Zinc oxide (ZnO), a semiconducting, photoconducting,

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1 2308 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 52, no. 12, december 2005 The Effects of ZnO Films on Surface Acoustic Wave Properties of Modified Lead Titanate Ceramic Substrates Sheng-Yuan Chu, Member, IEEE, Te-Yi Chen, and Walter Water Abstract Poly-crystal zinc oxide (ZnO) films with c-axis (002) orientation have been successfully grown on the strontium (Sr) modified lead titanate ceramic substrates with different Sr dopants by r.f. magnetron sputtering technique. Highly oriented ZnO films with c-axis normal to the substrates can be obtained under a total pressure of 10 mtorr containing 50% argon and 50% oxygen and r.f. power of 70 W for 3 hours. Crystalline structures of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of surface acoustic wave (SAW) devices with ZnO/IDT/PT (IDT, inter-digital transducer; PT, PbTiO 3 ceramics) structure were investigated. The devices with ZnO/IDT/PT structure shows that the ZnO film effectively raise the electromechanical coupling coefficient (k 2 ) from 3.8% to 9.9% of the device with the concentrations of Sr dopants of It also improves the temperature coefficient of frequency of SAW devices. I. Introduction Zinc oxide (ZnO), a semiconducting, photoconducting, piezoelectric, and optical waveguide material, shows a wide range of scientific and technological applications [1]. It belongs to a group of the hexagonal wurtzite, 6-mm symmetry. It is an n-type wide-bandgap semiconductor material and has a variety of potential applications [2], [3]. Zinc oxide films also have widely been used in the field of surface acoustic wave (SAW) devices and bulk acoustic devices due to its excellent piezoelectricity [4], [5], substantial electromechanical coupling coefficient, and good temperature stability. Zinc oxide films can be deposited by a variety of deposition techniques, such as sol-gel process [6], spray pyrolysis [7], molecular beam deposition (MBE) [8], chemical vapor deposition (CVD) [9], and sputtering [10] [14]. The most commonly used technique is the sputtering deposition method because it is possible to obtain good orientation and uniform films close to single-crystal mor- Manuscript received May 31, 2004; accepted April 19, The National Science Council of the Republic of China supported this research, under Grant No. NSC E The authors also acknowledge support from the Center for Micro-Nano Technology, National Cheng Kung University. S.-Y. Chu and T.-Y. Chen are with the Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan ( chusy@mail.ncku.edu.tw). W. Water is with the Department of Electrical Engineering, Tung Nan Institute of Technology, Taipei, Taiwan. phology, even on amorphous substrate or at low substrate temperature. For ZnO film SAW device applications, the control of the (002) preferred orientation of ZnO films is important because the acoustic velocity along the c-axis of ZnO is the fastest and its good piezoelectricity. It leads the device to obtain higher center frequency and larger electromechanical coupling coefficient. In previous work, we successfully prepared the oriented ZnO films with the c-axis (002) normal to the substrate on single crystal substrates, such as Si and quartz [15], [16]. We also have successfully deposited ZnO films with c-axis-preferred orientation on the ceramic substrates with preferred deposition condition [17]. Lead titanate is a perovskite-type ferroelectric material with a Curie temperature of 490 C; it is expected to be a useful material for piezoelectric applications [18]. However, pure lead titanate ceramics are very difficult to be sintered because of their large lattice anisotropy (c/a = 1.064). On cooling through Curie temperature, the large anisotropy of ceramic material becomes fragile. In addition, it is difficult to pole the ceramics with low resistivity ( Ω cm). We have resolved these problems by using several dopants to achieve modified lead titanate ceramics and retain good piezoelectric properties [19]. Since the 1970s, the applications of piezoelectric ceramics for SAW devices have been investigated [20] [26]. The modified lead titanate piezoelectric ceramics have potential for SAW device applications due to the ability to modify the composition to achieve a desirable combination of properties, such as high electromechanical coupling coefficient. We have reported that the Sr-doped modified lead titanate ceramic is a good potential material with high electromechanical coupling coefficient for SAW device applications [27]. Nakamura and Hanaoka [28] reported that ZnO films effectively increased the electromechanical coupling coefficient (k 2 ) of the SAW device with ZnO/IDT/lithium niobate (LiNbO 3 ) structure from 5% to 13.3%. However, none reportthe growthof ZnO films on the piezoelectric ceramic substrate that is the motivation of this research. In this paper, we continue our previous work [17] and prepare the Sr-modified PbTiO 3 (PT) system ceramics with composition (Pb 0.88 x Sr x Sm 0.08 )(Ti 0.98 Mn 0.02 )O 3 ; x = as the substrates, and fabricate SAW devices with ZnO/IDT/PT structure. Then, the SAW properties such as phase velocity, electromechanical coupling coefficient /$20.00 c 2005 IEEE

2 chu et al.: piezoelectric ceramics and saw devices 2309 Fig. 1. The X-ray diffraction patterns of ZnO films deposited on (Pb 0.73 Sr 0.15 Sm 0.08 )(Ti 0.98 Mn 0.02 )O 3 ceramic substrate. and temperature coefficient of frequency were investigated. II. Experimental Procedure A. Fabrication of Surface Acoustic Wave Devices The modified lead titanate powders with composition of (Pb 0.88 x Sr x Sm 0.08 )(Ti 0.98 Mn 0.02 )O 3 ; x = were prepared by a conventional mixed oxide method. Raw materials were mixed by pure reagent PbO, TiO 2,Sm 2 O 3, SrCO 3 and MnO 2 powders (>99.0% purity). The powders were calcined at 900 C for 2 hours, and excess PbO was added to counteract the volatilization of PbO during firing. After that, the powders were dried and milled with 8 wt% of a 5% PVA solution. Then, the powders were pressed into plates with dimensions: mm 3 for SAW measurements, and discs of 10 mm diameter and 0.8 mm thickness for bulk measurements, using a pressure of 25 kg/cm 2. Specimens were sintered isothermally at a heating rate of 10 C/minute at 1200 C for 2 hours. In order to fabricate the SAW devices, the plates were polished to a mirror finish on one side with surface roughness below 0.1 µm. Then, aluminum electrode patterns, 0.3 µm thick in the form of interdigital transducers (IDTs), were applied to the polished surface using the lift-off photolithographic process. The electrode finger pairs of IDT pattern are 15.5, the electrode overlap is 4 mm, and the delay-line distance is 1.6 mm. The IDT pattern of 20 µm width leads to a wavelength of 80 µm. B. Deposition of ZnO Films The ZnO films were deposited by r.f. magnetron sputtering system using sputtering power of 70 W and argonoxygen gas flow ration of 50% 50% with a sputtering pressure of 10 mtorr. The Li-doped ZnO target of 3 inches diameter was prepared by adding 1.5 mol% Li 2 CO 3 (99.99%) into the ZnO (99.9%) and firing at 900 C for 3 hours. Li-atom doping of ZnO involves their substitution for Zn atoms; they act as acceptors that compensate the donors (excess Zn atoms) to increase resistivity of ZnO films. The substrates were thoroughly cleaned with organic solvents and dried before loading in the sputtering system. The chamber was down to torr using a diffusion pump before introducing the premixed Ar and O 2 sputtering gases into the chamber through a precision leak valve and controlled by the main valve of diffusion pump. Throughout all experiments, the target was presputtered for 15 minutes under 100 W r.f. power before the actual deposition began to delete any contamination on the target surface to make the system stable and reach optimum condition. The electronic mass flow controller controlled the ratios of argon to oxygen. The substrate temperature ( 100 C) was monitored using a thermo-coupler attached near the substrate. The deposition rate of ZnO film using