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Analysis of Piezoelectric Boundary Acoustic Wave in Cu Electrode/Y-Cut X-Propagating LiNbO3 Substrate Structure Partially Covered With SiO2 Yiliu Wang, Student Member, IEEE, Ken-ya Hashimoto, Fellow, IEEE, Tatsuya Omori, Member, IEEE, and Masatsune Yamaguchi, Fellow, IEEE Abstract—This paper describes the existence of piezoelectric boundary acoustic wave (PBAW) propagating in a Cu electrode/Y-cut X-propagating (YX) LiNbO3 substrate structure partially covered with a SiO2 layer. In the analysis, two types of structures are taken into consideration: one is the so-called slotted structure with SiO2 pillars placed in the grating slots; the other is the so-called topped structure with SiO2 pillars placed on the top of grating electrodes. The top surface could be fully covered with an additional layer (like epoxy) to bridge the grating slots for encapsulation. Results show that SH-type PBAW begins to propagate in the slotted structure when the SiO2 thickness exceeds 0.3 wavelength. Strong electromechanical coupling factor K2 of 21%, and temperature coefficient of velocity (TCV) of −33 ppm/°C are obtained. In the topped structure, on the other hand, the boundary acoustic wave mode is not supported. Instead, the thickness resonance modes in the SiO2 pillar do exist. Comparison of the obtained results with those in the structure fully covered with the SiO2 layer indicates that, as for the PBAW mode, the slotted structure offers improved K2 but with worse TCV compared with the fully covered SiO2 structure.

I. Introduction

T

he temperature coefficient of frequency (TCF) is one of the most important factors for designing SAW devices. In general, highly piezoelectric substrate materials like LiNbO3 [1] and LiTaO3 exhibit negative and large TCF, and SiO2 is often deposited on the substrate to compensate (see Fig. 1) [2]–[5]. Recently, the SiO2/metal grating/LiNbO3 structure has received much attention because of its excellent SAW properties, namely, good TCF and large electromechanical coupling factor K2 [6], [7]. Nakai et al., discussed theoretically and experimentally how SAW properties change with the shape of SiO2 overlay (convex, concave, or flat) when Cu is chosen as a grating material [6]. The structure is also known to support the shear-horizontal (SH)-type piezoelectric boundary acoustic wave (PBAW) [8]–[12]. However, this structure brings one problem: frequency trimming is impossible. Once the PBAW

Manuscript received March 15, 2010; accepted January 1, 2011. This work is partially supported by TriQuint Semiconductor, Inc. The authors are with the Graduate School of Engineering, Chiba University, Chiba, Japan (e-mail: [email protected]). Digital Object Identifier 10.1109/TUFFC.2011.1844 0885–3010/$25.00

device is manufactured, a deviated center frequency cannot be adjusted using conventional trimming methods [13]–[15] because the PBAW properties are insensitive to a SiO2 top surface. Therefore, devices of this kind require precise manufacturing processes and suffer from larger variations in frequency than do conventional SAW devices. This paper describes the existence of PBAW in a structure in which the metal grating/piezoelectric substrate is only partially covered with SiO2. Fig. 2 shows two typical structures: (a) the SiO2 layer is only placed in the grating slots (so-called slotted structure), and (b) SiO2 is only placed on the top of the grating electrodes (so-called topped structure). Because the grating or substrate region is exposed, conventional trimming methods are readily applicable to frequency tuning, which is to use processes to add or remove material from the bottom to adjust the device’s operating frequency. Following the trimming process, one can fully cover the top surface with an additional layer (shown in Fig. 2 as the dashed box, a material with low acoustic impedance is recommended, for example, it could be epoxy [16]) to bridge the grating slots for encapsulation, namely, packaging. Because PBAW is not sensitive to the SiO2 top surface, this step should cause little variation in the PBAW propagation characteristics. In the following, we make an analysis of PBAW propagation in the structures shown in Fig. 2, and reveal how the wave characteristics change with the structural design, namely, the different allocation of the SiO2 pillars. II. Simulation and Analysis A. Simulation Steps In the simulation, YX-LiNbO3 and Cu were chosen as the semi-infinite piezoelectric substrate and the metallic grating, respectively, and Cu thickness hm was fixed at 0.2p (p = λ/2). The software SYNCO [17]was employed for the calculation of the input admittance Y(w) per period of an infinitely long IDT as a function of the relative frequency fp/VB, where VB = 4031 m/s is the slow-shear BAW velocity in YX-LiNbO3. For simplicity, the additional top bridging layer in Fig. 2 was not taken into account in the following analysis.

