High-Electromechanical-Coupling-Coefficient Surface Acoustic Wave Resonator on Ta 2 O 5 /Al/LiNbO 3 Structure Hidekazu Nakanishi  , Hiroyuki Nakamura, and Rei Goto Panasonic Electronic Devices Co., Ltd., Kadoma, Osaka 571-8506, Japan Received November 24, 2009; accepted February 22, 2010; published online July 20, 2010 In this paper, we describe a high-electromechanical-coupling-coefficient (K 2 ) surface acoustic wave (SAW) resonator on a Ta 2 O 5 /Al/LiNbO 3 structure for wide duplex gap applications. We analyzed the responses of a SAW resonator on a Ta 2 O 5 /Al/LiNbO 3 structure by finite element method (FEM)/spectrum domain analysis (SDA). We have clarified that the optimum Ta 2 O 5 thickness at which the high-performance SAW resonator without the Rayleigh-mode spurious response could be realized. The results of the simulation were in good agreement with those of the experiment. Also, the Ta 2 O 5 film has the advantage of decreasing the film thickness over the SiO 2 film. The developed SAW resonator shows excellent characteristics, namely, a low insertion loss and a high K 2 of 23%. By applying the SAW resonator in the filters, a ladder-type filter and a longitudinally-coupled double-mode SAW (DMS) filter with a low insertion loss and a large bandwidth were realized. We have therefore shown the feasibility of applying devices with a wide duplex gap such as Band I, IV, and X duplexers and a wide pass-band filter using the Ta 2 O 5 /Al/5  YX- LiNbO 3 structure. # 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.07HD21 1. Introduction The surface acoustic wave (SAW) duplexer is a key device in mobile phones for wide-band code-division multiple- access (W-CDMA) system. Therefore, excellent character- istics, namely, a low insertion loss, a high attenuation, and a high power durability are very important for SAW duplex- ers. Furthermore, the miniaturization of the SAW duplexer is necessary as mobile phones become smaller. As a standard of the Third Generation Partnership Project (3GPP), operation bands for the universal mobile tele- communication system (UMTS) exist. Each band can be separated into three groups from the viewpoint of relative duplex gap. The first group has a moderate duplex gap (Band V). The second group has a narrow duplex gap (Bands II, III, and VIII). The third group has a wide duplex gap (Bands I, IV, and X). The performance of SAW duplexers is primarily deter- mined by the properties of the substrate material structure and the filter design. In particular, the electromechanical coupling coefficient (K 2 ) and the temperature coefficient of frequency (TCF) of the substrate are strongly related to the pass-band width and the duplex gap of SAW duplexers. Therefore, an appropriate substrate needs to be chosen with respect to these features. The Band I, IV, and X systems have a large pass-band width and a wide duplex gap. Therefore, when we design SAW duplexers using a substrate with a small K 2 , it is difficult to ensure sufficient attenuation in the rejection band. Therefore, they have a disadvantage from the viewpoint of miniaturization because they need external circuitry to ensure sufficient attenuation. Thus, to realize small SAW duplexers with a low insertion loss and a high attenuation, a substrate with a high K 2 is required. In general, the 41  YX-LiNbO 3 and 64  YX-LiNbO 3 substrates are high K 2 substrates. However, it is very difficult to realize high-performance SAW duplexers because of leaky-type SAW. Recently, many researchers have attempted several methods of obtaining a substrate with a low loss and a high K 2 . The combination of a low-cut angle Y -cut LiNbO 3 substrate and a SiO 2 film 1,2) and the combination of a 15  YX-LiNbO 3 substrate and a Cu electrode 3) for wideband applications have been reported. For SAW duplexers with a wide duplex gap, a flattened SiO 2 film/Cu electrode/ LiNbO 3 structure 4) and a shape controlled SiO 2 film/Al electrode/LiNbO 3 structure 5,6) have been reported. How- ever, the Band IV and X systems have larger duplex gaps than the Band I system. Therefore, to realize better perform- ance for SAW duplexers, a SAW resonator with a higher K 2 is strongly desired. In this study, we propose the Ta 2 O 5 /Al/5  YX-LiNbO 3 structure for wide duplex gap application. The combination of a 64  YX-LiNbO 3 , quartz or langasite substrates and a Ta 2 O 5 film has already been reported. 7–9) However, the combination of a low-cut angle LiNbO 3 substrate and a Ta 2 O 5 film has not yet been reported for wideband application. Therefore, we investigate the optimum Ta 2 O 5 thickness for realizing a high-performance SAW resonator in view of phase velocity, K 2 , insertion loss, and Rayleigh- mode spurious response for the Ta 2 O 5 /Al/5  YX-LiNbO 3 structure. The developed SAW resonator shows excellent characteristics, namely, a low insertion loss, a high attenu- ation, and a high K 2 of 23%. 2. Simulation of and Experiment on Ta 2 O 5 /Al/5  YX- LiNbO 3 Structure First, we investigated the response of SAW resonators on the Ta 2 O 5 /Al/5  YX-LiNbO 3 structure by finite element method (FEM)/spectrum domain analysis (SDA). 10–12) Figure 1 shows a cross-sectional view of the Ta 2 O 5 /Al/LiNbO 3 structure of the simulation model. In the analytical structure, the piezoelectric substrate is a 5  YX-LiNbO 3 substrate. The interdigital transducer (IDT) electrodes consist of Al. The pitch of the IDT electrodes is 1.0 mm. The metallization ratio is 0.5. The Al electrode thickness normalized by the wavelength of the acoustic wave ( ) is 0:08 . Al Ta 2 O 5 LiNbO 3 Fig. 1. Cross-sectional view of the Ta 2 O 5 /Al/LiNbO 3 structure for simulation model.  E-mail address: nakanishi.hidekazu@jp.panasonic.com Japanese Journal of Applied Physics 49 (2010) 07HD21 REGULAR PAPER 07HD21-1 # 2010 The Japan Society of Applied Physics