Investigation of electrode pitch on resonant and capacitive characteristics in X-cut LiNbO 3 membrane-based laterally-excited bulk acoustic resonator

The laterally-excited bulk acoustic resonators (XBARs) have been gaining attention for their use as radio frequency filters due to their ability to resonate at high frequencies (>5 GHz) and wide fractional bandwidth. However, the limited cognition about the resonant characteristics of XBARs remains an obstacle for its commercial application. In this paper, the influence of electrode pitch ( P ) on resonant frequency ( f res ), quality factor ( Q ) and effective electromechanical coupling coefficient ( K eff 2 ) in X-cut LiNbO 3 membrane-based resonators are investigated for both the XBAR mode and first-order asymmetry mode ( A 1 ). The XBAR mode with f res above 8 GHz was excited at a metallization ratio ( η ) less than 0.5 when P was within 5 μ m to 10 μ m , as well as η less than 0.6 when P was within 15 μ m to 20 μ m . Meanwhile, we revealed that increasing P had negligible effect on the f res of XBAR mode but did produce a positive improvement on its K eff 2 and Q . In contrast, both f res and Q of the A 1 mode gradually declined with increasing P . In addition, we have demonstrated the disparity between simulated and calculated C 0 as a function of η , varying with P , to achieve a more accurate prediction of capacitance behavior in resonators. Overall, this work has demonstrated the complex relationship of resonant and capacitance properties of X-cut membrane-based resonators on the electrode pitch, which will aid in optimal designing of high-performance filters.