Surface acoustic wave properties of amorphous Ta ₂O ₅ and Nb ₂O ₅ thin films prepared by radio frequency sputtering

The fundamental acoustic properties of amorphous Ta ₂O ₅ and Nb ₂O ₅ thin films prepared by RF sputtering were evaluated using a Rayleigh-type surface acoustic wave (SAW) on 128° YX-LiNbO ₃ and a shear-horizontal-type SAW on 36° YX-LiTaO ₃. The elastic constants c ₁₁ and c ₄₄ of the thin films were determined from the measured phase velocities of two SAW modes with mutually perpendicular particle motion. Although the elastic constants of the Ta ₂O ₅ thin film were found to be approximately 14-17% larger than those of the Nb ₂O ₅ thin film, it was clarified that the Ta ₂O ₅ thin film has a stronger SAW trapping effect than the Nb ₂O ₅ thin film because the density of the former was measured to be 1.5 times higher than that of the latter. On the other hand, the Nb ₂O ₅ thin film has a smaller propagation loss for a Rayleigh-type SAW, a lower temperature coefficient of delay, and a higher refractive index than the Ta ₂O ₅ thin film.