Investigation of electrode pitch on resonant and capacitive characteristics in X-cut LiNbO3 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 LiNbO3 membrane-based resonators are investigated for both the XBAR mode and first-order asymmetry mode (A(1)). The XBAR mode with fres above 8 GHz was excited at a metallization ratio (eta) less than 0.5 when P was within 5 mu m to 10 mu m, as well as eta less than 0.6 when P was within 15 mu m to 20 mu m. Meanwhile, we revealed that increasing P had negligible effect on the fres of XBAR mode but did produce a positive improvement on its K-eff(2) and Q. In contrast, both fres 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 eta, 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.