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

Pure shear horizontal piezoelectrically active surface and bulk acoustic waves (SH-SAW and SH-BAW) exist along rotated Y-cuts, Euler angles (0/spl deg/, /spl theta/, 90/spl deg/), of trigonal class 32 group crystals, which include the LGX family of crystals (langasite, langatate, and langanite). In this paper both SH-SAW and SH-BAW generated by finite-length, interdigital transducers (IDTs) on langasite, Euler angles (0/spl deg/, 22/spl deg/, 90/spl deg/), are simulated using combined finite- and boundary-element methods (FEM/BEM). Aluminum and gold IDT electrodes ranging in thickness from 600 /spl Aring/ to 2000 /spl Aring/ have been simulated, fabricated, and tested, with both free and metalized surfaces outside the IDT regions considered. Around the device's operating frequency, the percent difference between the calculated IDT impedance magnitude using the FEM/BEM model and the measurements is better than 5% for the different metal layers arid thicknesses considered. The proportioning of SH-SAW and SH-BAW power is analyzed as a function of the number of IDT electrodes; type of electrode metal; and relative thickness of the electrode film, h//spl lambda/, where /spl lambda/ is the SH-SAW wavelength. Simulation results show that moderate mechanical loading by gold electrodes increases the proportion of input power converted to SH-SAW. For example, with a split-electrode IDT, comprising 238 electrodes with a relative thickness h//spl lambda/ = 0.63% and surrounded by an infinitesimally thin conducting film, nearly 9% more input power is radiated as SH-SAW when gold instead of aluminum electrodes are used.