11. Wafer Processing

The fabrication of silicon wafers for solar cells and modules is an expensive step in the processing chain. The technological development is therefore primarily driven by the need to reduce cost. The dominant wafering method is multi-wire sawing with a straight steel wire and an abrasive slurry consisting of polyethylene glycols (PEG) and SiC powders (loose abrasive sawing). Substantial cost reductions are possible with structured steel wires or wires coated with diamond particles (fixed abrasive sawing) and the replacement of PEG by water-based fluids. Apart from the cost, the wafer qualities such as thickness variations, roughness, subsurface saw damage, and fracture stability play an important role and have to be improved as well. These factors depend on many sawing parameters, which makes optimization a difficult task. The chapter describes the requirements on the sawing machines, the wires, the slurries, the wafer quality, and the experimental methods, which have been developed to characterize wafers and the consumables. The fundamental micromechanical sawing processes and models are also described. Their knowledge is helpful to improve the sawing process in a controlled way. Alternative wafering methods and their perspectives are presented briefly.