High-Electromechanical-Coupling-Coefficient Surface Acoustic Wave Resonator on Ta₂O₅/Al/LiNbO₃ Structure

In this paper, we describe a high-electromechanical-coupling-coefficient (K²) surface acoustic wave (SAW) resonator on a Ta₂O₅/Al/LiNbO₃ structure for wide duplex gap applications. We analyzed the responses of a SAW resonator on a Ta₂O₅/Al/LiNbO₃ structure by finite element method (FEM)/spectrum domain analysis (SDA). We have clarified that the optimum Ta₂O₅ thickness at which the high-performance SAW resonator without the Rayleigh-mode spurious response could be realized. The results of the simulation were in good agreement with those of the experiment. Also, the Ta₂O₅ film has the advantage of decreasing the film thickness over the SiO₂ film. The developed SAW resonator shows excellent characteristics, namely, a low insertion loss and a high K² of 23%. By applying the SAW resonator in the filters, a ladder-type filter and a longitudinally-coupled double-mode SAW (DMS) filter with a low insertion loss and a large bandwidth were realized. We have therefore shown the feasibility of applying devices with a wide duplex gap such as Band I, IV, and X duplexers and a wide pass-band filter using the Ta₂O₅/Al/5°YX-LiNbO₃ structure.