Evaluation of a substrate pretreatment for hot filament CVD of diamond
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The absolute concentration of methyl radicals (CH3) and the mole fraction of acetylene (C 2 H 2 ) are measured in a hot filament chemical vapor deposition (CVD) system both during and after an initial pretreatment that has been used successfully in microwave plasma and oxyacetylene torch CVD systems to produce more uniform and higher density crystal nucleation. The pretreatment technique, which consists of deposition for a relatively short time with a high input concentration of hydrocarbon in the feed gas, was studied for both methane (CH4) and C 2 H 2 as the input hydrocarbon diluted in H 2 . Scanning electron micrographs of diamond films deposited under the conditions studied indicate that the pretreatment using CH4 is not effective in increasing the crystal nucleation density, but is moderately effective in increasing the crystal size. The C 2 H 2 pretreatment has no apparent effect upon either the crystal size or nucleation density. The spectroscopic measurements suggest that the surface condition of the filament is the prominent factor affecting the gas phase chemistry both during and after the pretreatment stage.
I. INTRODUCTION The chemical vapor deposition of diamond thin films is accomplished by a number of different methods, including hot filament,1 dc plasma,2 microwave plasma,3 or rf plasma4 activation of the chemical species. All of these deposition techniques result in polycrystalline films with varying degrees of uniformity. No one has yet succeeded in reproducibly depositing large area, single crystal diamond thin films using CVD on a nondiamond substrate. Some studies have been made to determine the enhancement effect of various coatings and ex situ pretreatments of the substrate on the nucleation density in hot filament5 and microwave discharge6 systems. Recently, a number of researchers have reported a significant improvement in nucleation density and uniformity for films deposited in oxyacetylene torch7 or microwave discharge CVD systems8"11 by depositing a carbonaceous film as an initial in situ substrate pretreatment before proceeding to normal diamond deposition conditions. A detailed mechanism for the nucleation of diamond on the prism surface of graphite has been proposed12 that may explain the nucleation enhancement. This pretreatment may well represent a step toward the deposition of more uniform films. We report a study of a pretreatment technique similar to Refs. 9 and 10 in a hot filament diamond CVD system. During the deposition
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Present address: Department of Electrical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan. J. Mater. Res., Vol. 9, No. 4, Apr 1994
we measure the time evolution of both the methyl radical (CH3) density and the acetylene (C 2 H 2 ) mole fraction in an effort to understand the gas phase chemistry during such a pretreatment stage. II. EXPERIMENTAL The pretreatment in the microwave discharge systems typically consists of flowing a hydrocarbon-rich feed gas for a short time (=£30 min) and then switching to a much lower concentrati
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