PLD Elaboration of Piezoelectric ZnO Thin Film for 540 MHz Al/ZnO/Pt Bulk Acoustic Wave Resonator

Rafik SERHANE, Hammouche KHALES, Walid

AOUIMEUR a Division Microélectronique et Nanotechnologie, Centre de

Développement des Technologies Avancées (CDTA). 20 Août 1956, Baba Hassen, BP: 17, DZ-16303, Algiers

Algeria, E-mail : rserhane@cdta.dz

Samira ABDELLI-MESSACI, Slimane LAFANE Division Milieux ionisés et Laser, Centre de Développement des Technologies Avancées (CDTA). 20 Août 1956, Baba

Hassen, BP: 17, DZ-16303,Algiers, Algeria.

Abdelkader HASSEIN BEYa,b b Micro & Nano Physics Group, Faculty of Sciences,

University Saad Dahlab of Blida (USDB), BP. 270, DZ-09000 Blida. Algeria

Tarek BOUTKEDJIRT Equipe de Recherche Physique des Ultrasons, Faculté de physique, Université des Sciences et de la Technologie

Houari Boumediene (USTHB), BP 32, DZ-16111, El-Alia, Bab-Ezzouar. Alger. Algeria

Abstract—The ZnO thin films in Al/ZnO/Pt/Ti/SiO2/Si Bulk Acoustic Wave (BAW) resonators are realized by a reactive Pulsed Laser Deposition (PLD) technique. It is of most interest to improve their piezoelectric characteristics. These later are intimately linked to the microstructure, the texture and the growth conditions. In this work, the piezoelectric characteristics of ZnO thin films have been investigated. Wurtzite ZnO thin films were prepared on Pt(111) at different substrate temperatures (100-500 °C). The 200 nm of Pt on 20 nm Ti were prepared by Electron Beam Deposition (EBD) technique at room temperature on 2 µm SiO2 substrate. This SiO2 film has been realized by Silicon wet thermal oxidation step at 1100 °C. The top electrode of the BAW was made by thermal evaporation of Aluminum; the piezoelectric film was confined in a circle of 1.8 mm radius. X-Ray Diffraction (XRD) characterization showed that the deposited Pt (bottom electrode) has (111) preferential orientation. It also showed that the ZnO films were c-axis (002) oriented. The Scanning Electron Microscopy (SEM) of the realized ZnO thin films showed evidence of compact grains with honeycomb-like structure on surface and evidence of columnar structure on cross-section. The measurements indicate that all substrate temperatures are suitable to obtain a good quality of ZnO, but Ts=300 °C is the optimum. In order to evaluate the piezoelectric properties of the BAW, measurements of the electrical input impedance and admittance have been performed. They showed evidence of piezoelectric response. The resonance frequency was obtained at 524.5 MHz and the anti-resonance frequency at 540.9 MHz. The electromechanical coupling coefficient Keff

2 was evaluated to be 7.26 %, which is an indicator of a good piezoelectric response. From this measured values, the thickness d and the wave velocity Vp in the ZnO slab were deduced to be respectively 6.15 µm and 6451.35 m/s and the piezoelectric constant e33 was found to be equal to 1.21 C/m2.

Keywords—Piezoelectricity, Bulk acoustic wave, ZnO, Pulsed laser deposition, Thin films.

I. INTRODUCTION

Due to its simplicity combined with its flexibility in the choice of processing parameters, Pulsed Laser Deposition (PLD) is one of the best techniques used to prepare ZnO thin films. It has been applied to the synthesis and the deposition of

many compound films [1-4]. In this work, ZnO thin films are prepared for BAW transducers by PLD technique on Pt(111)/Ti/SiO2/Si substrate, at different substrate temperatures from 100 to 500 °C. The Pt/Ti films (200 nm/20 nm respectively), considered as bottom electrode were prepared by Electron Beam Deposition (EBD) technique at room temperature on 2µm SiO2 layer. This SiO2 film is performed by Silicon wet oxidation step. The BAW transducers are used to study the piezoelectricity of the deposited ZnO [5].

II. EXPERIMENT

A. Thermal Oxidation of Silicon

The oxidation step of (100) p-type Silicon is performed in an oxidation furnace, according to (Dry-Wet-Dry) process, as follows :

Load: from T=800 °C to 1100 °C, with heating rate of 5 °C/min, under gas flow of O2=500 sccm and N2=6000 sccm.

Dry: t=10 min at T=1100 °C with O2=6000 sccm.

Wet: t=9h10min at T=1100°C under (H2+O2)(5+3.5) slm.

