Molecular beam mass spectrometry and modelling of CH_4-CO_2 plasmas in relationwith polycrystalline and nanocrystalline diamond deposition

摘要

CH_4-CO_2 microwave plasmas have been studied by optical emission spectroscopy, microwave interfero-metry, Langmuir probing and molecular beam mass spectrometry. The variations of plasma parameters andthe concentration variations of both stable species and radicals in the plasma had been reported previouslyas a function of the power density; the influence of the total inlet flow rate is reported here. While the powerdensity influences directly the plasma kinetics, the flow rate changes the residence time in the plasma andthen the degree of conversion of the chemical system that is the extent to which the gas composition movestoward its steady-state composition. This is studied by modelling of plasma kinetics taking into account thecoupled fluid dynamics of the gaseous species and the gas-phase chemistry including electron dissociationand surface recombination at the reactor wall. The experimental and modelling studies are used forcorrelating: - the relative concentration of important hydrocarbon radicals and etching radicals in theplasma and the gradients of all these species in front of the surface; - to the deposition domain, the structure(polycrystalline or nanocrystalline) and the quality of diamond films, which is the ratio of spa to (sp~3 + sp~2)-hybridized carbon in the film. All results evidence the plasma kinetic effect on the diamond depositiondomain and the diamond deposition quality and structure, due to different degrees of conversion of thechemical system. The deposition of diamond coating from CH_4-0O2 is shown to be a versatile process thatpermits deposition of a great variety of diamond films. However it requires particular attention because ofthe variation of the deposition conditions and then diamond quality and structure of the deposits dependingon the extent of conversion of the inlet species to various intermediate and finally stable species formed inthe plasma chemical system.