Suppression of bulk wave radiation from leaky surface acoustic waves by loading with thin dielectric films

The bulk wave, which is radiated into the substrate from the leaky surface acoustic wave (LSAW), can be suppressed by loading the substrate with a thin film of dielectric such as tantalum pentoxide $(Ta2O5)$ and silicon dioxide $(SiO2).$ Rotated Y-cut X-propagating lithium niobate $(LiNbO3)$ is used for the substrate. For a certain range of the rotation angle, the attenuation of the LSAW can be suppressed by choosing the appropriate material and thickness for the film. For the free surface of 64° $Y−X$ $LiNbO3$ with $Ta2O5$ film, and the metallized surface of 41° $Y−X$ $LiNbO3$ with $SiO2$ film, the measured propagation losses decreased approximately to one-fourth to half of that of the sample without the thin film. It is shown that, even if the propagation path is a free surface, the particle displacement distribution in the LSAW is concentrated near the substrate surface by the loading with the thin film, as for the metallized surface. This result suggests that the bulk wave radiation in the LSAW excitation can be suppressed. The bulk wave radiation loss for the 41° $Y−X$ $LiNbO3$ with the $Ta2O5$ film decreased approximately to one-sixth of that of the sample without the thin film.