Modifications of radiofrequency capacitive discharge for deposition of carbon coatings

Goruppa, Alexander (2002). Modifications of radiofrequency capacitive discharge for deposition of carbon coatings. PhD thesis The Open University.



This thesis is an investigation of modifications of RF excited discharge for deposition of carbon coatings. Two separate discharge configurations were examined: an RF capacitive discharge with electron injection and removal by means of an emissive filament and a DC electrode, and an RF discharge with a hollow cathode powered electrode. Plasma characterization was conducted by means of an electrostatic probe and an energy and mass analysis probe. Interpretation of the electrostatic probe data in presence of RF harmonics in plasma has been discussed.
Electron injection and removal has been found to control strongly plasma potential and maximum of ion energy at the grounded electrode, reducing them to less than 10 V or increasing above 90 V accordingly. Related changes of electron temperature and density have been measured, with plasma density being increased up to an order of magnitude by electron injection. This effect has been linked with a regime, when hot filament instigates discharge inside of an electron source.
A model, based on the analysis of electron movement in an RF matrix sheath, has been developed to investigate an effect of the DC electrode on stochastic heating/cooling of electrons in the sheath. It has been demonstrated that an adequate heating of the EEDF tail and cooling of the bulk electrons could be produced by a combined effect of the sheaths of powered and grounded electrodes, assuming the multi-harmonic grounded sheath and non-equal plasma density at the sheath boundaries.
Another model, based on balance of conduction currents from RF plasma to the electrodes, has been aimed at investigation of discharge potentials. It has predicted correctly variation of time-averaged plasma potential with electron injection and removal.
Deposition experiments were conducted from both discharge configurations. Carbon coatings were analysed by means of Raman spectroscopy and scanning electron microscope (SEM). Raman spectra of coatings from the plasma with injected electrons revealed a polymeric-like nature of the coatings. SEM study of carbon films from the RF hollow cathode configuration demonstrated a wide variety of coating morphology: from porous films, consisting of separate particles, to nucleation of conical, spherical and cauliflower-like carbon phases.

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