Effects of periodically missing fingers and periodically shifted fingers on SAW propagation in quartz resonators

A finite element study of a 3-electrode finger periodic SAW structure of ST-cut quartz was studied. This multi-period analysis allows more interaction of the bulk acoustic wave modes with the surface acoustic wave modes. Plane strain assumptions were used to enable a two-dimensional analysis of straight crested acoustic waves in the structure. Adaptive finite element meshes were employed to reduce the size of the finite element problem. The results of this study may be useful for a full-scale finite element modeling of SAW resonators and filters. The effects of periodically missing or shifted fingers may be useful for generating/controlling leaky SAW. The physical difference between the modes shapes of the two fundamental SAW modes were compared and contrasted. The numerical results of frequency spectrum versus electrode geometry and electrode height, respectively, confirms empirical experience with these SAW modes. Possible couplings of the SAW modes with the BAW modes are shown. These couplings lead to SAW modes with some BAW mode characteristics. The couplings may be useful for improving frequency temperature behavior or generation of leaky SAW modes. On the other hand, they may lead to poor frequency temperature behavior or low Q devices.