Mechanisms of lifetime degradation in Si/ARC samples patterned by laser lift-off

摘要

Applications for laser patterning in Si photovoltaics include (ⅰ) patterning SiO_2 or SiN layers with openings for local contacts and (ⅱ) laser-doped selective emitter (LDSE) processes, in which the contact open is accompanied by the diffusion of dopants into a locally melted Si area. While contact open processes are best performed with UV wavelengths that can be strongly absorbed by the SiN or SiO_2 (allowing layer ablation with a minimum of Si heating), the Si melt depth required by LDSE requires irradiation at longer laser wavelengths where these antireflection coatings (ARCs) no longer absorb well. An optimized LDSE process must thus produce Si melting as well as the least amount of Si vaporization sufficient to lift off the overlying ARC. In this work, we investigate the mechanisms for lifetime degradation in Si(p-type, 100-oriented)/ARC samples resulting from 20 ns pulsed laser irradiation at 532 nm at fluences near the threshold for ARC removal. To differentiate between lifetime degradation induced by changes in the passivation layer vs. changes in the Si itself, samples were lifetime mapped after patterned laser irradiation and then again after a wet ARC strip and repassivation. Samples with ARCs of thermal SiO_2 and PECVD SiN typically showed some residual Si damage after irradiation at fluences sufficient for contact open. Interestingly, irradiation of the SiO_2 samples at lower fluences, between the threshold for Si melting and ARC removal, showed damage to the SiO_2 passivation, but no residual Si damage. Explanations for these observations and related results will be discussed.