ON THE EFFECT OF OXYGEN ON PLASMA CHEMICAL KINETICS IN CF4 + O2 AND C4F8 + O2 GAS MIXTURES

Abstract

In this work, we performed the comparative study of electro-physical plasma parameters, densities of active species and fluorine atom kinetics in CF4 + O2 and C4F8 + O2 gas mixtures with variable initial compositions at constant gas pressure and input power. The combination of plasma diagnostics by Langmuir probes and plasma modeling confirmed known features of plasma properties in individual fluorocarbon gases as well as allowed one to figure out key chemical processes determining plasma parameters in the presence of oxygen. It was shown that an increase in O2 content with a proportional decrease in the fraction of any fluorocarbon component a) causes relatively weak changes in electrons- and ions-related plasma parameters; b) results in more drastic (compared with the dilution effect) decrease in densities of fluorocarbon radicals due to their oxidation into CFxO, FO and COx compounds; and c) sufficiently influences both formation and decay kinetics of fluorine atoms. The non-monotonic (with a maximum at ~ 40-50% O2) change in the F atom density in the CF4 + O2 plasma repeats behavior of their formation rate after the contribution of processes involving CFxO и FO species. The monotonic increase (with a constancy region up to ~ 40-50% O2) in the F atom density in the C4F8 + O2 plasma contradicts with the change in their formation rate, but results from decreasing decay frequency in gas-phase atom-molecular processes. The predictive analysis of heterogeneous process kinetics was carried out using model-yielded data on fluxes of plasma active species. It was found that a) the addition of oxygen always lowers the plasma polymerizing ability; and b) the C4F8 + O2 plasma keeps the higher polymerizing ability at any feed gas composition.