Laterally Excited Bulk Acoustic Resonators (XBARs): Optimization Method and Application to Resonators on LiTaO3

This paper describes an optimization method for single-mode performance and suppression of spurious modes in a laterally excited bulk resonator (XBAR). The method is based on an analysis of the slowness surface of bulk acoustic waves propagating in the material of a thin plate. It was verified by application to XBARs based on different LT orientations close to the ZY cut. The simulated admittance functions confirmed low spurious modes in devices with optimized plate thickness and degradation of resonator performance with a deviation of a plate thickness from the optimal value. Analysis of the wave structure revealed its strong dependence on the plate thickness and the ratio between two components of mechanical displacement in a thin plate.