A Spurious-free Laterally-excited Bulk Acoustic Resonator with Ring-shaped Interdigital Electrodes

The suppression of spurious mode and the improvement of quality factor (Q) are hotspots in laterally-excited bulk acoustic resonators (XBARs), since spurious modes and Q constrain the performance of radio frequency (RF) filters. A feasible solution was proposed by constructing regions of different acoustic impedances in the metal-covered lithium niobate plate. In this paper, a spurious-free XBAR with ring-shaped interdigital electrodes (R-XBAR) was designed and fabricated based on a 350 nm Z-cut lithium niobate thin film. The R-XBAR was simulated using theoretical analysis and 3D FEM simulation models. And, the fabricated R-XBAR resonates at 5.85 GHz and achieves an electromechanical coupling coefficient ($K_{eff}^2$) of 20.6%. Compared to the conventional XBAR (Q
p
=498.68), the spurious modes in the R-XBAR are suppressed well, resulting in a 30% increment in Q value (Q
p
=654.22). Consequently, the R-XBAR shows a potential application for 5G filters.