3.1 Synthesis and purification of bulk semiconductors  (Page 2/7)

The overall reduction reaction of SiO ₂ is expressed in [link] , however, the reaction sequence is more complex than this overall reaction implies, and involves the formation of SiC and SiO intermediates. The initial reaction between molten SiO ₂ and C ( [link] ) takes place in the arc between adjacent electrodes, where the local temperature can exceed 2000 °C. The SiO and CO thus generated flow to cooler zones in the furnace where SiC is formed ( [link] ), or higher in the bed where they reform SiO ₂ and C ( [link] ). The SiC reacts with molten SiO ₂ ( [link] ) producing the desired silicon along with SiO and CO. The molten silicon formed is drawn-off from the furnace and solidified.

The as-produced MGS is approximately 98-99% pure, with the major impurities being aluminum and iron ( [link] ), however, obtaining low levels of boron impurities is of particular importance, because it is difficult to remove and serves as a dopant for silicon. The drawbacks of the above process are that it is energy and raw material intensive. It is estimated that the production of one metric ton (1,000 kg) of MGS requires 2500-2700 kg quartzite, 600 kg charcoal, 600-700 kg coal or coke, 300-500 kg wood chips, and 500,000 kWh of electric power. Currently, approximately 500,000 metric tons of MGS are produced per year, worldwide. Most of the production (ca. 70%) is used for metallurgical applications (e.g., aluminum-silicon alloys are commonly used for automotive engine blocks) from whence its name is derived. Applications in a variety of chemical products such as silicone resins account for about 30%, and only 1% or less of the total production of MGS is used in the manufacturing of high-purity EGS for the electronics industry. The current worldwide consumption of EGS is approximately 5 x 10 ⁶ kg per year.

Electronic-grade silicon (egs)

Electronic-grade silicon (EGS) is a polycrystalline material of exceptionally high purity and is the raw material for the growth of single-crystal silicon. EGS is one of the purest materials commonly available, see [link] . The formation of EGS from MGS is accomplished through chemical purification processes. The basic concept of which involves the conversion of MGS to a volatile silicon compound, which is purified by distillation, and subsequently decomposed to re-form elemental silicon of higher purity (i.e., EGS). Irrespective of the purification route employed, the first step is physical pulverization of MGS followed by its conversion to the volatile silicon compounds.

A number of compounds, such as monosilane (SiH ₄ ), dichlorosilane (SiH ₂ Cl ₂ ), trichlorosilane (SiHCl ₃ ), and silicon tetrachloride (SiCl ₄ ), have been considered as chemical intermediates. Among these, SiHCl ₃ has been used predominantly as the intermediate compound for subsequent EGS formation, although SiH ₄ is used to a lesser extent. Silicon tetrachloride and its lower chlorinated derivatives are used for the chemical vapor deposition (CVD) growth of Si and SiO ₂ . The boiling points of silane and its chlorinated products ( [link] ) are such that they are conveniently separated from each other by fractional distillation.