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The semiconductor and PV industries presently consume about 80,000 Metric Tons (MT) per year, the full current capacity of all polysilicon suppliers. These large users each produce millions of silicon wafers each month. This requires thousands of tons of polysilicon each month, which is used to produce the silicon ingots and wafers needed to fill large orders from the Integrated Circuit Industry and the Photovoltaics Industry.
These large users concentrate on supplying the top 50 –100 device manufacturers with high technology wafers, producing the larger silicon wafer diameters (200 and 300 mm), with exacting purity, geometric, electrical and crystal perfection standards. They also supply the photovoltaic cell producers with wafers sizes and crystallinity designed for high efficient, cost effective solar cells.
The large producers of silicon wafers, such as Shin-Etsu Handoti, MEMC, Wacker, Sumco, and KEM, supply over one hundred million silicon wafers to the large integrated circuit manufacturers, such as Intel, Texas Instruments, Motorola, AMD, Toshiba, and Samsung, so require thousands of metric tons (MT) per year of polysilicon. There are over 300 semiconductor companies with over 1000 IC fabrication lines using silicon wafers.
Czochralski Crystal Growth
The semiconductor wafer suppliers use the Czochralski (Cz) crystal growth process to convert polysilicon chunks into single crystal silicon ingots. In the Cz process, chunks of polysilicon are loaded into a quartz crucible in a Cz growth furnace. The crucible is heated to about 1400 deg. C., melting the polysilicon into a liquid. A small single crystal rod (seed) of the proper crystalline orientation is dipped into the rotating melt and the seed rod slowly withdrawn, while rotating. By controlling the temperature, pull rate, and rotation rates, perfect single crystals of 4, 6, 7, 8, or 12 inch diameter can be grown. The process takes about one and a half days for ingots for IC use, and a shorter time for ingots for solar cells, where the degree of crystallinity is not as critical.
The diameter of the crucible determines how much polysilicon can be loaded into the crucible. A crucible at 18 inch diameter will hold about 60 kg of poly, allowing growth of a 27 inch long 8 in. diameter ingot. A crucible at 24 inch diameter will hold about 150 kg of poly, allowing growth of a 67 inch long 8 in. diameter ingot.
The smaller diameter top and tail of the ingot are about 20% of the ingot length; and about 5% of the molten silicon is left in the bottom of the crucible, called potscrap. The tops and tails can be reused in another growth run or sold in the solar cell market. The potscrap can be processed, removing the attached quartz, and reused or sold in the solar cell market.
The solar cell wafer suppliers use the Czochralski crystal growth process, the Modified Cz crystal growth process (EFG, string, ribbon), or the Multi-Crystalline Growth Process (Directional solidification, casting) to produce silicon ingots, ribbons, or blocks for processing into round or rectangular wafers.