Metal Contamination in the Diamond Wire Sawing Process of Silicon and Influence on the Solar Cell Efficiency
In diamond wire sawing (DWS) of silicon wafers a steel core wire is used with diamonds embedded in a particular coating layer. This layer is either a nickel-based metallic layer or a resin-based layer. Due to the different properties of the layer material, the density of diamonds on the wire and the wire wear during the sawing process differ strongly between these two types of wire coatings. It was investigated, how the different coatings affect the metal contamination of monocrystalline silicon wafers produced in diamond wire sawing processes. The process parameters were chosen to obtain the same silicon contact length, which is a measure of the wire wear, for both types of wires. A rigorous analysis of the wires and the wafers with respect to the metal contamination was done. For Ni-coated wires, sum concentrations of Ni and Co in the range of 1016 at/cm³ were obtained in layers up to 12 mm below the wafer surface. For resin coated this concentration was significantly lower. PERC solar cells were fabricated from these wafers and the conversion efficiencies are compared.