Nanoscale investigation of the interface situation of plated nickel and thermally formed nickel silicide for silicon solar cell metallization

•Adhesion of metallization of fully plated nickel–copper contacts on silicon solar cells can be achieved by formation of nickel silicide at the cost of degraded cell performance.•Understanding of silicide growth mechanisms and controlled growth may lead to high performance together with excellent adhesion.•Silicide formation is well known from CMOS production from PVD-Ni on flat surfaces. Yet the deposition methods and therefore layer characteristics and the surface topography are different for plated metallization.•TEM analysis is performed for differently processed samples.•A nickel silicide growth model is created for plated Ni on textured silicon solar cells. In the context of nickel silicide formation from plated nickel layers for solar cell metallization, there are several open questions regarding contact adhesion and electrical properties. Nanoscale characterization by transmission electron microscopy has been employed to support these investigations. Interfacial oxides and silicide phases were investigated on differently prepared samples by different analytical methods associated with transmission electron microscopy analysis. Processing variations included the pre-treatment of samples before nickel plating, the used plating solution and the thermal budget for the nickel–silicon solid-state reaction. It was shown that interface oxides of only few nm thickness on both silicon and nickel silicide are present on the samples, depending on the chosen process sequence, which have been shown to play an important role in adhesion of nickel on silicide in an earlier publication. From sample pretreatment variations, conclusions about the role of an interfacial oxide in silicide formation and its influence on phase formation were drawn. Such an oxide layer hinders silicide formation except for pinhole sites. This reduces the availability of Ni and causes a silicide with low Ni content to form. Without an interfacial oxide a continuous nickel silicide of greater depth, polycrystalline modification and expected phase according to thermal budget is formed. Information about the nature of silicide growth on typical solar cell surfaces could be obtained from silicide phase and geometric observations, which were supported by FIB tomography. The theory of isotropic NiSi growth and orientation dependent NiSi2 growth was derived. By this, a very well performing low-cost metallization for silicon solar cells has been brought an important step closer to industrial intro