Multimode Photoacoustic Characterization of Subsurface Damage in Ground Thin Wafers

A multimode photoacoustic method has been applied for evaluating the degree of subsurface damage (SSD) in the ground silicon wafer. Mode conversion of laser-generated broadband guided waves is caused by wafer thinning. Bayesian inversion is applied to quantify the effective elastic constants of the SSD layer together with the corresponding uncertainty estimates from the experimental dispersion data of multimode guided waves. Laser ultrasonic experiments have been performed on the polished and ground silicon wafers with different thicknesses to determine the ability of the method to characterize SSD and the effect of acoustic mode conversion on the quantization results. The experimental results show that the mode of the laser-generated broadband guided waves will gradually change from Rayleigh wave to Lamb wave as the wafer is thinned. Both the Rayleigh mode and the A(0) Lamb mode can accurately estimate the effective elastic constants of SSD, but the A(0) Lamb mode leads to longer confidence intervals, which has been theoretically explained.