Characteristics of ZnO thin film surface acoustic wave devices fabricated using nanocrystalline diamond film on silicon substrates

Transcription

1 Available online at Diamond & Related Materials 17 (2008) Characteristics of ZnO thin film surface acoustic wave devices fabricated using nanocrystalline diamond film on silicon substrates Wen-Ching Shih a,, Mao-Jin Wang a, I. Nan Lin b a Graduate Program in Electro-Optical Engineering, Tatung University, No. 40, Sec. 3, Chungshan North Road, Taipei 104, Taiwan, ROC b Department of Physics, TamKang University, Taipei 251, Taiwan, ROC Received 16 August 2007; received in revised form 17 January 2008; accepted 23 January 2008 Available online 8 February 2008 Abstract The propagation characteristics of surface acoustic wave for the ZnO piezoelectric films and nanocrystalline diamond (NCD) films multilayer SAW devices on Si substrates were investigated. High surface acoustic wave velocity is achieved for a ZnO/NCD/Si multilayer structure excited by a bottomelectroded device configuration. The NCD films deposited on Si substrates (NCD/Si) not only possess smooth surface, but also show good compatibility with ZnO materials, such that [002] textured ZnO films can be directly grown on NCD/Si substrates without the necessity of using buffer layer. The surface acoustic wave velocity increased with the thickness of the NCD films. For the thickness of the ZnO and NCD films investigated, the 0th mode surface acoustic velocity in IDT/ZnO/NCD/Si structure achieves 5100 m/s, whereas the 1st mode SAW in the ZnO/IDT/NCD/Si structure reaches 8500 m/s Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond; Surface acoustic wave devices; High frequency electronics; Sputtering 1. Introduction Surface acoustic wave (SAW) devices possess marvelous frequency response characteristics, such as high band selectivity and low insertion loss for transmitted band. However, SAW devices fabricated on conventional piezoelectric materials such as quartz, LiTaO 3 and LiNbO 3, are limited in operation frequency due to low SAW velocity ( m/s) of the substrate materials. Such a deficiency can be overcome by using a layered structure consisting of piezoelectric thin films on a high SAW velocity substrates, such as ZnO/sapphire, which exhibits a high SAW velocity of 5000 m/s [1,2]. Even higher frequency layer SAW devices based on ZnO/diamond/Si structure have been successfully reported [4 9]. Diamond materials possess the highest SAW propagation velocity, 11,000 m/s and have great potential for fabricating high frequency layer SAW devices [3]. However, the polycrystalline diamond layers grown by hot filament CVD process usually possess very large grains and rough surface, which requires Corresponding author. Tel.: x ; fax: address: wcshih@ttu.edu.tw (W.-C. Shih). tremendous amount of mechanical polishing to improve the surface smoothness to a level compatible with the fabrication process for the SAW devices (Ra ~ 5 nm). Such a post-polishing process is time consuming and expensive. On the other hand, nanocrystalline diamond (NCD) films possessing nano-scaled diamond grains is advantageous over conventional diamond films with micron-sized grains in the very smooth surface, which is suitable for fabricating the SAW devices directly without the necessity of post-polishing process. Recently, they have been utilized for making the layer SAW devices and demonstrated high surface acoustic wave velocity [10 12]. In this paper, we explore systematically the effect of the characteristics of ZnO and NCD layers on the frequency response of the layer SAW devices with ZnO/NCD/Si structure. How the electrode configuration influence the performance of such a layer structure was also investigated and the related mechanism was explored. 2. Experimental The (100) oriented Si wafer was used as the substrate for fabricating the layer surface acoustic wave (SAW) devices. The /$ - see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.diamond

