Diamond coated wire sawing of silicon ingots into individual wafers accounts for 11 percent of solar cell production costs. This cost is primarily associated with generation of silicon powder which is a waste stream and requires energy intensive recycling strategies. Thinner wires can lead to minimal powder generation during wire sawing. However, thinner steel wires (smaller than 100 microns in diameter) break easily under the stress required for efficient cutting of silicon. This Grant Opportunity for Academic Liaison with Industry (GOALI) award supports scientific investigations on the use of carbon fiber wires smaller than 100 microns in diameter in diamond coated wire sawing of silicon ingots into wafers. Research results from this project will lead to reduced wastage of silicon and, therefore, reduced dollar-per-watt of silicon based solar cells.
The first research objective is to establish relationships between sawing conditions (such as wire velocity and normal force) and the chemical and structural changes of individual diamond particles coated on a carbon fiber wire. The changes observed in the diamond particles are softening (i.e., graphitization), cleavage, blunting, or complete removal from the carbon fiber wire. The approach to achieve this objective is to measure chemical and structural changes after sawing in diamond particles, carbon fiber wire, and generated silicon powder using micro-Raman spectroscopy. This technique can map chemical and structural changes in the diamond particles, carbon fiber wire and the silicon powder with micron scale spatial resolution. An industrial sawing machine, and single crystalline silicon ingots with a 15 cm x 15 cm square cross-section, will be used for sawing experiments. Wire velocities of up to 25 m/s and a normal force of up to 15 N will be used. The second research objective is to understand the effects of wear of diamond particles and carbon fiber on sawing rate. The approach to achieve this objective is to measure, in situ, wire bowing of the diamond coated carbon fiber wire as it performs sawing. This allows estimation of wire tension and frictional forces experienced by the wire inside the silicon kerf during sawing. As wear proceeds, wire bowing increases and so do frictional forces. Sawing rate will be measured as the vertical velocity of the silicon ingot as it presses down on the wire. Parameters such as carbon fiber wire diameter (from 60 µm to 100 µm) and diamond size (from 6 µm to 20 µm) will be varied.
