Recombination probability of oxygen atoms on dynamic stainless steel surfaces in inductively coupled O 2 plasmas

The authors have investigated the influence of plasma exposure time ( t ) on the Langmuir-Hinshelwood (i.e., delayed) recombination of O atoms on electropolished stainless steel surfaces using the spinning-wall method. They found a recombination probability ( γ O ) of 0.13 ± 0.01 after about 60 min of plasma exposure. γ O decreased to 0.09 ± 0.01 for t ⩾ 12 h and was independent of the O flux impinging onto the surface. These recombination probabilities are much lower than those obtained in plasma chambers exclusively made of stainless steel, but similar to values recorded in stainless steel reactors with large silica surfaces exposed to the plasma. Near real-time elemental analysis by in situ Auger electron spectroscopy showed that the stainless steel surface became rapidly coated with a Si-oxide-based layer ( Fe : [ Si + Al ] : O ≈ 2 : 1 : 9 for t = 60 min and 1:2:9 for t = 12 h ), due to the slow erosion of the silica discharge tube and anodized Al chamber walls. Thus, the recombination probability of oxygen atoms on stainless steel in plasma reactors with large amounts of exposed silica is largely determined by the amount of sputtered silica coating the chamber walls.