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Fig. 1. Cross-section of the SiO2 semi-infinite overlay structure.

Fig. 3. Effective velocities of resonance modes as a function of SiO2 thickness in the slotted structure.

TCF = TCV − α. Here, α is the thermal expansion coefficient of the substrate material in the acoustic wave propagation direction. For example, for X-crystallographic axis of LiNbO3, the value of α is about 15.4 ppm/°C [21]. B. Simulation Results and Analysis

Fig. 2. Cross-section of two kinds of SiO2 allocation structure: (a) slotted SiO2; (b) topped SiO2. Dashed box represents the additional top layer.

From the calculated IDT susceptance, the resonance frequency fr and anti-resonance frequency fa of resonance modes were determined. When two resonances are coupled, their intrinsic fr and fa were estimated by fitting the simple LCR model [18, pp. 131–137] having two acoustic  branches with the calculated Y(w). Then the electromechanical coupling factor K2 of the PBAW was evaluated using [19]

K 2 = (p f r/2f a)/ tan(p f r/2f a).

The temperature coefficient of velocity (TCV) of PBAW was estimated using [20]1

TCV = (f r

30 oC

- fr

25 oC)/(5f r 25 oC).

Note that TCF could be related to TCV according to the following equation [18, pp. 165–166]:

1 This is because the current numerical analysis takes into account only the tensor data change with temperature and omits the temperature influence on the length/width of the material.

Fig. 3 shows the effective velocity (Vr = 2frp) of resonance modes in the slotted structure shown in Fig. 2(a) as a function of the SiO2 thickness ha. In the figure, V1 (= 3766 m/s) is the shear BAW velocity in SiO2, On the other hand, V2 (= 4031 m/s) represents the slow-shear BAW velocity in YX-LiNbO3, determining whether the resonance modes are leaky or not; they are non-leaky when Va < V2, otherwise they are leaky. It is known that there are many modes in this structure, like the Rayleigh- and SH-type SAW, guided waves, or even the thickness resonance mode in the SiO2 pillar if the SiO2 thickness is relatively large. Moreover, from Fig. 3, it is quite clear that these modes couple with each other, making it quite difficult to distinguish modes from each other. In Fig. 3, if we focus only for the region in which ha/2p > 0.3, we can see one branch marked with asterisks where Vr tends to be constant (approximately 3400 m/s); the solid black circle branches represent the resonance effective velocities of other modes. Fig. 4 shows the displacement field distribution of the corresponding resonance mode, evaluated at the interface between the SiO2 pillar and Cu electrode when ha/2p = 1. It is very clear that the shear horizontal (SH) component is dominant and much greater than the shear vertical (SV) and longitudinal (L) components, and that its energy is well-confined along the boundary. Therefore, it can be concluded that this mode corresponds to the resonance of SH-type PBAW. Although some portion of energy leakage into the SiO2 could be ob-

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Fig. 5. Coupling factor K2 and temperature coefficient TCV of SH-type mode as a function of SiO2 thickness in the slotted structure.

Fig. 4. Displacement field distribution of the SH-type mode, evaluated at the interface between the SiO2 pillar and Cu electrode at x1 = 0.25p, with ha = 2p. In the figure, dash-dotted, solid, and dashed lines are for the shear vertical (SV), longitudinal (L), and shear horizontal (SH) components, respectively.

