1Graduate School of Engineering, Chiba University, Chiba, Chiba, Japan
2Murata MFG Co. Ltd., Nagaokakyo, Kyoto, Japan
3Murata MFG Co. Ltd., Nagaokakyo, Japan
4Taiyo Yuden Co. Ltd., Akashi, Hyogo, Japan
5Panasonic Electronic Devices Co., Ltd., Kadoma, Osaka, Japan
6NGK Insulators Ltd., Nagoya, Japan
Although SAW antenna duplexers are mass-produced and widely used in mobile phones, improvement of their temperature stability is strongly demanded without degrading the other performances. Achievable performances of SAW devices are inherently limited by the employed piezoelectric substrate. On the other hand, use of new piezoelectric materials is not desirable from industrial points of view because long years and investments are necessary to develop technologies and equipment to produce large size homogeneous wafers applicable to the mass use. Their inhomogeneity may badly influence the production yield. Thus possible solutions are limited for this request, and use of a single crystal LiNbO3 or LiTaO3 is mandatory as a member of the device structure. This paper reviews current status of research and development of temperature compensated RF SAW devices for the use in antenna duplexers. Currently two strategies are extensively studied. First one is based on deposition of a SiO2 layer on IDTs fabricated on a traditional LiNbO3/LiTaO3 substrate. Since the SiO2 layer influences not only the temperature characteristic but also the SAW excitation and reflection, the device configuration must be chosen properly. Two device structures were proposed: one employs a SiO2 film with the flattened top surface deposited on heavy electrodes, and another one employs a SiO2 film with the corrugated top surface deposited on relatively light Al electrodes. When LiNbO3 is used as a substrate, strong spurious resonances are observed, and proper structural design is necessary for their suppression. Another strategy is based on the wafer bonding of a single crystal LiNbO3/LiTaO3 with a stiff substrate having small thermal expansion coefficient. Since influence of the stiff substrate is small except the temperature stability, we can directly apply design and production tools established for conventional SAW devices using a LiNbO3/LiTaO3 wafer to the current purpose. After tight wafer bonding, LiNbO3/LiTaO3 should be thinned as much as possible to make the influence of the stiff substrate obvious. This is the most important hurdle for the device realization. Another important issue is suppression of plate modes caused by the bulk wave reflection at the boundary between the LiNbO3/LiTaO3 layer and stiff substrate. We will show what kinds of technologies are used to satisfy various W-CDMA specifications and how high performances are achieved using these technologies.