Thickness Effects on Acoustic Parameters of TiO2 Layers on SiO2, Ti, Al2O3, and Si Substrates

We investigated the effect of film thickness d on the acoustic response of titanium dioxide (TiO2) layers deposited on Ti, SiO2, Al2O3, and Si substrates. For each TiO2 thickness–substrate pair, we computed reflection coefficients and acoustic signatures under normal operating conditions of a conventional scanning acoustic microscope, then deduced the Rayleigh-wave velocity VR from spectral analysis of the oscillatory layer–substrate signatures. As d increased, VR either rose or fell, depending on the layer/substrate pair, and eventually approached a saturation value. For TiO2/SiO2 and TiO2/Ti, VR increased from those of the bare substrates (SiO2: 3415 m/s; Ti: 2965 m/s) toward 3830 m·s−1, the bulk TiO2 value. For TiO2/Al2O3 and TiO2/Si, VR decreased from the substrate values (Al2O3: 5700 m/s; Si: 4712 m/s) toward the same TiO2 saturation. These dispersion trends are consistent with stiffening (VR (TiO2) > VR (Substrate)) or loading (VR (TiO2) < VR (Substrate)) effects. The resulting VR–d dispersion charts provide theoretical reference trends relating thickness and Rayleigh-wave velocity for the idealized TiO2/substrate systems considered here.