served in Fig. 4, it is believed to be caused by the mode coupling of this SH-type PBAW with other modes (e.g., thickness resonance in the pillar); this energy leakage may be well suppressed by sandblasting the top surface of the SiO2 pillar [22]. Fig. 5 shows the estimated K2 and TCV of this SHtype mode. With an increase in ha, K2 decreases and TCV increases at first and then both become almost constant when ha/2p > 0.3. Their converged values of 21% and −33 ppm/°C might be those of SH-type PBAW. In [8], numerical calculation of the semi-infinite covered SiO2/Cu grating/ YX-LiNbO3 was done. Comparison of the slotted structure and the semi-infinite SiO2 structure, with Cu grating thickness being fixed at 0.2p, shows that SH-type PBAW K2 of slotted structure is 21%, which is greater than that of semi-infinite SiO2 structure (17%). Meanwhile, SH-type PBAW TCV of semi-infinite SiO2 structure is −2 ppm/°C, which is preferable to the −33 ppm/°C of the slotted structure. This reveals the trade-off relationship between the TCV and K2 characteristics in the SH-type PBAW, which could be understood by analogy to the SAW characteristics of the structure with SiO2 deposition [23]. For comparison, the same analysis was made on the topped structure shown in Fig. 2(b). Fig. 6 shows the calculated effective velocities, where V1 and V2 are the SiO2 shear wave and LiNbO3 slow shear wave velocities. In Fig. 6, there are four different main modes. The first two kinds represented by white triangle and solid square marks will not appear until the SiO2 thickness hb/2p is greater than

Fig. 6. Effective velocities of resonance frequency as a function of SiO2 thickness in the topped structure.

0.2; also these two modes are leaky in most of its existence region. Therefore, these two modes are of little interest. The remaining two modes have velocities that decrease with increasing the SiO2 thickness hb. Furthermore, they are non-leaky because their velocity is lower than V2. The asterisk mode has very small coupling factor (with resonance and anti-resonance frequency almost the same with each other), whereas the white circle mode has greater coupling factor. Hence, this white circle mode will be analyzed in the following. Fig. 7 shows the displacement field distribution of the resonance mode depicted in Fig. 6 as the white circle, evaluated at the Cu electrode edge when hb/2p = 0.5. It is very clear that, quite similar to Fig. 4, the SH component is the most dominant and even greater than those of the SV and L components. The only difference is that its energy is not confined along the boundary at all; on the contrary, the greatest displacement ap-

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LiNbO3 surface, but the resonance mode within the SiO2 pillar. This was confirmed by the calculation of the field distribution. III. Conclusions

Fig. 7. Displacement field distribution of the SH-type mode, evaluated at the Cu electrode edge at x1 = 0.25p, with ha = p. In the figure, dashdotted, solid, and dashed lines are for the shear vertical (SV), longitudinal (L), and shear horizontal (SH) components, respectively.

In this paper, we showed the existence of PBAW in structures in which the SiO2 only partially covers the metal grating/piezoelectric substrate. In the simulation, Cu and YX-LiNbO3 were selected as grating and substrate materials, respectively. Two kinds of partially covered structures, slotted SiO2 between the gratings and topped SiO2 on top of the gratings, were discussed. Their characteristics were also compared with the structure in which substrate and electrode are fully covered with SiO2 overlay. The result showed that SH-type PBAW propagates in the slotted structure. With SiO2 thickness greater than 0.3 wavelength, an electromechanical coupling factor K2 of approximately 21% together with a TCV of −33 ppm/°C can be obtained. On the other hand, the PBAW mode is not supported in the topped SiO2 structure. It is clear that temperature coefficient can be further improved by depositing SiO2 on the grating electrode as well, although thicker SiO2 results in better temperature coefficient and reduced coupling factor K2. Acknowledgments It is our pleasure to thank Dr. B. P. Abbott of TriQuint Semiconductor, Inc. for his fruitful suggestions and discussions. References

Fig. 8. Coupling factor K2 and temperature coefficient TCV of SH-type mode as a function of SiO2 thickness in the topped structure.

pears at the top of the SiO2 pillar. Therefore, it can be concluded that this mode cannot be the resonance of SH-type PBAW, but the resonance mode within the SiO2 pillar. Further investigation of this mode will be shown in the following. Fig. 8 shows the estimated K2 and TCV of the nonleaky mode (depicted with white circles in Fig. 6) with relatively strong coupling. With an increase in hb, K2 increases in the beginning, and then decreases monotonically. On the other hand, TCV is improved, and if hb/2p continues to increase, the TCV may approach approximately +85 ppm/°C, which corresponds to the TCV of shear BAWs in SiO2 [24]–[26]. These properties indicate that the mode is not PBAW propagating along the YX-

