar+ ions can be expected to

maintain their charge for a distance of 10 cm downstream [4].

Furthermore, the substrate is most often kept at room

temperature or close to it, the energy of the beam ensuring the

necessary rearrangement of the atoms in the surface of the film.

The original Aisenberg and Chabot's method [2] was

a direct IB one, meaning that the ionized carbon atoms

were directly sent to the substrate and deposited on it.

Other methods, developed since include: a mass-selected (or ­

separated) IB, permitting to select from the initial beam the

desired ionic species; IB sputter deposition, where IB is used

to sputter the carbon atoms from a suitable carbon or graphite

target in order to effect their deposition on a near-by

substrate; dual ion beam in which first beam is used either as

a direct IB transporting carbon or carbon-containing ions to the

substrate, or to sputter carbon atoms from a target which are

then deposited, the second beam being used to bombard the

substrate in order to improve the coating properties. In

a variant of the dual beam method, a laser beam or magnetron

can be used to vaporize carbon from the target instead of lB.

Table 1

Ion beam deposition of diamond and diamondlike coatings

N

Preparation Apparatus Substrate Characteristics Application Referencesconditions

1 2 3 4 5 6

Plasma from Direct ion beam 8i, stainless Transparent, well-adherent, hard, For thin-film [2)region (a) is two regions: steel. chern. resistant OLe, amorphous transistorsextracted to a) Plasma with and/or crystalline. Diamond isregion (b). sputtered C- indiCa\~d. k = 8-14; n > 2,0;The ions are atoms p ~ 10 n em,accelerated P=2.6-66 I'bar.by substr. b) Plasma con-voltage. stricted by

axial magnettcfield. P=10-moar . Substr.at -40V, and I

ac (RF)potential

Ion energy Direct ion Water-cooled Hard (MH = 18.5 GPa) , insulating, [81)50-11 eV beam metals, semi transparent, DLe composed mostly

conductor, of C-C bonds. n=2.3 at wavelengthinsulators. 5 um, Similar to RF plasma CVD

films.

C + . Direct ion MgO, Si. In undeflected ion beam neutral [86)H4 loons,ion beam beam and species content is nearly 30%.current magnetic Very smooth surface of film formeddensity 0'2 field to from deflected lB. Polycrystallineto 2 rnA/em . :Ieflect it. diamond film with large and fineSubstrate g~ains is -formed from deflectedvoltage and undeflec1ed IB, resp.200-600 V. Amorph~Us i~ carbon present.Td= 100-450°C. P = 10 -10 n em. MH = 70 GPa.

Table 1 continued

Co>

1 2 3 4 5 6+ + Mass-selected WC-Co R = 1.1 IllIl!h. Hardness, abrasive [10]C , CH3 I

CH+ ion beam, resistance and adhesion of coatingsC2~2+· 100 kV isotope prepared with C+ beam are superior.Ion energy separator Films prepared with hydrocarbon500-1000 eV. beams are br~ttle and adhere weakly.Max IB currentdensiZy 0.1rnA/emP = 10-6 mbar

Ar+ , ion Ion beam Cu, Ta, Si02.1 R = 119 nm/h. Simultaneously Heat [14 ]energy sputter. source rotated, sputtering of substrate was made- conductors1 keV, ion <l> 8 em. cleaned by with resulting removal/arrival for semicon-beam Pyro1itic sputtering rates of 0 (no sputtering), ductor devices;current graphite 0.j6 and 0.71. Density is 2.2 gl doped semicon-55 rnA, target em for simu1taneo~sly bombarded ductors. ,P = ~.4-5.3 30.5x30.5 em. films and 2.1 g/cm for sput\1r-xl0- mbar. Simultaneous deposited only films. p > 10 n em.

ion bombardment For 170 nm thick films and at 555possible. nm wavelength reflectance is 0.2,

absorb~ce °17, absorption ccef. is6.7xlO- cm- and transmittance 0.1.

Ar+Hg, up to Ion beam Substrate R = 100 nm/h. In resputtered [51]90% 2. Ion sputtering <l> cooled by area smaller by a factor of 1-3.5energy 500- 15 em source. liquid for ion beam accelerating voltage1500 eV, High purity nitrogen or of 600-1400 V. No effect of resput-typically carbon target water tering on film properties.1200 eV

4water cooled. Amorphous car~on ~ilms with up to

P=4x10- mbar. Ion beam 35 at % H, sp Isp bonding ratio isPartly <I> 5 em. :=::< 1.5, band gap 1.2 eV. Strongshadowed effe§t of H: d decr. from 2.25 to 1.6substrate to g/em , ME deer., but hydrogenated rfilmscontrol remaiYovery hard . . p incr. from 10resputtering. to 10 n em. Optical absorption

deer. with H, band gap incr. from-0.7 to 1.2 eV.

Table 1 continued

1 2 3 4 5 6

Ar or Ar+H2 Ion beam Si(lll) or With Ar ions: DLC, R = 30~-400 [55]ion beam sputtering ion fused quartz, nm/h; MH = 30 GPa, p = 10 n cm,mixt. Ion beam <I> 25 nun. sometimes n = 3.1energy 1200 High purity coated with With AI + Hi ion beam: DiamondeV. Ion beam graphite Ni or Ti. crystals O. -1 ~ in amorphouscurrent target. matrix~ R ": 300-400 nmlh;density 1 ~nd p = 10 n cm, n = 2.8, Eg = 1.04 eV.0.04 rnA/emat targetand subst.,respect.Td= . RT;P=6.5x10-5mbar in Ar,2.6 mbarAr+H2·

Sputtering: Dual beam Polished R = 550 nm/h. DLC with MH>9 Anti- [94]Ar+, 0.5-1 20 em ion beam sapphire, Ge, (Mohs) for Td <15°C, good adhesion reflectivekeV. 150 V, for sputtering Si, glass, except for Au and steel, good protective50 mAo pyro1ytic quartz, ZnS, chem. resistance. Absorption for coatingsIrradiation: graphite. Water AI, Cu, AU, wavelengths 600-1000 nm is belowAr+, 0.1-0.3 cooled target steel. 10%. Transmission (for 2-10 ~)

keV. and substrate Presputtered. >60%. n = 2 l1-3.1 for 350-800 nm.P=1.6x10- 4 p = 5-10x10 n em.mbar.TD S O°C.

CHt, (28 at %) Dual beam. Fused silica, R = 430 nmlh. F~lms with H/c Insulating [76]+ • 100

2ev,30 em <I> source Si. = 1; d=1.8 g/em , p =3.35x10 6, gates for

0.2 mA/em . for deposition, E = 0.34 eV, n=2.46. Films field effectIrradiation: 8 em <I> for a~ere well, are transparent transistors;209-6002ev, irradiation. and impervious. Transmittance doped semi-251lA/cm , between 800 and 1500 nm is conductors.P=2.6x10-4mbar 80-95%, S 10% reflectanceTd = RT between 400 and 1500 nm; 5-20%

absorbance between 800 and 1500nm.

...

Table 1 continued

'J'

1 2 3 4 5 6

Laser Focused CO2 Fused quartz R = 60 nm/h. ~orphou3 film [38Jevap~rati~n laser beam containing sp and sp bonded4xlO W/crn evaporates C-atoms. p j.~cr." with energy oflaser power powdered bombarding ions and decreasesdensity. graphite. with Td" P max ~ 1 n em. Trans-Ar+ energy Ar+ ion mittance increases with ion340-980 eV, source for energy.ion beam substratedellsity 20pA/ bombardment.cm .P=10-5mba r.Td from -115to 100°C.

+ Mass-selected Si(ll1) , Diamond films -0.7 ~ thick are Semi- [96JC f 120 e¥,400 IJA/crn . ion beam cleaned epitaxially grown with (111 ) conduc- ,Td = RT with dilute and (220) planes parallel tive

HF to Si(ll1) and (220), resp. devicesRotational spread around <111>normal is - 1.7°, mosaic blocksize ....15 run.

Designations: ac

MH

Td

alternating current; £g = optical gap; IB = ion beam; k = dielectric constant;

microhardness; R = deposition rate; RF = radio frequency; RT = room temperature;

deposition temperature.

Abbreviations: chern. = chemical; coef. = coefficient; deer. = decrease; incr. = increase;

mdxt. = mixture; resp., respect. = respectively: sputter. ~ sputtering;

subst., substr. = substrate.

16

Each method has its varieties. Table 1 contains examples

of the mentioned methods.

Direct ion beam

In this method an ion beam of controlled composition,

energy and flux is directed onto a substrate. The impacting

ions are used to supply both the deposition atoms and the

energy required for improved film formation.

The original Aisenberg and Chabot's apparatus (2) is

represented schematically in Fig. 5. There are two vacuum

regions (a high-pressure one and low-pressure one) separated

by a plasma constrictor opening used for differential pumping.

The diameter of the constrictor opening is about 1.5 rom.

The plasma source chamber is the smaller one where the higher­

pressure discharge (about 2.6x10-3 to 65x10-3 mbar) is used to

form the positive ions of materials such as carbon.

The discharge in the source chamber is used to introduce

the carbon atoms into plasma where the high-energy electrons

can ionize them. A hollow cavity in a carbon electrode is used

as one electrode and a carbon post in the center is used as the

other. Positive-ion bombardment is able to sputter carbon from

the electrode into the source plasma.

Both electrodes in the deposition source chamber are made

of carbon to reduce contamination. An axial magnetic field is

used to modify the electron orbits so that the discharge can

operate at lower pressures. The ions are extracted from the

source chamber by means of an auxiliary discharge drawn to an

anode located in the deposition chamber.

A constrictor is used to separate the source chamber and

the deposition chamber and to permit differential pumping so the

deposition chamber pressure can be maintained at 10-6 mbar. The

auxiliary discharge is designed so that use is made of the axial

magnetic field to direct the extracted plasma and to reduce the

space-charge spreading of the beam. By means of the extraction

arc, one has a low-pressure discharge (in the deposition

chamber) in series with the higher-pressure arc in the source

chamber.

17

In order to reduce film contamination, carbon inserts are

used in the constrictor and in the extractor anode.

The plasma in the deposition chamber can act as a source

of carbon ions as well as a source of electrons. A negative

potential applied to the substrate is used to attract the

positive ions. When insulating film are deposited, then an ac

or RF potential is applied to the substrate to attract the

necessary electrons for cancellation of the positive-ion

surface charge. The ion energy incident upon the substrate

can be adjusted by the substrate potential and by the amplitude

of the ac signal.

Before deposition, the substrate is cleaned by sputtering

for several minutes at -400 V; most of the substrate deposition

occurred at about -40 V. The ion current to the substrate is

approximately proportional to the extracted arc current. Film­

deposition rates of about 18 ~m/h were obtained.

Mass-selected ion beam

Greater control is afforded by the use of mass-selected

IB deposition, in which the beam is prefiltered by a mass

separator. This technique permits precise control over the beam

species and allows deposition in ultra high vacuum conditions

which minimize surface contamination.

Fig. 6 contains schematical representation of a mass­

selected IB apparatus (for negative C ions) [50].

The acceleration voltage of the deposited negative ion

beam was varied from 10 to 1000 V for a C- ion beam and from

40 to 400 V for C2- ion beam. The deposited area was about

1 cm2, the current density of the deposited ion beam was

30-100 ~A/cm2, and the deposition time was 1-2 h. The background

pressure in the deposition chamber was around 9xlO- 7 mbar during

deposition. Substrates used were n-type silicon wafers with

(100) plane and the resistivity of the wafers was 8-15 ohm em.

This method permits preparation of extremely pure films.

Its disadvantages are relatively low deposition rate, high cost

of the sophisticated mass-selected IB apparatus and, at least

in the existing systems, small size of the IB «3 cm2).

18

I><f-s

GASINLET..

TO HIGH VACUUMPUMP

Fig. 5: Schematic diagram of the direct ion beam depositionsystem. (1) carbon cathode; (2) carbon anode;(3) differential pumping and extraction aperture;(4) plasma ion extraction electrode; (5) electromagnet;(6) substrate located in high vacuum region [4] .

MICROWAVEPOWER SUPPLY

o-lOa 'II

. ACCELERATION •VOlTAGE SIPPLV

-01- -vTO PUMP

(e)

(a)

Fig. 6: Schematic diagram of a mass-selected negativeion beam deposition system. (a) negative ion source;(b) mass separator; (c) negative ion beam decelerationsystem [50].

19

This may restrict utilization of this method to some special

applications.

Ion beam sputter deposition

Ion beam sputtering differs from conventional plasma

sputtering in that the projectiles bombarding the target are

generated in external ion sources which are separated from the

deposition chamber by pressure stages.

The ion beam sputter deposition system is illustrated in

Fig. 7 [55]. The graphite disk target (purity 5N, 100 rom in

diameter) was bonded to the water-cooled holder. An electron­

bombardment ion source was employed. The ion energy and the ion

current were 1200 eV and 60 rnA, respectively. The ion-beam

diameter was 25 rom. The incident angle of the ion beam was about

30° from normal of the target. The substrate was placed near the

target as illustrated in Fig. 7. The surface of the substrate

was fixed almost parallel to the direction of the ion beam.

The ion beam sputtered the target and also grazed the

surface of the substrate. The ion current densities were 1 and

0.04 rnA/cm2 at the target and substrate, respectively. The ion

beam, which grazes (bombards) the substrate, modulates the

arrangement of the surface atoms in the deposited carbon film.

The momentum component normal to the surface of the substrate is

low but the overall momentum of the ions is high. The localized

atomic-scale activation is formed in lineal region (exhibits

some area). Therefore it is considered that the uniformity of

the film is better than that of the film deposited using the

assisted ions in the direction normal to the surface of the

substrate (not grazing incidence). The ions at normal incidence

make the localized atomic-scale activation in the pointed region

(small area).

Table 2 summarizes the sputtering conditions. Si (Ill) or

fused quartz plates of thickness 0.2-0.3 rom were used as the

substrates. The substrates were sometimes covered with Ni or Ti

thin film. It is believed that these metal films assist the

20

nucleation of the diamond. The substrate was not heated and kept

at room temperature.

Table 2

Sputtering conditions for film preparation [55]

Target

Target dimension

Ion source: Energy

Current

Beam apertureGas pressure

Target-ion source spacing

Growth rate

Graphite plate

100 mm in diameter

1200 eV

60 rnA

25 mm in diameter6.5xlO-5 mbar

250 mm

300-400 nm/h

The advantage of this method is that because of high

vacuum it produces pure carbon films (if the target material

is made of pure carbon). In addition, it is often possible to

bombard simultaneously the growing film on the substrate by the

ions from the beam, which may affect properties of the film.

The disadvantage is low deposition rate.

Dual ion beam

By using two different ion beam sources, a better control

of the deposition and sputtering conditions is ensured.

The apparatus is shown in Fig. 8 [40]. Two ion beams

sources of the Finkelstein type are attached to a deposition

chamber in such a way that one source is used to produce a flux

of sputtered particles from a solid target while the other

source allows the growing film to be bombarded with inert or

reactive ions of variable energy.

The target was pyrolytic graphite and was sputtered with

argon ions of energies 0.5-1 keV. A second source irradiated the

growing films with 0.1-0.3 keV Ar+. The optics in the primary

source are focusing tungsten grids; those in the co-bombarding

H, --->

21

I~.n·::~ ~~R'...... >..: Subslrate'«::e

toPump

Fig. 7: Schematic diagram of the ion beam sputterdeposition system [55].

on source

Inert gas or,eoelll!' Ion beam

C Wgel.·

/ Substrate holder

/ -: wostrctes

glass btll-Jor

reactsve Ion beam

5{Jutt.erro ma/enal

Ion source

pumping sy5tem

Fig. 8: Schematic diagram of the dual ion beamdeposition system [40].

22

source are graphite. Metallic contamination in the deposited

films was minimized by the graphite grids and by covering metal

parts with graphite shields. Table 3 summarizes the operating

parameters [94].

Table 3

Operating parameters for optimum transparent carbon filmsdeposition bythe dual ion beam technique

Sputtering source

Ar pressure

Presputter

Sputter

Co-bombarding source

Ar pressure

Presputter

Deposition

Chamber pressure

Deposition rate

Maximum substrate temperature

Target and sample angles to beams

Focused 20 cm Kaufman-type3.7x10-4 mbar

600 V, 700 rnA, 5 min

850 V, 840 rnA, neutralized

10 cm Kaufman-type2.7x10- 4 mbar

150 V, 100 rnA, 3 min

150 V, 50 rnA

1.2x10-4 mbar

550 nm/hS OOC

45°

Influence of ion beam deposition parameters on the film

properties

Ion energy is the primary parameter determining the

properties of the carbon films, inducing the formation of

dense, adhering, hard carbon films.

The ion energy range is rather wide and extends from

10 eV to 1.5 keY.

In the direct ion beam methods this range is usually from

40 to several hundred eV [4], although energies up to 1 keY have

been used for ionized molecule beams [116]. For mass-selected IB

the energy range from 10 eV to 1 keY is used [64]. For the

23

method using sputtering of the target material higher energies,

up to 1.5 keY are used. These energies provoke an enhanced

mobility of the atoms on the film surface, as well as their

shallow implantation, but they also cause a preferential

sputtering of the sp2 bonded atoms.

Incident flux and angle of incidence are parameters

influencing the film properties directly and indirectly

through secondary effects.

In the simplest set-up in the direct ion beam method,

in which the substrate is located directly within the beam

transport region, the bombarding ions are non selectively

extracted from the ion-source plasma. The beam is then a

combination of inert gas, deposition atoms, clusters,

molecular fragments, etc.

In order to achieve better control of ion beam

composition, magnetic field was used to deflect the beam

(CH4+ ions), thus avoiding the neutral species (amounting

to about 30%) and improving the film properties [86].

The best control of ion beam composition is achieved

using mass-selected methods.

The ion bombardment of the deposited film, as used in

dual-beam and (sometimes) sputtering methods is believed to

increase the diamondlike component by giving surface atoms

extra kinetic energy to move into sp3-bonded sites, and by

preferential removal by sputtering of amorphous or sp2-bonded

domains [4].

Species used for carbon deposition include carbon ions

(negative and positive), hydrocarbon radicals, carbon cluster

ions (Cn+) and non-carbon species such as hydrogen and argon.

In most carbon deposition processes there is an unidentified

mixture of several species.

Deposition from hydrocarbon species results in a high

conceptration of hydrogen in the films. Argon ions are used in

most "ion-assisted" processes (e. g. IB sputtering). Argon is

believed to preferentially sputter graphitic and amorphous

( Sp2) carbon, enriching the diamond (Sp3) constituent of the

film [64]. Hydrogen is believed to stabilize the sp3 carbon by

24

saturating dangling bonds and by preferentially etching sp2carbon.

The properties of the ion beam deposited films depend onthe process parameters used.

Composition and properties of the films depend on the ion

beam composition. carbon ions produce pure (hydrogen-free)

carbon films (DLC) and hydrocarbon ions produce hydrogen­

containing carbon films (H-DLC). Hydrogen content and properties

of H-DLC are nearly independent on the type of hydrocarbon, but

depend strongly on their composition, as well as on ion energy.

The virtual independence on hydrocarbon type is explained

(4) by hydrocarbon disintegration upon surface impact if the ion

energy is on the order of 100 eV.

The films are predominantly amorphous, but can have

extensive sp3-bonded short-range order [64,4]. However, poly­

crystalline diamond films can also be formed with crystallites

up to 5 ~m [4,86), so that the range from amorphous to the well

crystalline coatings is covered.

The films are chemically inert to all known solvents.

