Localised metallisation process for silicon solar cells

In photovoltaic field, specifically for the silicon solar cells, one of the today challenges is to replace the screen-printed silver contact lines by other processes or materials with a nanometric precision. An alternative method consists in the deposition of nickel-copper contacts by wet deposition process on silicon substrate, inside lines etched throughout the anti-reflective layer. In this research work, a four-step process was chosen: 1. Ni(P) chemical deposition, 2. annealing to form a silicide, 3. fresh Ni(P) layer to guaranty the barrier effect and 4. copper metallisation. In this study, the deposition mechanisms of electroless nickel and the annealing procedure to generate a 100 nm-NiSi layer are studied. The Ni diffusion dependence with the initial Ni(P) layer thickness is demonstrated, combining an overall approach based on top and cross section imaging (SEM) and chemical analysis (EDS) reinforced by an accurate diagnostic by XPS depth profiling. Particularly, XPS profiles performed enabled us to finely determine the depth diffusion of the NiSi alloy, allowing to optimize the process to satisfy both the adherence between Si and the Ni/Cu stack and a low contact resistance of the final lines. The elaboration of the complete stack NiSi/Ni(P)/Cu is then performed and characterized. The selectivity of the metallisation process (electroless nickel and electrolytic copper plating) is certified by XPS line profile showing only low overburden on the anti-reflective layer. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim