Investigation of Depth of Laser Damage to Silicon as Function of Wavelength and Pulse Duration

Abstract

When quantifying laser damage in silicon, two key parameters are of importance, namely the depth of the laser damaged region and the minority carrier lifetime in the laser processed region. In this paper, we investigate the depth of the electrically active laser damage as function of laser wavelength and laser pulse duration. By etch-back experiments, we find that the laser damage from picosecond laser pulses is confined to a considerably shallower region than what is the case for nanosecond pulses. This is as expected due to the longer available times for heat conduction experienced in the latter case. However, the depth of damage is also much shallower than what the linear optical absorption coefficient would suggest, pointing towards non-linear optical confinement. We also develop an analytical expression for the effective minority carrier lifetime measured on a wafer with a laser damaged region, and from this expression, we are able to give an estimate on the lifetime in the laser damaged region. Based on these findings, we develop an optimized laser process. Using a wavelength of 515 nm and a pulse duration of 3 ps, an effective lifetime of 1.8 ms is completely recovered after removal of just 240 nm of silicon from the wafer surface. The lifetime in the laser damaged region is in this case estimated to be on the order of 1 ns.