Raman studies of `pulse repetition effect' in laser processing of Si

Abstract

It is meaningful to avoid irreversible changes in laser processing of silicon wafer and find out the cause to which the laser induced damage threshold (LIDT) is decreased along with increasing frequency (>1 Hz). (Pulse Repetition Effect or PRE). We investigated the PRE in laser irradiation on c-Si wafer by Raman spectroscopy. Time-resolved Raman spectrometer was developed based on holographic notch filter, optical fibre and optical multichannel analyzer (OMA) which was applied to detect transient structure changes and stress in silicon wafer under laser shock. The surface morphology was monitored by CCD through a microscope when pulses were on. We applied 1.06 um Nd-YAG laser in our experiment to silicon wafer with and without silicon oxide top layer. Raman spectra was sampled in 100 us after laser triggering. Our system could easily detect the surface absorption changes by comparing the intensity before and after laser shock while the surface was not damaged. A reversible weak peak emerged around 500 cm-1 for c-Si with and without silicon oxide. It is attributed to grains under transient stress induced by laser which disappeared after seconds. No such peak is observed in sampling in 1 ms after laser triggering. Such emergence of grain states may accelerate the damage process, which will contribute to PRE.