Formation of methyl radicals from decomposition of methyl-substituted silanes over tungsten and tantalum filament surfaces.

The formation of methyl radical from the decomposition of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), over tungsten and tantalum filament surfaces has been systematically studied using vacuum ultraviolet laser ionization mass spectrometry. The methyl radical intensity increases with temperature for both filaments in the low-temperature region; however, beyond the optimum temperature, a gradual decrease in the methyl intensity was observed for MMS, DMS, and TriMS when using Ta, whereas the intensity reaches a plateau with W. This is due to the fact that Ta is more efficient in releasing surface-bound H and forming active sites, leading to the adsorption of methyl radicals on the metal surface in the high-temperature regions. The apparent activation energy for methyl radical formation from the dissociation of MMS, DMS, TriMS, and TMS molecules on both W and Ta filaments increases with the increasing number of methyl substitution. The dissociation process is believed to be initiated by the Si-H bond cleavage and followed by Si-CH3 bond breaking. The obtained low activation energy values for methyl radical formation in the range of 51.1-84.7 kJ·mol(-1) suggest that the ejection of CH3 radicals is accompanied by the formation of a Si moiety bound to the metal surface. Overall, TMS produces the least number of methyl radicals on both filaments with the highest activation energy. The numbers of methyl radicals produced when using MMS, DMS, and TriMS are similar, but MMS gives the lowest activation energy.