Dry: t=10 min at T=1100 °C with O2 flow of 6000 sccm.

Unload : from T=1100 °C to 800 °C, with a rate of 5 °C/min, under O2 gas flow of 500 sccm and N2 flow of 6000 sccm.

B. Deposition of Pt/Ti on SiO2 by EBD system

First, 20 nm of Ti is flash-deposited by electron beam deposition technique to favor the adhesion of Pt (Bottom electrode) on SiO2. The deposition is realized during 2min at the pressure p=3.4 10-7 mbar with current intensity I=0.208 A and voltage of 6.69 kV. The deposition rate is estimated to be 1.4 Å/s. Then, Pt was deposited, also by EBD on Ti for 11 min under the same pressure, with I=0.415 A, V=6.36 kV and rate of 3 Å/s. The surface morphology of the Pt grown films

275978-1-4673-5996-2/13/$31.00 ©2013 IEEE 2013 Joint UFFC, EFTF and PFM Symposium

(Fig.1.a) is analyzed by SEM (SEM-JOEL 6400) and the Pt films structure (Fig 1.b) is determined with Bruker D8 X-ray diffractometer (Advance (θ,2θ), Cu Kα and λ0=1.5406 Å).

C. Deposition of ZnO thin film by PLD

The Pt/Ti/SiO2/Si(100) substrate of 0.2µm/0.02µm/2µm/350µm thickness respectively is mechanically attached to a heating block of a PLD chamber at 10-6 mbar vacuum. The temperature is monitored by a power generator which keeps current and voltage at constant values. The target (compacted ZnO powder in rotation) is fixed on holder located at 4 cm far from the substrate holder. The pulsed KrF laser beam, (f=10 Hz, λ=248 nm, τ=25 ns), is focused on the target surface at a fluence of 2 J/cm2 and is directed at 45° with respect to its normal. The vacuum chamber is, then, filled with oxygen up to 10-2 mbar [6]. The deposition of ZnO is performed during 90 minutes at different substrate temperatures (100 to 500 °C).

D. Deposition of Al top electrode by thermal evaporation

For the realization of the BAW transducer, a circular top electrode (with 1.8 mm radius) is made on the surface of ZnO by thermal evaporation of Aluminum through a mask. The piezoelectric properties are deduced from electrical input impedance, measured by a HP8753ES Network Analyzer in harmonic regime.

III. RESULTS AND DISCUSSION

A. Pt/Ti thin film characterization

1) Surface morphology of Pt/Ti thin film Figure 1.a shows that the Pt thin film is uniformly

deposited on the SiO2 surface, the deposited Pt surface is smooth and no cracks are observed.

2) Film cristallinity of Pt/Ti thin film (Bottom Electrode)

The XRD patterns of Pt thin films gown on SiO2/Si substrate (Fig 1.b) showed that the Pt has a different crystalline orientations ([111], [200], [220] and [311]), but if we take into account the K X-ray, the [111] direction becomes the most predominant orientation.

B. ZnO thin film characterization

1) Surface morphology of ZnO thin film

Fig. 2.(a) and (b) show SEM images of the ZnO thin film deposited on Pt/Ti/SiO2/Si at 300°C, which corresponds to the best (002) ZnO peaks which appear on Fig 2.(c). Figures 2.a exhibits the SEM cross-section image of the ZnO thin film, indicating that this thin film is very dense and exhibits columnar grains growth along the c-axis. The growth of ZnO is almost epitaxial on Pt [7]. SEM results at this optimized temperature indicate that the c-axis of grains is uniformly perpendicular to the substrate surface. The surface morphology of ZnO film on Pt presents some roughness and shows a macroporous structure of honeycomb-type morphology.

2) Film crystallinity of ZnO thin film

Fig 2.c shows XRD patterns of ZnO thin films deposited on Pt at different substrate temperatures (Ts), the ZnO polycrystalline film of Wurtzite structure grows in a preferred (002) orientation (located at 2θ=34.48°) with P63mc space group [8]. It can be noticed that the substrate temperature plays an important role on the quality improvement of ZnO thin film structure. As Ts increases from 100 to 500 °C, the (002) peak magnitude increases up to its maximum at 300 °C, than, it shows a further decrease. The film realized at 300 °C exhibits the best diffraction peak. This may be attributed to the changing trend of the bottom electrode (Pt(111)) phase orientation which appears at 2θ=39.95° (Fig 2.c). To assess the quality of the thin films, the Full Width at Half Maximum (FWHM) is calculated by fitting the (002) ZnO peak at 34.48° with the pseudo-Voigt function (Fig.2.(d)). The FWHM is related to the grain size D by the Scherrer formula [9, 10] , that is :

