Cvd Diamond Particles

Most CVD diamond film is formed by initial nucleation that is followed by the growth of nuclei. However, Argonne National Lab pioneered to avoid the growth of nuclei, so the nuclei are continually added to join together to form nano-crystalline diamond film. The way to avoid nuclei to grow is to replace hydrogen gas by argon gas. Normally, hydrogen molecules are dissociated at CVD to form hydrogen atoms. The presence of hydrogen atoms can stabilize the sp3 bonds of carbon atoms and hence they allow diamond nuclei to grow. The unavailability of hydrogen atoms prompt nuclei to join one another. According to Argonne National Labs' analysis, C2 molecules were formed by pyrolysis of either CH4 or C2H2 (Fig. 5.17). The C2 molecules then join together to form nano-grained diamond.

Another way to deposit nanodiamond is to increase the rate of diamond nucleation. The nucleation rate can be boosted by seeding of the substrate (e.g. a silicon wafer) with nanodiamond particles. Alternatively, the nucleation rate can be promoted by applying a negative bias (e.g. 200 V) that allows the increase of the kinetic energy of impinging carbon ions. The nucleation rate can also be enhanced by increasing the methane-nitrogen ratio (e.g. from 1% to 5%). The growth rate of diamond can be suppressed by reducing the substrate temperature (e.g. from 900°C to 600°C).

Micron diamond particles may also be bonded by nanodia-mond film. For example, Diamonex has a product with micron diamond grains bonded to silicon wafer by a thin layer of diamond film with submicron grains (Fig. 5.18).

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