On the Mechanism of Homogeneous Decomposition of the Chlorinated Silanes. Chain Reactions Propagated by Divalent Silicon Species Mark T. Swihart* and Robert W. Carr Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: September 30, 1997; In Final Form: January 6, 1998 A mechanism for the homogeneous gas-phase decomposition of SiHCl 3 , SiH 2 Cl 2 , and SiH 3 Cl in hydrogen is derived from the results of ab initio molecular-orbital studies. It consists of 39 reversible elementary reactions among 25 species, including pressure-dependent unimolecular decomposition of the chlorinated silanes and secondary chemistry due to reactions of SiH 2 , SiHCl, and SiCl 2 with one another and with the chlorinated silanes. Rate parameters in the mechanism have been calculated based on results of ab initio studies using transition-state theory and unimolecular rate theories. This allows us to construct a reasonably complete mechanism that provides qualitative explanations for several features of dichlorosilane decomposition that have been presented in the literature, including observations on the presence and concentrations of SiCl 2 , SiHCl, and Si atoms. Several chain reactions in which the chain carriers are divalent silicon species have been identified. Introduction The chlorinated silanes, particularly dichlorosilane and trichlo- rosilane, are used as precursors for the chemical vapor deposition (CVD) of epitaxial silicon. At the high temperatures where this process is carried out, homogeneous decomposition of the parent molecules may play an important role by generating reactive species that lead to film growth. Secondary reactions can both accelerate the decomposition of the precursor and consume reactive intermediates that could otherwise lead to film growth. To understand the mechanisms of film growth and develop detailed, physically based models of silicon epitaxy from the chlorinated silanes, we must be able to assess which, if any, homogeneous reactions are likely to be important at the reactor conditions and what the rates of these reactions are. This has not, so far, been possible for silicon epitaxy from dichlorosilane or trichlorosilane. Here, we present a reaction mechanism and rate parameters for the homogeneous thermal decomposition of the chlorinated silanes. The mechanism is applicable to homogeneous decomposition of SiH 3 Cl, SiH 2 Cl 2 , and SiHCl 3 in H 2 . Silane decomposition mechanisms are understood in more detail and have been presented and discussed elsewhere. 1-3 Additional chemical species and reactions would be required to describe SiCl 4 decomposition. A number of observations on thermal decomposition of the chlorosilanes have been published, but rate parameters and decomposition mechanisms have not been experimentally established. Laser pyrolysis of trichlorosilane 4 was observed to give HCl and a white powder that was assumed to result from SiCl 2 polymerization. Rate parameters were obtained and were presumed to correspond to the elementary reaction SiHCl 3 f SiCl 2 + HCl. There are several observations of dichlorosi- lane decomposition in the literature, but both the rate parameters and the primary reaction products still remain somewhat unclear. In IR multiphoton dissociation experiments, Sausa and Ronn 5 detected electronically excited SiCl 2 . Walker et al. 6 decomposed monochlorosilane and trichlorosilane in static bulb experiments, and dichlorosilane both in static pyrolysis experiments and using a single-pulse shock tube. They concluded that the initial dichlorosilane decomposition products were SiCl 2 and H 2 , based partially on the fact that very little HCl was observed as a product in their shock-tube experiments. They interpreted their experimental results in terms of a reaction mechanism that differs in some respects from the one presented here. On the basis of that mechanism, they obtain rate parameters for the three decompositions. Kruppa, Shin, and Beauchamp 7 found that SiCl 2 and HCl were the primary products of vacuum flash pyrolysis of SiH 2 Cl 2 . Ban and Gilbert 8 observed SiCl 2 by mass spectrometry under silicon CVD conditions, and Ho and Breiland 9 observed SiHCl by laser-induced fluorescence in a silicon CVD reactor. Ho, Breiland, and Carr 10 observed that 193-nm photolysis of SiH 2 Cl 2 produced SiHCl. In pulsed laser- powered homogeneous pyrolysis experiments, 11 we measured reaction rates consistent with an activation energy for unimo- lecular decomposition near 75 kcal/mol but were unable to identify the primary reaction products. Ab initio calculations 12,13 predict that the dominant unimo- lecular decomposition path for dichlorosilane is SiH 2 Cl 2 f SiHCl + HCl, with an activation energy near 75 kcal/mol. CVD reactor models that included reactions in the gas phase 14,15 have assumed that the reaction was SiH 2 Cl 2 f SiCl 2 + H 2 . These models also used rate parameters that gave reaction rates that were orders of magnitude larger than the rates of the unimo- lecular decomposition reactions predicted from the ab initio calculations. This apparent discrepancy could be partially resolved if there are secondary reactions in the gas phase that accelerate the overall decomposition and convert SiHCl to SiCl 2 . Consideration of such mechanisms can also shed light on the varied experimental observations of dichlorosilane decomposi- tion and on related processes in the decomposition of monochlo- rosilane and trichlorosilane. In this work, we present a mechanism and rate parameters based on ab initio calculations for the unimolecular decomposi- tion reactions of the chlorinated silanes and subsequent second- 1542 J. Phys. Chem. A 1998, 102, 1542-1549 S1089-5639(97)03174-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/11/1998