Evaluation of SiO₂ Thin films on piezoelectric substrates using line-focus-beam ultrasonic material characterization system

SiO₂ thin films deposited on LiNbO₃ and LiTaO₃ using RF magnetron sputtering were evaluated using a line-focus-beam ultrasonic material characterization (LFB-UMC) system. From the measured velocity of a leaky surface acoustic wave (LSAW), c ₁₁ and c ₄₄ for a SiO₂ film were determined to be 0.755 ×10¹¹ and 0.289 ×10¹¹ N m^-2 assuming that the density and dielectric constant of the thin film were bulk values. The measured LSAW velocities are in good agreement with values calculated using determined values. The acoustical loss was evaluated from the difference between the measured propagation attenuation and the calculated leakage loss into water. However, the differences were 0.02-0.27 dB/λ, which were considered to be too large for the acoustical loss of SiO₂ thin films. By accurately measuring the density of a thin film and determining its elastic constant, it is possible that the acoustical loss of a thin film can be evaluated using an LFB-UMC system.