Nanosecond Laser Processing for Improving the Surface Characteristics of Silicon Wafers Cut Using Wire Electrical Discharge Machining

Wire Electrical Discharge Machining (WEDM) has been recently used to fabricate ultra-thin silicon wafers for applications in solar cells. However, the slicing of wafers induce thermal damage and deteriorate the surface of wafers, thereby requiring polishing to eliminate the damage. In order to reduce the polishing time, laser surface modification using nanosecond pulses is explored as a potential pre-treatment technique before polishing to reduce thermal damages incurred during the WEDM process. The process relies on the selection of appropriate parameters. Therefore, this work attempts at understanding the effect of various laser parameters on the surface to optimize the process for a nanosecond pulsed laser. Since the range of parameters to be used is entirely unknown, a numerical model is developed using the Finite Volume Method to arrive at a preliminary range of parameters to carry out the experimental work. The temperature profile obtained from the numerical model is then used to select the working range of laser power for the experiments. Experiments were carried out on WEDM sliced silicon wafers using the range of laser power obtained. Based on scanning electron microscope images, it was observed that the surface thermal damage depth reduced up to 50 % after laser processing. The thermal damage depth was observed to decrease with an increase in pulse repetition rate with a minimum at 50 kHz and increases thereafter. Besides, it was also observed that the residual stresses also reduced significantly at lower values of laser power and pulse repetition rate.