Young's Modulus and Poisson's Ratio Characterization of Tungsten Thin Films via Laser Ultrasound

The non-destructive examination of microstructural properties of thin films is of consistent significance in microelectronic applications and simulations. Here, a laser ultrasonic (LUS) method is used to determine Young's modulus E and Poisson's ratio v of thin films deposited on silicon substrates. In particular, we determine the longitudinal and transversal sound velocity of a surface acoustic wave (SAW) in a 695 nm tungsten layer on a silicon (100) substrate, from which E and v are derived. The setup uses laser pulses to generate SAWs on the sample of interest. By detecting the SAWs with an optical beam deflection method, it is possible to determine the frequency-dependent phase velocity. Fitting a theoretical model to the experimentally evaluated dispersion curve, the longitudinal and transversal sound velocities of the thin film are derived. (C) 2015 Elsevier Ltd. All rights reserved.