Accurate FEM/BEM-simulation of surface acoustic wave filters

The accurate analysis of surface acoustic wave (SAW) filter is very challenging task for SAW designers. Especially the prediction of losses in RF-filters employing leaky surface acoustic waves (LSAWs) with aperiodically arranged metallic electrodes such as DMS-filters is not possible with conventional CMO-, P-matrix or equivalent-circuit models, since these approaches ignore relevant second-order effects: neither the weak guiding of the LSAWs near the upper stop-band edge nor radiation into bulk-waves are accounted for correctly. However, bulk-wave radiation causes increased insertion loss (in particular near the upper pass-band edge) and deteriorates the steepness of the upper skirt. Therefore, it is a matter of special importance. We have developed and implemented an accurate model for acoustic tracks with the 2D approximation only: to describe the electro-acoustical properties of the piezoelectric substrate a boundary element formulation is used, which is based on a semi-infinite dyadic Green´s function, avoiding any compromises in the characterization of the physics of bulk-wave interactions. The mechanical behaviour of the electrodes is described with finite elements since this approach provides high flexibility with respect to the geometry and material composition. Due to the fact that the FEM/BEM model does not make any a priori assumptions concerning crystal cuts and/or wave types employed in the simulated device, it is widely applicable. Since well-adapted shape functions are used for the BEM-part of the model, the size of the discretized system is kept to a minimum, thereby enabling the simulation of acoustic tracks with up to 400 electrodes in reasonable CPU times on a PC. To guarantee an accurate description of real SAW-devices, a model for finite finger resistance as well as a tool for the description of the electromagnetic effects on the chip and in the package is attached to the FEM/BEM-model. Comparison between simulations and measurements on real SAW-devices show (i) very good agreement and (ii) proof the effectiveness of the developed simulation tool.