Laser Ultrasound Measurement of Diaphragm Thickness, Young's Modulus and Poisson's Ratio in an MEMS Device

Laser-generated Lamb waves, coupled with a large bandwidth Michelson interferometer, have been demonstrated to accurately measure the thickness of a microelectromechanical systems pressure sensor diaphragm in the [110] direction of a silicon wafer. Using the reassigned Gabor time–frequency method to produce group velocity dispersion curves, the technique facilitates the measurement of thickness, Young's modulus, and Poisson's ratio from just one noncontact measurement. In this paper, thickness was determined to be 35.01 μm ± 0.18 μm. For comparison, the thickness was measured using an independent optical technique, obtaining a value of 34.60 μm ± 0.27 μm. Values for Young's modulus and Poisson's ratio were also determined to be 163 GPa ± 11.7 GPa and 0.351, respectively, and these are in good agreement with values found in the literature.

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