Both "pure" (DLC) and hydrogen-containing (H-DLC) films have

been deposited on many different substrates: glass, quartz, Si,

NaCl, KCl, CaF 2, Ge, plastics, steel, other metals, nylon, paper

and other. DLC display the highest adhesion to carbide-forming

substrates (Si, Ge, Mo, W), but they adhere well to most

substrates if these are properly cleaned by ion bombardmentprior to deposition.

According to (64), the sp3 films are deposited on

substrates held at TdS70°C in the mass-selected IB method,

while graphitic films are obtained at Td~lOOoC. The explanation

offered is that under proper temperature conditions, energetic

carbon ions displace preferentially sp2 atoms, leaving the

sp3 atoms in their positions, but also that the host diamond

matrix imposes the bonding type of the deposited carbon

( "mold" effect).

Although similar'experimental results have been obtained

by other authors, a recent paper (86) in which polycrystalline

diamond has been reported at Td=100-450°C should be mentioned.

The density is typically about 2.4 g/cm3 for pure carbon,

25

and 1.8 g/cm3 for hydrogen-containing films.

Although the thin (100 nm) films are apparently stress­

free, thicker films tend to separate from the substrate,

probably due to internal compressive stress.

Both ion-beam OLC and H-OLC are unusually smooth, often

smoother than the substrate. Thin films reproduce the morphology

of the substrate [103].

The resistivity of OLC is on the order of 10 11_1012

ohm cm, while H-OLC have resistivity in the range 10 7_10 16 ohm

em, increasing with increasing H content.

Breakdown voltage of OLC is about 1x10 6 V/cm, and

. dielectric constant is about 8.

The OLC are optically transparent. IR absorption of both

OLC and H-OLC in the 1-10 ~m wavelength range is very low,

except at wavelengths where C-H vibrations appear.

The index of refraction is ~2 in the visible and IR parts

of spectrum for both OLC and H-OLC. Very high values (up to 3.1,

see Table 1) have also been reported.

In general, the advantages of the ion beam methods are

that diamondlike or diamond films can be formed under controlled

deposition conditions on the substrate surface made of various

materials at the ambient temperature. However, since accelerated

ions are employed, the formed diamond has many defects and it is

difficult to obtain diamonds having good crystallinity.

Furthermore, the deposition rate is intrinsically low because

the ion beam density can not be substantially increased.

Combination of properties of ion-beam-deposited OLC

and H-OLC (transparency, electrical insulation, high breakdown

voltage, high thermal conductivity, chemical resistance, barrier

properties, hardness, good adhesion to substrates, smoothness,

wear resistance), together with the fact that they can be

deposited at temperatures slightly above room temperature,

have led to their application.

An initial application for the transparent diamondlike

films were protective coatings for plastic lenses, while OLC of

controlled thickness have been used to improve transmission of

Ge IR optics [4].

26

However, wider application of IB deposited coatingsremains a challenge. This may include, in addition to various

optical elements (for high power lasers, for mirrors and

windows, optical sensors, decorative jewellery or silverware),

electronic devices (field effect transistors, semiconductors),

high power or high power density devices, protective coatings

for use in biomedical applications and fiberoptics, for

cutting tools etc.

Nevertheless, for all these and many other possible

applications, the coatings prepared by ion beam methods will

have to resist a competiton with coatings obtained by other,

cheaper methods.

27

3. CHEMICAL METHODS FOR PREPARATION OF DIAMOND AND

DIAMONDLIKE COATINGS

3. 1 CHEMICAL VAPOUR DEPOSITION

3.1.1 Conventional chemical vapour deposition

Since at low pressure (and temperatures) the thermo­

dynamically stable modification of carbon is graphite, the

question is whether, and if so, how diamond can be produced

under non-equilibrium conditions. The factors which determine

the modification that will be produced during the pyrolysis of

a hydrocarbon are, in addition to pressure and temperature,

composition of the gas phase, nature of the hydrocarbon used for

pyrolysis and availability of a diamond surface. The first three

parameters determine how far from the equilibrium heterogeneous

reactions of formation of diamond and graphite will take place.

Theoretical studies show [105] that in the simplest case

of growing diamond from supersaturated carbon vapour there is a

certain optimum supersaturation (i.e. ratio piPe' where P is the

optimum pressure and Pe is the equilibrium vapour pressure over

diamond) at which the ratio of probabilities of formation of

two-dimensional nuclei of diamond and graphite will be maximum.

At this supersaturation the unavoidable covering of the growing

diamond surface with graphite growth centres will be minimal.

However, under these conditions the diamond growth rate would be

too low, so that in order to increase it only to the lower limit

which allows its experimental detection (about 0.1 nm/h), it is

necessary to operate at much higher supersaturations, where

graphite evolution becomes considerable.

Availability of diamond surface is certainly a factor

which would favorize the diamond growth. As to the nature of the

hydrocarbon used, the obvious factor which should influence

graphite or diamond formation would be the type of bonds, the

tetragonal bonds being favorable for diamond growth.

28

Remains gas phase composition, the factor which finally

permitted to solve the problem of pure diamond growth under

non-equilibrium conditions.

Various experiments have been designed with the aim of

preparing diamond, many of them trying to solve the problem

using difference in kinetics of formation of diamond and

graphite. One such approach is based on a difference in kinetics

of formation of diamond and graphite nuclei in the pulse

crystallization [105]. In this method synthesis of diamond on

diamond seed crystals is effected from hydrocarbon gases, using

a periodical (pulse) change in supersaturation.

The time of formation of a critical (stable) diamond

nucleus should be shorter than the corresponding time for

graphite because of the orientation effect of the diamond

substrate. With suitably chosen pulse duration, the diamond

nuclei will reach the critical size and the graphite nuclei

will remain subcritical. Therefore, the unstable (subcritical)

graphite nuclei will disappear during pauses between the pulses

when supersaturation decreases, but the stable diamond nuclei

will remain and continue to grow during the next pulse.

Systematic research work on the growth of diamond in the

region of its thermodynamic metastability has independently and

almost simultaneously started in the USSR and USA in the second

half of 1950's [105].

Already in 1956 Spitsyn and Derjaguin succeeded in

building up local areas of a· natural diamond single crystal face

through thermal decomposition of gaseous CBr4 and CJ 4 under low

pressures «10-3 mbar) and at a temperature of about 900°C

[105].

The Eversole's patent [35] filed in 1958 describes the

growth of diamond on the diamond seed crystals by pyrolysis of

methane at low pressure (optimum range 0.20-2.6 mbar) and low

temperature (optimum range 1000-11000C). Weight gain with

respect to initial weight of the seed crystals was up to

0.76%/h. The pyrolysis was periodically interupted in order

to remove graphite which was simultaneously deposited, using

prolonged heating in hydrogen at 1000-1100 oC under 10-50 bar.

Instead of methane, methyl-group-containing compounds could

29

be used, but benzene, CC14 and hydrocarbons not containing

methyl groups did not produce diamond.

The diamond deposition was found to continue even after

considerable black carbon had been deposited.

Another paper [8] describing diamond formation by thermal

decomposition of methane used similar conditions to those of

Eversole (lOSO°C, 0.4 mbar). Methane was passed continously over

diamond seed crystals. The obtained deposits on the diamond

crystals (up to 23.7 wt% mass increase) were analysed by a

number of methods (chemical analysis, chemical etching, density

measurements, X-ray and electron diffraction, microwave

absorption, electron spin resonance and visual observations).

The authors consider that the evidence presented demonstrates

conclusively that diamond has been formed.

Thus, growth of diamond by CVD has been shown to be

possible. However, growth rate of diamond is low and the method

suffers from an unavoidable deposition of graphitic carbon which

must be removed by tedious procedures.

Nevertheless, after these experiments a number of

researchers have studied various aspects of diamond formation.

However, the most important problem of avoiding simultaneous

deposition of graphitic carbon was solved only recently.

Probably the most effective approach to obtain selective

growth of diamond consists in etching the non-diamond carbon

directly during the diamond film growth by introducing into the

gas phase an etching or purifying gaseous agent. Although oxygen

or other oxidising agents may be used [105], the methods using

atomic hydrogen have been applied most extensively because of

high reactivity of hydrogen and the fact that it reacts strongly

with graphite, but practically does not affect diamond [8].

This highly selective etching of graphite, is illustrated in

Fig. 9 .lIOS]. Thus, even if the growth rates of diamond and

graphite differ insignificantly (as represented in Fig. 9),

the overall effect will be growth of diamond, and total absence

of graphitic carbon.

Such a method has been described in [104] using a

"chemical transport method". Fig. 10 represents an apparatus for

the chemical transport [105]. The gas containing hydrocarbons

30

,~

<

I~

,

~:<

~

01"""""V

Graphite

Fig. 9: Relationship between the growth and etchingrates of diamond and graphite in the presence of areagent which can etch graphite selectively. Theresultant rates (hatched arrows) may correspond tothe growth of diamond and the etching of graphite[105] .

Gos inlet

'as

Corbon tube 1=:37Furnace

-<:1 SUbstrate/

Holder

Fig. 10: Schematic diagram of a chemical transportdeposition system [105].

31

(CH4, C2H6, C2H4, C2H 2) formed by reaction of graphite with

hydrogen at high temperature is transported to the diamond

substrate. A superequilibrium concentration of atomic hydrogen

obtained from H2 gas either by heating (several % H), or byelectric discharge (about 10% H) was introduced in order to

remove the graphitic carbon formed. At a substrate temperature

between 800 and 1200°C (the optimum temperature resulting in the

highest growth rate being 1000°C) a pure diamond was obtained

(without graphitic carbon) which could not be graphitized in

vacuum at 1500°C, in contrast to the diamondlike carbon.

To illustrate the great advantage of this method, the

authors have compared it with pyrolysis of methane (Table 4)

[104] .

Table 4

Synthesis of diamond by pyrolysis of CH4 and by chemicaltransport reaction technique (CTR)

Process characteristics

Growth rate at 10000C (~m/h)

Epitaxial layer thickness (~m)

Threshold temperature of

epitaxial growth (OC)Activation energy (kJ/mole)

Graphite deposition

Pressure in the gas phase (bar)

Pyrolysis of CH4

0.001

0.002

N850

250

Yes

0.001-0.5

CTR

Nl

5

N750

105

No

0.01-0.5

Properties of the diamond layers produced are very similar

to those of natural diamond (Table 5) [104].

32

Table 5

Some properties of diamond

Property

diamond

Synthetic

diamond

Natural

diamond

~225

5.81013_1015

0.35668±0.00001

100

2.40

85-110

~225

0.35664±0.00001

100

2.38±0.2

95±4

(nrn)

4-6(ohm ern) 10 12_1013

Lattice constant (22°)Carbon content (%)Refractive index

Electrical resistivity

Microhardness on (Ill) (GPa)

Transmittancy in the

UV range

Dielectric constant

Complexity of reactions taking place on the diamond

surface during the deposition has been discussed [105].

According to the authors, these processes involve adsorption

of atomic hydrogen and hydrocarbons, recombination of atomic

hydrogen, hydrogenization and de-hydrogenization of adsorbed

complexes, formation and decomposition of nuclei of the diamond

and non-diamond structure, desorption of hydrogen and hydro­

carbon molecules and their diffusion towards the carbon surface.

Because of high hydrogen atom adsorption rate, the diamond

surface is thought to be coated by hydrogen atoms to

a considerable extent even at a temperature of about 1000°C.

Thus, adsorption of hydrocarbon molecules takes place on the

hydrogenated diamond surface, which leads to increased surface

concentrations of complexes having tetrahedral coordination.

According to the authors, it is possible that the high diamond

growth rate (3 orders of magnitude higher than in the

(conventional) pyrolysis of CH4, see Table 4), is solely due

to this effect [105].

In addition to diamond substrate, other substrates have

been used. The diamond nucleation rate was found to be one or

33

FurnaceSilica mantle

W, filament

Substrale

,_-",,uj-j- Silica cellThermocouple

IAlumina rod

To~mpFeed gas

To vacuum To power supplygouge

Fig. 11: Schematic diagram of a hot filamentdeposition system [68].

34

In addition to diamond substrate, other substrates have

been used. The diamond nucleation rate was found to be one or

two orders of magnitude higher on carbide-forming substrates

(Si, Mo, W), than on non-carbide-forming ones (Cu, Au).

The rate of nucleation is higher on polycrystalline than

on single-crystal substrates and it starts on defects like

scratches, grain boundaries, dislocations, etc.

The doping by boron, introduced during diamond deposition

using gaseous B compounds, was found to produce at low B levels

lattice contraction and a p-type of conductivity. However, the

lattice parameter of a highly doped (up to 1 at% B) diamond is

almost equal to that of the substrate, which is probably a

consequence of presence of B in both substitutional and

interstitial positions (substitutional and interstitial B

produce contraction and expansion of the lattice, respectively)

[104].

3.1.2 Hot filament chemical vapour deposition (CVD)

of diamond coatings

This relatively simple and apparently efficient method of

diamond film preparation makes use of a hot filament (z20000C)

placed near the substrate. The role of the hot filament is

manifold:

- To decompose hydrocarbons. Starting from methane, the gas

phase in the equilibrium at 2000 0C should contain CH3, C2H and

C2H2, but CH2 and CH may be also formed by interaction of CH4with a hot filament [68].

- To form a considerable amount of atomic hydrogen.

Calculation by means of the equilibrium constant of the

reaction H2=2H for 2000°C and 45 mbar shows that about 20%

of H2 molecules are dissociated.

- To heat the substrate. Temperature of the substrate

surface heated only by the hot filament is in the range

500-1000°C, according to measurements. However, the real

substrate surface temperature must be considerably higher,

because the temperature was measured by a thermocouple placed

3S

at or near the substrate holder or (as in [16]) within the

substrate.

- To supply electrons which, if attracted by the positively

charged substrate (holder), bombard the substrate surface thus

activating it and increasing mobility of atoms in the surface.

However, application of such accelerating voltage was reported

only in some cases [100,16].

Thus, by using the hot filament method the conditions

essential for nucleation and growth of diamond are ensured.

A schematic diagram of the hot filament apparatus is shown

in Fig. 11 [68]. A tungsten filament which can be heated up to

2000 0e is placed in front of, or near the substrate. Gases can

be introduced between the substrate and the filament. Prior to

the deposition, the whole system, which is placed in a quartz

tube, is evacuated to 10-6 mbar with an oil diffusion pump.

The values of the experimental parameters are listed in the

Table 6 [16].

Table 6

Experimental parameters

Total gas pressure

Gas flow rate

eH4 concentration

Substrate temperature

Filament temperature

Substrate potential

Reaction time

30 mbar

0.9-1 l/h

1-12%

S80-880oe

2l00·e

200 V

3 h

Either Si (100) wafers or molybdenum films deposited on

stainless steel are used as substrates. The substrate

temperature is measured with a thermocouple placed inside (when

possible) or close to the substrate. The filament temperature is

controlled by measuring the filament resistance.

Table 7 contains data relative to preparation and

properties of diamond coatings.

36

Certain authors [68,45] used, in addition to the hot

filament, an external furnace to heat the substrate.

In some experiments [16,100] a positive potential of the

substrate holder was applied in order to accelerate electrons,

thus effecting an electron bombardment of the substrate.

In all cases low concentrations (0.5-12%, but typically

a few percent) of hydrocarbon gas (CH4' acetone, other organic

compounds) were used.

A wide range of pressure (0.7-1050 mbar) was used,

although in all the cases relatively narrow ranges were

apparently preferred, mostly at low pressure [16,68,45,100],

except in [92] where 1000 mbar pressure was used.

In a study concerning a more complete utilization of Raman

spectra for characterization of diamond films prepared by the

hot filament method [19], it has been shown that density of

defects in the diamond phase increases as pressure is increased

(the pressure range studied was from 6.5 to 130 mbar).

The filament temperature (Tf) and the filament-substrate

distance were found to be critical: no deposit was formed under

Tf=1730°C and under 1 ern distance [16]. However, higher Tf

(2000-2150 0C) combined with higher CH4 concentration (3%) led

to formation of graphitic carbon [68].

The hydrocarbon concentration in hydrogen was found to be

important: to obtain the diamond phase, the CH4 concentration

had to be not higher than 1% [16,68]. Similar concentrations

were used by other researchers [92,100], but considerably wider

range (0.5-10%) was reported to be suitable if some organic

compounds containing oxygen or nitrogen in addition to carbon

and hydrogen were used [45]. In this particular case a wide

range of pressure (1-1050 mbar) could be used. Because of the

wide ranges of parameters and high deposition rate (8-10 ~m/h),

the authors consider their method as promising for scaling-up.

In all the cases RHEED and SEM were used for

characterization of coatings; Raman spectroscopy was used in all

the cases except in [100]; other methods used were determination

of density [45], electrical characteristics [92], microhardness

[45,100] and boron-dopant concentration [92].

Table 7

Hot filament CVD of diamond coatings

W-.J

Preparation Apparatus Substrate Characteristics Application Referencesconditions

1 2 3 4 5 5

0.5-5% CH4 W-filament 5i-wafers, Por 1% CH~ in H~, Td>700-10000C, [58]in H2; cj> 0.15x30 nun, Mo-plates, T = 2000.0 ,P= 3.2-130 robar,P= 0.55- 20 turns above SiOji plates pK= 0.24 l/h, well defined diamond990 mbar; substrate, etc ed or crystals are obtained. Lower CH4PR= 0.24- heated by washed. conc. deer. number and size of12 l/h; external Vertical 5i02 crystals, and higher leads toTf 500- furnace (de reactor. graphite formation. Higher Tp at100°C, source) . high CH4 conc. favours graph~te.

measured at R deer. with incr. 5ubst-filamentsubstrate distance. Higher FR favours graphite.holder. Lower Td deer. number and size of cActual subst. crystals. At higher T~ blocky .temp. is particles formed. At ow P, smallhigher. ill-defined crystals; at severalTp= 2000°C. tens robar large and densely

populated crystals. At -1 bar onlysmall amount of deposit, non-diamond.C2Ht gives similar results.Nue eation rate much smaller onSi02 than on Si or Mo.

0.5-10% of Similar as in Si Under typical conditions (acetone Semicon- [45]acetone or [68]. + H2 at 133 robar, PR= 3-18 l/h, ductingother organic Tg= 550°C at subs. holder, Tp= material.compo in 200°C), polycrystalline diamond Electronics,H2. P= 0.001- film without non-diamond bonds is optical,1.05 bar. Ob~ained. R= 8-10 ~h; d= 3.52g/ devices.FR= 0.5-30 em ; MH= 100 GPa. The method isl/h. Td~ 500- considered promising for industri-800°C at subs. alization.holder.Tp= 2000°C.

Table 7 continued

wcc

1 2 3 4 5 6

1-2% CH4 in Similar as Sintered Nucleation and growth of diamond [100 JH2; in [116), but SiC plates; film with lattice constant a= 0.3561p= 40 mbar. Me as subst. Si, Mo, we, nm is more rapid than without appliedFR= 3 l/h. holder at TiC. voltage. R= 3-5 JllIl/h. MH= 90 GPa. OnTd= 850°C at positive Si-substrate both diamond and P-SiCsUbs. holder. voltage. are forme~~ Decomposition of CH4 andTF=.1900°C. H2 is accelerated byelectron-bombard-Vs= 150 V. ment.

1-12% CH4 in W-filament Si(lOO) wafer, For filament-substrate distance less Semicon- [16]H2 o placed in front Mo, deposited than 1 em, and TF<1730°C, no deposit ductorp= 30 mbar; of substrate on steel. is formed. For 1% CH4 in H2, FR= 0.9 (if doped),FR= 0.9- (max 1 em l/h; Td= 830°C; TF= 2100 oC, well- optics1.0 l/h. distance). All faceted di!F0nd crystals are obtainedTd = 580- is in a horiz. with no sp bonds. For 4% H2 and.Td=BOOoe; 730°C, both diamond and graphite are ~

TF= 1700- formed. To obtain continuous film,2100°C; the substrate must be polished withVs = 200 V. diamond paste.

