Modeling of diamond growth from a microwave plasma: C 2 H as growth species
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Diamond growth experiments were performed in a microwave plasma ball reactor on silicon wafers or on a molybdenum sheet provided with cones (stamped into the sheet with a punch). All substrates had been treated by scratching with diamond powder in advance. The gas mixture used was CH 4 /H 2 , sometimes with the addition of CO. Substrate temperatures ranged from 953 to 1428 K, pressures from 100 to 400 mbar, and microwave powers from 250 to 700 W. A strong preference of diamond growth was observed on the cones in the molybdenum substrates. This is interpreted as being caused by gas transport hindrance. The resulting deposition coefficient of the "active" species is about 0.1 under all conditions investigated. The deposition experiments on silicon substrates are numerically modeled in two steps. In the first step, temperature fields and electron density and energy distributions in pure hydrogen are calculated following the method described previously. The output of this first simulation step is taken as input data for the second step. The condition is applied that chemical reaction rates due to thermal or electronic activation and diffusional flows compensate each other at every point of the reactor. In this way stationary concentrations of the 13 species in 29 elementary reactions are computed and, from these, the expected deposition profile of diamond on the silicon substrate, assuming one of the carbon-containing species to be the "active" one. When the experimental deposition profiles are compared with the calculated ones, C2H as the "active" species gives the best match to all the experimental results. CH 3 and C 2 H 2 (and perhaps others) might contribute to the diamond growth to a limited extent only.
I. INTRODUCTION Over the last decade a large number of methods have been investigated by which diamond can be deposited from the gas phase under low pressure (at or below ambient). These methods will not be reviewed here (e.g., hot filament,1 microwave CVD, 2 welding torch,3 and dc jet 4 ). What is interesting is the fact that depending on the method used the diamond growth rate differs by orders of magnitude ranging from about 1 ^m/h 1 ' 2 to nearly 1000 fim/h,4 while the diamond quality seems to be more or less the same. A lot of research has been carried out with respect to the mechanism of the CVD diamond growth, especially regarding the "active" gas species acting as main diamond precursor, but an agreement has not been achieved up to now. Tsuda et al.5 performed quantum chemical calculations and concluded that CH 3 + ions are adsorbed at the diamond surface and finally bonded as diamond building units. A positively charged crystal surface would give similar results.6 Frenklach and Spear7 and Huang et al.8 proposed another growth process where C 2 H 2 is assumed to be the active species. The a)
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J. Mater. Res., Vol. 8, No. 9, Sep 1993
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same growth species, but another reaction scheme, w
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