Gas-phase chemistry and tungsten filament aging in hot-wire chemical vapor deposition with silacyclobutanes
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2009
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Abstract
118 nm vacuum ultraviolet (VUV) laser single photon ionization (SPI) coupled with time-of-flight mass spectrometry (TOF-MS) was used to examine the products from the primary decomposition of three silacyclobutane molecules, namely silacyclobutane (SCB), 1, 1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB), and 1, l-dimethyl-1-silacyclobutane (DMSCB), on a heated tungsten (W) filament and from secondary gasphase reactions in a HWCVD reactor. It was found that both SCB and DMSCB decompose on the filament to form Si-containing (silenes and silylenes) and C-containing ( ethene and propene) species. The formation of propene from SCB was demonstrated to be initiated by a 1,2-H migration to form n-propylsilylene, followed by an equilibrium with silacyclopropane, which then decomposes to propene. The production of propene and ethene from DMSCB was shown to occur via a diradical process involving the initial cleavage of a ring Si-C bond and ring C-C bond, respectively. The formation of ethene is favoured over that of propene for both SCB and DMSCB. TMDSCB decomposes on the filament to form methyl, dimethylsilene, and dimethylsilylene. It was demonstrated that the dominant secondary reactions with TMDSCB are the hydrogen abstraction reactions involving methyl radicals and subsequent biradical reactions, whereas reactions involving silenes and silylenes are more important with both SCB and DMSCB. Isomerization between silenes and silylenes was shown to occur in the HWCVD processes with all three molecules studied. Our theoretical calculations at singlet doublet coupled cluster (CCSD) level of theory showed that the isomerization between silenes and the corresponding silylenes via 1,2-H shift is energetically more favorable than those via 1,2- CH3 shift. The tungsten filament aging process when using SCB and TMDSCB as source gases in a HWCVD reactor was systematically studied. It was found that filament aging is caused by silicidation with SCB and by carburization with TMDSCB. The surface Si or C content was found to be dependent on the filament temperature and source gas pressure. The in-situ mass spectrometric measurements of the gas-phase reaction products in the process showed that filament carburization affects the consumption rate of the source gas and the intensities of gas-phase reaction products. It was demonstrated that H2 can etch the silicides from the aged filament at temperatures above 1900 °C.
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Bibliography: p. 165-174
Some pages are in colour.
Some pages are in colour.
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Tong, L. (2009). Gas-phase chemistry and tungsten filament aging in hot-wire chemical vapor deposition with silacyclobutanes (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/3080