trichlorosilane is converted into solid polycrystalline semiconductor grade (99.999% Si) by a thermoelectric decomposition (Figure IV.B.l.a.12) process described by the following: It should be noted that over 200 possible reactions exist for producing silicon material. Twenty of these are analyzed under contract to the ERDA (Energy Resource and Development Administration). Of this group, three appear to be the most promising and include the technique previously described. [2] CRYSTAL GROWTH Single crystal material is necessary to maximize the efficiency of silicon solar cells. Single crystal silicon ingots are grown from the high purity polysilicon. There are innumerable methods of growing single crystal silicon ingots of which many are simply modifications of each other. The most widely used method is the Czochralski method which uses a rotating crucible of molten silicon. A counter rotating crystal is dipped into the crucible and then slowly withdrawn from the molten liquid (Figure IV.B.l.a.13). The silicon adheres to the seed crystal and solidifies as the crystal is withdrawn from the melt. In this manner an ingot is "pulled" and retains the orientation of the seed. Another method which is sometimes used to produce ingots is the float zone technique (Figure IV.B.l.a.14) which employs a long vertical polycrystalline cylinder of silicon. In this process a small section of the polycrystalline silicon cylinder is melted and allowed to solidify. A seed crystal is placed at the top of the melt, and an r-f induction coil is moved slowly from the top of the cylinder downwards to melt the silicon. As the melted section cools, it crystallizes in the orientation of the single crystal. This method has the advantage that there is no crucible in contact with the silicon, therefore less contamination and defects within the crystal. Because of this fact, zone refined silicon is more radiation resistant than Czrochralski silicon. However, today's Czochralski crystal pullers produce large volumes of ingots with few defects and are the most commonly used method of growing silicon ingots. The crystal growing process produces a single crystal ingot up to 0.075 meters in diameter and more than one meter in length. [3] WAFER FABRICATION The silicon ingot is sliced perpendicular to its length near each end to form a cylindrical shape. A grinding operation is then used to eliminate the surface irregularities which resulted during the crystal growing operation. Next, the cylinder of
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