Fast FEM/BEM computation of SAW harmonic admittance and slowness curves

The combination of the finite element method (FEM) and the boundary element method (BEM) is widely used for the simulation of surface acoustic wave (SAW) devices. Although the periodic FEM/BEM method strictly applies only to the infinite periodic interdigital transducer (IDT), its high degree of accuracy and reasonable computational burden have made it the basis of many SAW parameter extraction programs. The FEM part of the computation is numerically the heaviest. The FEM/BEM is a spectral method and the FEM problem for the electrodes has to be solved for every frequency point. Moreover, when oblique propagation is considered, the FEM problem again has to be solved for every transverse wavevector. We summarize and illustrate an approximation method that was introduced recently (Laude, V. et al., IEEE Trans. Ultrason., Ferroelec., Freq. Control, vol.51, no.3, p.359-63, 2004) and that requires the solution of the FEM problem only for the central frequency of the bandwidth of interest. The method is further extended to oblique propagation. Examples of SAW slowness curves are presented, with a particular view at the deformation of the slowness curve induced at resonance and at the dependence with the metalization height.