New Approach for the Ablation of Dielectrics from Silicon Using Long Wavelength Lasers

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.