A program of research and development is addressing the feasibility of using magnetoplasmadynamic (MPD) sources in the chemical vapor deposition (CVD) of synthetic diamond films. Because of its unique combination of thermal, electronic, mechanical, and chemical properties, diamond has potential for use as a coating material in numerous engineering and scientific applications.

Figure 1. The Cathode Test Facility at NASA's Jet Propulsion Laboratory includes a vacuum chamber and associated equipment previously used to investigate the behavior of high-current, thermionic cathodes for high-powered electromagnetic thrusters. This schematic shows the arrangement of substrate and methane injector to the accelerator electrodes.

A large amount of research has been directed toward understanding and developing CVD process (including plasma-assisted CVD process) for the synthesis of diamond and diamondlike materials. The plasma-assisted CVD processes include some that involve dc-arcjet sources; the development of these processes has benefitted from extensive prior research on dc-arcjet thrusters for spacecraft. Rates of deposition that have been achieved by use of dc-arcjet sources have exceeded those achieved by use of other gas-activation sources.

Figure 2. Scanning Electron Microscope Image of diamond film is shown (approximately 2.5μm thick) grown over 180-minute test at low power using a mixture of 1.5 percent methane to hydrogen by volume.

The success of the dc-arcjet approach has led to speculation on the utility of other thruster-type plasma sources for CVD of diamond. There is a large body of data from previous research on the performance and plume characteristics of electric propulsion devices; these data are available to support continuing efforts to understand the reaction kinetics and growth chemistry of diamond. One logical extension of the prior research would be an assessment of electric thrusters, other than dc-arcjet thrusters, for their potential to increase rates of deposition even further. MPD thrusters are among those that could be considered.

Regarded as thrusters, MPD sources have been found to perform with low efficiency at power levels below a hundred kilowatts. However, some characteristics of the discharges and plumes from MPD sources indicate that these sources might be well suited to synthesis and deposition of diamond at rates higher and over areas larger than those achievable by use of dc-arcjet sources; the characteristics of particular relevance in this regard are higher levels of dissociation and ionization of gas in the cores of the plasma plumes, higher jet velocities, and the scaleability to higher power levels.

  • Parametric sensitivity (effects of discharge power, gas mixture, substrate biasing, and background pressure on substrate temperature, film quality, area of deposition, and rate of deposition);
  • Properties of the plasma plume (e.g., mean gas velocity, pressure, and temperature);
  • The degree of pyrolysis of methane (assessed by means of spectroscopy of visible emission lines of C, H, and CH), in comparison with corresponding published information for arcjets;
  • Gas-phase and surface chemistry (modeled computationally with the help of experimental and published data).

This work was done by James Polk, John Blandino, and David Goodwin of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Materials category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20668, volume and number of this NASA Tech Briefs issue, and the page number


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CVD of Diamond Using Magnetoplasmadynamic Sources

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This article first appeared in the August, 2000 issue of NASA Tech Briefs Magazine.

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