2 W.-C. Shih et al. / Diamond & Related Materials 17 (2008) substrates were cleaned by a RCA process, followed by the growth of nanocrystalline diamond (NCD) films using microwave plasma enhanced chemical vapor deposition (MPECVD) process (ASTeX, 2.45 GHz, 1.5 kw). The details of NCD film synthesis process were reported elsewhere [13]. The ZnO films were then sputter-coated on the NCD/Si substrates to serve as the piezoelectric films with the detailed sputtering conditions listed in Table 1. The structure of NCD was characteristics by using Raman spectroscope (RENISHAW, 514 nm Ar laser excitation). The plan-view and cross-sectional morphology of the films were examined by using field emission scanning electron microscope (FE-SEM, Jeol JSM-6500F) whereas the surface roughness was estimated using the atomic force microscopic micrograph (AFM, Digital Instruments). For SAW device fabrication, the interdigital transducers (IDT) with 5 μm line-width and line-to-line spacing were fabricated by using a lift-off technique in conventional photolithographic process. The structure of the SAW device is schematically illustrated in Fig. 1, where the top-electroded configuration (IDT/ZnO/NCD/Si) was used for exciting the 0th mode and the bottom-electrode one (ZnO/IDT/NCD/Si) was used for exciting the 1st mode surface acoustic wave for the considered normalized thickness of ZnO and NCD layers, which will be discussed in detail. 3. Results and discussion Table 1 Deposition parameters of ZnO film by RF magnetron sputtering Deposition parameters of ZnO film Target Li-doped ZnO target Target substrate distance 43 mm Substrate temperature 380 C Gas flow ratio (Ar/O 2 ) 1 RF power 178 W Working pressure 10 mtorr Deposition time 30 min Deposition thickness 1.2 μm Fig. 1. Schematic diagram of the surface acoustic wave device with layered structure: (a) top-electroded IDT/ZnO/NCD/Si configuration and (b) bottomelectroded ZnO/IDT/NCD/Si configuration. Fig. 2a shows the typical plan-view micrographs of the diamond films deposited on Si substrates, indicating that the diamond grains are very small (b50 nm) and form clusters of sub-micron size. The cross-sectional micrograph shown in Fig. 2b illustrates that the thickness of the NCD films, about 5 μm, is rather uniform. Detailed examination using AFM (Fig. 2c) indicates that the root-mean-square (rms) surface roughness is very smooth, around 14.5 nm, which is smooth enough for growing the piezoelectric films needed in layer SAW devices. Fig. 2d shows the typical Raman spectra obtained from the NCD films grown on silicon substrate. Broadened diamond peak is generally observed due to the decrease in grain size to nm-scale, and also to an increasing concentration of a variety of growth defects, including point defects, twins, internal stresses and sp 2 admixtures [14,15]. The peak obtained at around 1350 cm 1 is assigned as D band and had been reported by many researchers to be the characteristic of nano-sized diamond grains. Peaks at 1150 and 1480 cm 1 are affirmed to be transpolyacetylene, presenting in the grain boundaries of NCD films [16]. The other main peak usually observed in the NCD is at around 1550 cm 1 and is assigned as G band, which is again the characteristic of nano-sized diamond grains. The c-axis textured ZnO films can be directly deposited on the NCD coated Si substrates (NCD/Si) without the necessity of using any buffer layer. Fig. 3a shows that the ZnO films are [002] preferentially oriented. The full width at half maximum (FWHM) of the ZnO(002) diffraction peak is about 0.22, which indicates that the texture characteristics of the ZnO films is good enough for exciting the surface acoustic wave [17,18]. The plan-view SEM micrograph for ZnO films is shown in Fig. 3b, indicating that the grains are about 500 nm in size and are distributed uniformly, whereas the cross-sectional micrograph in Fig. 3c reveals that the ZnO films, about 1.2 μm in thickness, contain columnar grains with very good uniformity in thickness distribution. Detailed examination by AFM (Fig. 3d) reveals that the rms surface roughness is about 18.5 nm, which is very close to that of the NCD films. These results imply clearly that the morphology of the ZnO films inherited that of the NCD films. The surface of the NCD films grown by MPECVD is so smooth that the subsequently deposited ZnO films is also smooth enough for directly fabricating the interdigital transducers (IDT). No post-polishing process is required for both the NCD and the ZnO films. The ZnO films contain densely-packed and [002]-textured columnar grains, which possesses good piezoelectric properties and can excite the surface acoustic wave efficiently. However, it is not clear whether or not the presence of large proportion of

