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 2GHz 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 (K-2) and good temperature coefficient of frequency (TCF). An SiO2/Al/LiNbO3 structure meets these two requirements, it also supports a large electromechanical coupling factor (K-2). 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 5 degrees Y-X LiNbO3 substrate. We applied the proposed technique to the development of a miniature W-CDMA duplexer with the packaged size of 2.5mmx2.0mm. The duplexer exhibited the excellent performance.