Laser-ultrasonic measurements of thin-film properties using surface acoustic waves

Abstract

High-frequency, laser-ultrasonic methods to evaluate the mechanical properties of thin films have been developed. The approach is based on the optical generation and detection of surface acoustic waves (SAWs) to determine the frequency dependence of the phase velocity (dispersion relation). Broadband or variable-frequency, quasi-plane wave SAWs are generated by a line-focused, 200-ps pulsed laser. The out-of-plane SAW displacement amplitudes are measured using a path-stablized Michelson interferometer with line-focus detection. The apparatus incorporates differential photodiode detection with a −3-dB bandwidth of 40 kHz–800 MHz. With this system, several displacement waveforms over a range of propagation distances are acquired and used to determine the SAW dispersion. Dispersion relations up to 200 MHz are presented for a variety of specimens, including a series of Si wafers with TiN films 0.2–1.3 micrometers thick and containing residual stress. The results illustrate the applicability of this technique to various conditions such as stiffening and loading films and isotropic and anisotropic substrates. The experimental dispersion curves obtained with this method are compared to theoretical predictions to evaluate the film elastic properties.