High-frequency bulk acoustic wave resonators with thickness shear and extensional modes in LiNbO3 thin plates with acoustic multilayers

Currently, wide-bandwidth and high-frequency acoustic wave filters are required. Focusing on LiNbO3 (LN)-based bulk acoustic waves (BAWs), we optimized the LN orientation for solidly mounted BAW resonators (SM-BAWRs). The velocities and coupling factors (k2) of the thickness shear (TS) and extensional (TE) modes in SM-BAWRs showed the same orientation dependence as self-supported BAWRs, but with velocities roughly half and k2 about three-quarters of the self-supported type. The 167°Y LN TS mode offers wide fractional bandwidth (FBW) without spurious responses, while the 36°Y LN TE mode provides higher velocity with low spurious responses. SM-BAWRs in the TS mode from 2.9–3.6 GHz (fa) achieved 14%–15% FBW and 47–68 dB impedance ratio (ZR), and in the TE mode from 4.2–11 GHz, 3.8%–8.9% FBW and 29–54 dB ZR. To achieve high-performance SM-BAWRs above 8 GHz, further improvements in electrode design and measurement methods are required.