the above sputtering parameters is 0.5 µm per hour. The thickness of ZnO film reaches 1.5 µm after3hours deposition. C. Electrical and Acoustic Properties Measurement The bulk densities of the sintered bodies were measured by the Archimedes method. In order to measure the electrical properties, the samples were electroded then poled in a heated oil bath at 150 C with a field of 50 kv/cm for 15 minutes. The dielectric and piezoelectric properties were measured by using an impedance analyzer (HP4294A, Hewlett-Packard, Palo Alto, CA). Piezoelectric properties were calculated from the resonance measurement method

3 2310 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 52, no. 12, december 2005 TABLE I The Parameters Derived from XRD Analysis of ZnO Films. FWHM Grain size c Stress (degree) (nm) (nm) (dyne/cm 2 ) [29]. The Curie temperature was calculated by measuring the dielectric behavior as a function of temperature using an impedance analyzer (HP4192, Hewlett-Packard, Palo Alto, CA). The dependence between sputtering conditions and the physical structure of the films (crystalline structure and microstructure) were investigated by X-ray diffraction (XRD, CuK α radiation λ = Å), scanning electron microscopy (SEM, Hitachi-S KeV) and atomic force microscopy (AFM, DI NS3a-MMAFM with a DI NS3a controller). Assuming a homogeneous strain across the films, the grain size of film may be calculated from the full width at half maximum (FWHM) of the (002) peak by the Sherrer equation: D =(0.9λ/β cos θ), where λ( nm) is the X-ray wavelength and β is the FWHM in radians [30]. The c-axis lattice constant can be obtained by the formula: 2d sin θ = nλ, andthe biaxial stress was calculated from the parameter c : σ = [c c 0 /c 0 ], where c 0 ( nm) is the strain-free lattice constant [31]. The frequency response of the SAW device was measured by using a network analyzer (HP 8714ES, Hewlett- Packard, Palo Alto, CA). The experimental phase velocity was obtained from the equation v = f 0 λ, where f 0 is center frequency and λ is the wavelength. The experimental k 2 was obtained from the equation [32] k 2 = (π/4n G a /B) f=f0,wheren is the number of IDT fingers, and G a and B are radiation resistance and susceptance at the center frequency, respectively. The temperature coefficient of frequency (TCF) was determined from measurements of the shift of center frequency as temperature from 25 C to 80 C, using the following equation TCF = ( 1/f 0(25 C)) ( f0(80 C) f 0(25 C)) /(80 25). III. Results and Discussion A. XRD, SEM, and AFM Analysis of ZnO Films Fig. 1 shows the XRD patterns of ZnO film deposited on PT ceramic (x =0.15) with strong c-axis (002) orientation. X-ray examination of piezoelectric films has been a major tool for determining the uniformity of crystalline structure [33]. The higher degree of orientation of a ZnO film, the higher electromechanical coupling coefficient of the SAW device will be obtained [34]. The parameters of ZnO films that derived from XRD analysis are shown in Table I. From the (002) diffraction peaks, the c-axis length is estimated at nm. It is almost the same as that of ZnO single crystal, nm, which indicates that the ZnO films have high-quality crystallinity. The other peaks belong to the PT-ceramic substrates, and all of them belong to the perovskite phase. Surface morphology of the preferred oriented ZnO films were investigated by scanning electron microscopy and atomic force microscopy techniques. Fig. 2 shows the SEM top view and cross-section view of ZnO film deposited on PT-ceramic (x =0.15). The structure of Fig. 2(b), which looks columnar, was proved to have a preferred c-axis orientation. The AFM photograph of ZnO film is shown in Fig. 3. The analysis from AFM illustrates that the average roughness of the ZnO film grown on PT ceramic is about 93 Å. B. Piezoelectric and Dielectric Properties of Ceramic Substrates The piezoelectric and dielectric properties of our modified PT ceramic substrate are shown in Table II. The densities of the sintered substrates are high and larger than 95% theoretical