Measurement Technique for Elastic and Mechanical Properties of Polycrystalline Silicon-Germanium Films Using Surface Acoustic Waves and Projection Masks

Using Rayleigh surface acoustic waves (SAW), the Youngs modulus, the density and the thickness of polycrystalline Silicon-Germanium (SiGe) films deposited on silicon and SiO2 were measured, in excellent agreement with theory. The dispersion curve of the propagating SAW is calculated with a Boundary Element Method (BEM)-Model based on Greens functions. The propagating SAW is generated with a nanosecond laser in a narrowband scheme projecting stripes from a mask on the surface of the sample. For this purpose a glass mask and a liquid crystal display (LCD) mask are used. The slope of the SAW is then measured using a probe beam setup. From the wavelength of the mask and the frequency of the measured SAW, the dispersion curve is determined point by point. Fitting the BEM-Model to the measured nonlinear dispersion curve provides several physical parameters simultaneously. In the present work this is demonstrated for the Youngs modulus, the density and the thickness of SiGe films. The results from the narrowband scheme measurement are in excellent agreement with separated measurements of the thickness (profilometer), the density (balance) and the Youngs modulus (nanoindenter).