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

Abstract

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 BBr 3 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 SiN x :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 screenprinting pastes to p + emitters through a SiN x :H layer.

Summary of the 5th Workshop on Metallization for Crystalline Silicon Solar Cells

“…In this model, contacts grow in localized areas through openings of the SiNx:H coating, and SiNx:H acts as a mould during the growth of the Ag/Al contacts spots ( Fig. 1) [12][13][14][15]. Furthermore, it was shown that increasing the Al content limits the densification of the screenprinted metal and leads to modified glass distribution in the interface structure [16,17].…”

Crystalline Nature of Metal Spikes and Silicon Inclusions in Ag/Al Screen-Printing Metallization

“…The wafers were treated in different ways to realize different surface structures: Wafers were either etched in NaOH to realize plane (100)-oriented surfaces or alkaline textured to obtain a pyramidal surface texture with (111)-oriented pyramid planes or plasma textured [16]. After cleaning, a 50-Ω/ emitter was diffused in a BBr 3 -based open tube furnace process.…”

“…However, up to now, most studies dealing with the microstructure of screen-printed Ag/Al contacts were based on scanning electron microscopy (SEM) analysis and, therefore, lack of information about the crystallographic structure and orientation of the Ag/Al contact spots [11], [15], [16]. In addition, the spatial resolution of SEM energy-dispersive X-ray (EDX) spectroscopy is limited to >1 μm as a result of the large activation volume of the electrons in a bulk sample.…”