Preservation of Si Surface Structure by Ag/Al Contact Spots : An Explanatory Model

In the past years, the contact formation of Ag screen-printing pastes to n+ emitters has been profoundly investigated and at least in parts explained. However, p+ emitters cannot be contacted well with standard Ag pastes. It has been shown that adding Al to Ag screen printing pastes leads to lower contact resistances. Therefore different mechanisms must play a role in the contact formation process. The role of Al and the exact mechanism of contact formation of these Al containing Ag screen-printing pastes have not been well understood up to now. A drawback of Al containing pastes is that metal spikes growing into the Si wafer can be deep enough to corrupt the space charge region and contact the base thus shunting the pn-junction. A better understanding of the contact formation process is necessary to enable the development of improved screen-printing pastes with a reduced probability of shunting. In this work the influence of differently structured Si surfaces on the contact formation to a BBr3 based boron emitter is investigated. The Ag/Al contact spots that grow into the Si surface show the same surface structure as the surrounding Si. It is concluded, that the SiNx:H layer acts as a mould of the Si surface for the growth of the contact spots. The presented observations are then explained by a recently introduced model for the contact formation of Al containing Ag screen-printing pastes to p+ emitters through a SiNx:H layer.