silicon is sliced repeatedly with a centerless diamond saw to produce individual wafers (Figure IV.B.l.a.15). The saw kerf may be as much as 0.500 mm and is a function of the diameter of the wafer. The wafers are typically rough cut at 0.250 mm minimum to minimize breakage from saw vibrations. Handling losses require that the wafers from large diameter ingots be cut proportionally thicker. Thin wafers are obtained by lapping the 0.250 mm wafers down to the desired thickness. Because of these factors, the solar cell area per unit weight of silicon ingot does not increase by producing wafers thinner than 0.750 mm although thinner wafers are required to minimize the weight of the solar arrays of the satellite power system. [4] SILICON LOSS/RECLAMATION The end pieces of the silicon ingot which were cut off are reused since they represent a significant amount of pure material. Such is not the case with the dust from the sawing operation because its surface is quickly oxidized. Reclamation of the dust has not been economically feasible thus far but will be in the future. The kerf loss for a 0.500 mm saw cut amounts to 66 percent of the volume of the cylinder of silicon. Lapping and polishing the wafers down to 0.100 mm results in the loss of another 20 percent of volume. When the rectangular cells are fabricated from round wafers, a further decrease in volume from edge loss is another factor. Figure IV.B.l.a.16 shows that 30 percent of each wafer, which will be diced onto 20 mm x 20 mm solar cell blanks, is lost because of this reason. This further reduces the maximum useful volume of an ingot to only 9.3 percent. To this must be added additional unrecoverable losses from handling, testing, etc. A complete listing of the ingot losses is shown in Table IV.B.l.a.2. It is clear that while the ingot process can produce excellent quality wafers, in its present state-of-the-art it is not amenable to the low-cost, high-volume production of silicon necessary for the SPS. (b) SILICON SHEETS There has been a considerable effort to develop methods of growing silicon in thin sheets. The advantage of such a method would be the elimination of sawing, lapping, and polishing operations which are necessary for the silicon blanks. The process could be a continuous flow, and it has been estimated that a cost reduction of 10 to 100 times could be realized if it is successful. One of the oldest sheet growth processes is the dendritic web technique (Figure IV.B.l.a.17). In this method, a molten film of silicon is pulled from the melt by being suspended between two dendrites. So far, this method has produced a better crystal structure with fewer defects than other silicon sheet processes. However, its growth rate is slower than the EFG (edge-defined film-fed growth) described below and the webs are somewhat thicker.
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