Heats of formation of SiHmFn calculated by a b i n i t i o molecular orbital methods

Abstract

Reliable heats of formation are known for SiH_n and SiF₄, but experimental data for some of the other perfluorosilanes and the mixed SiH_mF_n compounds are less reliable or unavailable. Because of the importance of these compounds in chemical vapor deposition (CVD) and etching silicon and because of the continued uncertainty in some of the heats of formation, we have extended our previous calculations [J. Phys. Chem. 8 8, 6254 (1988)] on the ΔH⁰_f of these compounds. The present calculations have been carried out at a higher level of theory [Mo/ller–Plesset perturbation theory including single, double, triple, and quadruple (MP4SDTQ)] with larger basis sets [6‐31G(2d,2p), 6‐31++G(d,p), 6‐31++G(2d,2p)] for the entire set of molecules. To establish accurate theoretical estimates of ΔH⁰_f for SiH_n and SiF_n additional calculations were performed on SiX, SXY, SiH₃, and SiH₂XY (X,Y=H,F) at the MP4SDTQ level with even larger basis sets [6‐31G(3d,3p), 6‐31G(df,p), 6‐311G(d,p), 6‐311G(2df,2p), 6‐311++G(2df,2p)]. Based on the experimental ΔH⁰_f for SiH_n and SiF₄ and the calculated ΔH_r for isodesmic reactions, SiF is found to be 9 kcal/mol more stable and SiF₂ is found to be 12 kcal/mol more stable than the currently accepted values; the calculated ΔH⁰_f of SiH_n and SiF₃ agree with the value advocated by Walsh. The theoretical estimated heats of formation (298 K, ideal gas) are: SiH, 89.9±2; SiF, −14.2±2; SiH₂, 65.7±2, SiHF, −37.8±2; SiF₂, −153.0±2; SiH₃, 47.7±2; SiH₂F, −44.9±2; SiHF₂, −142.6±2; SiF₃, −240.7±2; SiH₃F, −85.8±2; SiH₂F₂, −186.3±2; SiHF₃, −288.2±2 kcal/mol.