Thickness measurement of a thin-film layer on an anisotropic substrate by phase-sensitive acoustic microscope

Complex V(z) curves for single thin-film layers on anisotropic substrates are studied both experimentally and theoretically, and the application of V(z) measurement to the determination of film thickness on anisotropic substrates is discussed. Complex V(z) curves for aluminum layers (with thicknesses between 0.5 and 2 mum) on a silicon wafer have been calculated. The inverse Fourier transform of the V(z) curves, which corresponds to the reflection coefficient, shows sharp changes at critical angles of pseudosurface waves, pseudo-Sezawa waves, and Rayleigh surface waves. These critical angles strongly depend on the thickness. Complex V(z) curves for these specimens have been measured using a phase-sensitive acoustic microscope with a point focus lens at 400 MHz. The critical angles of the surface waves obtained from the measured V(z) curves are in good agreement with those obtained from the calculated V(z) curves. On the basis of this result, it is shown that the V(z) measurement is applicable to the determination of film thickness on an anisotropic substrate.