Reactant: sat. W-filament 5i(100) wafer Semiconductive B-doped polycrystal- Electronic [92]solution of placed above polished with line diamond films without non- componentsB203 in substrate. diamond paste. diamond bonds detectable by RamanCH30H mixed are obtained. Max B conc. in thewith acetone, fiLms is = 1000 ppm. Slight shift+ H2, 2% of Raman peak with B onc. is found.reactant; p linearly decr. with B conc. forp= 1 bar, = 3 orders of magnitude.FR= 3 l/h,Td= 850°C,TF= 2240°C.

Designations: FR = flow rate; MH = microhardness; R = deposition rate; Td = deposition

temperature; TF = filament temperature; Vs = substrate voltage.

Abbreviations: compo =

sat. =

compound; conc. =

saturated; subs.,

concentration; deer. = decrease; max. = maxLmum;

subst. = substrate; temp. = temperature.

39

Well-defined diamond coatings, consisting of micro­

crystals, were reported in all cases, with little or no sp2

bonds present, at least in a certain (chosen) range of working

conditions.

In order to increase nucleation rate, the substrate was

polished with diamond paste [16,68]. A positive voltage applied

to the substrate holder was found to increase the deposition

rate and decomposition of CH4 and H2 [100].Influence of deposition conditions on the diamond

formation was investigated in [16,68]. Rather similar optimum

conditions were found in both cases: 1% CH4 in H2'

Td=700-1000oC, Tf=2000°C, P=13.2-130 robar, FR=0.24-0.9 I/h.

Higher CH4 concentration and 20 times higher FR resulted in

graphite formation [68]. An increase in pressure to 1 bar led

to a small amount of deposit in which no diamond was found [68].

An increase in Tf led to graphite, if CH4 concentration was

also increased, while Td affected mainly the morphology of the

diamond crystals in the coating [68].

The B doping of the diamond coatings was studied in view

of their utilization as semiconductors [92]. Well defined

diamond coatings wLth different B concentrations up to 1000 ppm

were produced. Electrical resistivity of the coatings decreased

linearly with B concentration. A rather wide interval of the

activation energy obtained from a plot of current vs. liT (0.12­

0.20 eV in agreement with 0.17-0.18 eV found by others) is

expected to be reduced in a single crystal film.

The substrate nature was found to affect nucleation rate,

in agreement with the results found for the diamond coatings

prepared by other plasma deposition methods (see e. g. microwave

plasma CVD 3.2.3.).

The reactions taking place in the hot filament method are

not fully understood. It is generally thought on the basis of

thermodynamical estimations that CH4 is decomposed through two

decomposition reactions. one is the direct thermal decomposition

near hot filament (CH4 = CH3 + H) and the other is the

abstraction reaction with atomic hydrogen, generated by H2decomposition on the hot filament (CH4 + H = CH3 + H2) [1].

40

However, by thermal decomposition of CH4 at high

temperatures (>1600°C) a quantity of acetylene is produced

which, in turn, leads to deposition of graphitic carbon.

In order to suppress production of acetylene, a modified

hot filament apparatus has been designed [1] in which an upward

stream of hydrogen prevents CH4 from coming to the hot filament,

other working conditions being similar to those in the already

described procedures. The diamond films thus obtained contain

less amorphous ( Sp2) carbon and their crystallinity is better.

The authors express opinion that separation of atomic hydrogen

production from CH4 decomposition may permit a better control

of process parameters in the diamond synthesis.

Thus the hot-filament technique was found to be suitable

for producing polycrystalline, virtually "pure" diamond coatings

(without sp2 bonds) having the structure and properties close to

those of natural diamonds. In particular, microhardness values

are within the interval obtained for the natural diamond. Less

strict requirements regarding conditions of deposition and high

deposition rates attained make the method attractive from the

point of view of its industrialization.

A method combining hot filament, elect~on beam and plasma

CVD has been developed to grow diamond films from gas mixtures

containing CH4' H2 and Ar [60]. The apparatus and procedure aims

at creating favourable conditions for high nucleation first, and

then for a high rate of growth.

The conclusions are not much different from those obtained

by other researchers. Diamond can be deposited at 640-800 oC,

at 6.5-40 mbar with 0.24-2.0 vol% CH4 in H2, but temperature and

pressure should vary to obtain optimum conditions.

The nucleation density is enhanced by scratching the

substrate surface with diamond powder, but nucleation is also

initiated without it. With respect to sapphire, silicon takes

much longer time to form nuclei. Diamond layer grown on Si has

a good adhesion, but transition layer is necessary on sapphire

because of big difference in thermal expansion coefficients.

The deposition rate obtained is 1 ~m/h.

41

3.2 Plasma chemical vapour deposition

3.2.1 Direct current plasma CVD of diamond and diamondlike

coatings

The most simple method of activation of a gas phase is to

produce a glow discharge using a high dc voltage. This may be

the reason why the first successful experiments on plasma CVD

of DLC were made with such a discharge [105].

A schematic diagram of the experimental set-up, as used in

[89], is shown in Fig. 12. A glass cylinder 7.5 cm in diameter

and 30.5 cm long contained a dc glow discharge. The cathode was

a water-cooled graphite rod. The gas entered through a hole near

to the cathode.

Typical discharge conditions were 600 V and 0.1 A.

A coaxial magnetic field produced by the solenoid helped to

stabilize the discharge, which was operating in the abnormal

glow discharge region at a pressure of about 65 ~bar.

The plasma density was increased near the anode by

an insulating disc. The plasma was extracted through a hole

2.0-2.5 mm in diameter in the anode and passed through the

positively biased grid with an aperture of about 6.0 mm

diameter. A second grid which was negatively'biased in the

range 100-2000 V accelerated the ions towards the substrate

while repelling the electrons.

This arrangement resulted in an ion beam of fairly uniform

energy and permitted the investigation of the effect of ion

energy on film properties. Since the diamondlike coating is an

insulator, a floating potential is established at the surface of

the substrate. It could be varied by changing the voltage of the

grid. No thermionic filament was used. The pressure in the

deposition chamber was in the 10-4 mbar range.

The gas consisted of roughly equal amounts of C2H2 and

CO2, sometimes with some addition of argon. The gas pressure

increased by about 50% when the discharge was started.

Table 8 illustrates the experiments and results obtained

with this type of discharge.

..... . ..'--..:..-

42

,....... ~

--7

--B

TOPUMP

Fig. 12: Schematic diagram of a dc plasma depositionsystem: (1) insulating glass cylinder; (2) cooledgraphite cathode; (3) gas inlet; (4) magnetic fieldsolenoid; (5) metal anode; (6) insulating anode cover;(7) positively biased grid; (8) negatively biased grid;(9) substrate [89].

TableS

Direct current plasma CVD of diamond and diamondlike coatings

..w

Preparation Apparatus Substrate Characteristics Application Referencesconditions

1 2 3 4 5 6

C2H~+5% Ar; vert. with Ti alloys, Ni. Black, ~eatureless deposits, [117]P=l -26Il-bar. upper anode. High tensile R= 1,8 um/mnxh , MH>28 GPa onGlow Air-cooled and stainless Ti-aloy, but could not bedischarge substrate steels <I> 12.7 x measured on other substrates.witb 0.5-2.0 fastened to 7-10 IDm, Good adhesion of thin films;rnA current. cathode. cleaned by Ar thick films bubble, and are

ion bombardment. split and peeled away.FiLm nature not studied.

Ct~ + CO2 Vert. Negatively Ge, AI, 5i, R=l~/h; DLe coating on Ge [89]( : ); biased grid quartz, glass, contains 2-2.5 at %0 and

"l30ll-bar. (100-2000 V) NaCl, steel, =10 a\O% H).fB:= 12-30 GPa; ,accelerates cleaned chemi- p =10 -10 q em. n=2.0-2.3.ions toward cally and by CO2 removes sp bondssubstrate sputtering preferentially.

using Ar or CO2

0.3-4% CH4 Vert. subs. 8i, a-A1 203 Well-defined polycrystalline [107Jin H2; mounted on diamond film. Nucleus densities260 mbar; water-cooled for diamond-powder-scratchedFR= 1,2 l/h. upper electrode surface and mirror-polishedTf 600- (anode) surface of 8i02 are-equal.80°C. lkV, 4A/em2 R= 20lJlll!h. Amorphous carbon(800°C is obtained if Td<600°C or ifwithout substrate is mounted on cathode.cooling) . MIl= 100 GPa.

Designations: FR = flow rate; MH = microhardness; R deposition ratei,Td deposition temperature.

Abbreviations: subs. substrate; vert. vertical.

44

The coatings obtained by dc plasma CVD are comparable

to those prepared using the RF plasma. The essential conditions

determining the nature of the deposit are composition of the

initial gas mixture and substrate temperature.

In the papers dealing with mixtures of the C2H4 with

5% Ar [117] and C2H2 with C02 (1'1) [89] and with substratetemperatures probably close to room temperature, diamondlike

coatings were formed. Their properties (microhardness up to

30 GPa, electrical resistivity up to 1012 ohm cm, refraction

index up to 2.3) are similar to those of the hard coatings

obtained with RF discharge, which is probably due to a high

ion bombardment (high accelerating voltage). On the other hand,

in the paper [107] reporting preparation of diamond films,

a gas mixture consisting of 0.3 to 4% CH4 in H2 and substrate

temperature above 600°C were used. These conditions are

necessary to prepare diamond by hot filament and plasma CVD

methods (see 3.1.2 and 3.2).Some interesting points in this'paper are: the use ·of

electron bombardment of the substrate, which was also reported

in some papers dealing with hot filament method [16,100];

conclusion that scratching of the 6i substrate surface with

diamond powder does not affect the nucleation rate, which is

contrary to the generally accepted theory; the use of relatively

high pressure (265 mbar) , which is considerably different from

the range of pressures most often used in the plasma CVD

methods.

Craig and Harding [26] used dc magnetron plasma sputtering

of the carbon coatings on a metal cathode in an atmosphere of

Ar-C 2H2. Dissociation of the C2H2 molecules in the plasma at

a low pressure (1.5-400 ~bar) results in the formation of

reactive hydrocarbon and hydrogen species which subsequently

strike the cathode, the substrates or the chamber wall.

At the high C2H2 injection rates employed (up to 13.2

l/h), a hydrogenated carbon layer forms over the surface of the

stainless steel cathodes with the associated deposition of the

diamondlike film on the substrates.

The DLC formed was the front surface antireflection layer

and insulating matrix of a graded (carbon-metal) cermet-copper

45

solar selective surface operating as a solar absorber IR

reflector tandem fur all-glass evacuated tubular collectors.

The deposited films contain a large amount of hydrogen

(up to 42.6 at%) and up to 6.7 at% of oxygen and have a low

density (0.98-1:6 g/cm3 ) . Their refraction index (for film

thickness greater than 80 nm) is 1.8-1.9 in the wavelength range

0.3-2.5 ~m, and electrical resistivity is above 10 7 ohm cm.

The optical band gap (Eg) increases with H/C ratio

(from 1.1 eV at H/c=0.2 to 2.0 eV at H/C=0.8).

The authors conclude that the basic OLC structure is

a random network of C-atoms in tetrahedral coordination with

nearest neighbours.

The deviations of bond lengths and angles from diamond

lattice values are minor, althougth the distribution of second

and further neighbours is highly disordered. The tetrahedral

carbon network is modified by C=C bonds and the inclusion of

hydrogen of the order H/C=0.5 in monohydride and dihydride

bonding configurations. After heat-treatment at 500°C, during

which large quantities of H2 and CO evolve, the film becomes

thermally stable which is believed to result from residual

Hand 0 atoms occupying fixed sites in the OLC network

characterized by an absence of near-neighbour H atoms.

Thus, it can be concluded that the dc plasma cva is

a method which can be successfully applied for both OLC and

OC preparation. The fact that this method has been relatively

little used is associated with difficulties involved in

neutralization of the surface charge and in ensuring uniform

thickness of the layers over a surface area greater than

a couple of cm2.

3.2.2 Radio Frequency (RF) Plasma chemical vapour deposition

RF plasma deposition of diamondlike coatings

By far the most widespread deposition process employed to

deposit diamondlike carbon films makes use of a low pressure RF

46

glow discharge in hydrocarbons or their mixtures with inert

gases. After the pioneering work of Holland and Ojha [46], this

method rapidly became popular allover the world.

There are several reasons why the RF discharge has been so

widely investigated. An important advantage is that standard,

commercially available equipment intended for preparation of

thin films by cathode sputtering can be used. Another advantage

comes from the fact that by using the RF plasma one of the

greatest problems involved in deposition of dielectric films on

various substrates, including the non-conducting ones, can be

successfully solved. The RF excited plasma exposes the substrate

to positive ions and to electrons alternately and thus prevents

any net charge accumulation on non-conducting deposits.

Reactor system

A schematic diagram of the deposition set-up as used in

[21] is given in Fig. 13. The reaction vessel 35 cm in diameter

and 22.5 cm in height is made of stainless steel. The electrodes

are water-cooled stainless steel discs 12.7 ern in diameter (area

about 127 cm2). The interelectrode spacing is 6.5 cm.

The discharge is sustained in CH4 or C6H6 or their mixture

(5 vol % and 7.6 vol % respectively) with helium by means of

a 13.56 MHz RF power source. The power is coupled to the excited

electrode via an L-type capacitive matching network for the high

frequency discharge (13.56 MHz), the other electrode being

grounded. All the substrates (glass or silicon) are set on the

powered electrode. The discharge pressure is measured with

a capacitance manometer which drives a trottling valve to keep

a constant pressure during all deposition cycle which may last

for several hours. The entire system is pumped down with

a mechanical pump which gives an ultimate vacuum of 1.3 robar.

The high purity gases are used without further purification.

The benzene vapor is obtained by the vaporization at room

temperature of spectroscopically pure liquid benzene.

The general deposition conditions are summarized in the

Table 9 [21].

47

Table 9

Range of deposition conditions

Gas Negative self bias(V) Pressure(~bar) Power density(w/cm')

100-400

100-700

53-530

67-800

0.08-0.8

0.2-0.6

Another apparatus which can be used for plasma deposition

of carbon films is schematically represented in Fig. 14 [59,18].

It is basically identical to a RF diode sputtering system and

differs from the system already described [21] mainly in that

only one electrode (cathode) is used, the reactor chamber

representing the other grounded electrode. The substrate is

placed on the 4 in. diameter electrode (cathode) which is

capacitively connected to a 13,56 MHz RF generator. The RF glow

discharge is sustained in a hydrocarbon atmosphere. Practically

any hydrocarbon gas or vapor can be used.

The hydrocarbon ions generated in the glow space are

accelerated across the ion sheath towards the cathode (Fig. 15).

Growth of carbon films proceeds at the cathode-mounted substrate

by condensation of these energetic hydrocarbon particles.

The impact energy of the hydrocarbon ions is determined by the

sheath potential, i.e. essentially the bias voltage, and the

hydrocarbon pressure which determines the mean free path.

Due to the different mobilities of ions and electrons in

the plasma (since inertia of the ions is high, they are (unlike

the electrons) unable to follow the rapid changes of the RF

potential and "see" only the constant (bias) potential), a high

potential drop is formed across the cathode dark space.

The time-averaged value of this sheath potential is given by the

sum of negative self bias voltage of the cathode and the plasma

potential (VS=VB + Vp).In an asymmetric system with small capacitively coupled

cathode and large grounded anode the plasma potential is reduced

to a small fraction of the RF amplitude. Thus, the bias voltage

is a good approximate measure of the sheath potential. It can be

48

wain

g.~,~-W ~inlet ~==;;UILIT==9

water :::==:)J

Fig. 13, Schematic diagram of a RF diode plasmasputtering system [21].

Pyrometer

Zn SeWindow

Substrote

-,

Choke

ROot5~

PumpRF-Molchlng

Unit

Turbomolecular

Pump

RF-Powermeter

RF· Ge n er o t cr

Pressurize 1Goses

Vo l v e

Fig. 14: Schematic diagram of the RF plasmadeposition system [18].

49

Average time distributIon

01 voltage wltI'IIll discharge

VoLioge waveform ot

capocltIVel)' coupt e d electrode

-v

tCapacltlyel)'coupled

et e c tr oce

hmolll

Vs Vp VB

GlowSpa c e

D's lance

Grounded

electrode

IlofgcJ

·v

Fig. 15: Spatial and temporal potentialdistribution in a capacitively coupled RFdischarge system with asymmetric electrodearea [59].

1{2(w/P)

6

Fig. 16: Plotl~~ the self bias potential againstparameter (W/P ).Full circles CH4; full squares C6H6;empty circles CH4-He; empty squares C6H6-He [22].

50

measured by means of an inductive coupled voltmeter. Figure 15

shows the spatial and temporal variation of the potentials in

an asymmetric RF discharge.

Variables controling deposition rate and film properties

The measured values of VB are presented in Fig. 16 as a

function of (w/p)I/2 [22]. It can be seen from the figure that

linear relation exists, but also that VB is higher for benzene

than for methane and that dilution of both gases with He also

causes an increase of Va'

Thus the variables controling energy of the ions

bombarding the substrate for a given gas mixture are the self

bias voltage (VB)' or the power and the gas pressure. However,

geometry of the discharge, in particular the ratio of the

powered electrode area and the area of the grounded part of

the apparatus, must also be considered as a variable determining

energy of the ions.

The negative bias (most often self bias) potential used

by different authors varied in the range 0-1400 V. The RF power

range was 10-1000 W (most often in a lower values range).

Different working pressures in the range 0.5-13000 ~bar were

used, but most often lower pressures up to 100-130 ~bar.

Consequently, the essential parameters, determining

energy of the ions impinging on the substrate and controlling

properties of the deposited films, are bias voltage (VB) and

gas pressure. Unlike film properties, deposition rate depends

significantly on the starting hydrocarbon gas.

Structure and bonding of RF deposited carbon coatings

Generally the coatings produced by the RF discharge

contain both sp3 and sp2 hybridized carbon atoms. Also great

majority of authors report formation of amorphous films

[109,6,46,91,25,17,57]. Certain authors [25,17] state that at

low power, or rather low power density, polymerlike, soft films

51

are produced. As the power increases, polymeric nature is

changed into hard, diamondlike coatings. According to [25]

hard coating consists of DL clusters embedded in polymer

matrix. Further power increase leads to graphite-like coatings

[17].

In several papers an abrupt transformation from soft to

hard films has been reported [109,57,82]. The transformation has

been found to occur at a certain W/P ratio [109,82]. According

to [57], the gas pressure at which the transformation takes

place is different for different gases: 90 ~bar for pure CH4'

13 ~bar for pure C2H4' which may be a consequence of different

number of C atoms in the molecule, but 1300 ~bar for a

50%CH4/50%H2 gas mixture.

The reported hard films, although most often found to be

amorphous, also contained crystalline phases [109,46,57], which

have not always been identified. Some of the diffraction rings

found in the paper [109] reporting formation of several

crystalline phases might be attributed to diamond, but others

indicate presence of unidentified phases, possibly new forms

of carbon.

Amorphous films with some small isolated unidentified

crystallites were found in [46]. The hard films deposited on Si

substrate from CH4 were found to contain diamond particles, but

only if the Si substrate was natur~lly oxidized [57]. According

to the authors, oxygen seems to enhance the diamond growth

because oxygen atoms can eliminate graphitic structures

disturbing the growth of diamond.

