Axial Temperatures and Electron Densities in a Flowing Cascaded Arc
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AXIAL TEMPERATURES AND ELECTRON DENSITIES IN A FLOWING CASCADED ARC:
J.J. Beulens, M. de Graaf, G.M.W. Kroesen, and D.C. Schram, University of Technology, Dept. of Physics, P.O.Box 513, 5600 MB Eindhoven.
INTRODUCTION Since about 1985 a cascaded arc is used as a particle source in the deposition machine described by Kroesen [la,lb]. This method of deposition showed to be very fast and efficient to grow amorphous carbon films (a-C:H), varying from graphite and diamond to polymers [1,2]. The most important difference of this method, with respect to R.F. techniques, is that the three most important functions of a deposition process, as there are dissociation/ionization, transport and deposition are spatially separated. The dissociation takes place in a cascaded arc burning on argon. The temperatures in the arc are about 10000-12000 K. At the end of this arc hydrocarbons are injected which are then dissociated and ionized effectively. At the end of the arc the plasma expands supersonically into a vacuum vessel. That means that the plasma cools down and the formed hydrocarbon fractions are transported towards the substrate, where an amorphous carbon film can grow. The quality of the films depend mainly on the amount of energy available for each injected carbon atom. The behavior of the refractive index as a function of this energy could be a confirmation that in our deposition method the carbon ions rather than radicals govern the deposition process [1,3,4]. Therefore the cascaded arc is investigated numerically and experimentally in order to improve the ionization efficiency. The conservation laws for mass, momentum and energy for both the electrons and the heavy particles are solved 2 dimensionally by a control volume numerical method with a non A ,•staggered grid. By Fabry Perot interferometry heavy particle temperatures, electron temperatures and electron densities as a function of the axial position in the cascaded arc are measured. The obtained numerical results are compared to the experimental data, obtained by the optical Fabry Perot diagnostics.
EXPERIMENTAL SET UP The cascaded arc (fig.1) used in this work, consists of a stack of ten water cooled copper plates insulated electrically from each other Fig.1
Outline of the cascaded arc as a particle
by
plastic
(PVC)
source.
spacers
and
0-rings, three tungsten-thorium cathodes on one end and an anode on the other. Through the copper plates and the anode plate there is a bore of 4 mm diameter, forming a cylindrical channel Mat. Res. Soc. Symp. Proc. Vol. 190. @1991 Materials Research Society
312
of 6 cm long. The argon gas is fed through mass flow controllers and then injected at the cathode side. The gas or plasma is extracted through a nozzle in the anode plate which is mounted on a vacuum vessel. The pressure in the vessel is about 1 mbar, whereas the pressure in the arc is about 0.5 bar. This means that the plasma will be extracted supersonically. For the experiments the cascaded arc has two special features to measure the plasma pressure and to co
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