Experimental and modeling study of O and Cl atoms surface recombination reactions in O-2 and Cl-2 plasmas

In low-pressure plasmas commonly used in materials processing, plasma wall interactions 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 O-2 and Cl-2 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, gamma, can vary following plasma exposure due to surface conditioning. In Cl-2 plasmas, gamma was also found to depend on the Cl-to-Cl-2 number density ratio, a mechanism ascribed to a competition for adsorption sites between Cl and Cl. We have also determined the recombination rates of Cl atoms in Cl-2 high-density plasmas sustained by electromagnetic surface waves by comparing the measured degrees of dissociation of Cl, to those predicted by an isothermal fluid model. For a reactor with large SS and quartz surfaces exposed to the plasma, gamma values and their dependence on the Cl-to-Cl-2 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.