Finite element simulation and experimental study of laser-generated surface acoustic waves on determining mechanical properties of thin film

Abstract

The laser-generated surface acoustic wave (LSAW) nondestructive testing (NDT) technique is a promising method to characterize the mechanical properties of thin films. In this study, based on the thermoelastic mechanism, a finite element method (FEM) is put forward to simulate the LSAW in the film/substrate structure, and the effect of the temporal and spatial distribution of the Gaussian pulse laser on the Rayleigh-type SAW signals is revealed. For the SiO2 and low dielectric constant (low- k) dense Black Diamond™ (SiOC:H, BD) films with the thickness of 500 and 1000 nm, the typical displacement waveforms of SAW at a series of probing points along the propagation direction are obtained. By analyzing the full width at half maximum (FWHM) of the signal, the optimal NDT experimental conditions for laser are determined with the minimum possible pulse rising time and the linewidth less than 10  μm. Based on the FEM simulation result, the LSAW NDT experiment is carried out and the dispersion curve of SAW is calculated to characterize Young's modulus of the SiO2 and low- k samples. It is found that the experimental results are in good agreement with the simulation results. This study verifies the validity of FEM simulation of LSAW in layered structures containing thin film and that the laser parameters determined by FEM fit perfectly in characterizing the mechanical properties of thin films.