COM parameters for thick metal and partially buried electrodes extracted from a mixed FEM/BEM numerical model

SAW devices can be accurately modeled with a coupling-of-modes (COM) formalism. Modifications to the COM parameter relations are used for modeling STW devices. However this is less accurate near the upper part of the stopband due to coupling to SSBW. Partially burying the metal electrodes increases the separation between STW and SSBW, improving the accuracy of the COM analysis, however both electromechanical coupling (conductance) and electrostatic (capacitance) terms are found to vary with buried depth. COM parameters are also found to vary with electrode buried depth for SAW. The goal is to find an accurate method for modifying the COM parameters to take into account the effects of electrode thickness and buried electrode depth for both SAW and STW devices. The harmonic admittance of a metal electrode was computed using a mixed FEM/BEM Numerical Model for 0.20 to 0.80 line width to period ratio (a/p), 0 to 10% normalized metal thickness (he/p), and 0 to 5% normalized buried depth (hb/p). Additional computations were made for raised electrodes with 1 to 5% normalized height. An effective permittivity was then obtained which, when substituted into the calculation for thin metal, results in the same value for the imaginary part of the harmonic admittance. A polynomial fit was then made in terms of a/p, he/p, and hb/p to obtain an effective permittivity polynomial. It was found that the capacitance for both SAW and STW devices is accurately predicted using this effective permittivity polynomial in the P-matrix models. It was also found that COM parameters relating to the electromechanical coupling for both SAW and STW devices are accurately predicted by using the effective permittivity polynomial with he/p=0. Measured admittances for STW resonators on quartz with various values for a/p, he/p and hb/p are compared with modified COM analysis using the effective permittivity polynomial equation.