Kinetic aspects of the contact formation by glass containing silver pastes
The glass addition in a silver paste has a key role for the adhesion of the silver track, on one hand, and for a homogeneous formation of an interfacial microstructure showing a sufficient number and size of silver precipitates, on the other hand. In our study, time and temperature dependencies of the sintering density and formation of interfacial layers were examined by interrupted firings of metallized wafers. The cross sections of intermediate states of the sintered microstructures of the model pastes were visualized by focused ion beam preparation and SEM. While the silver finger becomes densified a distinct fraction of the paste glass reacts at the interface in terms of silver dissolution, transport, precipitation and emitter etching, while the layer is growing continuously in thickne ss. Thereby, the ohmic resistivity of the glassy interface layer is simultaneously lowered by Ag-precipitation and increased by thickness. The P-doped emitter layer is partly etched by the reactive glass melt at the interface. In other words, the contact resistivity of the P-doped silicon surface is increased continuously. The specific contact resistivity of the interface can be referred to as a superposition of the specific resistances of the silver enriched glassy interface layer and the etched, phosphor doped Si-surface. By optimizing powder geometry, silver densification characteristics and sinter onset of the glass phase it is possible to control the active amount of paste glass at the interface paste/wafer. The effects found on a model paste were examined in praxis on the example of new PbO free paste compositions for the front side metallization of solar cells.