Microscopic thick film contact formation mechanism to p(p+) Si and influence of the metal contact on the Si substrate - lessons learned from the KONSENS project

Abstract

The German research project KONSENS investigated the contact formation of screen-printed contacts to crystalline silicon. Special focus was set on contact formation to boron emitters using Al containing Ag thick film pastes and on the formation of local Al contacts. The contact resistances to B emitters using Al containing Ag thick film pastes show dependence on surface topography, i.e. the surface texture. Regarding paste composition, we found that contact formation may be influenced by the amount of Al, as well as by the amount of glass frit. The SiNX:H layer serves as a mask during contact formation preserving the Si surface structure above the contact points. A small hole within the SiNX:H layer allows for material exchange and thereby facilitates contact formation. This means that the contact spots show the same surface morphology as the Si. Additionally, we find that the contact spots are crystalline and consist mainly of Ag. The pn-junction can be smeared out at points of direct metal-silicon contact. Our data suggest that the Al from the Ag thick film paste does not lead to local Al doping underneath the contact spots. Our findings can be explained by a model for contact formation of Al containing Ag thick film pastes to B emitters. The formation of Al rear contacts is influenced by the Al particle size in the Al thick film paste. Smaller Al particles allow for a more homogenous full area Al BSF. On PERC rear structures smaller Al particles in the Al thick film paste result in a thicker local Al BSF and less maximum spread distance of diffused Si in the Al thick film paste.