Comparison of picosecond laser sources for SiN_x ablation with subsequent nickel silicide formation by Excimer Laser Annealing (ELA) for high efficiency silicon solar cells

摘要

We compare two types of laser ablation for ARC removal on polished and textured surfaces. Selective ablation with limited impact on the underlying substrate is performed with short wavelength picosecond sources working at relatively low repetition rate (＜200 kHz). By adapting wavelength and fluence, the SiN_x could be removed efficiently with slight change initial topography. Crystal damage is detected whatever the laser parameters but could be reduced using low fluence in UV regime. The second ablation process uses ultra-high repetition rate picosecond laser (80 MHz) and targets both SiN_x ablation and over-doping of the initial n~+ emitter. The thermal effect induced by the short duration between pulses performs simultaneously SiN_x removal and selective emitter structure with with deep dopant profiles and low surface concentration. We investigate the correlation between the post-ablation properties and a nickel silicidation process using Excimer Laser Annealing of a thin layer of Ni. A reference process is first described on pyramid topography without pre-ablation of SiNx. It is demonstrated efficient formation of SixNiy compounds on the silicon substrate depending and laser fluence. The similar silicidation process is transferred on sample after the SiN_x ablation step. A continuous nickel silicide layer is observed but its thickness distribution reveals non-uniformity over the pyramids due to post ablation roughness.