High Purity Silicon 184

High-purity polycrystalline silicon (polysilicon) is the starting material for nearly all semiconductors, and is the largest dollar volume chemical consumed by the electronics industry. Consumers melt the polysilicon and draw it into single-crystal ingots, then slice it into silicon wafers that are used to fabricate a variety of semiconductor devices.

Polysilicon with impurities in the low ppm range, is one of the purest elements manufactured commercially. In fact, the purity Is often measured by a practical test rather than chemical analysis--a resistivity in the 100-1000 ohm-cm range is required for semiconductor applications. Polysilicon that does not meet semiconductor-grade specifications may be suitable for sale as solar-grade polysilicon for less-demanding photovoltaic device manufacture.

During the late 1970s, the U.S. Department of Energy sponsored research programs to develop processes for making lower-cost polysilicon for solar cells. The proposed processes were less complicated and were expected to produce polysilicon containing a higher level of impurities than was allowable for semiconductor applications. However, interest in, solar-grade silicon has waned and none of these processes has been commercialized. Information on the proposed processes is relatively meager, and they have not been included in this report.

This report includes a review of the technology for making trichlorosilane (TCS), a key intermediate in the manufacture of polysilicon, the means of purifying TCS, and ways of converting it to silane. Processes are reviewed for producing polysilicon by decomposition of TCS or of silane.

We have prepared evaluations of an integrated process for producing polysilicon by decomposition of TCS (similar to the Siemens process), and of an integrated process for producing polysilicon by decomposition of silane (similar to a process practiced by Union Carbide).