Kinetics of the reactions involving CF₂ and CF in a pure tetrafluoromethane plasma: I. Production of CF₂ and CF via electron-impact dissociation

Abstract

The kinetics of the production and loss of CF₂ and CF radicals in a glow discharge in pure CF₄ is investigated by the laser-induced fluorescence method. The effective rate constants for electron-impact dissociation of CF₄ molecules along the pathways toward CF₂ and CF radicals are determined within a wide range of the reduced electric field (80–250 Td). It is shown that, along with the direct electron-impact dissociation of CF₄, the radicals are also produced via the dissociation of the C_xF_y polymer fluorocarbon particles that form in the plasma. A detailed analysis of the kinetics of the radical production and loss in a modulated discharge made it possible to evaluate the contribution of the electron-impact dissociation of CF₄ to the production of radicals and, consequently, to determine the dissociation rate constants $k_{CF_2 }$ and k _CF. A comparison of the obtained $k_{CF_2 }$ and k _CF values with the results of calculations by the Monte Carlo method and the literature data on the cross sections for electron-impact dissociation of CF₄ molecules enabled the normalization of these cross sections in the threshold region and the construction of the model cross sections for the electron-impact dissociation of CF₄ into neutral products. The calculated cross sections allow a satisfactory description of the experimental results throughout the entire range of E/N under study. A significant scatter (up to 100%) in the experimental data on $k_{CF_2 }$ and k _CF at low values of E/N is related to the considerable contribution of the C_xF_y polymer molecules (and, probably, C_xF ⁺_y ions and fluorocarbon grains) to the production of CF₂ and CF radicals both in the plasma volume and on the surface of a fluorocarbon film covering the discharge tube wall.