p+ Emitters on n-Type c-Si dry-etched with nitrogen trifluoride and passivated with ALD aluminum oxide

The photovoltaics industry is expected to slowly transition from p-type mono-Si cells to n-type mono-Si over the next several years. Diffusion of boron into silicon to fabricate a p+ emitter can result in an efficiency-reducing boron rich layer (BRL) which can be removed reliably with an NF₃ dry-etch process. The emitter should be passivated with a film that does not result in minority carriers being attracted to its surface. Boron was diffused from a-Si films deposited by PECVD on both sides of n-type Si substrates of 3.3 ohm-cm. The emitters were dry etched with nitrogen trifluoride (NF₃). Best results for effective lifetime (198 μs) and emitter saturation current (31.6 fA/cm²) were realized on a sample etched to 89 Ω/□ from 71 Ω/□, then passivated with 10 nm of Al₂O₃ and 70 nm of SiN_X and subsequently annealed in N₂/H₂ @ 450°C for 60 minutes.