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Diamond Synthesis

Dateline: 11/23/98

By Alan Bruzel

Diamond and graphite are crystalline forms of carbon, but diamond occupies an exalted position in the world of materials. One or two of its virtues would be remarkable enough for a single substance, but diamond displays a wealth of unique and useful properties.

Diamond is unreactive and exhibits little corrosion potential marking it as a good candidate for structures requiring chemical inertness. It conducts heat better than other compounds – its thermal conductivity is twice that of copper – and its thermal coefficient of expansion is less than that of borosilicate glass. It is also a good electrical insulator. Diamond thus possesses three valuable characteristics of a heat sink. It draws off excess heat from its surroundings, it shows little resultant alteration of its shape, and it accomplishes this task without affecting conduction through nearby circuits.

Diamond is also transparent to wavelengths of light ranging from the ultraviolet to the far infrared regions. (It has but two minor infrared absorption bands.) Along with all of these assets are diamond's better known qualities of unsurpassed hardness and exceptional mechanical strength, which have made this substance sine qua non among cutting tools, reamers, and drill bits.

All of the aforementioned attributes are present in both naturally occurring diamonds and in diamonds synthesized by the chemical vapor deposition (CVD) process. Although less spectacular than diamonds produced geologically, CVD allows a much finer control of the final product's composition. CVD of diamond films takes place in reactors operating between 700 to 1000 ^oC under low pressures (4 to 40 kPa; 30 to 300 mm Hg) in an atmosphere of 1 to 2 percent methane in hydrogen gas. The highly reactive components formed (atomic hydrogen and hydrocarbon radicals such as carbenes) attach to a high melting substrate, usually silicon wafers. A slow deposition process (about one micron per hour) yields diamond films with higher diamond to graphite ratios and better crystallinity than diamond films manufactured at faster growth rates (hundreds of microns per hour).

Anticipated innovations resulting from CVD diamond technology are: hollow diamond tubes (formed by removing the core from diamond-coated fibers), diamond-coated surgically implantable devices, diamond-coated gears with reduced lubrication needs, diamond-coated tungsten carbide tool inserts, and diamond-coated infrared optics for use in instruments exposed to corrosive environments.

Recommended Web resources for additional information:

Affordable Industrial Diamond Manufacture
CVD diamond production process described by P1 Limited, CVD diamond manufacturers.

Bristol University CVD Diamond Group
This interdepartmental diamond film research facility is an excellent source of information.

CVD Diamond
Background and applications from Fraunhofer-Institut für Angewandte Festkörperphysik.

CVD Diamond at Heriot-Watt University
Diamond deposition onto metallic and semiconducting substrates.

Molecular Manufacturing: Adding Positional Control to Chemical Synthesis
Article by Ralph C. Merkle, Xerox Corporation, describing proposed construction of molecular computers utilizing diamond semiconductors.

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