20 30 40 50 60 70 80

0

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2000

2 (°)In

ten

sity

Pt[111]

K

Pt[200]

K

Pt[220]

K Pt[311]

K Pt[222]

K

Pt/Ti on SiO2

Pt[200]

Pt[220]

Pt[311]

Pt[222]

Pt[111]

Figure 1. Characterization of Pt/Ti thin film deposited on SiO2/Si substrate, (a)-SEM Image of Pt layer and (b)- XRD patterns.

a)

b)

276 2013 Joint UFFC, EFTF and PFM Symposium

0

0

0 .9

. co s( )F W H M

Eq

where λ0=1.5406 Å is the CuKα wavelength and θ0 the Bragg angle corresponding to (002) ZnO peak.

The FWHM values are around 0.12°, which means a high quality of the thin films deposited by PLD [11]. The grain size is around the 67 nm. The c0 lattice constant of ZnO hexagonal unit cell is calculated from the Brrag relationship using 1/dhkl

2=4(h2+hk+k2)/3a02+l2/c0

2 [12] (h=0, k=0, l=2), c0 is close to the theoretical value which is 5.207 A° [13].

C. Electrical characterization

The representation of the input impedance magnitude in Fig.3 shows that the fundamental resonance in thickness mode vibration is observed at fr=524.5 MHz and the anti-resonance frequency appears at fa=540.9 MHz. Its magnitude at resonance is 5.5 Ω, whereas at anti-resonance, it becomes 8.36 Ω. These values which are close to each other are due to the high attenuation of the acoustic wave caused by the Silicon substrate (350 µm) and by the fact that the opposite Si face of the transducer has a rough surface. The phase representation (dashed line) shows that the resonator behaves like a capacitance just before resonance. It switches to an inductive

behavior between the resonance and the anti-resonance. Then, it becomes capacitive after the anti-resonance frequency.

By fitting the magnitude of the electrical input admittance Y=1/Z by the equation |Y|=2πfC0 we can deduce the static capacitance C0 of the device. In this case we found C0=132.28 pF. The active area of the device is about S=10.18 mm2. Therefore, the deduced thickness from this static capacitance (d=ε0εrS/C0, [14, 15]) is d=6.15 µm, where εr=9.03 is the relative permittivity of ZnO. The velocity Vp of the propagating wave is related the anti-resonance frequency by [16]:

2a

V pd

f Eq

which gives Vp=6451.34 m/s. This value is in agreement with the theoretical one (Vp=6401.9 m/s). If in addition, the density kg/m3 is considered, then the elastic stiffness can be deduced c33=Vp

2=238.07 GPa.

The effective electromechanical coupling coefficient [17], Keff, is related to the resonance and the anti-resonance frequencies by the equation:

2 1. tan ( )

2 2efff fr rf fa a

K Eq

(b)

0 20 40 60 80 100 100

200

300

400

500

0

5000

10000

15000

T (°C)

2 (°)

Inte

nsity

ZnO [002]

Pt [111]

Pt [222]ZnO [004]

100 200 300 400 500

0.11

0.12

0.13

0.14

0.15

T(°C)

FW

HM

(°)

Pt

Figure 2. Characterization of ZnO thin films deposited on Pt/Ti/SiO2/Si substrate; (a)- SEM Cross-section and (b)- SEM surface morphology of

sample realized at 300°C. (c)- XRD patterns at different temperatures and (d)- FWHM of (002) ZnO peak versus substrate temperature.

ZnO Pt/Ti SiO2

a)

b)

(c)

(a)

(d)

277 2013 Joint UFFC, EFTF and PFM Symposium

The previous values of fr and fa lead to Keff2=7.26 %. We

also define the quality factor at resonance Qr and at anti-resonance Qa by impedance phase derivation [18]:

( )2rf Zr

f f f r

Q

and ( ).

2af Za

f f fa

Q

Eq

The quality factors deduced from the phase curve in Fig.3 at resonance and anti-resonance are respectively Qr=1125.6 and Qa=856.51, which reflect a good piezoelectric response.

Finally, the piezoelectric constant e33 is linked to the electromechanical coupling coefficient and the elastic constant c33 as follow:

33 33/33eff eK c Eq

where ε33=εrε0 (ε0 is the dielectric permittivity of vacuum). The piezoelectric constant e33 is determined as 1.21 C/m2, which is closed to its value in [19].

