Kinetics of Diamond-Like Film Growth Using Filament-Assisted Chemical Vapor Deposition
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been combustion flame synthesis, plasma torch deposition, plasma-assisted and filament-assisted chemical vapor deposition (FACVD). The latter is the focus of this work. During FACVD a hydrocarbon (or other carbon containing species, such as an alcohol) diluted in a carrier gas (typically hydrogen) is passed over an electrical filament and the thermal cracking products are brought in contact with a substrate resulting in the growth of a film. A lot of effort is currently being devoted to understanding the gas-phase and surface chemistry underlying the growth of diamond films by FACVD. The gas-phase chemistry appears to be more tractable, when compared to the surface chemistry. It involves radical chain reactions and recombination reactions of atomic hydrogen and hydrocarbon radicals produced by the cracking of the precursor and the carrier gas near the hot filament. Measurements of gas-phase species during FACVD of diamond have been reported [e.g. 6-8] and models of the growth kinetics based on gas-phase chemistry have been developed [e.g. 7,9,10]. On the other hand, the surface chemistry is not very well understood, mainly due to the difficulty of in situ probing of surface kinetics during FACVD. Nevertheless, the energetics of surface reactions have been studied theoretically and mechanistic models of the process have been proposed [e.g. 111]. A transport model of the process has also been developed [12] and was coupled to the gas-phase mechanism developed by Harris [91 to predict upper-bound, diffusion-limited growth rates. In this paper, a kinetic model of the FACVD of carbon films that includes both gas-phase and surface reactions is proposed. The kinetic model is coupled to a transport model of a stagnation flow FACVD reactor describing flow, heat and mass transfer. The resulting reaction- transport * Author to whom correspondence should be addressed. 151 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society
model of the process can be used for estimating film growth rates and compositions. Observations from growth experiments in a stagnation flow FACVD reactor and from similar experiments reported in the literature [1] have been used to fit unknown rate constants of surface reactions. The proposed model3 can yield estimates 2of both the total growth rate of carbon and the growth rates of diamond (sp ) and graphitic (sp ) phases. It can also provide predictions of species concentrations in the gas phase and on the surface. EXPERIMENTAL A schematic of the experimental stagnation-flow FACVD reactor is shown in Figure 1. The gases enter the stainless steel reactor via a 1/2" stainless steel tube, and flow past the hot filament towards the (100) Si substrates, which were preseeded with diamond powder. Diamond and diamond-like carbon films were grown using CH 4 diluted in H2 at P=26 Torr. Typical substrate temperatures (Ts) ranged from 600 to 950 'C, filament temperatures (Tf) from 1700 to 2200 °C and inlet concentrations of CH 4 from 0.5 to 1.5%. Filament temperatures were measured using a pyromet
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