Rayleigh-mode spurious analysis for non-flat SiO2/Al/LiNbO3 structure by using FEM/SDA

The UMTS Band I and Band IV systems have a wide frequency interval 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 a good temperature coefficient of frequency (TCF). Although a SiO₂/Al/LiNbO₃ structure meets these two requirements, it also supports a certain number of unwanted spurious responses. As previously discussed by the authors, the Rayleigh-mode spurious can be suppressed by controlling the cross-sectional shape of a SiO₂ overlay deposited on resonator electrodes. Here, we analyzed the Rayleigh-mode spurious behavior of a non-flat SiO₂/Al/LiNbO₃ structure by using FEM/SDA. In this study, we applied the extended FEM/SDA to the non-flat SiO₂/Al/LiNbO₃ structure. The SiO₂ structure has a trapezoidal shape, which we developed for suppressing the spurious responses. The analysis indicates that the spurious level due to the Rayleigh-mode decreased with a decrease in the average width of the trapezoidal region. Furthermore, the results of simulation agree well with those obtained experimentally. We have analyzed the behavior of Rayleigh-mode spurious on the non-flat SiO₂/Al/LiNbO₃ structure. It was shown that the spurious level changes in both the shape and mass of SiO₂ on resonator electrodes.