Numerical simulations of microacoustic resonators and filters

摘要

This dissertation discusses numerical simulations of microwave acoustic resonators and bandpass filters employed in wireless telecommunication systems. In the first part of the dissertation, tailored finite element method (FEM) software with efficient numerical techniques is implemented and applied in the modeling of thin-film bulk-acoustic wave resonators (FBARs). Simulations with 3D FEM models of FBARs are carried out to investigate the effect of the electrode shape on the spurious resonances that often are present in the electrical response. The modeling results are validated through comparison of simulated and measured mechanical vibration amplitudes. The usability of the FEM tool is further demonstrated in simulations of a resonator design that features a clean electrical frequency response, free of spurious resonance peaks caused by anharmonic modes. Additionally, a method is proposed for determination of the elastic constants of a piezoelectric thin-film material. The technique is based on fitting of the computed dispersion curves of Lamb-wave modes to those measured from an FBAR using a scanning laser interferometer.In the second part of this dissertation, numerical simulations are used to study propagation properties of longitudinal leaky surface acoustic wave (LLSAW) mode under periodic electrode array on YZ-cut lithium niobate (LN). A combined FEM/boundary element method is employed to compute the electric admittance of one-port synchronous LLSAW resonators. Simulations and experiments are used to derive the dependence of the resonator resonance frequencies and Q values on the electrode dimensions. Ladder-type bandpass filters exploiting the LLSAW mode are implemented on YZ-cut LN in the frequency range from 2.5 GHz to 5.2 GHz, with fundamental mode LLSAW resonators as building blocks. The results demonstrate that the high phase velocity of the LLSAW mode on YZ-cut LN allows inexpensive fabrication of wide-band, low-loss filters up to 5 GHz using conventional optical lithography.The third topic considered is comprehensive modeling of a SAW duplexer, including electromagnetic modeling of the ceramic package. The parasitic capacitive and inductive couplings in the package are obtained using rigorous computation, allowing one to estimate the package effects on the duplexer performance.