4E-2 2D-FEM Modeling of Mirror-Type FBARs Based on an Adapted Set of Material Parameters

Abstract

Film bulk acoustic resonators (FBARs) show many unwanted "spurious modes". They all are energy-trapped modes and therefore arise - in addition to the main resonance - as ripples in the resonator's admittance. Furthermore, non-trapped modes reduce resonator quality due to leaking energy to the environment of the resonator. Since the excitation of both spurious and non-trapped modes has been identified as one of the main factors limiting the performance of high-quality FBAR-based filters and duplexers, there is a high demand for the accurate prediction of the appearance of these unwanted modes. Starting point of our simulations was the adaptation of a material parameter set. In order to do that we fitted calculated dispersion curves to measured dispersion characteristics in a very wide frequency range by varying stiffness constants systematically. Based on the resulting material data set we iteratively performed 2D-FEM simulations in order to improve our mesh, adapted the viscous losses, and added lumped elements to account for dielectric and resistive losses. The final FEM-model in combination with the optimized material parameter set shows an excellent agreement of measurement and simulation