Studies of chlorine-oxygen plasmas and evidence for heterogeneous formation of ClO and ClO2

Abstract

Plasma and surface diagnostics of Cl2/O2 mixed-gas inductively coupled plasmas are reported. Using trace rare gas optical emission spectroscopy and Langmuir probe analysis, electron temperatures (Te) and number densities for Cl atoms (nCl), electrons (ne), and positive ions were measured as a function of percent O2 in the feed gas and position in the plasma chamber. Adsorbates on and products desorbing from a rotating anodized aluminum substrate exposed to the plasma were detected with an Auger electron spectrometer and a quadrupole mass spectrometer. Te and ne increased with increasing percent O2 in the plasma, while nCl fell off with O2 addition in a manner reflecting simple dilution. Cl atom recombination probabilities (γCl) were measured and were found to be a nearly constant 0.036±0.007 over the range of Cl2/O2 mixing ratios and Cl coverage. Large yields of ClO and ClO2 were found to desorb from the surface during exposure to the plasma, ascribed predominantly to Langmuir–Hinshelwood reactions between adsorbed O and Cl.