A Finite Element Analysis of Second Order Effects on the Frequency Response of a SAW Device

Electrode perturbation of the surface may cause significant second order effects on the frequency response of surface acoustic wave (SAW) devices for high frequency applications. These effects in a Y-Z lithium niobate filter are analyzed using a recently developed finite element model. In this model, the equations of wave propagation in piezoelectric materials are discretized using the Galerkin method, in which an implicit algorithm of the Newmark family with unconditional stability is implemented. The Rayleigh damping coefficients are included in the elements near the boundary to reduce the interference of reflected waves. The frequency response of the filter is obtained through the Fourier transform of the impulse response, which is solved directly from the finite element simulation. The analyses have shown that for aluminum electrodes of fixed thickness, the second order effects on the frequency response become increasingly significant as the device is scaled smaller to have higher central frequencies. Typically, for aluminum electrodes of thickness 2000 Å, the frequency response is significantly distorted for the device of the central frequency at above 2 GHz. Quantitative analyses of the second order effects in the device of various center frequencies provide insights for designing high performance SAW devices for high frequency applications.