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Yiliu Wang (S’08) was born in 1982 in Jiangsu, China. He received his B.S. degree in electrical engineering in 2003 from Tsinghua University and his M.S. degree in signal and information process from the Institute of Acoustics, Chinese Academy of Sciences, in 2006. In March, 2010, he graduated from Chiba University with a Dr. Eng. degree; his research focused on the numerical simulation of piezoelectric boundary acoustic waves and BAW duplexer nonlinearity evaluation. He is now working on RF SAW/BAW devices in Nihon Dempa Kogyo.

615 Ken-ya Hashimoto (M’83–SM’01–F’05) was born in Fukushima, Japan, on March 2, 1956. He received his B.S. and M.S. degrees in electrical engineering in 1978 and 1980, respectively, from Chiba University, Japan, and his Dr. Eng. degree from Tokyo Institute of Technology, Japan, in 1989. In 1980, he joined Chiba University as a Research Associate, and is now a Professor of the university. In 1998, he was a Visiting Professor at Helsinki University of Technology, Finland. In the winter of 1998/1999, he was a Visiting Scientist of the Laboratoire de Physique et Metrologie des Oscillateurs, CNRS, France. In 1999 and 2001, he was a Visiting Professor at the Johannes Kepler University of Linz, Austria. In 2001, he served as a guest co-editor of the Special Issue on Microwave Acoustic Wave Devices for Wireless Communications of the IEEE Transactions on Microwave Theory and Techniques. He also served as a publicity co-chair of the 2002 IEEE International Ultrasonics Symposium. He was appointed to a member of the speaker’s bureau of the IEEE MTT Society. He served as an International Distinguished Lecturer of the IEEE UFFC Society during the term between July 2005 and December 2006. He has also served as a Distinguished Lecturer of IEEE Electron Device Society since 2007. His current research interests include simulation and design of various high-performance surface and bulk acoustic wave devices, acoustic wave sensors and actuators, piezoelectric materials, and RF circuit design. Dr. Hashimoto is a Member of the Institute of Electronics, Information and Communication Engineers of Japan, the Institute of Electrical Engineers of Japan, and the Acoustical Society of Japan.

Tatsuya Omori (M’98) was born on April 1, 1967. He received his B.Eng. degree in electronics engineering in 1989 from Kogakuin University, Tokyo, and M. Eng. degree and Ph.D. (Eng.) degree in 1991 and 1994, respectively, from Chiba University. He joined Fujikura Co. Ltd., Sakura, in 1994 and he engaged in research and development of optical fiber cable and leaky coaxial cable. He has been employed at Chiba University as a research associate since 1998. His current research includes optical fiber sensors, SAW device design, and preparation of piezoelectric thin films and their applications. Dr. Omori is a Member of the Institute of Electronics, Information and Communication Engineers of Japan, and the Institute of Electrical Engineers of Japan.

Masatsune Yamaguchi (SM’84–F’01) was born in 1944 in Nagoya, Japan. He received his B.S. degree in electrical engineering in 1967 from the Nagoya Institute of Technology, and M.S. and Dr. Eng. degrees, both in electrical engineering, in 1969 and 1972, respectively, from the Tokyo Institute of Technology. He joined Chiba University, Chiba, Japan, as research assistant in 1972 and was promoted to be associate professor in 1975. From 1980 to 1982, he worked as research assistant in the Department of Engineering Science, Oxford University, England. After he became professor of Electrical and Electronics Engineering of Chiba University in 1987, he also served as director of the university computer center (1992–1996) and university library (1996–1998), and dean of the faculty of engineering (1998–2002). He also served as a member of university council (1995–2002) and university management council (2005–2006). He retired Chiba University in 2010 and is now professor emeritus. He has worked on the propagation of acoustic waves, signal processing and sensor devices based on microwave acoustics, and the preparation of piezoelectric and magnetostrictive films. Dr. Yamaguchi is a fellow of the Institute of Electronics, Information and Communication Engineers of Japan, and a member of the Institute of Electrical Engineers of Japan, the Acoustical Society of Japan, and the Royal Institution (GB).