High growth rate diamond synthesis in a large area atmospheric pressure inductively coupled plasma
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High growth rate synthesis of polycrystalline diamond using atmospheric pressure arc discharges and flames1"5 is a promising alternative to the more conventional lower pressure plasma and hot filament synthesis techniques.6'7 Growth rates of approximately 200 /im/h are routinely achieved in direct current (dc) plasma arcs,1"3 and in cases of slightly subatmospheric pressure, growth rates approaching 1 mm/h have been observed.8 In atmospheric pressure dc plasma arc deposition, diamond growth occurs primarily over the region where the thermally pinched arc is in contact with the substrate (usually the anode in the transferred arc mode) and the deposit is irregular and nonuniform.3 High power transferred and nontransferred arcs suffer from additional disadvantages such as cathode material contamination, suggesting that applications of such synthesis strategies may be limited to the production of industrial grade polycrystalline powder for use as abrasives or polycrystalline films for use as coatings on tool inserts. Matsumoto et al.3A was the first to demonstrate diamond synthesis in an atmospheric pressure electrodeless inductively coupled plasma (ICP) discharge. Since then, there has been very little study of the use of such so-called "thermal" ICP technology for diamond synthesis, although such a plasma is preferred over the dc arc in the synthesis of diamond coatings that contain relatively low levels of impurities. In addition, inductively coupled atmospheric pressure plasmas can be generated over larger volumes than their dc counterparts. This makes it attractive for large area coating applications. Such plasmas, however, have a number of
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http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 11, Nov 1990
Downloaded: 13 Mar 2015
disadvantages for laboratory and industrial applications in that they are intrinsically noisy, generating radio frequency (RF) interference at the operating frequency (typically 4-27 MHz), and can establish relatively high (1-3 kV) plasma potentials.9
II. BACKGROUND Perhaps most important to the understanding of the deposition chemistry and possible mechanism associated with diamond synthesis in atmospheric pressure plasmas is the fact that they can be generated with electron number densities sufficiently high to ensure that the electron energy distribution function (EEDF) is very near Maxwellian in shape. This considerably simplifies the gas phase kinetics in that rate coefficients for many likely important electron collision-induced chemical reactions (i.e., e + CH4 -> e + CH3 + H) can be estimated without having to solve directly the Boltzmann equation for the EEDF. In addition, the pressure is high enough so that it is often a good approximation to assume that the electron and heavy species have equal translational temperatures. The assumption of a Maxwellian EEDF and electron-heavy species thermalization is generally not valid for low pressure glow discharges, making the analysis of such plasmas (now frequently employed for diamond synthesis) quite difficult. Even at a
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