Suppression of transverse mode spurious in SAW resonators on an SiO2/Al/LiNbO3 Structure for wideband CDMA applications

The wideband CDMA (W-CDMA) system at 2 GHz has a wide duplex gap between transmitting (Tx) and receiving (Rx) bands. When developing a small-sized SAW duplexer for this application, one needs a substrate with a large electromechanical coupling factor (K²) and good temperature coefficient of frequency (TCF). An SiO₂/Al/LiNbO₃ structure meets these two requirements, it also supports a large electromechanical coupling factor (K²). However, certain number of unwanted spurious responses. They are categorized into two types; one is caused by the Rayleigh mode and the other by the transverse mode. This paper proposes a new technique to suppress the transverse mode spurious appearing in the SAW resonators on the SiO₂/Al/LiNbO₃ structure. In the technique, the dummy electrodes are only length-weighted in the form of a triangle, which selectively scatter higher-order transverse mode spurious. In addition, the SiO₂ overlay is selectively removed from the dummy electrode region for enhancing the SAW energy confinement in the active electrode region. This technique was applied to an SAW resonator fabricated on a 5deg Y-X LiNbO₃ substrate. We applied the proposed technique to the development of a miniature W-CDMA duplexer with the packaged size of 2.5 mm times 2.0 mm. The duplexer exhibited the excellent performance.