Effect of subsequent annealing of the coatings on their

structure, in particular bonding in the coatings, has been

rather extensively studied. Results obtained in an analysis

based on the IR spectra of the coatings deposited from toluol

[36] show that at lower annealing temperature (Ta from 100 to

350°C) total intensity of C-H bond vibrations abruptly drops and

the ratio of intensities I( Sp3)/I( s p2) slightly increases with

with Tal at higher temperatures (350-700°C) the intensity ratio

rapidly increases because the H atoms bonded to sp2 C atoms are

more rapidly annealed than H atoms bonded to sp3 carbon.

52

In a similar study in which vacuum annealing of hard

DL coatings was performed at different Ta up to 600°C [59],it was found that sp3 sites and olefinic sp2 sites exist.

The composition of the as-deposited films containing 68% sp3

sites, 30% sp2 and 2% sp1 sites is s1igtly changed upon

annealing to 300°C, but above this temperature hydrogen is

evolved and drastic change in the bonding occurs.

The work based on the photoemission spectra of DL films

obtained from toluol [95] confirms the previously mentioned

conclusions [59] in that the sp3 bonding decreases at Ta>350oC.However, the sp3/sp2 ratio was found to reach saturation at

about 500°C. In addition, it is concluded that the sp3 content

increases up to Ta=350oC.An interesting experimental result is that properties

of the hard films deposited from different hydrocarbons under

identical conditions, are independent of the nature of the

starting hydrocarbon [59,6], even although hydrocarbons with·

different types of bonding (such as benzene and methane) are.

used [59]. This is even more surprising knowing that these

hydrocarbons behave differently in the RF discharge (e.g.

different ionic species are formed) [59]. The explanation of:

these experimental observations is that the relatively high

energy of the plasma leads to efficient fragmentation of the'

hydrocarbon gases, so that small fragments, such as C, C2 or

CH may be regarded as the actual precursors for DL film growth

[59].

This is not true, however, when soft, polymerlike films are

considered. Namely, properties of soft films generally depend

on the starting hydrocarbon gas, because under such conditions

larger fragments of the hydrocarbon molecules can be

incorporated into the film [59].

Influence of deposition conditions on coating properties

Deposition rate values were reported in a number of papers

[46,32,33,6,21,31,95]. The values vary in a wide range, from·

0.03 ~m/h [33] to 72 ~m/h [95], but the majority of them are ·of

53

the order of several ~m/h. The deposition rate was found to

increase with RF power, bias voltage and hydrocarbon pressure

[33]. Addition of inert gases (Ar,He) which stabilize plasma by

increasing its conductivity, sharply reduces deposition rate

[120]. For hydrocarbon gases having the same structural form,

deposition rate was found to increase with molecular weight [6].

According to the paper [21], mass deposition rate should

be distinguished from thickness rate, the former increasing

linearly vith p1/2 and the latter increasing quasi-linearly with

the product VBP. This difference is explained by variation in

film density, which was found to increase linearly from 1.3 to

1:6 9/cm3 with the reduced parameter VBP-1/ 2. Since the ion

energy is also proportional to this parameter (18), it follows

that densification of the layers is the result of the ion

bombardment.

In an apparatus employing inductive coupling of the RF

source and thus permitting control of the bias power

iridependently of the RF power, use of an additional magnetic

field produced an increase in deposition rate by a factor 5 to

10, reaching about 45 ~m/h and permitting to obtain reasonable

values even at a low pressure (eg. 2.9 ~m/h at 0.5 ~bar pressure

[31] .

Amorphous carbon films can contain large amounts of

hydrogen, exceeding 50 at% [59]. A number of methods were used

to determine hydrogen content, but those most widely used are

IR absorption and nuclear reaction analysis. The former methods,

aithough detecting only chemically bonded and not chemisorbed

or trapped hydrogen, can show not only (relative) hydrogen

concentrations, but also details of chemical bonding. Hydrogen

not chemically bonded to carbon, which may amount to more than

30%, may be trapped by ion implantation and burial from bond

breakage during growth, and causes a compressive stress which

can result in disintegration of thick films (>1 ~m) [47].

A number of researchers studied correlations between

hydrogen, content and/or its bonding and other coating properties

[47,25,120,59,57,90,36,31]. It is logical to expect such

correlations, because hydrogen as a bond-terminating atom,

piays central role in determining the coating properties [59].

54

Low ion bombardment was found to produce hydrogen-rich

polymeric films [33,48,25], but as the power was increased,

H content and polymeric component decreased and the diamond

component increased, resulting in higher index of refraction,

lower optical gap, higher hardness and scratch resistance, but

also in higher internal compressive stress [25]. Interrelations

between hydrogen content, bonding and some physical properties

for polymerlike and hard films are presented in Table 10 [59].

Table 10Bonding, hydrogen content andsome propertiesof carbon coatings

Property

sp3:sp2:spl

CH3:CH2:CH[H] total (at%)

Optical gap (eV)

Refractive index

Density (g/cm 3 )

Carbon

Hard

68.30:2

0:40:60

27

1.1

2.1

1.9

coating

Polymerlike

53:45:2

25:60:2

50

3.0

1. 65

1.3

It follows from the table that po1ymerlike coatings

contain about twice as much hydrogen as the hard ones, but

also that bonding of hydrogen and properties of the layers

are different for the two layer varieties.

The substrate temperature (TdJ also influences H content,

as evidenced by a decrease in IR absorption with increasing

Td (120]. In a study which used an apparatus with inductively

coupled RF discharge, the H content was found to decrease

rapidly above Td=350°C (maximum Td used in the study was

550°C), while percentage of graphitic sp2 bonds increased [31].

In another study it was found that in a narrow interval

of Td (350_380°C), polycrysta1line coatings consisting of

55

carbine and graphite are obtained, while the coatings prepared

at 620-800 oC consist of carbine, defected diamond and graphitic

carbon [125].

Hydrogen content and optical gap were found to be

inversely related to microhardness [90], in agreement with the

mentioned results of other authors.

Generally, increased hardness of coatings is followed by

an increase in density, electrical resistivity and index of

refraction, as well as a decrease in H content and optical gap

values. Fig. 17 [59], showing dependence of film properties on

the self-bias voltage, illustrates behaviour of all these

properties except for electrical resistivity.

According to different authors (Table 11) the soft

(polymer1ike) films with hardness 4 on the Mohs' scale [87] have

low density (1.3 g/cm3) [59], low refraction index (1.60-1.65)

[59,34], and large optical gap (3.0 ev) [59]. Hard films with

hardness of 30-46 GPa [120,6,57] or 7-8 on the Mohs'scale

[17,87] are characterized by high density (1.9-2.12 g/cm3)

[59,6,17,57], high electrical resistivity (10 9 - 10 13 ohm em)

[48,17,57,87,6,31], high index of refraction (2.0-2.2)

[109,120,59,34], optical gap between 1.1 and 0.8 eV [59] and

high breakdown strength (>10 6 ev) [39,87].

Kobayashi et al. [58] have measured sputtering rates of

various diamondlike and diamond films prepared by RF plasma

deposition and by hot filament methods, respectively. Behaviour

of sputtering rate of the DLF is similar to that of H content

of the films. Both increase with increasing CH4 pressure during

the deposition and decrease with increasing power. On the other

hand, sputtering rate depends on (decreases with) the film

hardness, the ratio of sputtering rates of the films being

directly related to the ratio of their hardness. That this

dependence is extended to diamond films follows from a

comparison of sputtering rates of the DL films and the diamond

ones: the former are sputtered 3 times more rapidly than the

latter, which can be compared to their hardness (35 and

70-100 GPa, respectively).

Since the early successful attempts to produce DLC it has

been observed that the layers exhibit compressive stress and,

56

45

~40

~~..J 350~

~ 30I

"'---~~

25

~ \> 0

Q) 2.0

\~

CL 1.8<{ 1.p

'~'-'-' 1.4-cu;:: 1.2

~-o-c...CL1.00

x 2.2 ..--0/w0 2.1 ~z .»:2.00:: Iu, 1.9w0::

1.8

~

"'E 2.0 0 0

~ 1.9~ 1.8 0

>- 1.7>--in 1.6Zw 1.50~

00.. 0'-' 10 0 0~

(Il0(Il 8w

z0 60::<{I

04

I0:: 0 200 400 600 800 1000u::E BIAS -VOLTAGE (V)

Fig. 17: Properties of diamondlike filmsdeposited from a 13.56 MHz glow dischargeof benzene at 3 Pa vs. substrate biasvoltage [59].

Table 11

Radio frequency plasma CVD of diamondlike coatings

Preparation Apparatus Substrate Characteristics Application Referenceconditions

1 2 3 4 5 6

CjH1g,40- RF 13.56 MHz. Glass slides Amorphous films with some Hard-wearing [46]

1 O~ ar: One electrode placed on the small isolated unidentified coatings.Input power insulated from powered RF crystallites. R=1.5to Optical100-300 w. ground and target Al 8.1 ~/h for RF p~wer absorptionVB from -430 connected via electrode.Max. 100-300 w. p = 10 ohm em. filters.to -750V. blocking capaci- glass TO +70°C.

tor to a ~-match- Cleaned chemical-ing network and 1y and then bya grounded RF sputtering in Ar.supply

C4H1 0, 13.56 MHz RF Ge single At2~ow w/p ratio (1-2 wi [48]6.6 or 130- crystal on em Torr) polymer~c films.800 IJ.bar, water-cooled At w/p = 40 w/cm Torr,20-420 W Al-target, amorphous carbon films with

cleaned by no absorption between 2 andsputtering 20~ba~ n=1.9-2.0 and

p =10 ~ ohm em. At higher w/p,target temperature rises andgraphitic film is obtained.

C4HtO(n and Standard RF Glass or steel, Amor~hous fi~, d=1.9-2.0 [ 6]iso , C3HS 1 sputtering cleaned g/cm , p =10 ohm em, MH=34C2H6,CH4· module chemically and GPa. Properties independ. onPressure ad- then by Ar the gas used. Excel. adhesion.justed to give plasma. Placed For gases having the samea canst. nega- on water-cooled structural form, prod. ratetive bias of cathode. incr. with molecular weight.900V on the Etching mechanism dependscathode fed strongly on the etc~ant gas.with RF powerof 160 W

'"-.J

Table 11 continued

1 2 3 4 5 6

C4H1 0 RF 13.56 Glass, placed on Films produced at low R are [15]S~bar MHz water-cooled more DL-highly insulating50-600 W cathode; etch and DL photoelectron spectra.

cleaned in Ax High R films resemble10 min graphiteo_ Large amount

of ° found.

99,99% C3HS' 13.57 MHz RF Si02, Al 203, Transparent insulating DL [39]26~bar. 5i on water- films. R=0.43-0.76 ~/g.

VB=400-700 V. cooled Cu Dielectric strength 10V/cm, MH=30-50 GPa. Opticalabs. coef . incr. with decreas-ing wavelength (in the range200-600 nrn). Presence of un-paired spins suggests sp2bonding. The ~LC is a

2randomnetwork of sp and sp -bondedC with relative fractionsdepending on the preparationconditions.

Ethylene, 13,56 MHz RF To obtain good R~0.03-4 ~/h, incr. with [33]0.5-130 ~bar, power coupled adhesion of VB and P of C2H4. Sliding100-1000 W RF via matchbox deposited layers, .friction caef. ~ncr. withpower, Vij= terminating degreased Si, RH (N2 atmosphere) from10 to 14 0 V. in blocking glass and steel 0.01 to 0.30, but shows

capacitor. placed on a hysteresis. MH is low forCathode-to- water-cooled low VB and high P of C2H2'anode distance support, cleaned but very·high for other70 mm. by presputtering conditions. The layers

in Ar plasma at exhibit high compres.5 mbar, 400 V stress.bias voltagefor 15 min.

'J'ex>

Table 11 continued

encc

1 2 3 4 5 6

C4Hl O' C2H6, 13.45 MHZ RF Glass slides; Predominantly amorphous [109]C3HS' CH4· KCl and KBr of films. With incr. RF power-

{laO} orienta- to depos. pressure ratio,tion (cleaned there is (at 100 W/Torr)or polished) i abrupt transition from soft,8i wafers. low n to hard, :n ~ 2 films.Placed on water- Several crystalline phasescooled anode found by TEM are cubic, somesupport. of them are unknown.

CH~, Ct4 RF with a Glass slides; R = 3 ~ Ih for 10 W/em2Torr Anti- [17]Cs 6' 5- frequency in polished Ge With incr. power density, reflective2 0 IJ.bar; MHz range. discs; polished films change from polymer-like coatings2-p.S wi 8i wafers. (soft) to graphite-like. Hard for Ge.em Torr. Cleaned in films ~~ohs hardness 8) with

oxygen discharge. p = 10 n em in intermedi~te,,

range. Density 1.2-2.0 g/cm .n incr. with VB from 1.7 to2.0. The short wavelengthopt. edge shifts to longerwavelengths with incr. VB'

Similar. to Similar to [46]. Glass slides. From W/P = 10 4 to S·102w/Torr~ Anti-- [S2][46], Butane. n decr.

2slowly,but from 4·10 reflect-

to 2'10 rapidly. With lower ingW/p, films have lower absorp-, coatingstion, contain more Haand are for Sisofter. The soft/har transition solaris at -100 W/Torr. Reflectivity cells.of the films prepared at 200W/Torr on Sidrastically reducesits reflectivity (-0.1% at 0.55~ wavelength).

Table 11 continued

"'o

1 2 3 4 5 6

CH4 plasma 13.56 MHz RF Single crystal Transparent amorphous films with Optical [l1B 1wafers of Si, less than 20% of tetrahedral C-C coatingsGaAs and InP; bonds. H ~ontent·~ 40%. Er=2.7ev. especial-

No change in properties b ly forannealing to 500-600°C, but ), >500nm.graphitization at higher T.H content drops at 900°C to5%, but surface H remains.

CH~, Ct~' RF 13 MHz. Glass, quartz, OLe film surfaces contain onl¥ C, [B7]C3 g' 3 6; Symmetrical single crystal with occasional traces of O. Egde ias 0 diode-reactor, 5i-wafers, depends strongly on Wand biasto -400 V; connected to RF KBri Au on voltage. Eg values up to 2.7 eV25-200 W via impedance- quartz (from CH4). MH up to 7 Mohs. E

matching and H content are in~~sely gnetwork. related to MH. P >10 ,n ern; ,

dc breakdown strength >B.0·10 6V/cm. IR vibr,tional frequenciestypical of sp bonding, and(weak) C-H absorption found.

CH4 (5vol%) 13.56 MHz RF. Glass or Si, R up to 2.9 ~/h. V¥ ~ncr. [21]or C6H6 Reaction vessel pla~ed.on the linearly with (W/P) / . VB is(7.6 vol %) 35 cm dia., powered higher for C6H6 and for He-in He. For 22.5 em high. electrode. diluted gases. Mass depos 2ateCH4: 53-530 12.7 cm dia. increases linearly with pi/ anduber , O.O~- water-cooled thickness rate (R) incr. with VBP.10.B W/em ; stainless steel Little effect of W. Incr. in RFfor CaHf electrodes, power density leads to lower depos.80-27 J.1 ar, spaced 6.5 cm. thickness. Film is densified by0.2-0.B W/cm. ion bombardment.

Table 11 continued

'"

1 2 3 4 5 6

CH4, CHa(5%)

~s foregoing 8i; glass High ion bombardment (IB) 1ea~s Protective [25]-He, C6 6 to lesser H-content and to sp~; coatings(7.6%) low IB ~roduces C-H

2bondsin sp

-ne , ; CH3, sp CH2 and sp CH2, the films70-700 ~bar are of Polymeric nature. n=1.88-2 ..56Td <60°C in pure CHa, iner. with VB E0-100 W between O. and 1. 8 eV. polymgric0-0.79 w/cm2 component deer. with W from 63 toVB = 0-700 V. 53% and diamond iner. from 23 to

33%. Graphite-like component is9-13%. High hardness and scratchresistance. High internal compres-sive stresses, ioer. with deer.H content.

CH +H • RF 13.56 MHz Two 0.2 mm dia Soft and hard films were obtained [57] ,

C§a4+~a; W probes. at higher and lower gas pressure,( 0-10 % Cleaned by Ar resp. R has a max. near the phase

~~66 W.sputtering. transition P (0.09 mbar for pure

CH4, 1.3 mbar for 50% CH,). SimilarTd= 200°C. beliaviour of CH (431 nm) H (656 nm)

optical emission intensity ratio.The soft /hard film transition forp~re C!H4 occurs at 13 ~bar. Diamondpartie es were found in hard filmsdeposited on Si from CH4, but onlywith naturally oxidized surfjce.Hard films: l~ensity 2.12g/cm ;p = 2.24'10 n cm~ n= 2.5,MH = 46 GPa.

Table 11 continued

'"'"

1 2 3 4 5 6

C2H~ Modified RF 8i, Ge, ZnS, Sheath thickness depends on p-1/2. Highly [120]vr 00- parallel plate ZnSe, fused Ar/He stabilize plasma, but sharply efficient1 00 V; reactor. silica. The reduce R. Hydrocarbons stabilize anti-P=1-100 llbar Water-cooled samples glow- plasma and have little effect- on R. reflective

electrodes discharge- At P>80 llbar C2H2 discharge fairly coatings35 mm dia. cleaned in Ar stable. n=2.0; C-H vibrations. for IR13.56 MHz at 400-~00 V reduced at higher substrate temp. opticalcathode and 10- _10-1

~ range 950-1100 V for optimum·MH. elementscapacitively mbar for 5-10 ove 1100 V a partially degraded (Si, Ge)coupled to min. DLe film with low IR transparency.power supply. At low v~ polymeric soft films.

Efficien 'durable antireflectivecoatings obtained. Reactive ionetching in oxygen plasma "tunes"thickness of the layer to aparticular frequency. ,

e

C H CH RF Si, quartz By annealing DL films between Ta [36](~otuOl) 100 and 350°C, total intensity of200 llbar; C-H bond vibration~ drops2abruptlyT~= 20-300°C; and the ratio I(sp )/I(sp ) slight-R cathode ly increases with T _; in th3 secon~

current stage (350-700 0C), ~he I(sp )/I(sp )50 rnA ratio rapidly increa~es because

H atoms bonded to sp C-atoms aremore annealed. The increase in DLproperties above T = 400°C isascribed to CH3-ra&icals-rich plasma(from toluol) - these favour the DLegrowth.

Table 11 ccntinued

1 2 3 4 5 6

CD4-D2 a) RF-source Mo,. polished Annealing studies. At Ta = 500-800°C [65]mixture b) Vacuum and cleaned a well-defined M02C film is formed4.7 I'bar. furnace, up to (50 nm thick at T = 665°C for 50 min).Bias -200 V. 1000°C for C atoms diffuse tGrough already formed100 nm thick annealing carbide and carbide grows at the MoC film. interface.

C H CH RF, 13.6 MHZ Crystalline Si R= 65-72 I'ffi/h. Higher Td favourize sp3 [95](~o1uol) formation. ~ealing at Ta = 200-800°C270 I'bar, results in sp increa~e up to 350°C,Td= 20-280°C but above this Tat sp decresases and

then saturates.

Ar with Twin electrode Si (111) , Simultaneous gr~wth of OLe on Si-sub- [42]0.2-10% system: one, cleaned by strate from plasma activated hydro-CH4 or C2/H4- coupled to RF sputtering, carbon, and tantalum sputtering and ~15 robar. r supply (13.56 water-cooled its deposition. C-deposition occursV - MHz), is used partly through sputtering of C-~500 V. as Ta sputter deposited on Ta target. Amorphous

target (t:athode) hydrogenated films. No significantinfluence of the C-source gas or Tacontent on chern. structure. Carbidicand polymeric states. 2 new C-states:crystalline carbidic·and "transitionstate" carbidic.

