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A Phenomenological Model for the Contact Formation of Ag/Al Screen-Printing Pastes through SiN x :H Layers

The process of contact formation of Al containing Ag screen-printing pastes to p emitters is investigated by an electron microscopic analysis of etched-back contacts (aqua regia and hydrofluoric acid) and polished contact cross-sections. Ag/Al contact spots grown into the Si surface are detected below inhomogeneously structured areas of the bulk contact that contain Al. In addition, Si accumulations are found in the Ag/Al phase in these parts of the contact after contact formation. It is concluded that during the firing process Ag and Al are transported from the bulk contact to the Si surface, where Ag/Al contact spots grow. The solved Si diffuses in the parts of the contact containing Al. Despite the diffusion of Si, Ag and Al, the SiNx:H layer below the inhomogeneous part of the contact and especially above the Ag/Al contact spots is not completely etched away, as is shown by EDX measurements using top view and cross-sectional samples. Based on the observations made by SEM and EDX analysis, a phenomenological model for the exchange of Si, Ag and Al through holes in the SiNx:H layer and the growth of Ag/Al contact spots below the SiNx:H layer is presented.