IV. CONCLUSION

A 540 MHz BAW resonator has been fabricated as an Al/ZnO/Pt/Ti/SiO2/Si structure. The ZnO thin films of the devices were prepared by the PLD technique at different substrate temperatures, from 100 to 500 °C. The XRD characterization showed that the deposited ZnO films were c-axis (002) oriented (Wurtzite structure) and indicated that the substrate temperature of 300 °C was the optimum condition to obtain well crystallized ZnO on Pt buffer layer. The SEM characterization showed evidence of compact grains on the surface and evidence of columnar structure on the cross-section, which was uniformly perpendicular to the substrate surface. The electrical measurements showed an evident piezo-electrical response at the resonance frequency of 524.5 MHz with an anti-resonance frequency at 540.9 MHz. The values of quality factors were Qr=1125.6 and Qa=856.51 respectively. The electromechanical coupling coefficient Keff

2 was evaluated as 7.26 %, which means a good piezoelectric response. Finally, the thickness d and the longitudinal wave velocity Vp

of the ZnO slab were deduced as 6.15 µm and 6451.353 m/s respectively. The piezoelectric constant e33 was evaluated to be 1.21 C/m2.

ACKNOWLEDGMENT

The authors wish to express their gratitude to Mr. Fethi Metehri for his help in the clean room and they would like to acknowledge Mrs A. Lafane-Smaali for her contribution in elaborating the ZnO target of the PLD system.

REFERENCES [1] F.S. Hickermell, Proc. IEEE 64, 631 (1976)

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[3] F. Cracium, P. Verardi, M. Dinescu, G. Guidarelli, “Reactive pulsed laser deposition of piezoelectric and ferroelectric thin films”, Thin Solid Films. 343-344 (1999) 90-93.

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[5] Z.L. Wang, The new field of nanopiezotronics, Materials Today, 10 (No.5) (2007) 20-28.

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[7] Y. Yoshino, Y. Ushimi; H. Yamada, M. Takeuchi, “Zinc oxide piezoelectric thin films for bulk acoustic wave resonators”, (2003), Murata Manufacturing Co., Ltd., 2-26-10 Tenjin, Nagaoka-kyo, Kyoto, Japan.

[8] J. Goulon, N. Jaouen, A. Rogalev, F. Wilhelm, C. Goulon-Ginet, C. Brouder, Y. Joly, E.N Ovchinnikova and V.E Dmitrienko, “Vector part of optical activity probed with x-rays in hexagonal ZnO”, J. Phys. Condens. Matter. 19 (2007) 156201.

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[11] Y. Yoshino, M. Takeuchi, K. Inoue, T. Makino, S. Arai, T. Hata, “Control of temperature coefficient of frequency in zinc oxide thin film bulk acoustic wave resonators at various frequency ranges”, Vacuum. 66 (2002) 467-472.

[12] E. A. Irene, Electronic Materials Science, Wiley, New Jersey, 2005, p.22.

[13] E.H. Kisi and et coll., acta cristallographica C vol.45, (1989) p.1867

[14] J.G. Gualtieri, J.A. Kosinski, and A. Ballato, “Piezoelectric materials for acoustic wave applications”, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 41 (1994) 53–59.

[15] A.F. Wright, “Elastic properties of zinc-blende and wurtzite Aln, GaN and InN”, J. Appl. Phys., vol. 82 (1997) 2833–2839.

[16] R. Serhane, T. Boutkedjirt, A. Hassein-Bey, “Electromechanical Response Simulation of Film Bulk Acoustic Wave Resonator”, Proc. IEEE (2012), DOI: 10.1109/ICM.2012.6471442.

[17] S.R. Naik, J.J. Lutsky, R. Reif, C.G. Sodini, “Electromechanical Coupling Constant Extraction of Thin-Film Piezoelectric Materials using a Bulk Acoustic Wave Resonator”, IEEE UFFC. Vol.45, No.1 (1998) 257-263.

[18] K.M. Lakin, G.R. Kline and K.T. McCarron, “Hight-Q Microwave Acoustic Resonators and Filters”, IEEE TRANS. Vol.41, No.12 (1993) 2139-2146.

[19] M. Catti, Y. Noel, R. Dovesi, “Full piezoelectric tensor of wurtzite and zinc blend ZnO and ZnS by first-principles calculations”, J. Phys. Chem. Solids 64(2003) 2183-2190.

4 4.5 5 5.5 6

x 108

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)

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x 108

-15

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Figure 3. Impedance magnitude and phase of the realized BAW structure.

278 2013 Joint UFFC, EFTF and PFM Symposium