Production and loss of CF 2 and CF radicals at the polymerization in CF 4 plasma

Abstract

The production and loss kinetics of CF2 and CF radicals in the CF4 glow discharge were studied with using the time— resolved LIF technique. The effective rate constants of the CF4 dissociation into the chanhels with CF2 and CF production were determined in a wide range of reduced electric field: 80-250 Td. It is shown that besides of the CF4 dissociation by direct electron impact there is also another source conditioned by fluorocarbon plasma polymerization processes. The detailed analysis of growth and decay dynamics of radical concentrations at the discharge modulation has allowed us to separate different channels of the radical production and consequently to determine the constants of CF4 dissociation rate by electron impact KCF2 KCF and A comparison of received KCF2 and KCF with calculations made on the base of solving the Boltzman equation by Monte-Carlo technique with using the available data on electron scattering cross-sections on CF4 molecules, has in part allowed us to renormalize the partial dissociation cross-sections near the threshold and to build up the model cross-sections of neutral dissociation of CF4 by electron impact It is shown, that the polymerization processes substantially contribute (particularly at low values of E/N) to production of radicals, as in a plasma volume, and on a surface of the discharge tube. The analysis of the obtained data on characteristic frequencies (times) of these processes has allowed us to study mechanisms of fluorocarbon's polymerization in CF4 plasma in conditions of high relative concentration of fluorine atoms and low ion energy. So it is shown, that with increasing pressure, when the concentration of polymer fragments CxFy in bulk of plasma is comparable with concentration of simple fluorocarbon radicals CFx (x=1-3), the dissociation of these fragments by electronic impact and the reactions of unsaturated fluorocarbon fragments CnF2m+1 as in volume, and on a surface of a fluorocarbon film, covering discharge tube walls become the main production channels of CF2 and CF. Since with lowering pressure a major channel of CF2 and CF destruction is connected with their heterogeneous losses on tube walls, the surface loss probabilities of CF2 and CF radicals in conditions of the fluorinated fluorocarbon film surface are determined. It is shown, that the most probable mechanism of heterogeneous loss of CF2 and CF under these conditions is the surface recombination of chemisorbed fluorine atoms Fch and physadsorbed radicals CFxPh (x=1-2) through the formation of an activated surface complex. The analysis of the experimental data with using this approach has allowed us to estimate the approximate values of activation energies for the formation of the surface complexes Fch. CF2ph and Fh. CFPh - 750 ± 70 K and 1030 ± 100 K respectively.