Laser optoacoustic non-destructive method of thickness measurement of subsurface damaged layer in machined silicon wafers

Abstract

In the present work we propose the laser optoacoustic (OA) method for nondestructive measurement of the thickness of damaged layer in a machine cut silicon wafer. It is based on different mechanisms of laser excitation of ultrasound in monocrystalline silicon – the concentration-deformation mechanism and in a damaged layer – the thermoelastic one by absorption of Q-switched Nd:YAG laser pulse at the fundamental harmonic. Due to the uniform heating of a damaged layer during the laser pulse action the amplitude of the compression phase of the excited OA signal is proportional to the layer thickness. The rarefaction phase of OA signal arises by absorption of the rest of laser energy in monocrystalline silicon beneath the damaged layer. Comparison of the ratio of phase amplitudes with the scanning electron microscopy measurement of damaged layer thickness has shown it linear dependence vs. thickness within the variation of thickness and the corresponding spread of OA signal amplitudes. This provides the possibility of non-destructive laser optoacoustic measurement of the thickness of the damaged layer in silicon wafers. The minimum detectable value of the damaged layer thickness is of the order of 0.15-0.2 micron.