Non-destructive characterization and evaluation of thin films by laser-induced ultrasonic surface waves

Abstract

Ultrasonic surface waves induced by a pulsed laser act as high-frequency elastic oscillations propagating along the target materials surface. The velocity of these propagating waves is effected by the elasticity and density of the material. These surface waves have a unique property in that their depth of penetration in the material decreases with an increase in their frequency. At higher frequencies, the velocity of the surface waves are effected if the film or near-surface region composition differs from the substrate material. Generating surface waves using short laser pulses has allowed for non-destructive measurements to be performed over a wide frequency range. This in turn results in a spectrum of velocity frequencies which, when compared to known film and density characteristics using a fit program, can be used to characterize surface composition and thin films. This velocity spectrum can be used to determine Young's modulus, density, and thickness of the film. The ultrasonic thin film analyzer has been developed for use in industrial test laboratories and research institutes.