Identification of hydrocarbon precursors to diamond in chemical vapor deposition using carbon monoxide reagent
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Diamond films were grown by microwave plasma-assisted chemical vapor deposition using mixtures of 13 CH 4 and CO. Mass spectrometry was used to identify CO, CH 4 , and C 2 H 2 as the stable gaseous products in the reactor exhaust gas. By comparing gaseous 13 C compositions with that of the diamond films, the efficiency of diamond growth from methane (possibly via the methyl radical) is found to be about two orders of magnitude higher than that for carbon monoxide. Most of the diamond that is formed from the CO reagent results from the conversion of CO to hydrocarbons. The conversion of CO to hydrocarbons is attributed to activation of CO by high-energy electrons in the plasma.
I. INTRODUCTION Diamond film synthesis by chemical vapor deposition (CVD) typically employs a gas mixture of hydrogen and a small amount of methane, which is the carbon source. Activation of this gas mixture produces a variety of hydrocarbon species, such as acetylene, methane, ethylene, and methyl radical.1 Considerable evidence exists for a major role of methyl radical as a gas phase diamond precursor for microwave plasma2"4 and hot filament5"7 CVD reactors. Carbon monoxide has also been found useful as a carbon source for diamond by plasma-assisted CVD.8"12 Advantages of a CO/H 2 feed gas over a CH 4 /H 2 feed gas include a higher film growth rate8'9'13"16 and the fact that oxygen is incorporated in the reagent. Oxygen species in the gas phase can improve both the growth rate and quality of diamond films.1'16"23 The CO/H 2 mixture may also be advantageous in coating W C - C o alloy tool blades.14 When a CO/H 2 mixture passes through a plasma, a small fraction of the carbon monoxide is converted to methane and acetylene.13'20'24"26 Based on indirect evidence, these hydrocarbon species have been suggested to account for diamond growth.13'15-17'20'27'28 In addition, oxygen-containing species such as HCO, formed on the surface from adsorbed CO, have been proposed to contribute to diamond growth.15'16 In the present work we have used carbon-13-labeled methane mixed with CO in the feed gas to distinguish the roles of CO-containing species and hydrocarbon species in the formation of diamond in a microwave plasma reactor. Raman spectroscopy was used to determine the 13 C content of diamond films, and mass spectrometry was used to determine the concentration and 13C content of stable gaseous species downstream of the plasma. This study provides the first experimental proof that most of
the diamond grown from CO reagent in a microwave plasma reactor originates from a hydrocarbon precursor.
II. EXPERIMENTAL The diamond deposition flow tube reactor and substrate preparation procedures have been described in detail elsewhere.21 The reactor consists of a 12.5 mm diameter fused quartz tube that passes through an Evenson resonant microwave cavity. Films were deposited on single crystal 7 X 13 mm2 Si(100) substrates that were seeded by abrading the surface with ^0.5 /nm diamond powder. Substrates were positioned parallel to the horizontal reactor tube a
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