Role of microstructure on the oxidation behavior of microwave plasma synthesized diamond and diamond-like carbon films
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Oxidation kinetics of microwave plasma assisted CVD diamond and diamond-like carbon (DLC) films in flowing oxygen were evaluated in the temperature range of 500 to 750 °C and were compared with those of graphite and natural diamond. The diamond and DLC films were prepared using CH 4 /H 2 ratios of 0.1, 0.25, 0.5, 1.0, and 2.0%. The films deposited at 0.1% ratio had a faceted crystalline structure with high sp3 content and as the ratio increased toward 2%, the films contained more and more fine crystalline sp2 bonded carbon. The oxidation rates were determined by thermal gravimetric analysis (TGA), which shows that the films deposited at ratios of 2, 1, and 0.5% oxidized at high rates and lie between the rates of natural diamond and graphite. The oxidation rate decreased with lower CH 4 /H 2 ratio and the films deposited at 0.25 and 0.1% exhibited the lowest oxidation rates associated with the highest activation energies in the range of 293-285 kJ/mol • K. The oxidation behavior of microwave plasma assisted diamond films was similar to that of DC plasma assisted CVD diamond films. The results suggest that the same mechanism of oxidation is operational in both DC and microwave plasma assisted diamond films and is probably related to the microstructure and preferred orientation of the crystallites.
I. INTRODUCTION
Diamond is one of the most technologically important materials because of its very attractive properties such as high thermal conductivity, large band gap, high breakdown voltage, excellent wear resistance, and transparency to visible light, infrared light, and microwaves. The engineering applications of diamond had to await the development of a cost effective technique for depositing thin diamond films. This approach involves synthesis of diamond films at low pressures and relatively low temperatures by activating hydrocarbon gases in the presence of atomic hydrogen. Activation is commonly achieved by the assistance of DC glow discharge, RF or microwave excitation, thermal excitation by hot filaments, and extremely high temperature ionization of the gases using arc discharges and torches. A majority of effort in the film growth area has been with low pressure plasma assisted chemical vapor deposition (CVD),1"5 the most popular being the processes assisted by hot filament and DC, RF, or microwave plasma. The polycrystalline diamond films are typically grown in the 700-1000 °C range, although more recent efforts are aimed at depositions in the 350-500 °C range.6 The a)With
Sandia National Laboratories, Livermore, California 94551-
0969.
J. Mater. Res., Vol. 5, No. 11, Nov 1990
grain size and orientation, defect types and densities, and the chemical state of carbon (e.g., sp3 or sp2) and its distribution within the film relate to the deposition technique and the process parameters. The properties of the films, in turn, depend on the microstructure, and there has been substantially increased research activity in understanding structure-property relationships of the diamond films. One of the properties tha
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