value. It shows that the dielectric constant (ε r ) increases correspondingly with the increase of Sr, but the loss factor (tan δ) changed little. The results of the thickness coupling factors (k t ) indicate that k t value increases at first and reaches the maximum value of at Sr = 15 mol% as the Sr additive increases. The k t values for x = are larger than It is obvious that the substrate with a high thickness electromechanical coupling coefficient (k t ) perhaps makes it with a higher surface electromechanical coupling coefficient (k 2 ) for SAW applications. The planar electromechanical coupling coefficient, k p, is about as Sr dopants changed. C. Surface Acoustic Wave Properties Fig. 4 shows the frequency response of the SAW device with ZnO/IDT/PT (x = 0.15) structure. The center frequency is MHz; it leads to a phase velocity of 2732 m/s. The insertion loss is about 18.8 db. The phase velocity as a function of the amount of Sr dopants is shown in Fig. 5. It shows that the phase velocity increases correspondingly with the increasing Sr additives from 2591 m/s to 2894 m/s for the devices with IDT/PT structure. Considering the devices with ZnO/IDT/PT structure, the ZnO films deposited on the substrates are helpful in rising the phase velocity of SAW devices till x = The ZnO films reduce the phase velocities of the SAW devices as x>0.15. It is due to the velocity of ZnO films at about 2724 m/s and lower than the PT substrates as x>0.15. Fig. 6 shows the electromechanical coupling coefficient (k 2 ) of the SAW devices versus the amount of Sr additives. The k 2 value increases at first and reaches the maximum value of 3.77% as Sr = 15 mol% with the increasing Sr dopants, then dropped for the SAW devices without ZnO films on them. The devices with ZnO/IDT/PT structure have similar results, and it shows that the ZnO film effectively raises the electromechanical coupling coefficient (k 2 ) from 3.77% to 9.86% of the device as x =0.15. According to [28], the magnitude of k 2 is highly dependent

4 chu et al.: piezoelectric ceramics and saw devices 2311 Fig. 2. The SEM photographs (a) top view (b) cross-section view of ZnO films deposited on (Pb 0.73 Sr 0.15 Sm 0.08 )(Ti 0.98 Mn 0.02 )O 3 ceramic substrate. (bar = 1 µm) TABLE II The Dielectric and Piezoelectric Properties of PT Substrates. X ε r tan δ k t k p N t (Hz-m) N p (Hz-m) Fig. 3. The AFM photographs of ZnO films deposited on (Pb 0.73 Sr 0.15 Sm 0.08 )(Ti 0.98 Mn 0.02 )O 3 ceramic substrate. on the relationship between the signs of strains generated in two piezoelectric media. If the strains of ZnO films and PT ceramics have the same sign, they contribute constructively to SAW excitation. Thus, higher coupling factors are obtained when the preferred oriented ZnO layer is on the surface of PT ceramic. Fig. 7 shows the TCF of the SAW device as a function of Sr additives. It is obvious that the Sr-doped piezoelectric substrates have negative TCF values, and its absolute values increase correspondingly with the increasing Sr additives. The ZnO films deposited on the substrates improve the TCF values of all SAW devices. The phase velocity and electromechanical coupling coefficient of our sample are higher than other commercial Pb-based ceramics [20], [24], [25] and are potentially good material for piezoelectric applications. Fig. 4. The frequency response of the SAW device with ZnO/IDT/PT (x =0.15) structure. IV. Conclusions The preferred oriented ZnO films deposited on PT ceramic substrates with c-axis normal to the surface have been demonstrated using the sputtering method. Highly oriented films with c-axis normal to the substrate can be obtained by depositing under a total pressure of 10 mtorr containing 50% argon and 50% oxygen and r.f. power of 70 W for 3 hours. The film surface examined by AFM and SEM exhibit smooth morphology and are dense, respectively. According to the experimental results of SAW properties, we showed that the preferred oriented ZnO films effectively raise the electromechanical coupling coefficient (k 2 )from3.8%to9.9%ofthedeviceasx =0.15. The ZnO films also improve the TCF values of all SAW devices. The SAW devices with ZnO/IDT/PT structure have a high

