Silicon ingot casting: Heat Exchanger Method (HEM)/multi-wire slicing: Fixed Abrasive Slicing Technique (FAST), Phase IV. Quarterly progress report No. 2, April 1, 1980-June 30, 1980

Abstract

Silicon ingot size cast by HEM has been extended to 34 cm x 34 cm x 10 cm. A 20 kg ingot has been solidified at 3 kg/hr with no crucible attachment or ingot cracking problems. Another ingot of 26 kg weight has also been solidified. The heat treatment used to develop a graded structure caused cracking on the inside surface of the first large crucibles. The thermal conditions were altered to minimize high gradients and the cracking was eliminated. A high degree of single crystallinity has been maintained as the size of the ingots has been increased. A graphite retainer made out of flat plates was used to produce an ingot with flat sides and rounded curves. It is now possible to electroplate diamonds only on the cutting edge of the wire. The advantages associated with diamonds on the cutting edge only are lower kerf, improved accuracy by improved seating in the support rollers, and less degradation of the rollers. This has resulted in less wander of wires and will reduce costs by using less diamonds and less degradation of rollers. The main failure mechanism of wires - diamond pullout - has been minimized by using filler diamonds to prevent erosion of the nickel matrix. It has been shown that an electroplated wirepack can be used to slice three 10 cm diameter silicon ingots without significant diamond pullout. IPEG analysis of value added costs of sheet formation using conservative and optimistic extension of HEM and FAST technologies yields $27.05/m/sup 2/ ($0.191/w) and $13.49/m/sup 2/ (0.095/w), respectively. Assuming cost goals of other tasks are met, the projected costs are $0.654/w, conservatively, and $0.539/w, optimistically, for photovoltaic modules.