Table 11 continued

'"..

1 2 3 4 5 6

CH4, C2H4, RF, inductive 5i single Considerably increased plasma density [31]or CaH2 I .Ar i coupling crystal, and 20% reduced electron T (for50-SOW; 1000 W glass, P<l~bar) by additional magnetic0.5-30 I1bar. metal field. Lower P increases ion energy.Additional Additional magnetic field increases Rmagnetic by a factor of 5-10 with C2H2 for midfield pressures. DLCproperties essentially20 gauss. independent on chem. nature.Bias 0-200 V. Electrical conductivity sJ20ngly incr.

with Tg above 350·C (p.<10 ohm em atTd>350 C). This variation iscorrelated with IR C=C absorption(incr. at Td>3500C) and H-con2ent(deer. at Td>350°C). Thus, sp incr.above 350·C. ,

C3H2, P=O.l- 13.5 MIIz RF Si(l11); For Td up to 350°C amorphous coatings. [125]1 0 mbar. Si coated by For Td = 350-380·C po1ycrysta11ineTf20- 8i02 coatings consis~ng of carbyne and1 OO·C. graphite, p =10 ohm em. For Td =

620-800·C po1ycrystalline coat~ngs

of carbyne, defected diamond andgraphitic carbon.

CHf

RF for film Quartz,- Transparent DL film with high Ion [41]0, W/em. preparation. Si cryst. bombardment breaks C~~ and C-C· bonds,Protons of 700 V Van de at RT H diffuses out and sp bonds are100 and 5~0 Graff formed.keV and N acceleratoroflg801~ev!2 for ion10 10 em . bombardment.fluences. .

Table 11:

Designations: dc = direct current; EgMH = microhardness; R =

RH = relative humidity;

VB = self-bias voltage;

watt and tungsten).

= optical gap; IR = infrared; = wavelength;

deposition rate; RF = radio frequency;

Ta = annealing temperature; Td = deposition temperature;

W = power (note that the same symbol is used to designate

Abbreviations: abs. = absorption; chern. = chemical; coef. = coefficient; compres. = compression;

canst. = constant; deer. = decrease; depose = deposition, deposited;

dia = diameter; excel. = excellent; incr. = increase; independ. = independent;

max. = maximum; opt. = optical; prod. = production; resp. = respectively.'"ct

66

therefore, if it is sufficiently high (e.g. in thicker films),

the layers tend to delaminate or peel off the substrate.

It has been established that the films containing the highest

concentration of sp3 bonded carbon exhibit highest stress,

while the polymerlike films are low-stressed [25].

The interpretation of the stress seems to be still rather

obscure. Heavy ion bombardment during deposition is an apparent

cause, but structural and compositional effects are superposed,

as is indicated by a maximum in the stress values at a certain

bias voltage, i.e. at a certain ion energy [34].

Adhesion of the films is strong to carbide-forming

substrates, which can be explained by the experimentally found

formation of an interfacial carbide layer on crystalline Si and

Ge [59].

A very broad range of electrical resistivity values (from

1 ohm cm to 10 14 ohm cm [32,15,98,46,30,7,6] was found for RF

plasma deposited coatings, but values in the range 10 9_10 12 ohm

cm seem to be most frequently reported. Dependence on deposition

rate, i.e. on the discharge power, has been found, the lower the

power (and, consequently, the lower the deposition rate), the

higher the resistivity; however, different resistivity intervals

were found by different researchers for similar power or

deposition rate.

The substrate temperature (Td) was found to have a

pronounced effect on electrical resistivity [52,83], the latter

decreasing rapidly with increasing Td' A decrease in resistivityfrom 1011 to 10 3 was produced by temperature increase of only

50 0C (from 250 to 300 oC). This effect, as well as the effect of

substrate potential on resistivity (resistivity decreases with

VB) is related to hydrogen content which decreases with

increasing VB and Td [83].

The hydrocarbon gas (CH4, C2H6, C2H2 or C2H4) influences

the resistivity, which differs up to a factor of about 3 for

the coatings prepared from methane and ethylene at Td=250oC

(resistivity(methane)/resistivity(ethylene) > 10 3) [52].

Good insulating properties of the coatings are ascribed

to saturation of dangling bonds by hydrogen [34].

67

For very thin films « 50 nm) electrical resistivity was

found to increase exponentially with decreasing film thickness

[44]. This effect is due to the fact that thinner films are

closer to the ideal amorphous state. Doping of the coatings with

elements of group III and V of the periodic system was effected

using B2H6 (for B) and PH3 (for Pl. Doping with N, Ta and Ru has

also been reported [34].

Optical properties, namely absorption, transparency and

reflection, are essential from the viewpoint of application for,optical components. It has been found that films produced at

different self-bias voltage change their colour from pale yellow

to black, showing that the short-wavelength optical absorption

edge (Eg) shifts to longer wavelength with increasing VB [17].Low absorption in the red and infrared part of spectrum,

and its virtual disappearance above the wavelength of about

1.5 ~m, is suitable for use of the films as antireflection

coatings for Si and Ge. Absorption measurements at 10.6 ~m on

the coatings prepared from C2H4' C2H6 and CH4 on Ge, have given

values of a few percent per ~m, indicating suitability for the

application as antireflection coatings on Ge [17]. This was

confirmed for the DL films on Ge showing high transparency in

the 3-12 ~m range, but also for a 2.5 mm-thick blank quarter­

wave DLC layer on both sides, which had transparency 96-97% in

the 3-5 ~m range [120]. Using the fact that colour of the DL

films depends on their thickness a "colour chart" [80] was

made for thickness ranging between 30 and 270 nm on Si-wafers

prepared by RF plasma decomposition from butane. An additional

interesting characteristic of DL layers is their low friction,

with sliding friction coefficients varying from 0.01 to 0.3,

depending on (increasing with) relative humidity [32,33,34].

In the experiments of sliding friction of steel against DLC­

coated Si [34], the friction coefficient had a value of only

0.02 in vacuum or very dry nitrogen. An increase in relative

humidity from 10 to 99% was found to lead to an increase of

friction coefficient from 0.05 to 0.3. An explanation of these

results is based on the fact that DL layers evolve hydrogen when

heated above 300°C, so that there is gas effusion during sliding

68

of the steel against the layer, the gas providing a "cushion" on

which the steel ball can skim.

Instead of using cooled substrate, as reported in majority

of the papers dealing with RF plasma deposition, higher

substrate teperature combined with C2H2 plasma resulted first

(at Td up to 350°C) in amorphous coatings, then in poly­

cristalline films consisting of carbyne and graphitic phase

(Td between 350 and 380°C), while at Td between 620 and 800°Ccarbyne, defected diamond and a graphitic carbon were produced

[125]. According to the authors, carbyne is the result of

deposition of the -C=C-radicals produced from acetylene and

their subsequent polymerisation. At higher Td the radicals

dissociate and presence of H atoms allows deposition of the

diamondlike carbon. The authors conclude that in order to obtain

better defined carbyne and diamond films (without graphite) it

is necessary to apply an accelerating voltage on the substrate.

In addition to the high-frequency RF plasma (frequency in

the MHz range, typically 13.56 MHz), used by a great majority

of authors, some authors have used a low-frequency RF plasma

with a frequency range 25-125 kHz [22,110].

These two plasma types differ essentially with respect to

the ability of ions to follow the voltage changes: in the high

frequency RF the ions can not follow the electrical field

changes, but in the low-frequency RF the ion flux reflects time

dependence of the plasma potential. Plasma pressures are also

quite different according to the studies by emission spectro­

scopy [22]. However, in spite of these differences, properties

of the carbon films, obtained with high impact energies of the

film forming particles are independent on the type of discharge

used [22].

RF plasma CVD of diamond coatings

Although many researchers have used RF plasma CVD to

deposit carbon coatings, the coatings obtained had properties

which approached more or less to those of diamond, but never

69

reached them. It is only recently that preparation of diamond

coatings by means of this method is reported [69,121,74].

Comparing the conditions of diamond preparation using

the microwave plasma and hot filament CVD methods (see 3.2.3

and 3.1.2, respectively) with those of diamondlike coatings

preparation using the RF plasma CVD, it can be seen that the

main difference consists in substrate temperature range and

initial gas (and, consequently, plasma) composition.

The substrate temperature in both microwave plasma .and

hot filament methods was between 700 and 1000°C, while in the

RF plasma it was not far from the room temperature.

Concerning the initial gas, it consisted in the case of

the former methods of a low concentration of a hydrocarbon in

hydrogen, resulting in high concentration of atomic hydrogen

in the plasma, while in the RF plasma method no hydrogen was

present in the initial gas mixture. It is these two parameters,

substrate temperature and gas/plasma composition, that are

changed in the mentioned recent RF plasma studies [69,121,74],

making them similar to those in the microwave plasma and hot

filament methods and enabling the authors to produce diamond

coatings.

A schematic diagram of the apparatus used for diamond

coatings deposition (Fig. 18) [121] shows that it contains

the same main components as those used to obtain diamondlike

coatings. RF power is supplied from a 13.56 MHz generator

through an impedance matching network terminating the working

coil. The substrate is brought to the necessary temperature

(700-1000 0C) by inductive heating and energy transfer of the

plasma, i.e. without any additional heating.

The hydrocarbons used were CH4, CO, CH30H, C2H50H,

CH3COCH3' C6H12 and C6H6 and their concentrations in hydrogenwere up to 2%. In order to increase atomic hydrogen content in

the plasma, high RF power was used.

The substrates used were Si wafers, Mo and quartz/silica.

Their polishing [69] and, in some cases, preliminarly coating

by a DL layer [74] resulted in a higher nucleation density [69].

Table 12 contains data relative to RF plasma deposited

diamond coatings.

70

Z 10

~ water r

'7 13.56MHz

? ke000000

t; 5 20000000

•

j.8

N ~

:c 011 u

1, quartz tube2, wark call3, substrate (0<8<90")4. diaphragm gauge5. schulz gauge6. matching bax

7. RF power supply8, forward oouer9. reflected power10. dilute 90S

11. reactant 9as

Fig. 18: Schematic diagram of an RF inductionplasma CVD apparatus [121].

71

The methods used to characterize the deposits were SEM

[69,121,74], electron diffraction [69,121], Raman spectroscopy

[121], XPS [121], Auger spectroscopy [74], IR spectroscopy

[121], measurement of electrical resistivity and microhardness

[69] .Growth rate and dependence of crystal shapes on deposition

conditions were found to be similar to those in the hot filament

method. A suitable substrate temperature was found to be 700­

800·C (above this temperature graphite appears) [121], 700­

1000·C [74] and below 950·C [69].

Properties of the obtained diamond films are high hardness

(80-90 GPa [121], 70-120 GPa [69]), high electrical resistivity(10 8_1 010 ohm cm) [69] and good adherence [69].

According to [74] RF plasma CVD has the advantage over

microwave system in that it can be easily scaled to large volume

and high power.

In a variant of RF plasma CVD [108], CH4 is not introduced

into the hot plasma region, but downstream from a He discharge.

A low pressure (around 13 ~bar) and low RF power (80 W) havebeen used. Metastable He, formed in the inductively coupled

discharge region is transported towards the substrate, and

diluted CH 4/He mixture (2-20%) is separately introduced into

the substrate region. The excited He atoms interact with CH4

molecules, causing dissociation into CHx or CHx + radicals, which

condense on the substrate heated up to 7l5·C. Short residence

time (i.e. high low-rate, amounting to 30 l/h) and low CH4

concentration prevent any gas phase polymerization, so that

gas-solid reactions predominate. Diamond layers are produced,

but they contain disordered graphitic carbon.

In conclusion, the RF plasma CVD can be used to produce

diamond coatings. In order to do so it is necessary to have low

concentration of hydrocarbon in hydrogen and high substrate

temperature (700-1000·C). High power results in higher atomic

hydrogen concentration. Polishing with diamond paste enhances

nucleation.

However, the available literature data are insufficient

to permit definite conclusions about the advantages or

disadvantages of this method with respect to other methods

Table 12Radio frequency plasma CVD of diamond coatings

Preparation Apparatus Substrate Characteristic Application Referencesconditions

1 2 3 4 5 6

0.2-1% CH4 13.56 MHz, 5i-wafers,; Mo; On polished substrates poly- [69]in Hr 1 kW RF silica glass crystalline diamond films areP=5- 0 mbar. generator~ plates, placed formed. At P<lO mbar, smallFR = 3 llh Working coil on a silica amount of SiC is codeposited,Td<950oC around reactor boat. (5i from silica tube). At lowW = 0.5-1 kW tube. T and P, R is low and {Ill}

crystal faces; at high T and P,polyhedral crystals wiJl> {lV}and {100} faces. p =10 -10n em. High hardness. Wellad.h~rent films.

CH30H,C2H~OH, 13.56 MHz Si, quartz, Diamond is synthesized both as Protective [12lJCHaCOCH3, 6H6' 0.5 kW RF Mo, treated particles and films. Suitable coatings,C6cli2, mixed generator by ultrason in substrate T is 700-800°C. At heat sinks,w~ H~ SiC or diamond higher T graphite appears. semicon-(0.03- vo1% paste. High nucleation density ductorof C-compd.) Substrate heated (treatment with diamond paste) devices.P=27-66mbar. up to 1000oC. leads to diamond fi~ contain-(mainly 40 ing non-diamond carbon. Filmmbar) hai:dness (Knoop) 80-90 GPa.Td=700-l000oC Activated carbon source andW = 490 W. atomic H are essential for

diamond formation. These areincreased by high ~ power.

-oJ

'"

Designations: FR = flow rate; R = deposition rate; RF = radio frequency; Td = deposition temperature;

W = power (note that the same symbol is used to designate watt and tungsten).

II As foregoingor coated withDLC. Inductivelyheatedsubstrate.

Table 12 continued

1

I Typically:0.5% CH4 inHZ'P=Z6 mbar,Td = 700­llJOOoC

II CH4 ( l. Z%)or CO (5%)in HZ'P=Z6-53mbarTd=850­950°C

Z

13.56 MHzRF inductivelycoupled toCH4 / HZ plasma.Quartz tubereactors.cleaned.I 500 WII 3 kW

3

I Si-waferssubstrate (ongraphiteSllsceptor)Polished orroughened,

4

I For Td = 850-1000°C nuclea­tion occurs on graphiticcarbon. For Td S 700°C (nographite susceptor) faceteddeposits obtained. Diamondconfirmed by Auger spectra.

II OLe promotes nucleation ofdiamond from CH4. Welldefined individual facetedparticles or polycrYstallinefi~s are obtained from COon quartz and OLe-coated Si.

5

Heat sinks,high-temp.semiconductordevices,opticalwindows,abrasives.

6

(74)

...,w

Abbreviations: compd. compound; temp. temperature.

74

of diamond coating preparation (coating quality, possibility

of using different substrate materials etc.)

3.2.3 Microwave plasma chemical vapour deposition (CVD)

of diamond and diamondlike coatings

Diamond coatings

Microwave plasma CVD of carbon coatings, a new method

developed during the last several years, enabled researchers

from different centers in the world to obtain well-defined

diamond layers. Indeed, one of the very first methods found

to produce well crystallized diamond used a 2.45 GHz microwave

power source to produce a glow discharge plasma in mixturees

of 1-3% methane in hydrocarbon and at gas pressures of 10 to

80 mbar [53].

Several crucial parameters are combined in the method:

- A suitable gas mixture, consisting of a low concentration

of a hydrocarbon gas (CH4) in hydrogen (H2);

- High ionization efficiency of the plasma (higher than in

the diode-type sources [13]) providing suitable species for

diamond deposition, in particular high concentracion of atomichydrogen;

- Substrate temperature in the range 700-1000°C;

- Substrate surface containing seed diamond particles,

carbides or carbide-forming metals.

The fact that microwave (MW) plasma CVD (MWPCVD) is an

electrodeless discharge was stated as an additional advantage of

the method, because no (otherwise unavoidable) contamination is

introduced by the electrode material. However, according to [11]

the MWPCVD produces very reactive species (like atomic hydrogen)

which can react with the Si02 tube wall placed in the MW cavity,

as well as with the substrate and its holder, thus introducing

impurities into the deposit.

Typical apparatus and working conditions are described

below [11]. The MWPCVD system is comprised of several smaller

units as shown in Figs. 19 and 20 which include: the gases

75

(5% CH4 in H2' H2 and Ar), the gas flow control, the MWgenerating equipment, the reaction chamber, the pressure control

and the pump. The two major components of the system discussed

below are the MW generating equipment and the reaction chamber.

The MW system is shown in Fig. 19. The microwave generator

may be operated at powers between 0.1 and 1.5 kW at a frequency

of 2.45 GHz. Confining the microwaves is a set of waveguides

attached to the generator. Four of the waveguide components have

additional functions. First, the isolator allows microwaves to

pass through from the generator, but protects the generator by

absorbing the reflected microwaves. Secondly, the reflected

power is monitored by the power monitor. Minimization of the

reflected power is required, and is achieved by adjusting the

next waveguide component, the three-stub tuner. Finally, the

microwaves interact with the reaction chamber in the water­

cooled applicator.

The reaction chamber consists primarily of a vertically­

mounted fused quartz tube, 3.8 cm o.d. and 76 cm long, which

confines the plasma. Figure 20 is an illustration of the

reaction chamber, but also includes the gas flow and pressure

control systems. The reactant gases, methane (CH4) and hydrogen

(H2), flow through the gas inlet at the top of the reaction

chamber. Also, located at the top of the reactor is a glass

viewport used to observe the progress of the deposition and to

measure the substrate temperature with an optical pyrometer.

Because of the interference of the plasma, the substrate

temperature obtained may not be the actual temperature and

caution must be taken.

Flowing from the inlet, the CH4 and H2 reactant gases

enter the fused quartz tube where the gases interact with the

microwave energy in the applicator which creates a plasma

confined within the quartz tube. A film is deposited by the

activated gases onto a single-crystall silicon (100) substrate.

The height of the substrate in relation to the plasma may

be adjusted by an alumina substrate holder assembly. Also, the

substrate is heated only by the interaction with the microwave

power and the plasma; no external heating is used. The reactant

gases finally flow out of the bottom of the reaction chamber

76

GAS FLOWCONTROl.

MICROWAVEGENERATOO

ISOLATORo MICROWAVE

AF'PLICATOR(WATER-COOL£D)

REACTIONCHAMBER

.L

POWER MONITOO;INDICATOR--------,

III

Fig. 19: Schematic diagramgenerating equipment [11].

of a microwave

GLASSVIEWPORT

CAPACITANCE:~A.wlo!..EIQl: I

,,,,

MICROWolVl;'APPl.lCATOR{WATER COOLEOI

FUSro QuARTZTUSING

'"Ih'l..ET

, i :.c",,, I I

·~I

GJ I J

~It;;\ MASS FLOfi iL.:J METER !

HEIGHTADJUSTA8LESU8STRATE

PROCAAM PRI;SSUREDISPlAY CONTROl.llR

D-Q

b'='=frr;n.--K'7V77 TO

FF"-J--'-~?,;--;;~,*,~ PUMf

Fig. 20: Schematic diagram of a microwave plasmaCVD reaction chamber, gas flow control and pressurecontrol [11].

77

where the pump maintains the vacuum in the chamber.

The flow rates of the gases are controlled primarily by

the mass flowmeters. The pressure of the reaction chamber is

monitored by a capacitance manometer. The manometer pressure

reading is compared with a set point value at the pressure

controller. The controller sends a signal to appropriately

adjust the throttle valve to obtain the set point value.

