Surface acoustic wave methods to determine the anisotropic elastic properties of thin films*

We have developed experimental and analytical methods to measure the anisotropic elastic properties of thin supported films. In this approach, surface acoustic waves (SAWs) were generated using a line-focused laser. Waves with frequency components up to 400 MHz were detected by a Michelson interferometer. Dispersion relations for the SAW phase velocity were calculated from displacement waveforms acquired with source-detector separations of 5-15 mm. To determine film properties from the dispersion relations, we developed a new inversion algorithm based on the delta-function representation of the fully anisotropic, elastodynamic Green function for wave propagation. Our methods were first applied to an elastically isotropic aluminium film on an isotropic fused silica substrate. The results show the validity of our methods and were in good agreement with literature values. The results also illustrate various aspects of measurement uncertainty. The same SAW methods were used to examine a series of titanium nitride films on single-crystalline silicon wafers. The inversion results assuming orthotropic elastic symmetry indicated that c 11 increased and c 13 decreased with increasing film thickness. Values for the film thickness determined by our analysis were in good agreement with destructive measurements of the actual thicknesses.