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 LiNbO 3 (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.

The loss reduction of a longitudinal-type leaky surface acoustic wave (LLSAW) by loading with a dielectric thin film with a higher velocity than the substrate is proposed. An aluminum nitride (AlN) thin film was adopted as a high-velocity thin film, and the propagation properties of an LLSAW on an X36°Y-LiNbO3 (LN) substrate were investigated. First, the elastic constants c 11 and c 44 of an amorphous AlN (a-AlN) thin film deposited by RF magnetron sputtering were determined from the measured phase velocities of two SAW modes with mutually perpendicular particle motion, and they were 78 and 96% of those of a single-crystal AlN thin film. Next, from the theoretical calculation for the LLSAW on X36°Y-LN using the determined constants, it was found that the LLSAW attenuation can be reduced to zero by loading with an a-AlN thin film. Then, the propagation properties of the LLSAW on X36°Y-LN were measured by using an interdigital transducer pair. It was found that the losses due to bulk wave radiation can be reduced by loading with an a-AlN thin film.

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