Prediction of Elementary Reaction Mechanism for the CVD Process in Si2Cl6-H2 System using Semi-Empirical Molecular Orbital Method

From our previous experimental study of the Si chemical vapor deposition in Si2Cl6-H2 system, the following elementary reaction mechanism was proposed: decomposition of Si2Cl6 to produce SiCl2, which becomes the adsorbent; Si2Cl6(g) -> 2SiCl3(g) -> SiCl4(g) + SiCl2(g) (-> SiCl2(a)), and hydrogen reduction reaction of the absorbed SiCl2 to produce Si(s). The preliminary quantum chemistry study for this reaction mechanism has shown that the reaction is so infeasible that the above-mentioned elementary reaction mechanism is unreasonable. The most feasible and appropriate reaction mechanism has been shown as follows: (Step 1) Si2Cl6(g) -> SiCl4,(g) + SiCl2(g) (Step 2) SiCl2(g) +a -> SiCl2(a) (Step 3) SiCl2(a) + H2(g) -> SiCl(a) + 2HCl(g) (Step 3-1) SiCl2(a) + H2(g) -> Si-2HCl(a) (Step 3-2) Si-2HCl(a) -> Si(a) + 2HCl(g) where, a is the active site on silicon surface. On the assumption that step 3 is the rate-determining step, this elementary reaction mechanism can explain the experimental observation in the rate expression on the concentration dependence with respect to Si2Cl6 and H2.