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.