Loss reduction of longitudinal-type leaky surface acoustic wave by loading with high-velocity thin film

Abstract

A longitudinal-type leaky surface acoustic wave (LLSAW) has large inherent attenuation owing to the continuous radiation of two bulk waves. The attenuation of an LLSAW can be reduced by loading with an aluminum nitride (AlN) thin film with a higher bulk wave velocity than the substrate. In this study, to further reduce the attenuation of the LLSAW, we investigated the propagation properties of an LLSAW on an X-cut 36° Y-propagating LiNbO3 (X36°Y-LN) substrate with an AlN thin film deposited by an RF magnetron sputtering system with a long-throw sputter cathode, in which the substrate is not exposed to plasma directly, by measurement using an interdigital transducer (IDT) pair with a wavelength λ of 8.0 or 4.8 µm. The insertion loss and bulk wave radiation loss into the substrate from the IDTs were markedly decreased by loading with the AlN thin film. When the film thickness was 0.250 λ for λ = 8.0 µm, the measured propagation loss decreased approximately tenfold from 0.28 dB/λ for the sample without the film to 0.03 dB/λ with the film. Furthermore, the temperature coefficient of delay was also reduced by loading with the AlN thin film.