5 2312 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 52, no. 12, december 2005 electromechanical coupling coefficients that makes them suitable for broad-band SAW filter and SAW gas sensor applications (the frequency range: 10 MHz 150 MHz). References Fig. 5. The phase velocity versus amount of Sr additive for SAW devices with IDT/PT and ZnO/IDT/PT structures. Fig. 6. The electromechanical coupling coefficient versus amount of Sr additive for SAW devices with IDT/PT and ZnO/IDT/PT structures. Fig. 7. The temperature coefficient of frequency versus amount of Sr additives for SAW devices with IDT/PT and ZnO/IDT/PT structures. [1] C.M.FransandV.D.Pol, Thin-filmZnO-propertiesandapplications, Ceram. Bull., vol. 69, pp , [2] M. Rajalakshmi and A. K. Arora, Optical phonon confinement in zinc oxide nanoparticles, J. Appl. Phys., vol. 87, pp , [3] A. Onodera, N. Tamaki, K. Jin, and H. Yamashita, Ferroelectric properties in piezoelectric semiconductor Zn1-XmxO (M = Li, Mg), Jpn. J. Appl. Phys., vol. 36, pp , [4] O. Yamazaki, T. Mitsuyu, and K. Wasa, ZnO thin-film SAW devices, IEEE Trans. Sonics Ultrason., vol. 27, pp , [5] F. S. Hickernell, Zinc oxide films for acoustoelectric device applications, IEEE Trans. Sonics Ultrason., vol. 32, pp , [6] M. N. Kamalasanan and S. Chandra, Sol-gel synthesis of ZnO thin films, Thin Solid Films, vol. 288, pp , [7] F.D.Paraguay,W.L.Estrada,D.R.N.Acosta,E.Andrade, and M. M. Yoshida, Growth, structure and optical characterization of high quality ZnO thin films obtained by spray pyrolysis, Thin Solid Films, vol. 350, pp , [8] K. Nakamura, T. Shoji, and H. B. Kang, ZnO film growth on (011 over-bar 2) LiTaO 3 by electron cyclotron resonance-assisted molecular beam epitaxy and determination of its polarity, Jpn. J. Appl. Phys., vol. 39, pp , [9] T. Minami, H. Sonohara, S. Takata, and H. Sato, Transparent and conductive ZnO thin films prepared by atmospheric-pressure chemical vapor deposition using zinc acetylacetonate, Jpn. J. Appl. Phys., vol. 33, pp , [10] S. Maniv and A. Zangvil, Controlled texture of reactively RFsputtered ZnO thin films, J. Appl. Phys., vol. 49, pp , [11] M. S. Wu, W. C. Shih, and W. H. Tsai, Growth of ZnO thin films on interdigital transducer/corning 7059 glass substrate by two-step fabrication methods for surface acoustic wave applications, J. Phys. D: Appl. Phys., vol. 31, pp , [12] K. Y. Hashimoto, S. Ogawa, A. Nonoguchi, T. Omori, and M. Yamaguchi, Preparation of piezoelectric ZnO films by target facing type of sputtering method, in Proc. IEEE Ultrason. Symp., 1998, pp [13] K. B. Sundaram and A. Khan, Characterization and optimization of zinc oxide films by RF magnetron sputtering, Thin Solid Films, vol. 295, pp , [14] N. Croitoru, A. Seidman, and K. Yassin, Some physical properties of ZnO sputtered films, Thin Solid Films, vol. 150, pp , [15] W. Water and S. Y. Chu, Physical and structural properties of ZnO sputtered films, Mater. Lett., vol. 55, pp , [16] S. Y. Chu, W. Water, and J. T. Liaw, A study of love wave acoustic sensors in ZnO/quartz structure, Integr. Ferroelect., vol. 44, pp , [17] S. Y. Chu, T. Y. Chen, and W. Water, The investigation of preferred orientation ZnO growth of ZnO films on the PbTiO 3 - based ceramics and its application for SAW devices, J. Cryst. Growth, vol. 257, pp , [18] T. Takahashi, Lead titanate ceramics with large piezoelectric anisotropy and their application, Ceram. Bull., vol. 69, pp , [19]T.Y.Chen,S.Y.Chu,S.J.Wu,andY.D.Jung, Effectsof strontium on the dielectric and piezoelectric properties of Smmodified PbTiO 3 ceramics, Ferroelectrics, vol. 282, pp , [20] C. C. Tseng, Elastic surface waves on free surface and metallized surface of CdS, ZnO, and PZT-4, J. Appl. Phys., vol. 38, pp , [21] M. Kodama, H. Egami, and S. Yoshida, Fabrication of temperature stabilised piezoelectric ceramic for surface acoustic wave application, Jpn. J. Appl. Phys., vol. 14, pp , 1975.

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