3 392 W.-C. Shih et al. / Diamond & Related Materials 17 (2008) Fig. 2. (a) The plan-view and (b) cross-section SEM micrographs, (c) the AFM micrographs and (d) the Raman spectrum of the NCD layer (5.0 μm in thickness). grain boundaries in NCD films will impose deleterious effect on the propagation behavior of surface acoustic wave. To investigate such a phenomenon, the interdigital transducer (IDT) with 5 μm in line-width and line-to-line spacing, as illustrated in Fig. 4a, is fabricated either on top of ZnO films or in between the ZnO and NCD films (cf. Fig. 1), which are designated as top-electroded and bottom-electroded layer SAW devices, respectively. Fig. 4b reveals the transmission frequency response (S 21 ) of the top-electroded layer SAW devices with IDT/ZnO (1.2 μm)/ncd (5.0 μm)/si configuration. The center frequency (f 0 ) occurs at MHz, which corresponds to a SAW velocity around 5100 m/s. The SAW velocity of these top-electroded layer SAW devices is larger when the NCD films used for fabricating the devices is thicker (open symbols, Fig. 5). To facilitate the comparison, the same IDT structure was fabricated on SiO 2 -coated silicon substrates, the ZnO (1.2 μm)/sio 2 (0.2 μm)/si. Fig. 4c indicates that the SAW velocity of such a structure can only reach 3852 m/s (f 0 =193.2 MHz), a typical value for IDT/ZnO/SiO 2 /Si layer SAW structure. Moreover, the propagation loss of the former is appreciably smaller than the latter. Higher SAW velocity for the ZnO/NCD/Si substrates, as compared with the ZnO/SiO 2 /Si ones, is apparently resulted from the high SAW velocity characteristics of the NCD films. However, the SAW velocity thus obtained is still pronouncedly lower than the acoustic wave velocity reported for the diamond materials ((V SAW ) diamond =11,000 m/s) [3]. The probable explanation for such a phenomenon is that, for the considered thickness of ZnO and NCD layers, the surface acoustic wave excited in top-electroded device configuration is 0th mode. This assumption is based on the calculation reported earlier [19]. According to the report, phase velocity of the 0th mode surface acoustic wave is markedly smaller than that of the 1st mode SAW (around 50% 60%) when the kh ZnO is small (b1.5), where H ZnO is the thickness of the ZnO layer and k=2π/λ; λ is the wavelength of surface acoustic wave under investigation. Moreover, the calculation indicated that for topelectroded configuration, the electro-mechanical coupling coefficient (K 2 ) of the 0th mode SAW is larger than that of the 1st mode SAW for kh ZnO b1.0. Therefore, the 0th mode SAW is excited due to larger K 2 -value, resulting in smaller SAW phase velocity, for the considered thickness of ZnO and NCD layers used for fabricating the device. To increase the phase velocity of the ZnO/NCD/Si layer SAW, it is apparently necessary to excite the 1st mode SAW rather than the 0th mode one. The possible route for exciting the 1st mode surface acoustic wave is, according to Nakahata's calculation [19], either to increase the kh ZnO -value over 1.5 or

4 W.-C. Shih et al. / Diamond & Related Materials 17 (2008) Fig. 3. (a) XRD patterns, (b) the plan-view and (c) cross-section SEM micrographs and (d) AFM micrographs of the ZnO thin films (1.2 μm in thickness) deposited on NCD (5.0 μm in thickness)/si substrate. to use bottom-electroded configuration (ZnO/IDT/NCD/Si) for the SAW devices. We adopted the 2nd approach. The bottomelectrode layer SAW structure (ZnO/IDT/NCD/Si, cf. Fig. 1b) was fabricated using the same electrode pattern. Fig. 4d illustrates that the central frequency of the layer SAW device thus made increases pronouncedly to MHz, which corresponds to a SAW velocity of 8500 m/s. Again, the SAW velocity is larger for the device fabricated on the thicker NCD film, as illustrated in Fig. 5 (solid symbols). The phenomena that 0th mode surface acoustic wave was excited for the top-electroded layer SAW device configuration, whereas 1st mode surface acoustic wave was excited for the bottom-electroded layer SAW device configuration is in accord with the theoretical calculation [19]. However, the SAW velocity deduced from the NCD-based layer SAW devices is still much smaller than those derived from the polycrystalline diamond-based devices [19], not mentioned is the theoretical SAW velocity for single crystalline diamond materials [3]. It seems that low acoustic wave propagation characteristics of the underlying materials, the Si substrates, has imposed pronounced effect on the effective acoustic wave velocity of the layer SAW devices, as the thickness of ZnO and NCD layers are smaller than the wavelength of the surface acoustic wave excited. Moreover, the presence of large proportion of grain boundaries, which is presumed to be the mixture of sp 2 and sp 3 C C bonds [20], might also retard pronouncedly the propagation of surface acoustic wave. Such an argument is supported by the phenomenon that higher SAW velocity was obtained if the thickness of the NCD films used for fabricating the layer SAW is thicker (cf. Fig. 5). Such a phenomenon implies that, when the wavelength of the surface acoustic wave is small enough in high frequency devices, the surface acoustic wave can be further increased. Further works on the possibility of utilizing the full velocity of NCD films for layer SAW devices are on going. The other silent feature inferred in the utilization of NCD as layer SAW structure is that the other kind of inexpensive substrates such as quartz or amorphous SiO 2 can be used as substrates for synthesizing the high frequency layer SAW devices, as the NCD films can be grown practically on any substrates at relatively low deposition temperature, ~ C. 4. Conclusions Characteristics of surface acoustic wave (SAW) propagation on a layer structured device, including ZnO and nanocrystalline diamond (NCD) films was investigated. The surface of NCD films is so smooth (~14.5 nm, rms) that the (002)-textured ZnO films with small surface roughness (~18.5 nm, rms) can be

