Design and Fabrication of a 7-GHz Bandpass Filter Based on Solidly Mounted A1-Mode LiNbO3 XBAR With Dielectric Bragg Reflectors

Abstract

Achieving both strong acoustic confinement and wafer-level manufacturability remains a key challenge for A1-mode laterally excited bulk acoustic wave resonators (XBARs), particularly for conventional suspended structures requiring membrane release. This work presents a solidly mounted XBAR (SM-XBAR) based on a 128° Y-cut LiNbO3 thin film and an asymmetrically optimized SiO2/Ta2O5 Bragg reflector for suppressing substrate-side acoustic leakage. A finite-element model considering the dispersive characteristics of A1 Lamb waves is employed to optimize the reflector structure, resulting in a broadband acoustic stopband covering 6–9 GHz. The optimized reflector improves the series quality factor ${Q}_{s}$ from 100 to 575 and the parallel quality factor ${Q}_{p}$ from 185 to 650 while preserving effective electromechanical coupling coefficient ${k}_{\textit {eff}}^{{2}} \gt 17$ %. To validate practical applicability, a fourth-order ladder filter is implemented, achieving a center frequency of 6.74 GHz, a 10% fractional bandwidth (FBW), and more than 40-dB out-of-band rejection. The proposed solidly mounted architecture eliminates membrane release and provides a manufacturable and mechanically robust solution for sub-8-GHz RF filtering applications.