Laser plasma diamond
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Diamond-like films containing no hydrogen can be deposited from laser plasmas that are highly ionized. Growth rates of 0.5 /im/h over 100 cm2 areas have been realized on untreated substrates of a variety of materials including Si, Ge, ZnS, glass, and plastics. Measurements of optical properties and mass densities support the identification of this laser plasma diamond as a conglomerate of very fine grains of diamond polytypes in a matrix of other carbon forms.
I. INTRODUCTION
Diamond-like carbon (DLC) films which contain no hydrogen offer some unique advantages when used in infrared (IR) applications.1 There are no CH absorption bands with which to contend and scattering losses are minimized by the extremely small sizes of the component diamond grains. The growth of these films is continuously self-seeding at room temperatures, and substrates require no preliminary scratching or nucleating. Tribological applications are equally favored by the smooth optical finishes generally displayed and by the ability of such films to conform to complex shapes being coated. The preparation of thin films of diamond-like material that are hydrogen-free seems best accomplished with a laser plasma source. In 1985 Nagel and coworkers2 first reported the use of a laser ablation source of carbon ions to produce diamond-like films at high rates of growth approaching 0.3 ju,m/h. They determined a critical threshold intensity of 5 x 1010 W/cm2 on the carbon feedstock, above which DLC was condensed from the carbon plasma and below which only soft, graphitic layers were deposited that resembled those produced by the thermal evaporation of carbon. Working well below that threshold Richter et al.3 and Sato and coworkers4 reported some diamond-like properties in films grown from laser evaporated material, but neither optical absorption data nor hardness measurements were shown to support those conclusions. The ranges of the properties of evaporated carbon are quite large5 and there was the definite possibility that an insufficient set had been considered in concluding a diamond-like structure for those products of the low intensity ablation of carbon. This was confirmed more recently when Sato et al.6 reported that their original material4 was found to have actually been hydrogenated DLC containing 30% hydrogen. To date the Nagel criterion has not been clearly violated by any low intensity results that depend only upon laser power for ablation. 2398
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 11, Nov 1990
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The introduction of the laser plasma source of carbon ions in 1988 ignited an explosion of effort in the production and characterization of unhydrogenated DLC. First used unknowingly by Wagal et al.1 to produce diamond-like films at intensities slightly below the Nagel criterion, it was first described by Collins et al.,s and recently rediscovered by Krishnaswamy and coworkers.9 In each case the critical concept has been the use of a high current discharge confined to the path of the laser ignited p
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