On the optimization of a dc arcjet diamond chemical vapor deposition reactor
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On the optimization of a dc arcjet diamond chemical vapor deposition reactor S. W. Reevea) and W. A. Weimer Chemistry Division, Naval Air Warfare Center, China Lake, California 93555
D. S. Dandy Department of Chemical Engineering, Colorado State University, Fort Collins, Colorado 80523 (Received 14 June 1995; accepted 27 October 1995)
Based on results from chemical kinetic model calculations, a method to improve diamond film growth in a dc arcjet chemical vapor deposition reactor has been developed. Introducing the carbon source gas (CH4 ) into an AryH2 plasma in close proximity to the substrate produced diamond films exhibiting simultaneous improvements in quality and mass deposition rates. These improvements result from a reduced residence time of the methane in the plasma which inhibits the hydrocarbon chemistry in the gas from proceeding significantly beyond methyl radical production prior to encountering the substrate. Improvements in growth rate were modest, increasing by only a factor of two. Optical emission actinometry measurements indicate that the flux of atomic hydrogen across the stagnation layer to the substrate is mass diffusion limited. Since diamond growth depends upon the flux of atomic H to the substrate, these results suggest that under the conditions examined here, a low atomic H flux to the substrate poses an upper limit on the attainable diamond growth rate.
I. INTRODUCTION
Experimental measurements and chemical kinetic modeling results obtained previously in this laboratory have identified CH3 as the dominant diamond growth precursor in our dc arcjet reactor.1 Based on conclusions drawn from that work, an optimization strategy was developed to simultaneously improve the quality and growth rate of the diamond films produced in this reactor. The gas feed system was modified from introducing the gas feed into the plasma gun as a mixture (3% CH4 in H2 ), to one identical except that the CH4 feed was injected into the plasma separately at a position midway between the gun and the substrate.2 Herein we will refer to the former method as the premixed feed and the latter as the remote CH4 feed. The concept behind the remote CH4 feed method was to limit the exposure time of the CH4 to the hot plasma, thereby allowing sufficient time for CH4 to react to produce CH3 via CH4 1 H ! CH3 1 H2 upstream of the substrate while forcing subsequent CH3 consumption reactions to occur downstream of the substrate.2 The goal was to determine the feasibility of increasing the CH3 flux to the substrate while simultaneously reducing the flux of all other hydrocarbons by taking advantage of the high gas flow velocities produced in the reactor and the finite reaction kinetics of the hydrocarbon chemistry.1–3 Diamond films a)
Current address: Department of Chemistry, Arkansas State University, State University, Arkansas 72467.
694
J. Mater. Res., Vol. 11, No. 3, Mar 1996
grown using the remote CH4 feed exhibited a significant enhancement in quality as evidenced by Raman
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