HEATS OF FORMATION OF SIHMFN CALCULATED BY ABINITIO MOLECULAR-ORBITAL METHODS

Reliable heats of formation are known for SiHn and SiF 4, but experimental data for some of the other perfluorosilanes and the mixed SiHmFn 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. 88, 6254 ( 1988) ] on the ΔHf0 of these compounds. The present calculations have been carried out at a higher level of theory [Møller-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 ΔHf0 for SiH n and SiFn 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 AH f0 for SiHn and SiF4 and the calculated ΔHr 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 ΔHf 0 of SiHn 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; SiHF 2, - 142.6 ± 2; SiF3, - 240.7 ± 2; SiH 3F, - 85.8 ± 2; SiH2F2, - 186.3 ± 2; SiHF3, - 288.2 ± 2 kcal/mol. © 1990 American Institute of Physics.