Partially Etched Piezoelectric Film Filled with SiO₂ Structure Applied to A1 Mode Resonators for Transverse Modes...

With the arrival of the Fifth Generation (5G) communication era, there has been an urgent demand for acoustic filters with a high frequency and ultrawide bandwidth used in radio-frequency (RF) front-ends filtering and signal processing. First-order antisymmetric (A1) lamb mode resonators based on LiNbO₃ film have attracted wide attention due to their scalable, high operating frequency and large electromechanical coupling coefficients (K²), making them promising candidates for sub-6 GHz wideband filters. However, A1 mode resonators suffer from the occurrence of transverse modes, which should be addressed to make these devices suitable for applications. In this work, theoretical analysis is performed by finite element method (FEM), and the admittance characteristics of an A1 mode resonator and displacement of transverse modes near the resonant frequency (f_r) are investigated. We propose a novel Dielectric-Embedded Piston Mode (DEPM) structure, achieved by partially etching a piezoelectric film filled with SiO₂, which can almost suppress the transverse modes between the resonant frequency (f_r) and anti-resonant frequency (f_a) when applied on ZY-cut LiNbO₃-based A1 mode resonators. This indicates that compared with Broadband Piston Mode (BPM), Filled-broadband Piston Mode (FPM) and standard structures, the DEPM structure is superior. Furthermore, the design parameters of the resonator are optimized by adjusting the width, depth and filled materials in the etched window of the DEPM structure to obtain a better suppression of transverse modes. The optimized A1 mode resonator using a DEPM structure exhibits a transverse-free response with a high f_r of 3.22 GHz and a large K² of ~30%, which promotes the application of A1 mode devices for use in 5G RF front-ends.