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Microstructure analysis of the interface situation and adhesion of thermally formed nickel silicide for plated nickel-copper contacts on silicon solar cells
For low cost high efficiency fully plated NiCu contacts on silicon solar cells, a thermal formation of nickel silicide is commonly performed for adhesion promotion. Since the results of this strategy have been found to be ambiguous, and as the interface situation is not understood, a microstructural analysis of the metal-silicon interface after thermal annealing of plated nickel layers on silicon solar cells has been carried out. The point of failure upon performing solder and peel testing on sintered contacts has been identified to be at the silicide-nickel interface. Microstructural analysis shows that void formation can be observed at this interface. The mechanisms of silicide and void formation from plated layers on solar cells are studied taking into account different electrolytes and pretreatments. The cavities can be removed by selective nickel etching and replating nickel onto the formed silicide after silicide pretreatment. Excellent adhesion of up to 2.5 N/mm has been measured, which represents the force of silicon wafer breakage rather than true metal adhesive force.