Markus König

Development of a high-throughput fine line metallization process using CFD-simulation

In order to enhance dispensing technology towards an industrial application in Silicon Photovoltaics, in particular throughput rate has to be increased. For this reason, a novel parallel high precision fine line dispensing unit is currently being developed at Fraunhofer ISE providing one nozzle per contact finger and a central Paste supply. In order to ensure a homogeneous paste distribution to all nozzles, the influence of paste rheology on the flow profile of the dispensing nozzles was analyzed. An analytical comparison of two different dispensing pastes with water gave a good insight on the influence of paste rheology on flow patterns inside the dispensing nozzles. Furthermore, numerical CFD-simulation (CFD: Computational Fluid Dynamics) was used to investigate different nozzle geometries and finally print head designs. In various iteration steps, the influence of fabrication tolerances especially concerning the nozzle geometry was isolated and print head designs were optimized based on CFD towards maximum process stability. In the meantime, process optimization using a single nozzle approach led to an average finger width below 35 μm, confirmed by several characterization methods.

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.

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

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.

Contacting p+ emitters with Al containing Ag screen printing paste : a microscopic analysis

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.