Typical reaction conditions are given in Table 13.

Table 13

Preparation conditions

Methane concentration (vol.%)

Hydrogen concentration

Total flow (l/h)

Total pressure (mbar)

Power (kW)Temperature (0C)

0.5 - 5

remainder0.6 - 12

1 - 13

0.3 - 0.5

below 700

Very similar MW units have been described in other

papers [106,122,53,101,43,84,119,12]. The same is true for

the substrate: Si was used in all the cases.

A number of authors used a diamond-seeded or polished

substrate [119,84,122,111,112]. It is stated that the best

substrate for diamond growth is the diamond itself, because

the lattice match promotes quick nucleation and growth of a

continuous layer [84]. It was experimentally found [119,84]

that seeding or polishing of the substrate with diamond

particles produces dense diamond films in a few minutes,

while several hours are needed to grow large individual

crystals without them.

It is also reported that dense, well-defined diamond

layers can be obtained on diamond [12], but also on P-SiC [12]

and Si-wafer [78,106,11,53,43,56,12], i.e. without any diamond

78

present. This is explained by the lattice matching of P-SiC

(also formed on the surface of Si) with respect to diamond [12].

The experimentally found nucleation on highly-oriented graphite

[12] is also ascribed to the presence of P-SiC, which is formed

by reaction of graphite with Si coming from the Si02 walls of

the reactor due to etching by atomic hydrogen.

On a Ni surface only amorphous carbon was obtained, but

nucleation of diamond becomes possible if gaseous Si species are

present [12]. Fused silica [106,12] and Ti [84] were also used

as substrates. It follows from these and other experimental

results that carbide-forming elements are suitable for diamond

nucleation.

Dilute mixtures of CH4 in H2 were used by majority of the

authors, the CH4 concentration varying from 0.1 to 10%. Working

pressures used were in the range 1-120 mbar, while the substrate

temperature was varied from 600 to 1200 oC.

Unlike in the other papers, preparation of diamond films

from the vapours of different organic compounds (CH4, CH30H,C2H50H, CH 3COCH3) has also been reported [111,112]. The diamond

films obtained contain sp2-bonded carbon, as evidenced by the

Raman spectroscopy (1550 cm- 1 peak), but its amount can be

reduced by choosing suitable working conditions: one of the

alcohols as a starting compound, low concentration in H2 (O.5%),

6-26 mbar pressure, Td=850-950°C.

The role of total pressure has not been investigated above

120 mbar. According to Kawarada [126], above 5.3 mbar deposition

of crystalline diamond predominates, while at lower pressures

the deposits contain significant quantities of graphite or SiC.

It has been also reported [12] that pressure between 50 and

120 mbar, combined with other optimum experimental conditions

(Td=975-1000°C, CH4/H2=0.2-0.5%, FR=6 I/h), rather than

1-13 mbar used in an earlier study [11] allows formation of

well defined diamond crystals.

Concerning the deposition rate, it was found to vary

in a broad range, from 30 nm/h to 3 ~m/h, i. e. two orders

of magnitude (see Table 14). It depends on CH4 concentration,

increasing with it, but contamination with sp2 carbon also

increases [84]. It was found that an addition of less than

Table 14

Microwave plasma eVD of diamond coatings

...,'"

Preparation Apparatus Substrate Characteristics Application Referencesconditions

1 2 3 4 5 6

1-3% CH4 in MW generator 8i wafers 3 h depqsition produces well- [53]H2; 2.45 GHZi vert. defined diamond crystals,P=10-80 mbar; Si02 tube ~ (100) or (111 ) faces. Poly-W=O.3-0.7 kWi 40 rom reactor hedrons are formed at lowerTf800- CH4 conc. and lower W. Max.100°C. R= 3lUll/h. Raman scattering

shows well defined diamondline and unknown broad peak.

0.5-5% CH4 MW generator Si (100) R=30-60 nm/h. Diamonds films Optical [11]in H • 2.45 GRz. contain 1-2 at % Si and applicationsP=l-f3 mbar; 0.1-1.5 kW; several % H. Depending on ,FR= 0.6-12 Vert. ,fused deposition conditions, polycr.1/h; quartz tube with at. structure of diamond,W=0.3-0.5 reactor. or amorphous DC are obtained.kWi Complex growth mechanismTd<700°C. involves formation of ~-SiC

buffer layer and the defectstructure of diamond. Opticaltransparency over widespectral range is probable.

CRfR2:Ar= As foregoing Si single IR absorption spectra of the Radiation- [106]1: 00:80; crystal; deposited diamond film in the hardened semicon-P=8mbarj Fused silica range 2.5-25 ~ show- trans- ductor devices;FR=5 l/h; mittance from 93 to 85% and high-T electronics;W=400 W; a min. of -68% at 13.3 lUll. substrates forTd=700°C. Evidence is provided for high density

space-charge-limited current high speed inte-in presence of traps. grated circuits;

UV light-emittingdiodes and lasers

Table 14 continued

1

5-10% CH4 inH .F~':' 7.9 1/hTd =850°C

0.5-5% CH4in H2.P= 1-130 mbarFR= 3.6 l/hTd= 700­1IOO°C.

CH4 in HZ

2

MW generator2.45 GRz;vert. quartztube <I> 60_

MW generator2.45 GHz.Horizontalquartz tubereactor<I> 40 JIDII.

MW plasma

3

8i wafers

(100) Siwafers onBN holder,vertical;polished withSiC powder.

Si

4

R=1.5 ~/h; well defined diamondparticles with diam. 5-10 ~m.

Ev6n d~stribution for ¢ >2~,10 lern • MH>35 GPa. p = 10 _lOlln em. Transmiss~on enhancementeffect = 18% in the IR range4-10~. Pronounced IR abs. at12.8 ~. For 5% CH4, amorphouscarbon and graphite almost nonexistent, at higher CH4 conc.non-diamond bonding incr.Excellent chem. inertness.

Dense diamond fi~s, consisting"of= 10~ particles, each growing fromone point of the substrate. R=0.75~h. Relatively weak adhesion,probably depends on number of contactpoints (i.e. small particles shouldadhere better). Intensity ratio ofCH (314.41 nm) and H (486.13 nm)must be between 5 and 20 andTd >700°C to obtain diamond. CHxand C~radicals are responsiblefor d1amond and graphite formation,respectively.

Morphology depends on CH4 cone. At0.3% CH4 triangular (Ill) faces; at0.4% (100), the square feature ofwhich becomes more prominent at 1%CH4. At >1.6% CH4 the film is micro­crystalline. Dur~ng film growth at1.Z% CH4 morphology undergoes cyclicevolution of higher-order growth andappearance of (100) faces.

5

Semicon­ductivediamonddevices

6

[43]

[78]

[56]

ex>o

Table 14 continued

co~

1 2 3 4 5 6

Optimal Similar to Diamond, highly Diamond octahedra (micro) crystals [12]condo : that in [11] oriented are obtained on diamond substrate;0.2-0.5% CH4 graphite; R ~ 11ll1l/h. Nucleation readily obtainedin H . Si, j9-SiC, 5i02 1 on diamond, SiC, Si. Difficultp~ 53:120'mbar Ni. nucleation on SiO

I"Only amorphous. C

FR~ 6 l/h on Ni, but possib e if gaseous 5iTf 975- species are present. Nucleation on100°C graphite possible due to presence ofw~ 300-400 W P-SiC on its surface. Deposits are

well-crystallized diamond, containing0.2 at % Si and 435 at ppm H;a = 0.3565 run. Morphology depends ondeposition conditions.

Very dilute Si..mi.lar to Single crystal R:S 11ll1l/h. R can be incr. by incr. CH4 Coatings [84]mixture of that in [53]. 5i seeded with conc., but contamination also iocr. for (

CH4 with Vert. Si02 fine diamond Po1ycr. diamond layers, with grains cuttingH2 . tube 4> 34 rom particles; up to 10 1llIl. Morphology depends on tools,Tg ~ 800- reactor. Ti growth conditions and substrates. semicon-100°C Raman givrs a single sharp peak at duative(heated by -1330 cm- . Max thickness 70 1llIl. deVices,plasma) High tensile stress leads to curvature optical

of the substrate, sometLmes to dis- elements.in~eg~ation. Very strong adhesio~.

. Free standing layers, up to -1 emobtained by dissolving substrate.Lyers produced from CHaconc. <0.3%are generally white a~ translucentand almost withou~ sp bonding. Abovethis CH4 cone. sp appears and thesamples darker. Absorption of 1 IllIlthick layer -1%. Hardness too high" tobe measured. Durable,. Problem; highoptical" scatter losses .

.

Table 14 continued

ce

'"

1 2 3 4 5 6

0.2-5% CH4 MW generator Si (100) r (111 ) With~ut surface polishing with diamond Tribology; [119Jin Hr 2.45 GHz, and (110), pOWder, large, -individual crystallites semicon-p = .7- vert. quartz, 7.5 em q" obtained after several hrs. Seeding or ductor52 mbar tube 10 em q,; chem. cleaned, polishing with diamond powder promotes heatFR = 1.8- heater of then etched formation of dense D films within few sinks;6 l/h. substrate in HF. min. Addition of <0.3 vol % H~O incr. protectiveTq~600- (1100°C) . Seeding or R by an order of magnitude bu only optical100°C. polishing with if CH4 cone. is incr. to 5%. Highly coatings

diamond.· defected, spherical cryst. are for IRobtained at 650~950°C from 2% CH4 at transmitting27 mbar, but low-defected regular windowscryst. with <1% CB4 0 Pref. orientation(110) perpend. to substrate. The D-filmsare composed of cryst. with a typicaldia. of several J.I..IIl. No non-diamondLmpurities detected. A few dislocations (

and twins present. Two phases detectedI

at the subst./DF interface: {i-SiC anda FCC with a = 0.346 nm, believed to be

- diamond with Si-~purities. Raman showsvirtually pure d~amond with littleindication of sp bonding.

CH4 in ~' As in [11] As-doped Si Diamond nucleates at discrete sites. A IR window [122]p= 120 ar wafer, etched continuous film or layer is developed and lensTd= 1000°C with HF/conc. as cryst. grow and meet forming

HN03, 3:1, continuous surface. X-rays show diamond,pohshed with Raman shows DF, but also a non-diamonddiamond powder. carbon w~ich is present in greater

quantity near the substrate. Microstruc-ture varies over the coating thickness.n=2.36-2.73. IR optical abs. coef. incr.with deer. wavelength.

Table 14 continued

1 2 3 4 5 6

CH~, CH30H MW 2.45 GHz Fused Si-wafers Diamond film with particles having [111]~ tH or 400 W. Horiz. roughened with characteristic facets £rom alcohols,

~ OCH3, quartz reactor 40 IJ.lll particle or ball-like fro~ CH4 or acetone.O. % in H2. tube, <I> 36- size diamond Films contain sp bonded carbon. Morep= 2.6- 40 mm. powder. uniform thickness obtained at low P.26 mbar. R incr. with P, but depends on theFR = 6 l/h. compound used: higher R is obtainedTg = 850- from alcohols.90°C.

2% CH~in H2 Cavity-mode <I> 5 cm Si- Diamond fiLms without non-diamond [49]P=5 mbar MW discharge. wafer, carbon obtained, consisting of 1-2 ~FR = 12 l/h <I> l4.8x17.8 em roughened with crystals. Nearly continuous film inTd = 500°C. vert. cavity. diamond powder the central (1 em <1» part of the

Substrate (3 IJ.lll particles). wafer. R = 0.06 IJ.lll/h (compared toholder cooled 0.8 IJ.lll/h at Td = 1000°C). Qualityby means of of lOOO°C film is far inferior.gas lines.

ccco

Designations: FR = flow rate; IR = infrared; MH = microhardness; MW

Td"= deposition temperature; W = power (note that thewatt and tWlgsten).

microwave; R = deposition rate;

same symbol is used to designate

Abbreviations: abs. = absorption; at., at = atomic; chem. = chemical(ly); coef. = coefficient;

conc. = concentration; condo = conditions; cryst. = crystallites; decr. = decrease;

dia., diam. = diameter; horiz.= horizontal; incr. = increase; max. = max~um;

min = minimum; perpend. = perpendicular; po.Lycz , = polycrystalline; pref. = preferr'ed;

vert. = vertical.

84

0.3% H20 provoked an increase of deposition rate by one order of

magnitude, although only if CH4 concentration was also increased

[119]. This is thought to be due to production of OH and 0 which

aid decomposition of CH4, the oxygen simultaneously preventing

(by etching) formation of non-diamond deposits.

In all the papers formation of diamond was reported, its

structure varying from amorphous to polycrystalline [11], often

with well defined particles (crystals) [53,43,84,119,56,78,12]

of up to 10 ~m diameter [43,84,78], the diamond being identified

by electron diffraction [11,119,53], reflection high-energy

electron diffraction (RHEED) [78], X-ray diffraction [122,78,

12], SEM and/or TEM [119,84,53,56,12], Raman spectroscopy

[84,119,122,43,53,12].

Crystallinity, morphology and preferred orientation were

found to depend on deposition conditions [119,84,53,56,12],

namely CH4 concentration, substrate nature and power.

In addition to deposition temperature which must be

higher than 700°C, intensity ratio of the emission lines of CH

(at 314.41 nm) and Hp (at 486.13) was found to have a crucial

importance for diamond deposition [78]. This ratio must be

between 5 and 20. Generally, CHx and Cx radicals are considered

resposible for diamond and graphite formation, respectively

[78] .

In order to reduce the substrate temperature with respect

to the typically used range (700-1000°C) and thus allow use of

lower-temperature-melting substrate materials, the substrate

exposed to a new cavity-type plasma was cooled by means of the

nitrogen gas flowing through a pipe embedded in the substrate

holder [49]. The deposition temperature was reduced to 500°C,

which resulted in a substantially lower deposition rate

(0.06 ~m/h, as compared to 0.8~m/h obtained at 1000°C under

otherwise similar conditions), but well defined diamond films

almost withough non-diamond carbon were obtained. According to

the authors, good quality of the films is a consequence of a

high hydrogen ion and atom density in the plasma and the low

growth rate.

In another study [93] a discontinuous microwave plasma was

used with repeated cycles, in which the substrate was heated up

85

to a final temperature (between 500 and 800°C), held at it for

a certain time and then cooled to room temperature (cooling

intervals are not considered to be within the cycles).

The authors operate with the term llaverage temperature It

during the cycle which is lower than the final temperature and

amounts to about 400°C. This technique enabled the authors to

control diamond nucleation and to obtain films with fine grain

size and surface roughness of the order of 5 to 20 nm, thus

reaching the optical transparency above 60% in the 0.6-2 ~m

wavelength range, comparable to natural diamond.

The complex growth mechanism was found to involve

formation of a "buffer layer" on the substrate/layer interface

[11,119]. This is composed of ~-8iC and a face-centered cubic

phase with a=0.346 nm, which is believed to be defected diamond

containing 8i impurities. It has been suggested [119] that these

phases might facilitate the transition from pure 8i to pure

diamond through intermediate compounds.

The adherence of the diamond layers to the substrate is

generally excellent, but associated with high tensile stresses,

leading to the curvature of the substrate and sometimes to

disintegration [84]. Interpreting results obtained by means of

the X-ray topographic technique [85], the authors conclude that

narrow cracks, as well as residual strain (due to the coherent

contacts with the substrate) are present in the 20 ~m thick

diamond film. It was reported [78], however, that the adherence

of a layer grown on the 8iC-polished-8i-wafer was poor, because

in this case each diamond particle was found to be in contact

with the substrate only at the point which was the vertex of

the single crystal.

Microhardness of the diamond layers was reported to be

higher than 35 GPa [43], or too high to be measured by Knoop

indenter [84].

A number of papers is concerned with optical properties

of the diamond layers, primarily in the IR region [106,11,43,

122,119,84]. Transmittance in the range from 2.5 to 25 ~m is

high, amounting to 85-93% and showing a minimum of 68% at

13.3 ~m [106]. Transmission enhancement effect of about 18%

86

was found in the range 4-10 ~m, pronounced absorption being at

12.8 ~m [43).

IR optical absorption coefficient was found to increase

with decreasing wavelength [122). Because of the nature of the

polycrystalline morphology of the layers, they have optical

scatter losses suspected to be a primary source of the energy

loss with unpolished CVD diamond coatings [122).

Optical properties in the visible part of the spectrum

depend on the presence of non-diamond carbon phases, which in

turn depends on deposition conditions. The layers grown from low

CH4 concentration «0.3%) are generally white and translucent

and their Raman spectra show almost complete absence of sp2

bonding. Higher CH4 concentration leads to darker and less

translucent layers, in which the presence of sp2 bonds is

apparent in the Raman spectra [84).

The effective refractive index of the diamond layers was

found to vary from 2.36 to 2.73 [122).

Electrical resistivity of the diamond film was reported to

vary from 10 8 to 1011 ohm cm, decreasing with the amount of

amorphous carbon [43).

By using a mixture of CH4 (0.3%) and diborane (0.136-4.80

ppm) with hydrogen under the conditions used in [53), B-doped

diamond films showing semiconductive properties were epitaxially

grown on diamond single-crystal substrates [54). The B/C ratio

in the films is related to B concentration in the starting gas

mixture. The electrical conductivity of the B-doped films

increases exponentially with substrate temperature.

The diamond films were found to have excellent chemical

inertness [43), which agrees with the finding that the diamond

layers are durable [84).

In conclussion, it can be said that virtually pure diamond

layers can be grown by the MWPCVD method, with a maximum

deposition rate of the order of 1 ~m/h. The essential deposition

conditions that must be satisfied to obtain diamond deposit are

the gas mixture consisting of hydrogen with a low CH4

concentration at pressures 50-120 mbar and substrate temperature

in the range 700-1000°C (according to [49), the substrate

temperature of 500°C combined with a dense plasma can be used).

87

Although the presence of diamond seed particles on the

substrate is not essential, it certainly has a favourable effect

on the nucleation and growth of the diamond layers. Presence of

carbide forming elements is also suitable for diamond formation.

The optimum values of deposition parameters, in particular

CH4 concentration, remain to be determined. In order to do that,

a better knowledge of the mechanism of nucleation and growth and

of its dependence on the processes in the plasma seems to be

necessary.

Diarnondlike coatings

Although majority of papers concerning the MW plasma CVD

of carbon coatings describes preparation of diamond, formation

of diamondlike coatings has also been reported [70,72,101]

(see Table 15).

The working conditions used were not suitable for "pure"

diamond formation, either because the initial gas mixture did

not contain hydrogen, or because deposition temperature was too

low. However, in one of the papers [70] the deposit obtained

from 5% CH4 in H2 at 700°C contained 1 ~m diamond particles,although their analysis showed presence of a large amount of

hydrogen and small amount of graphitic carbon.

In a study intended to clarify the role played by

hydrogen [72], the DLC formed was exposed to hydrogen plasma.

The graphitic (Sp2 bonded) carbon contained in the coating was

sputtered by plasma to form hydrocarbons and carbon molecules,

but the sp3 bonded atoms were not sputtered. This is in

agreement with earlier conclusions [104]. Formation of CH3radicals with sp3 bonds is considered by the authors to be

resposible for diamond formation, as well as the adsorption

of atomic hydrogen on the deposited carbon.

