Analysis of SAW filters using finite-difference time-domain method

Abstract

A number of analytical and numerical techniques have been developed to model surface acoustic wave (SAW) filters. However, due to the future wireless communication applications, the present modelling schemes are difficult to accurately predict the characteristics of SAW filters with complex interdigital transducer (IDT) and fabricated on multilayered substrates. Furthermore, none of them except the direct finite element method (FEM) can accurately predict the performance of SAW filters for high frequency applications when second order effects become significant. The goal in this dissertation is to extend the finite-difference time-domain (FDTD) technique, one of the most powerful tools in computational electromagnetics, to analyze the characteristics of SAW filters. In this method, the partial derivatives of quasi-static Maxwell's equations and equation of motion are discretized to centered finite-differences. Furthermore, the perfectly matched layer (PML) boundary condition is applied to reduce the spurious reflections. The numerical model was applied to analyze the characteristics of unapodized SAW filters fabricated on zinc oxide (ZnO) substrates with different number of electrodes and different separations between the centers of input and output IDT. The surface skimming bulk wave (SSBW) interference was studied. The proposed method was further extended to analyze the characteristics of a ZnO/IDT/Diamond/Si layered SAW filter. The wave-front images of the acoustic waves were simulated and the phase velocities of the Rayleigh and 1st Sezawa waves were extracted from the time domain responses by using the Prony's method. Furthermore, the velocity dispersion of the layered structure and the massloading effects of the IDT were studied. The simulated results are in good agreement with the existing experimental data, indicating that the FDTD method is an appropriate approach for modeling SAW filters.