Suppression of transverse-mode spurious responses by selectively SiO2 removing technique for SAW resonators on a SiO2/Al/LiNbO3 structure

A SiO₂/Al/LiNbO₃ structure has a large electromechanical coupling factor (K²) and good temperature coefficient of frequency (TCF) for the UMTS Band I application. 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. On the other hand, the transverse-mode spurious responses are commonly suppressed by IDT (Interdigital Transducer) apodization, and the length weighting for dummy electrodes selectively could scatter higher order transverse-mode spurious responses. In this paper, the spurious responses of the SAW resonator on a SiO₂/Al/LiNbO₃ structure are analyzed by the laser probe system. And, 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 transverse-mode spurious behavior was dependent on the velocities (v_i and v_d) in the IDT and the dummy regions. When v_i <; v_d, the higher-order transverse-mode spurious response could be suppressed. Furthermore, the SAW energy confinement could be enhanced in the IDT electrode region by selectively SiO₂ removing. The transverse-mode spurious responses were successfully suppressed without degrading the SAW resonator performances.