Table 15

Microwave plasma CVD of diamondlike coatings

ex>ex>

Preparation Apparatus Substrate Characteristics Application Referencesconditions

1 2 3 4 5 6

5%CH4 in H2; MW 9.enerator (111) B-doped In CH4 or CH4~He graphite obtained; [70]Ar or He; 2.45 GHz; P ~ 5i wafers, CHt-Ar plasma produces OLe with aP=1.3 mbar; hor.i:.z.quartz 400 ~ thick, li tle graphite. In CH4-HSDLC withFR=0.9 l/h; tube <I> 18 JDIII, chemically particles up to l~ was 0 tained. ThisW= 150 W; 0.8 m long cleaned. contains large amounts of H as c~Tf 700-800°C reactor. fragments. OLe from CH!-Ar conta~n

( OO°C in small amount of C~, ut Cx (x=1-8)case of are present. Compos tion of plasma isCH4+H2) . rather similar to that of the deposits. ,As foregoing. As foregoing. As foregoing. The electron energy in H2 plasmas [ 72]

,The coating exceeds dissociation energy of H2obtained with molecule.s. CZ~2 is most abundant species.5% CH4 in Ar Only graphit~c carbon contained in thewas exposed coating is sputtered by plasma to formto H2 plasma. hydrocarbons or carbon molecules.

C6Hg, 1 mbar, MW Si, quartz R= 7.2-13.2 ~/h. d=1.7-l.8 g/cm3, incr. [lOlJTd= 0-200°C (polished) with Va. H content decr. from 40 at% forVB=100-1200 V VB = 1 0 V to 20 at% for VB= 800 V, MH

incr. with ~ from 10 up to max. of29 GPa for

9.B=800 l ! , then deer.; p

between 10 and 10 . n cm; n = 1.7.Eg deer. from 1.9 eV for VB = 100· V to1.72 for VB= 800 V; H is primarilybonded to sp3 carbon. The filmspr2pared at V~= 100 V consist of planarsp -hybridize mostly 6-membered ringsin~erconnected by a small number ofsp bonded C. At VB= 800 V the filmsco~sist of strongly deformed mostlysp structure with wrinkled rings.

Table 15:

Designations: Eg = optical gap; FR = flow rate; MH = rnicrohardness; MW = microwave;

R = deposition rate; Td = deposition temperature; VB = self-bias voltage;

W = power (note that the same symbol is used to designate watt and tungsten).

Abbreviations: deer. decrease; incr. increase; max. maximum.

co

'"

90

3.2.4 Thermal plasma chemical vapour deposition

In the last several years a new technique of diamond

coating preparation has been introduced, using thermal plasma.

By using high power dc, ac, RF or MW discharge, a high­

temperature plasma is obtained.

A substantial difference of such high-temperature plasma

(with gas temperature preferably higher than 2300°C, usually

about SOOOOC) from low-temperature plasma (with gas temperature

~1300°C) is in pressure and concentration of active species

(ions or radicals) which are high in the former and low (maximum

about 10%) in the latter [73]. High temperature is achieved by

using high power. A high concentration of active (excited)

chemical species in thermal plasma is caused by a combination of

collisions with electrons accelerated by the electric field and

collisions of thermodynamically activated species themselves.

Therefore, the rate of diamond growth in the thermal plasma is

high (of the order of 100 ~m/h), compared to that in the low­

temperature plasma (maximum of the order of 1 ~m/h).

Because of high temperature, both degree of decomposition

of reactant gases and atomic hydrogen content are high. By

placing the substrate at an appropriate distance from the plasma

and/or applying water cooling, a temperature range 600-1200°C is

obtained, so that conditions for diamond deposition are

achieved.

Synthesis of well-defined diamond films was effected by

means of thermal plasma [71]. This was obtained by using a

plasma torch (4 MHz, 60 kW) at 1 bar with methane introduced

into Ar carrier gas. The substrate was water-cooled Mo plate,

the temperature of which was estimated to be 700-1200°C. The

deposition of polycrystalline diamond film was effected on the

diamond-paste-polished substrate, while polyhedral diamond

crystals 10-30 ~m in size were obtained without polishing.

Very high deposition rate (60 ~m/h for films, 180-300 ~m/h

for single-crystal diameter) were obtained.

Rather similar technique and results have been reported

[61] for a de thermal plasma (dc arc discharge). A mixture of

91

CH4 and H2 is fed between cylindrical anode and cathode rod of

the plasma torch and a plasma jet is generated by dc arc

discharge around the torch nozzle (Fig. 21).

The plasma jet is directed to water-cooled substrate (Si,

Mo, Pt, SiC or silica), the temperature range of which is 500­

1200°C. Polygonal diamond crystals are grown in this temperature

range (below and above this temperature a diamonlike film and

graphitic or amorphous carbon are obtained, respectively). Well­

crystalline and good-quality diamond layer can be formed at a

growth rate of 80 ~m/h.

Even higher deposition rate (200-250 ~m/h) has been

reported [5] for similar deposition conditions (130-1000 mbar,

typically 260-520 mbar, 1% ethanol in H2' substrate temperature

800-900 0C). The polycrystalline diamond films show rugged (rock­

like) surface. The films do not contain non-diamond carbon.

Thus, thermal plasma method appears to be suitable for

diamond deposition. In particular, very high deposition rates

are obtained, differing by at least one order of magnitude

from those obtained by other methods, and high-quality diamond

layers containing no non-diamond carbon are produced.

The method has, however, some disadvantages. It is

difficult to achieve a proper control of deposition temperature.

The films have poor adhesion to the substrates and the film

thickness is not uniform [71].

A variant of thermal plasma is described in a recent

patent [73]. The plasma is generated by dc, ac (50 Hz), RF or MW

discharge in a hydrocarbon gas, an inert gas (Ar) or hydrogen,

either alone or in a combination. If Ar or hydrogen is employed

as plasma-generating gas, a carbon-containing compound (CH4'

C2H6' C3H8' C4HI O' C2H4' C6H6' polyethylene, polypropylene,polystyrene, or an organic compound containing 0, N, halogens,

Sf such as an alcohol, acetone, an amine, methyl chloride,

thiophene or triethyl phosphine; CO or CO2 in combination with

hydrogen may be also employed) must be injected into the plasma

separately.

The pressure of the plasma-generating gas is in the range

10-4-1 bar. The lower the pressure, the lower deposition rate

92

of diamond. The higher the pressure, the more cumbersome the

handling of the pressure container becomes.

The plasma is adiabatically expanded using an orifice ornozzle into a zone where diamond growth is expected, because

under these conditions the gas is rapidly and uniformly cooled

and the substrate temperature (400-l700·C, preferably 750­

llOO·C) and the growth zone temperature can be readily

controlled to a uniform level [73].

As the substrate either a metal (Mo, steel, Sil, ceramics

(alumina), diamond single crystal, or other material can be

employed. The resulting deposition rate is of the order of

100 ~m/h.

93

H2+C~~41llf__II_~D.C.powersupply

Anode--- - ',v ///, ,//

/

Cathode-------.----- -Plasma jet

SUbstrate-~_~'~======l--

Cooling water

Substrateholder

Fig. 21: Schematic diagram of a dc thermal plasma("plasma jet") CVD apparatus [61].

94

4. CHARACTERISTICS (PROPERTIES) OF DIAMOND AND

DIAMONDLIKE COATINGS

A brief summary of structure and properties of the

diamondlike and diamond coatings is presented here. The relevant

literature for each property which has not been mentioned in the

foregoing text and tables is given only.

Structure and bonding

[88,27,28]The structure of the coatings grown by all the method.

varies between amorphous and the well-crystallized diamond

structure and the bonding is characterized by presence of both

sp3 and sp2 bond types, the percentage of sp2 varying from about

45% in polymerlike coatings, via 30% in hard DLC, to nearly 0%

in diamond layers.

The physical methods (sputtering and ion-beam methods)

have been mostly used to prepare diamondlike coatings, although

the possibility of controlling the deposition parameters to the

highest degree permits to obtain materials with structure and

bonding ranging from diamondlike to pure diamond (pure from both

chemical and bonding standpoints), although with many defects in

the latter.

Using the high energy of the ion beam to bombard the

deposited film, the diamondlike character of the latter is

increased both by confering to surface atoms extra kinetic

energy to move into sp3-bonded sites, and removing selectively

by sputtering amorphous or sp2-bonded domains [4].

The DL films are predominantly amorphous, but can have

extensive sp3-bonded short-range order [64,4].

Polycrystalline diamond with crystals up to 5 ~m have also

been obtained.

In the films grown by conventional CVD method without

atomic H (under the conditions suitable for epitaxial growth)

the films had essentially diamond structure, i.e. structure of

the substrate. However, since the used CVD conditions had to

95

differ (for practical reasons) from those ideally suited for

epitaxy, a graphitic component was codeposited, and

subsequentlly removed (in a separate procedure). The bonding

in the final films was sp3 possibly with a small portion of

sp2 bonds.

In the method using atomic hydrogen, it removed

selectively the sp2 bonds during the deposition, so that

the resulting diamond was virtually free from sp2 bonds.

In the hot filament method, the working conditions

(high substrate temperature, presence of atomic hydrogen, low

hydrocarbon concentration) are suitable for diamond formation.

The bonding is sp3 with little or none sp2 bonds present, and

the structure is that of diamond.In the plasma CVD methods both diamondlike and diamond

coatings were produced.

The diamondlike coatings cover a rather wide interval of

properties, presumably related to their structure. The

polymerlike films produced with low energy of the ions can not

be regarded as diamondlike because of their properties caused

by their structure. They contain about 50 at% H and larger units

of the starting hydrocarbons can be incorporated into their

structure. In addition, their sp2-hybridized-carbon content

is about 45%.

The hard DL films, unlike the polymerlike ones, do not

depend on the nature of the starting gas because this is

decomposed in the plasma almost down to single atoms [59].

However, similar to polymerlike coatings, they contain large

amount of hydrogen, one part of which is bonded to C-atoms.

The suggested models of the structure of amorphous

materials can be divided in two basic groups - microcrystalline

models and models of a random network [105]. The latter group,

according to which the random character of the network is a

result of a statistical distribution of angles between bonds,

is more widely accepted.

With increasing energy of the ions in the plasma

bombarding the substrate, the amorphous structure is changed

into a quasi-amorphous with crystallites which are of the order

of 0.5 nm [44]. The coatings incorporating larger crystal

96

domains are also often formed.The other end of the interval, diamond coatings, have a

structure varying from amorphous to polycrystalline with defined

crystals up to 10 ~m in size. In addition to crystals containing

detectable graphitic component, apparently "pure" diamond

coatings have been reported in a number of papers.

Microstructure

In general, diamondlike coatings show microstructure

without particular characteristics. The layers are compact,

without visible cracks, pores and do not have a pronounced

surface roughness.

The diamond layers with crystallite dimensions on the

order of microns have a more pronounced surface roughness.

The layers with small crystallites can be smoother.

The ion-beam prepared diamond layers are unusually smooth

and are featureless under SEM.

Density and porosity

[114,124,9,30]

Density of the sputtered DL films is in the range

2.1 - 2.2 g/cm3 •

Density of the ion-beam prepared diamondlike films

depends on hydrogen content and is typically 2.4 g/cm3 for

pure carbon and 1.8 g/cm3 for hydrogen-containing films.

The soft, polymerlike films prepared by RF plasma method

have low density (typically 1.3 g/cm3 ) . The hard films prepared

by RF plasma are characterized by higher density (typically

1.9-2.12 g/cm3). Since these films contain both sp3 and sp2

bonds, their properties (including density) might be expected to

be between those of graphitic carbon and diamond. The relatively

low density values even compared to graphite (2.26 g/cm3 ) are

because of the relatively high hydrogen content and disordered

structure.

97

The linear relationship between density of RF plasma

coatings and the parameter v B/pl/ 2 (determining energy of the

ions impinging on the substrate, see 3.2.2) has been found [18]

for density range between 1.5 and 1.8 g/cm3. In a new review

article [59], a non-linear dependence has been reported to existbetveen density and VB (pressure P being constant) for a density

range between 1.5 and 2.0.

The OLC films produced by dc magnetron sputtering from Ar­

C2H2 mixture had a wide density range, from 0.98 to 1.6 g/cm3.

The papers describing diamond coatings preparation usually

do not report density values, but on the basis of other

characteristics it can be concluded that these should be close

to the diamond density (3.5 g/cm3) at least for well defined

crystalline layers.

Porosity of the OL coatings is very small and sometimes

layers 1 ~m thick are obtained which show no porosity at all;

they are also pinhole-free. Such properties are precondition for

protective coatings.

Mechanical properties

[114,124,7,83]

Adhesion of the DL coatings (produced by RF plasma) is

strong to diamond or carbide forming substrates, which is

explained by an interfacial carbide layer formation. Adherence

is less on other materials. Adherence of diamond layers produced

by RF or MW plasma is generally very good and even excellent.

Stress in the coatings is generally reported to exist.

Low-thickness films do not exhibit stress, but thicker OL films

tend to separate from the substrate because of the internal

compressive stress, as found for the ion-beam prepared layers.

Diamond layers prepared by MW plasma method showing excellent

adhesion, develop high tensile stress and cause curvature or

even disintegration of the substrate.

Sliding friction was found to be very low for diamondlike

coatings prepared by RF plasma method. Sliding friction

coefficient values were found to increase with relative humidity

from 0.01 to 0.3. In vacuum or very dry nitrogen it is 0.02, but

98

increase in relative humidity from 10 to 99% leads to an

increase of friction coefficient from 0.05 to 0.3.

Microhardness of polymeric coatings obtained by RF plasma

method is low (4 on the Mohs scale). DL coatings obtained by dc

plasma have microhardness up to 30 GPa, and those prepared by RF

plasma (according to different authors) 7-8 (Mohs) or 30-46 GPa.

Diamond coatings obtained by conventional CVD, RF plasma,

MW plasma and thermal plasma methods have similar hardness.

The values reported are mostly in the range from 70 to 120 GPa,

i.e. virtually identical to natural diamond hardness, although

lower values (35 GPa) have also been found.

Electrical properties

The electrical resistivity values reported for diamond

coatings prepared by different methods (ion beam, CVD,

dc plasma, RF plasma, MW plasma are within 1_1015 ohm cm range.

The diamondlike coatings obtained by ion beam methods have

resistivity in the 10 11_10 12 range, but a wide range, extending

from 10 7 to 10 16 ohm cm was found for hydrogen-containing DLC.

Electrical resistivity of diamondlike coatings prepared by

dc plasma is 10 10_10 12 ohm cm. This interval can be extended

(10 3_10 15) varying the deposition temperature, the higher the

deposition rate, the lower the electrical resistivity.

A very broad range of electrical resistivity values

(from 1 ohm cm to 10 14 ohm cm was found for RF plasma deposited

coatings, but values in the range 10 9_10 12 ohm cm seem to be

most frequently reported. Dependence on deposition rate, i.e. on

the discharge power, has been found, the lower the power (and,

consequently, the lower the deposition rate), the higher the

resistivity; however, different resistivity intervals were found

by different researchers for similar power or deposition rate.

The substrate temperature (Td) was found to have a

pronounced effect on electrical resistivity, the latter

decreasing rapidly with increasing Td. A decrease in resistivity

from 1011 to 10 3 was produced by temperature increase of only

500C (from 250 to 300 oC). This effect, as well as the effect of

99

substrate potential on resistivity (resistivity decreases with

VB) is related to hydrogen content which decreases with

increasing VB and Td'The hydrocarbon gas (CH4, C2H6' C2H2 or C2H4) influences

the resistivity, which differs up to a factor of about 3 for the

coatings prepared from methane and ethylene at Td=250oC

(resistivity (methane)/ resistivity (ethylene) > 10 3).

Good insulating properties of the coatings are ascribed to

saturation of dangling bonds by hydrogen.

For very thin films « 50 nm) electrical resistivity wasfound to increase exponentially with decreasing film thickness.

This effect is due to the fact that thinner films are closer to

the ideal amorphous state.

Doping of the coatings with elements of group III and V of

the periodic system was effected using B2H6 (for B) and PH3(for Pl. Doping with N, Ta and Ru has also been reported.

Breakdown strength of diamond like films obtained by ion­

beam and RF plasma methods has a value ~106 V/cm.

Dielectric constant for DL films obtained by ion-beam

method is 8 and for diamond films obtained by CVD method is 4-6

(compared to 5.8 for natural diamond).

Optical properties

[75,99]

Index of refraction (n) of polymer1ike films is 1.6-1.65,

but for DLC it was found to vary in a broad range (1.8-2.8)

although majority of papers report values in the range 1.8-2-3.

A linear relation was found between nand VB/P (for the range

1.8-2.15), the slope of the straight line being changed at

n=2.05.

The n values found for diamond layers are 2.38 if

obtained by CVD and 2.36-2.73 when obtained by microwave

plasma (the n value of the natural diamond is about 2.40).

Optical gap (Eg) of DL films amounts to 3.0 eV for

polymerlike films, 0.40-0.74 eV for the DL films prepared by

sputtering in Ar atmosphere. Eg=1.2 eV was found for the films

obtained by ion beam sputtering (in vacuum). The DL films

100

obtained by dc plasma and RF plasma have Eg values in the range

from 0.8 to 1.8 eV, although most often reported values are from

0.8 to 1.1 eV. The Eg is reported to increase with hydrogen

content of the film, and to decrease with deposition

temperature, especially above 250°C.

Transparency of the polymerlike films is good in the

visible and IR (high Eg values). Transparency of the DL films is

generally good in the IR; thin DL films are transparent also in

the visible.

For the diamond coatings, transparency in the visible part

of spectrum depends on the presence of non-diamond (graphitic)

phases, the less the sp2 bonding, the higher transparency. The

diamond coatings, prepared by CVD were found to be transparent

even in UV (wavelengths above 225 urn), similar to natural

diamond.

minimumis generally very good.

range is 85-93%, showing a

Transparency in the IR

Transparency in the 2.5-25 ~m

(68%) at 13.3 ~m.

Absorption of DL films in IR (and in the red part of the

visible spectrum) is very low, both for ion beam and RF plasma

prepared films, becoming almost zero for wavelengths above

1.5 ~m. The adsorption is low for both hydrogen-containing and

no-hydrogen films prepared by the ion beam method.

Absorption of diamond films prepared by MW plasma is low

in the IR, but absorption coefficient increases with decreasing

wavelength.

Thermal properties

Room temperature thermal conductivity values for ion beam

prepared DLC greater than 700 W/mK have been measured [4] (for

comparison, room temperature thermal conductivity of natural

diamond, the highest of all known materials, is about 2000

W/mK).

101

Chemical properties

Diamondlike coatings are chemically resistant to most

acids, bases and solvents [4].Diamond layers are stable even to the most aggressive

chemical agents (HN03+H2S04 (1:3), H2S04+K2Cr207' aqua regia,

concentrated HF).

102

5. APPLICATIONS

The major useful properties of the diamond and diamondlike

films include the following. i) transparency; ii) electrical

insulation; iii) high breakdown voltage; iv) high thermal

conductivity; v) chemical resistance to most acids, bases and

solvents; vi) barrier properties; vii) hardness; viii) good

adhesion to many substrates; ix) wear resistance; x) low

coefficient of friction; xi) deposition on substrates at

slightly above room temperature. The ion beam methods of DLe

preparation can provide, in addition, remarkably smooth films.

The methods of the diamond coating preparation developed

up to the present permit preparation of monocrystalline and

polycrystalline coatings. The monocrystalline films can be

produced at a slow rate (of the order of 1 ~m/h), while

polycrystalline coatings have been prepared at various rates

ranging from .several tenths of ~m/h up to hundreds ~m/h.

Properties of diamond coatings are much closer to those of

natural diamond than are properties of diamondlike coatings and

therefore field of their potential application is very broad.

Most of applications of diamond and diamondlike coatings

can be grouped under the categories of optical, electrical,

thermal, chemical and mechanical. Fig. 22 shows possible

application of the diamond films in relation to their properties

[105] and gives an idea of applications of both diamond and

diamond like coatings. The following text contains more data.

