The experimental and theoretical characterization of SAW modes on ST-X quartz with a zinc oxide film layer

Abstract

The Y-rotated, X-propagating, (ST-X), cut of quartz is used extensively for the development of surface acoustic wave (SAW) devices where temperature stability is of primary importance. Besides the commonly used generalized SAW (GSAW) mode, this crystal cut also supports a pseudo-SAW (PSAW) and high velocity pseudo-SAW (HVPSAW) mode. All three predicted SAW modes have been observed in ST-X quartz using an interdigital transducer configuration, with and without a zinc oxide film layer, and the SAW compared with theoretical. There was excellent agreement between experimental and theoretical velocity values without the ZnO film layer. There was good agreement between experimental and theoretical velocity dispersion characteristics in the presence of the ZnO film layer for the GSAW mode. The viscosity values for quartz were introduced into the complex elastic constants to theoretically predict and correlate with propagation loss for the three SAW modes with and without the ZnO films. The trends predicted by the theoretical calculations were strongly evident in the measured data particularly in the PSAW and HVPSAW modes where the presence or absence of the ZnO film represented an order of magnitude difference in the propagation loss. The frequency dependencies of the measured propagation losses correlated well with the predicted frequency dependencies. Differences in absolute loss values between theory and experiment were attributed to diffraction losses, air loading, and the absence of viscosity values for the ZnO film.