| AVS 54th International Symposium | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS1-WeA |
| Session: | Plasma-Wall Interactions |
| Presenter: | L. Stafford, University of Houston |
| Authors: | L. Stafford, University of Houston J. Guha, University of Houston V.M. Donnelly, University of Houston |
| Correspondent: | Click to Email |
We investigated the desorption of O2 molecules from electropolished stainless steel surfaces following Langmuir-Hinshelwood (L-H) (i.e. delayed) heterogeneous recombination of oxygen atoms, using the spinning-wall technique. In this experimental set-up, a cylindrical section of the wall of an inductively coupled plasma reactor is rotated and the surface is periodically exposed to the plasma, a mass spectrometer, and an Auger electron spectrometer in separate differentially pumped chambers. Measurements can be performed over a wide range of O flux by varying the absorbed power (50-600 W) and total gas pressure before plasma ignition (1.25-20 mTorr). When the substrate is spun with the plasma on, a large increase in the mass spectrometry signals and Auger chamber pressure is observed with increasing rotation frequency. This increase results from O atoms that adsorb on the stainless steel surface when it is in the plasma and then recombine over the ~1-15 ms period probed by changing the rotation frequency. L-H recombination probabilities of O atoms were determined by extrapolating the O2 signals to t = 0 (i.e. infinite rotation frequency). For example, at an O atom flux of 1017 cm-2s-1 obtained at 5 mTorr pressure and 600 W plasma source power, we have found a recombination probability of 0.035. This value is lower than those previously reported for O on stainless steel (e.g. 0.07 by Mozetic and Zalar, Appl. Surf. Sci. 158, 263 (2000), and 0.14 by Kiehlbauch and Graves, J. Vac. Sci. Technol. A 21, 660 (2003)). It is, however, similar to those previously obtained for O on anodized aluminum under comparable plasma conditions, where a Si-oxide layer was observed to form on the surface, due to the slow erosion of the quartz discharge tube. In the present study, analysis by Auger electron spectrometry during the recombination measurement also showed that the stainless steel surface became rapidly coated with a Si-oxide layer (Fe:Si:O atomic concentration ratios of ~1:2:5). This suggests that the recombination kinetics of oxygen atoms in this plasma reactor is determined by the coating on the walls, and not as much by the wall materials.