Optical

An initial application for the transparent diamondlike

coatings was as protective coatings for plastic lenses. The

hardness of the coatings, good adhesion to plastics, high

transmission in the visible (water-clear to slightly yellow in

colour), and the low deposition temperature made this attractive

application. The experiments performed indicated that

diamondlike coatings could indeed improve scratch resistance

of these lenses [99].

103

Properties Applications

Hard

Low triet Ion

Hleh thermal conductivity

Eloctrlc Insulator

IHoot rostsuvc

I l.orqe bond 90P

Abrasive coatings tor cuttinqtools

Heat sinks tor electronicdevices

Microwovo power devices

RF electronic devices

HI9h bote mobility

I Low dielectric constant Jr-----:===========~r------J Hi<;Jh-spccd electronic devices

'------------'

IAdd resistive

IRadiation resistive

ITransparent

IHi9h refractive index

Electronic devices for severeinvironments such as in spoce orIn rocteor roactors

Eleetro- opticct devices

Fig. 22: Properties and some possible fields ofapplication of diamond coatings [105].

104

A low coefficient of absorption in the IR makes

diamondlike films very promising as antireflecting, and also

erosion-and corrosion-resistant coatings for IR optics.

Furthermore, the index of refraction of diamondlike coatings

can be varied in the range 1.8-2.1 by changing deposition

parameters, e.g. under the RF plasma conditions. This allows

production of single-layer antireflection coatings for optical

materials which have refraction indices in the range of about

3.24-4.41. Very important materials for IR techniques are Ge,

Si and GaAs with n values of 4.0, 3.4 and 3.34, respectively.

One of the most important applications of the diamondlike

films are as coatings for germanium IR optical elements. Owing

to the possibility of making its refractive index n equal to 2,

diarnondlike film is an ideal antireflection as well as

protective coating for Ge with refractive index of 4 [17].

It has been shown that it is possible to increase transmission

of IR radiation (at 5 ~m wavelenth) from 49% for uncoated

Ge windows to 96% for windows coated by diamondlike films of

proper thickness on both sides (Fig. 23) [48,47].

Diamondlike coatings can be used to reduce reflection

losses of Si solar cells. The effect of enhanced infrared

transmission for Si wafer coated with a thin diamondlike film

is illustrated in Fig. 24. The high hardness of the diamondlike

films also imparts abrasion protection to these infrared optical

materials.

The use of ion beam diamondlike films as protective

coatings for optical elements in high power lasers was suggested

for a number of reasons. Included were adhesive strength of the

coating to optical materials such as glass and CaF2' low optical

absorption throughout the visible and infrared spectral regions,

expected abrasion protection, and the capability to form a

hermetic coating with chemical resistance to acids, bases and

solvents [4].

Plasma formation at the surface of mirrors and windows

inside the laser cavity also causes problems in high power

lasers. Small defects on the surface of laser optics can be the

initiation sites for the breakdown plasma, which then degrades

105

a.s a.e ... s.o , .o 7.' , .e s.c W ra " 16 lli 20

eo a..f::{Cc/a-C

"S"60,-..j',,.~

o«oc asoo coco ace "'" . '"'" '000 "'" J~OO ".,. ~ '00

....~\"rnr.b.'r (=_1)

Fig. 23: The transmittance of uncoated Ge, and Gecoated on both sides with one- quarter-lambdathick diamondlike films for 5 ~m wavelength [48].

./DLe-COATED SI

'00

80wuz

E60

•~z• 40">-~

20

02,5 3,0

BLANKSI

4.0 5.0WAVELENGTH (MICRONS)

6,0 7,0 B,O

Fig. 24: The transmittance of silicon wafer coatedwith 60 nm thick diamondlike film, and uncoated Siwafer [4].

106

these optical surfaces and reduces the power threshold for

subsequent use. In some cases, diamondlike films can be

smoother than the substrate surface. Thus, it is believed that

a diamondlike coating on the intracavity optics can increase

their breakdown threshold by providing a smoother surface and

reducing protrusions that would otherwise enhance breakdown [4].

Because diamondlike films are resistant to chemical

attack, they are candidates as protective coatings for laser

optics which corne in contact with corrosive gases. However,

the various forms of hard carbon are susceptible to attack

by reactive ions and atoms of oxidizers such as fluorine and

oxygen.

Another proposed optical application for diamondlike

films is as transparent protective coatings for optical disks.

The diamondlike coating is hermetic barrier and will protect

the recording medium from degradation and extend the archive

lifetime [4].

Optical sensors immersed in corrosive fluids or

environments can also benefit from the protection provided by

thin optically transparent diamondlike films applied over the

exposed surfaces.

There have been suggestions that transparent diamondlike

films on softer polished gem stones would give them the surface

hardness of diamond, while retaining the appearance of the gem.

Another related application that has been suggested is as

transparent protective coatings on decorative silverware and

jewellery to prevent the tarnishing and reduce the need for

cleaning.

High index of refraction and transparency of diamond films

suggest its application for optical components for visible and

IR spectral regions (windows, lenses) [105,11,74,122,45,119].

Electrical

If the electrical properties (intrinsic high resistivity,

wide band gap) and thermal properties of true diamond material

can be approached by diamondlike coatings, then there will be

many advantages to using diamondlike films in device

107

applications. Diamondlike films with high resistivity,

>10 11 ohm cm, have already been deposited. Ion beam deposited

diamondlike films can exhibit a thermal conductivity greater

than 700 W/mK [4]. This combination of high thermal conductivity

and high electrical resistivity makes diamondlike films

attractive as a dielectric layer for high temperature devices.

However, diamondlike films with a band gap greater than about

2 eV have not yet been demonstrated [4].

As an example of the use of ion beam deposited diamondlike

films in electronic device applications, thin film silicon field

effect transistors (FET) with insulating diamondlike films were

fabricated (Fig. 25).

Possibility of application of diamond coatings for

electronic devices is pointed out by a number of authors

[45,84,56,23,74,106,105]. Radiation hardened semiconductor

devices, high-temperature electronics, high-density, high-speed

integrated circuits, UV-light emitting diodes and lasers have

been mentioned [106].

Among the possibilities of application of diamond

coatings, their doping leading to production of p- and n-type

semiconductor layers has been mentioned [105,92].

It is expected that diamondlike films will play a special

role in applications where the power density is high. An example

of this will be found in the next generation of high density

integrated circuits. These devices will permit shorter signal

transit distances and reduce communication time between

transistors and memory cells, resulting, in higher computation

rates. Because of the ability of the resulting devices to

operate at higher temperatures, cooling requirements can be

reduced and the operating lifetime can be much greater.

This will be important in the new supercomputers where the

high power density currently makes sophisticated cooling

systems a requirement [4].

Another important application can be for semiconductor

circuits used in space enviroments. Here, the high temperature

capability will permit lighter cooling systems to be used.

Also, the radiation resistance of diamondlike devices will

permit a reduction in the weight of radiation shields.

108

COPLANAR FET

QA.TE {AI)

Si (P) 10000 - em

INVERTED COPLANAR FET

Fig. 25: Geometry of active thin film field effecttransistors fabricated using ion beam depositedn-type silicon and ion beam deposited diamond-like film as the intermediate insulator film [4].

109

Both of these features can permit the weight and cost of

satellites to be reduced, and increase operating life.

This radiation resistance property will also be important

for a variety of military applications and nuclear power

plant electronics.

Thermal

The power dissipation of a solid state device such as a

microwave oscillator or power transistor can be significantly

increased if it is mounted on a substrate (such as copper) that

permits thermal energy to be efficiently transferred away from

the device.If the device must be electrically isolated from the

cooling substrate, it is desirable to use an insulating layer

with high breakdown voltage (so that it can be made thin) and

high thermal conductivity.

Because diamondlike films (i) can exhibit high thermal

conductivity, (ii) can be made thin while providing the

necessary electrical isolation, and (iii) do not require

intermediate bonding layers (which may cause phonon impendance

mismatch) to adhere to most substrates, they should find use as

insulating layers in many high power or high density device

applications.

Provided that diamondlike films can be made thick enough,

they may find use as heat spreaders. However, because of the

thickness required (usually >100 ~m) this application will

probably be better fulfilled by polycrystalline diamond films.

High thermal conductivity of diamond coatings in

combination with high electrical resistivity, good heat

resistance and large band gap open possibilities of their

application as heat sinks in various electrical components

[105,74,106,119).

110

Chemical

The chemical resistance to most acids, bases and solvent

permits the use of diamondlike films as protective coatings for

various applications.

The inert nature of diamondlike films suggested its use

as a blood compatible coating for use in the artificial heart

program. Additionally, use of diamondlike films as tissue

compatible coatings for other artificial organs or biological

implants such as hip joints and plastic replacement arteries

or veins used in bypass operations has been suggested [4].

The roles of in-vivo protective coatings are (i) to be

biologically inert to blood contact, (ii) to prevent harmful

components (e.g. plasticizers, monomers, filters) of the

artificial organs from leeching into the blood, and (iii) to

prevent blood components from leeching into the plastic to

degrade the performance and lifetime. Because the chemical

composition is similar to organic material, and since the dense

lattice provides an excellent diffusion barrier, diamondlike

films were considered nearly ideal coatings.

It was also expected that the smooth surface and inert

nature of diamondlike films would hinder the formation of blood

clots. unfortunately, it was found that when plastic substrates

had surface structural defects or inclusions larger than 100 nm,

a diamondlike coatings of about 100 nm thickness could not cover

these defects, and blood clots would eventually form.

This result suggested that thicker protective coatings

and/or much smoother test substrates were required. At the time

of the tests, plastic substrates without defects could not

always be supplied. If artificial hearts made of plastics are to

be practical in the long term, then plastic surfaces without

clot nucleating defects are needed, even if the plastic material

and diamondlike coating are intrinsically blood compatible [4].

Diamondlike coatings have been shown to be an effective

hermetic seal (moisture barrier), capable of extending the lifeof optical fibers.

111

An ion beam coating system was designed and built to

permit in-line deposition of diamondlike films onto an optical

fiber after it was drawn, but prior to application of the

protective polymer coating. With this system, diamondlike

coatings up to a thickness of about 30 nm were applied.

The films could not be made thicker because of the limited

discharge power and cooling capability in the coating section,

and because the fiber could not be drawn too slowly.

This 30 nm diamondlike coating improved the fiber

strength as determined by measurements of the breaking strength

as a function of strain rate (dynamic fatigue). These data

permitted calculation of parameters that predicted an

improvement in strength under constant strain (static fatigue)

relative to the case where only polymer coatings were used [4].

The insulating protective diamondlike coating has

advantages over metallic coatings when interferences due to

electromagnetic pulses occur.

Because of their quasi-amorphous structure, diamondlike

films are almost without pores of sizes which would be large

enough to allow passage of corrosion-active gases and liquids.

A diamondlike coating 0.1 ~m thick noticeably reduced

permeability to the ions of an electrolyte solution and with

1 ~m thick coating the protective action is greatest and hardly

changes at greater thickness. Various authors [7,123] have

reported complete stability of diamondlike films tenths to a few

~m thick to strong acids, alkalies and organic solvents, as well

as their ability to protect a number of materials (AI, Si,

glass, etc). against corrosion-active agents.

A 60 nrn thick diamondlike coating has been shown to be an

efficient barrier against water vapour penetration into the

surface layer of fused quartz, so that preservation of strength

of fibre optics elements can be ensured for a long time [102].

A similar example of protection of the output window of a

semicondu~tor laser against degradation caused·by atmospheric

moisture and oxygen has been reported. The service life of the

protected laser device amounted to about 6000 h [105].

112

Applications of diamond coatings for electronic devices

(as in space or nuclear reactors), based on their chemical and

radiation resistance, have been mentioned [106,105].

Mechanical

Because of the hardness, smoothness and low friction

coefficient od diamondlike coatings, it was anticipated that

these films on the edges of cutting and razor blades would

improve performance.

Experimental results showed that diamondlike coatings on

industrial cutting blades reduced the cutting force required to

slit paper by more than a factor of four, presumably by reducing

friction between the paper and the blade. Second, the coated

blades could cut about 100 times longer than the uncoated blades

before the same cutting force was reached. It is anticipated

that diamondlike films on other cutting tools (e.g. knives,

scalpels, microtomes) can improve their cutting abilities, and

extend the period of sharpness under use [4].

Diamondlike coatings on certain machine tools used under

high force or load may also provide some degree of protection.

However, if high temperatures occur at the cutting edge, the

diamondlikefilms will be rapidly degraded by oxidation. Also,

diamondlike films begin to decompose by losing hydrogen at about

400°C. The diamondlike coatings are probably not appropriate for

application on tool bits for machining iron or steel, because

chemical reaction between the carbon film and iron will occur.

Machining non-ferrous materials and ceramics is a possibility.

Another important application of diamondlike coatings will

be for wear protection of magnetic recording disks and thin film

media. An example is a procedure in which a 40 nm diamondlike

film was used to protect a Zn-Mn ferrite head against wear for

a long-term operation, and which could be applied to all types

of magnetic heads for video recording units [123].

Diamondlike coatings have already demonstrated the ability

to protect Winchester disks from head crashes. The diamondlike

coating also protects the disk against chemical reaction with

113

lubricating fluids. There will be many other wear-related

applications (e.g. ball bearings, bearing races, valves, seals,

etc.) which can benefit from the application of a smooth, low

friction diamondlike coating.

The rigidity of diamondlike films has resulted in a novel

application associated with loudspeakers. Because of the

stiffness of the diamondlike film relative to the added mass,

the response of coated titanium tweeters in the high frequency

range (20-35 kHz) is significantly improved compared to that of

uncoated tweeter [4].

The obvious possibilities of using diamond films as hard,

wear resistant coatings for cutting tools and other applications

have been mentioned [84,74,105].

114

6. CONCLUSION

The investigations lasting for several decades in which

many researchers allover the world have been involved have

established working conditions for fabrication of carbon

coatings having various properties ranging from low-density,

soft, high hydrogen content, polymerlike, to denser, hard

coatings with less hydrogen.

The latter, diamondlike coatings can be produced by a variety

of physical and chemical methods, but RF plasma CVD methods

have been mostly used. The apparatus (standard, commercial

RF systems), experimental conditions used (low pressure plasma,

variety of substrates held at ambient temperature or close to

it, variety of hydrocarbons mixed with Ar or He) and attractive

properties of DLC (transparency, high electrical resistivity,

high breakdown voltage, high thermal conductivity, chemical

resistance, hardness, adhesion to many substrates, wear

resistance), have made them attractive for many applications.

The diamondlike coatings can be also produced by other

plasma CVD methods (dc or MW plasma). Common to all chemical

methods used for diamond like coatings preparation are low

substrate temperature, high hydrocarbon gas concentration

(up to 100%) and no hydrogen in the initial gas mixture.

Physical methods (sputtering and ion beam) have been

used too. The ion beam methods in particular, permitting

greater control of deposition parameters than any other method,

can be used to produce deposits with desired composition and

properties. The deposits obtained are generally similar to those

prepared by other methods, but certain important differences

exist, e.g. their exceptional smoothness.

The diamond coatings have first been produced already

3.5 decades ago but interest of researchers was stimulated only

much later when they have been produced by plasma CVD methods.

Today diamond coatings can be produced by a number of

methods.

The plasma CVD methods use dc, RF or MW commercial

apparatus. Working conditions used for diamond coating

115

preparation .common to all these methods are a low concentration

of hydrocarbon in hydrogen, high ionization efficiency of the

plasma providing suitable species for diamond formation, in

particular high concentration of atomic hydrogen, substrate

temperature most often in the range 700-l000°C; it is desirable,

but apparently not indispensable to have a substrate surface

suitable to promote diamond nucleation (containing diamond seed

particles, or consisting of diamond, a diamond-lattice-matching

carbide or carbide-forming metal).

A simplified picture of the processes leading to diamond

coating formation would be as follows.

The role of the plasma is to decompose and ionise

hydrocarbon and to form atomic hydrogen in superequilibrium

concentration (with respect to substrate temperature). The ions

and fragments consisting mainly of carbon are deposited on the

substrate, the temperature of which ensures the surface

mobility. The atomic hydrogen preferentially reacts with

sp2 bonded atoms and removes them, leaving and stabilizing

sp3 bonded C atoms on the surface.

Nucleation is favorized by presence of diamond seed

crystals or a carbide with a crystal lattice matching that

of diamond.

The role of the hot filament (~2000°C) placed at an

appropriate distance from the substrate (heated to 700-1000°C)

is essentially the same as the role of plasma. The same is valid

for thermal plasma, in which very high temperature (5000 0C)

leads to both high degree of decomposition of hydrocarbon and

production of atomic hydrogen. The substrate temperature is

again in the same range as in the other methods.

By means of the physical methods, diamond coatings can

be deposited on non-carbon substrates kept at or near room

temperature, using energetic carbon ions, preferably without

accompanying neutral particles.

It is clear therefore that the factors playing essential

role in the chemical deposition methods (high substrate

temperature, presence of atomic hydrogen) do not exist in the

case of physical methods. Their role must therefore be played

116

by the energetic particles, but the mechanisms of their action

are rather poorly understood.

The ion bombardment increases the diamondlike component

by giving surface atoms extra kinetic energy to move into sp3

bonded sites, and by preferential removal by sputtering of

amorphous or sp2 bonded domains. According to a recent

"s ubplantation model" for mass-selected ion beam deposition,

a dense sp3 matrix evolves from hyperthermal carbon ion

impingement due to preferential displacement of sp2 bonded atoms

and due to the "mold" effect of the host matrix which imposes

the structure to be formed. According to the model, the optimum

energy range for the diamond evolution is about 60-180 eV, and

substrate temperature ~70°C. Higher substrate temperature

(~100°C) would lead to evolution of graphitic films due to

C-substrate diffusion and ion mixing.

With structure varying from amorphous to polycrystallyne

with crystals up to 10 ~m, the reported properties of diamond

coatings also vary, but can be made close to those of natural

diamond.

The existing applications of diamondlike coatings are

already numerous (antireflecting/protective coatings for optical

elements, Ge in particular, coating of cutting tools to increase

their performance, wear protection of magnetic recording

elements, coatings on loudspeakers to improve their response,

insulating coatings on silicon field-effect transistors).

Many other applications are possible, although some of

them require certain improvements in diamondlike

characteristics.

Prospects for a wide application of diamond coatings seem

real. Certain characteristics of diamond coatings are better

than those of diamondlike coatings (higher thermal conductivity,

higher heat and chemical resistance) and can be therefore more

suitable for certain applications (e.g. in high power devices,

for machine tools used under high force or load and thereby

heated up to high temperatures, for better corrosion protection

etc.) .

117

However, diamond coatings can not replace diamondlike

coatings in all applications. Properties of diamondlike coatings

are different from those of diamond, but also can be varied

in a certain range to make them optimum for a given application

(e.g. refractive index can be made 1.84 or 2.0, which are

optimum values for antireflecting coatings on Si and Ge,

respectiveLyj .

Thus, both diamond and diamondlike coatings may be

expected to have their applications. Which material will be

chosen for a given application will depend on its

characteristics, but also on the conditions of its preparation

(e.g. high temperature used in the majority of methods of

diamond preparation restricts the choice of substrate

materials), possibility to be produced on sufficiently large

surface area and/or with sufficient uniformity of thickness,

economic and other factors.

118

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Scientific Series of the International Bureau

1 Fundamentos do Metodo de Correntes ParasitasD. Stegemann

2 Diamond and Diamondlike CoatingsS. Marinkovic and Z. Marinkovic with S. Krawczynski

ISBN 3-89336-044-1