Pure and Applied Chemistry

Abstract: In low-pressure plasmas commonly used in materials processing, plasma–wall inter-actions play a crucial role in the evolution of the plasma properties both over time and across large-area wafers. We have recently studied the heterogeneous recombination of O and Cl atoms on reactor walls in O₂ and Cl₂ plasmas through both experiments and modeling. The Langmuir–Hinshelwood (i.e., delayed) recombination was investigated using a “spinning-wall” technique in which a portion of the substrate surface is periodically exposed to an inductively coupled plasma and to a differentially pumped chamber where either Auger electron spectroscopy (AES) or line-of-sight mass spectrometry (MS) is used to detect surface and desorbing species. In this paper, a review of the various effects driving the O and Cl atoms recombination dynamics on anodized aluminum (AA) and stainless steel (SS) surfaces is presented. It is shown that recombination probabilities, γ, can vary following plasma exposure due to surface conditioning. In Cl₂ plasmas, γ was also found to depend on the Cl-to-Cl₂ number density ratio, a mechanism ascribed to a competition for adsorption sites between Cl and Cl₂. We have also determined the recombination rates of Cl atoms in Cl₂ high-density plasmas sustained by electromagnetic surface waves by comparing the measured degrees of dissociation of Cl₂ to those predicted by an isothermal fluid model. For a reactor with large SS and quartz surfaces exposed to the plasma, γ values and their dependence on the Cl-to-Cl₂ number density ratio were consistent with those obtained from the rotating substrate technique. Similar values were obtained for plasmas sustained in a quartz discharge tube. It is expected that for plasmas sustained in or adjacent to a silica tube or plate, the Cl atoms recombination coefficient becomes independent of chamber wall material due to reactor seasoning, producing a silicon-oxychloride layer.