Recovery of lower-boiling silanes in a CVD process

A process for the deposition of semiconductor-grade silicon by reductive chemical vapor decomposition of a precursor silane selected from a group consisting of chlorosilane, dichlorosilane, and trichlorosilane, the process comprising

• A. feeding the precursor silane as a vapor with hydrogen gas into a deposition vessel containing a heated substrate of semiconductor-grade silicon;
• B. forming and depositing semiconductor-grade silicon on the substrate;
• C. passing effluent gases from the deposition vessel to a means for separating a mixture enriched in lower-boiling silanes from the effluent gases;
• D. combining the mixture enriched in lower-boiling silanes with additional tetrachlorosilane, the proportions of the mixture and the additional tetrachlorosilane being controlled so that there is present in the combination less than about 1.0 mole of hydrogen bonded to silicon per mole of total silicon;
• E. passing the combination of the mixture enriched in lower-boiling silanes and the additional tetrachlorosilane through a bed of a solid catalyst, the catalyst being effective in disproportionation of hydrogen-containing silanes and chlorine-containing silanes and being essentially free of water;
• F. facilitating disproportionation of hydrogen-containing silanes and chlorine-containing silanes to produce a stream that is reduced in content of silane (SiH.sub.4), chlorosilane, and dichlorosilane and increased in content of trichlorosilane;
• and
• G. isolating and separating the trichlorosilane.