FEM/BEM impedance and power analysis for measured LGS SH-SAW devices

Pure shear horizontal surface acoustic waves (SH-SAW) exist on rotated Y-cuts, Euler angles (0°, 0, 90°), of trigonal class 32 group crystals, which include quartz and the LGX family of crystals (langasite, langatate, and langanite). This pure SH-SAW mode has a number of interesting propagation characteristics. Along selected LGX orientations, interdigital transducers (IDTs) used to generate the SH-SAW also excite a significant amount of shear horizontal bulk acoustic wave (SH-BAW) that propagates almost parallel to the surface, with a slightly different phase velocity. We use combined finite and boundary element methods (FEM/BEM) to calculate IDT admittance, while considering both the SH-SAW and SH-BAW contribution. The ratio of transduced SH-SAW power to total input power is analyzed as a function of device geometry and IDT metallization material and thickness. Typically, for a 20λ IDT, the FEM/BEM simulations indicate that about 40% of input power is converted to the SH-SAW when the area outside of the IDT is coated with an infinitesimally thin conducting film. Both our simulations and experimental results consistently indicate that the ratio of SH-SAW to total input power strongly depends on metal type and thickness. The SH-BAW angular plots obtained with the FEM/BEM analysis confirm that the mode excited by the IDTs propagates nearly parallel to the surface. Measured device impedances agree extremely well (within 5%) with the FEM/BEM IDT impedance calculations accounting for both SH-SAW and SH-BAW generation.