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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 515nm and a pulse duration of 3 ps, an effective lifetime of 1.8ms is completely recovered after removal of just 240nm of silicon from the wafer surface. The lifetime in the laser damaged region is in this case estimated to be on the order of 1ns. [Copyright &y& Elsevier]

Abstract: Laser ablation of dielectrics from silicon substrates represents a useful technique for e.g. the creation of local contacts. However, these dielectrics are transparent at the laser wavelengths normally employed for silicon solar cell processing, i.e. the first, second and third harmonics of solid state lasers (1064, 532 and 355nm). As a result of this, the ablation is indirect, and follows from energy deposition in the silicon rather than in the dielectric. This mechanism introduces defects in the silicon substrate, an effect which is detrimental to solar cell performance. Attempts have been made to limit the extent of the laser damage, by going to shorter wavelengths and shorter pulse durations. In this work, we suggest an alternative route to low-damage ablation of dielectrics by application of long wavelength laser pulses from e.g. CO2-lasers. At wavelengths above approx. 8μm, we find absorption bands in many of the dielectrics applied in solar cells. Simulations show that it may be possible to keep the silicon temperature below melting temperature, while reaching vaporization temperature in the dielectric. Experiments using laser pulses at 9.3μm with a duration of approx. 100ns show, however, that the silicon substrate experiences melting. We conclude that even shorter pulses must be applied for the method to be successful. [Copyright &y& Elsevier]