Optimization of frame-like film bulk acoustic resonators for suppression of spurious lateral modes using finite element method

Film bulk acoustic resonators (FBARs) are comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. The portion of the piezoelectric film included between the overlap of the electrodes forms an acoustic cavity. The primary vibration mode of this cavity is that in which sound waves propagate in a longitudinal direction perpendicular to the plane of the electrodes. Unfortunately, the electric field also excites other spurious lateral vibration modes, These spurious lateral resonances near the main resonance of the resonator show up as a ripple in the pass band of filters. Frame-like FBARs have been proposed to eliminate the generation of the spurious lateral modes. The frame-like FBAR structures comprise a frame-like electrode confining a central area. The width and thickness of the frame-like electrode are arranged so that the displacement relating to the piezoelectrically excited strongest resonance mode is substantially uniform in the central area. In such a structure, the spurious resonances have often only a weak coupling. We apply the finite element method to investigate the effect of the frame-like FBARs on the suppression of lateral modes and find the optimal design of frame-like electrodes.