Monitoring of adhesion for plated metallisation: why busbar pull tests are not sufficient

Copper plating can reduce the consumption of silver for silicon photovoltaic manufacturing, whilst also offering the potential to increase cell efficiency by way of reduced shading due to very narrow fingers and contacting silicon surfaces with low phosphorus concentrations. However, it can be challenging to plate busbars and fingers that have sufficient adhesion to the silicon. To date adhesion has typically been assessed by busbar pull tests however we propose that this measurement does not consider the properties of copper-plated fingers and may not be a good indicator of whether fingers may dislodge or peel with subsequent processing or during module fabrication. In this paper we investigate the factors of silicon interface roughness and plated copper properties using a combination of busbar pull tests and stylus-based adhesion measurements. We show that average 180° pull test forces of 2.1 N/mm can be achieved when a UV ps laser is used to ablate the silicon nitride, however ensuring strong finger adhesion is a far more complex problem with no accepted standard to determine what is “sufficient” adhesion. Although use of a ps laser to ablate the silicon nitride can result in plated metal adhering so strongly to the silicon that fragments of silicon are broken off with the finger when it is dislodged by the stylus, use of fast plating rates can result in reduced finger dislodgement forces and excessive finger peeling whereas busbar pull test forces are largely unchanged because the increased plating current is directly mostly through the fingers of the plated metal grid during light-induced plating. The plating of busbars and fingers on a cell presents challenges for uniform silicon-metal adhesion and this paper highlights the importance of finger adhesion measurement for process quality control for nickel/copper plating of p-type silicon cells in a manufacturing environment.