The Origin of Background Plating

In the last years efforts were made to overcome the widespread screen printing metallization technique to reduce shading of the front side, increase the finger conductivity, and improve the contact properties of the metallization of crystalline silicon solar cells. A promising approach is plating via wet chemical metal deposition. Hereby, the metals nickel as contact layer and copper or silver as conduction layer are alternatives to the thick film screen printing process. One challenge using this technique is to avoid the unwanted background plating where the metal is not only deposited on the opened emitter structures but also partly on the isolating surface of the silicon nitride layer.In this paper a detailed overview of the origin of background plating is given, documented by scanning electron microscope pictures. By cutting a groove in the surface of the sample and milling the unwanted metallization as well as the silicon underneath in 12.5 nm steps with a focused gallium ion beam, it was possible to investigate the cross sections by EDX and reveal different origins for the background plating behavior.