Suppression of transverse-mode spurious responses for saw resonators on SiO2/Al/LiNbO3 structure by selective removal of SiO2

A SiO₂/Al/LiNbO₃ structure has a large electromechanical coupling factor (K₂) and good temperature coefficient of frequency (TCF) for applications as a SAW duplexer of the Universal Mobile Telecommunications System (UMTS) Band I. However, the SiO₂/Al/LiNbO₃ structure also supports two unwanted spurious responses; one is caused by the Rayleigh mode and the other by the transverse mode. As the authors have previously discussed, the Rayleigh-mode spurious response can be suppressed by controlling the cross-sectional shape of a SiO₂ overlay deposited on resonator electrodes. In this paper, a new technique to suppress the transverse-mode spurious responses is proposed. In the technique, the SiO₂ overlay is selectively removed from the dummy electrode region. The spurious responses are analyzed by the laser probe system. The results indicate that the spurious responses in question were hybrid modes caused by the coupling between the main (SH) SAW and another (Rayleigh) SAW with different velocities. The hybrid-mode spurious behavior was dependent on the velocities in the IDT and the dummy regions (v_i and v_d). The hybrid-mode spurious responses could be suppressed by selectively removing SiO₂. Furthermore, the SAW energy confinement could be enhanced in the IDT electrode region when v_i <; v_d. The transverse-mode spurious responses were successfully suppressed without degrading the SAW resonator performances.