Study on the subsurface damage depth in machined silicon wafers by the laser-ultrasonic method

This work aims at applying the laser-ultrasonic method for nondestructive evaluation of the depth of the subsurface damage in machined silicon wafers. It is based on different mechanisms of laser excitation of ultrasound by absorption of Q-switched Nd:YAG laser pulses at the fundamental wavelength: the concentration–deformation mechanism in the single-crystalline silicon and the thermoelastic one in the damaged layer. Due to the uniform heating of the whole damaged layer during the laser pulse action the amplitude of the compression phase of the laser-induced ultrasonic signal is proportional to the damaged depth. The rarefaction phase of this signal arises by absorption of the rest of laser energy in the single-crystalline silicon beneath the damaged layer. The empirical relation between the depth of the subsurface damage and the ratio of the amplitudes of compression and rarefaction phases of the laser-induced ultrasonic signal can be fitted by a linear function within the depth variation and the corresponding spread of the signal amplitudes. This relation can be used for in situ quantitative nondestructive evaluation of the depth of the subsurface damage in machined silicon wafers; the minimal reliably detectable depth is estimated at the level of 0.15–0.2 μm.