Pseudo-SAW propagation on layered piezo-substrates: experiments and theory including film viscosity

The pseudo-SAW (PSAW) characteristics of sputtered glass films on substrates of 41° and 64° Y-X lithium niobate (LiNbO₃) and 36° Y-X lithium tantalate (LiTaO₃) have been measured from 30 MHz to above 1.0 GHz. Glass films in the 500 nm to 2000 nm thickness range were deposited by RF diode sputtering. Using suitably located thin-film aluminum interdigital electrode patterns at the film surface and interface, the PSAW velocity, propagation loss, resonator capacitance ratio (C_m /C_o), and temperature coefficient of frequency (TCF), were measured. The properties of these layered structures were simulated using the matrix method including layer viscosity. The glass film material was assumed isotropic with finite viscosity (frequency dependent stiffness) in the simulations, whereas the substrates were assumed to have zero viscosity. The frequency dependent measurements and theory were well correlated. The theoretical results were sensitive to the choice of material constants, both film and substrate. The propagation loss was dominated by leaky-wave loss and the inclusion of film viscosity did not substantially alter the propagation loss properties