Measurement technique for the density, the thickness and Young’s modulus of thick polysilicon films using surface acoustic waves

Abstract:
Using Rayleigh surface acoustic waves (SAW), the density, the thickness and the young’s modulus of ~20micrometer thick polysilicon films deposited on silicon are measured, in excellent agreement with theory. The dispersion curve of the propagating SAW is calculated with a Boundary-Element (BEM)-Model based on Green’s functions. The propagating SAW is excited with a picosecond laser in a narrowband scheme projecting lines from a mask on the surface of the sample. Then the surface vibrational velocity of the SAW is measured using a Probe-Beam- Deflection-setup. From the wavelength defined by the mask and the frequency of the measured SAW, the dispersion curve is determined point by point. Fitting the BEM-Model to the measured non-linear dispersion curve provides several physical parameters simultaneously. In the present work this is demonstrated for three properties of the polysilicon film: Young’s modulus, thickness and density. The properties obtained are in good agreement with recent literature data.