Investigation of gas phase chemistry of alkylsilane molecules in a hot wire chemical vapor deposition reactor

Date
2007
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Abstract
Hot-wire chemical vapor deposition (HWCVD) is a new and promising technique to fabricate device quality thin films. It is believed that gas-phase reactions play an important role in the deposition of high quality films. In this work, three alkylsilane molecules, namely tetramethylsilane (TMS), trimethylsilane, and hexamethyldisilane (HMDS), were used as single source gases in a HWCVD reactor for SiC film deposition. 118 nm vacuum ultraviolet (VUV) laser single photon ionization (SPI) coupled with time-of-flight (TOF) mass spectrometry were used to study the decomposition products of all three molecules on the hot filament made of tungsten or tantalum under collisionfree conditions and subsequent gas-phase reaction products formed in a HWCVD reactor. It is found that alkylsilane molecules decompose to radicals on the hot filament. Methyl radicals are produced from all three molecules studied. Hydrogen abstraction reactions between the methyl radical and the abundant parent molecules produce more radicals in the HWCVD reactor. Subsequent biradical combination reactions constitute the main secondary gas-phase reaction pathways, resulting in the formation of many alkylsubstituted silanes and silyl-substituted alkanes. In addition, it is believed that several disilacyclobutane compounds have been produced through the cycloaddition of the unsaturated silene intermediates. This provides strong evidence for the existence of the reactive silene species in the reactor. The Si-C bond is found to be the major bond connection in the gas phase reaction products. With Si-Si bonds in HMDS and Si-H bonds in trimethylsilane, more Si-Si bond connections are present in the backbone of the high mass reaction products. Due to the existence of both Si-Hand C-H bonds in trimethylsilane, experiments with its isotopomer, (CH3) 3 SiD, have demonstrated that two H-abstraction reactions exist between CH3 radical and the parent molecule, yielding two secondary radicals with the same mass. This leads to the observation that trimethylsilane has the most complicated gas phase chemistry of the three molecules studied. A study of the effect of TMS partial pressure has shown that source gas decomposition on the filament and secondary gas-phase reactions become less efficient with a decrease in the source gas pressure in the reactor. The experiments with TMS using both W and Ta filaments indicate that the filament material does not affect the decomposition pathway of source gas on the filament. Results with a 0.25% TMS in He mixture suggest that Ta may have a slightly better catalytic capability than W in decomposing TMS.
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Bibliography: p. 125-133
Some pages are in colour.
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Citation
Li, X. (2007). Investigation of gas phase chemistry of alkylsilane molecules in a hot wire chemical vapor deposition reactor (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/1401
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