5 394 W.-C. Shih et al. / Diamond & Related Materials 17 (2008) Fig. 4. (a) Typical optical micrograph of the aluminum interdigital electrodes (IDT), and the transmission frequency response characteristics (S 21 ) of the top-electroded layer SAW devices with the configurations of (b) IDT/ZnO (1.2 µm)/ncd (5.0 µm)/si and (c) IDT/ZnO (1.2 µm)/sio 2 (0.2 µm)/si; and (d) those of the bottomelectroded layer SAW devices with the configurations of ZnO (1.2 µm)/idt/ncd (5.0 µm)/si. deposited directly on the NCD films, without the necessity of using any buffer layer or post-polishing treatment. The SAW velocity estimated from the layer SAW structure is 5100 m/s for 0th mode and is 8500 m/s for 1st mode surface acoustic wave, which are markedly larger than that of the ZnO films directly deposited on SiO 2 /Si substrates. Acknowledgments This work was sponsored by the National Science Council under the contrast number of NSC E The authors deeply appreciated the financial support. References Fig. 5. Variation of the SAW velocity with the thickness of NCD films in topelectroded layer SAW devices with the IDT/ZnO/NCD/Si configurations (open symbols) and bottom-electroded layer SAW devices with the ZnO/IDT/NCD/Si configurations (closed symbols). [1] Nuri W. Emanetoglu, George Patounakis, Shaohua Liang, Chandrasekhar R. Gorla, Richard Wittstruck, Yicheng Lu, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 48 (2001) [2] Jin-Bock Lee, Myung-Ho Lee, Chang-Kyun Park, Jin-Seok Park, Thin Solid Films (2004) 296. [3] K. Yamanouchi, N. Sarurai, T. Satoh, Proceedings IEEE Ultrasonics Symposium, 1989, p [4] H. Nakahata, A. Hachigo, S. Shikata, N. Fujimori, Proceedings IEEE Ultrasonics Symposium, 1992, p [5] S. Shikata, H. Nakahata, K. Higaki, A. Hachigo, N. Fujimori, Y. Yamamoto, N. Sakairi, Y. Takahashi, Proceedings IEEE Ultrasonics Symposium, 1993, p [6] H. Nakahata, K. Higaki, A. Hachigo, S. Shikata, N. Fujimori, Y. Takahashi, T. Kajihara, Y. Yamamoto, Jpn. J. Appl. Phys. 33 (1994) 324. [7] L. Le Brizoual, T. Lamara, F. Sarry, M. Belmahi, O. Elmazria, J. Bougdira, M. Remy, P. Alnot, Phys. Status Solidi 202 (2005) [8] S. Fujii, Y. Seki, K. Yoshida, H. Nakahata, K. Higaki, H. Kitabayashi, S. Shikata, Proceedings IEEE Ultrasonics Symposium, 1997, p [9] H. Nakahata, K. Higahi, S. Fujii, A. Hachigo, H. Kitabayashi, K. Tanabe, Y. Seki, S. Shikata, Proceedings IEEE Ultrasonics Symposium, 1995, p. 361.

6 W.-C. Shih et al. / Diamond & Related Materials 17 (2008) [10] B. Bi, W.S. Huang, J. Asmussen, B. Golding, Diamond Relat. Mater. 11 (2002) 677. [11] F. Benedic, M.B. Assouar, F. Mohasseb, O. Elmazria, P. Alnot, A. Gicquel, Diamond Relat. Mater. 13 (2004) 347. [12] O. Elmazria, F. Benedic, M. El Hakiki, H. Moubchir, M.B. Assouar, F. Silva, Diamond Relat. Mater. 15 (2006) 193. [13] Y. Liu, C. Liu, Y. Chen, Y. Tzeng, P. Tso, I. Lin, Diamond Relat. Mater. 13 (2004) 671. [14] K. Fabisiak, M. Maar-Stumm, E. Blank, Diamond Relat. Mater. 2 (1993) 722. [15] M. Hiramatsu, C.H. Lau, A. Bennett, J.S. Bokova, N. Salk, B. Gunther, Thin Solid Films 407 (2003) 18. [16] A.C. Ferrari, J. Robertson, Phys. Rev. B 63 (2001) [17] W.C. Shih, M.S. Wu, J. Cryst. Growth 137 (1994) 319. [18] M.S. Wu, W.C. Shih, W.H. Tsai, J. Phys., D, Appl. Phys. 31 (1998) 943. [19] H. Nakahata, A. Hachigo, K. Higaki, et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Control 42 (1995) 362. [20] P. Keblinski, D. Wolf, F. Cleri, S.R. Phillpot, H. Gleiter, MRS Bulletin (1998) 36 (September).