Probing surface reactivity in plasmas- is it valid to use a constant reaction probability model?
ORAL · Invited
Abstract
Surface-catalyzed recombination of atoms and free radicals play a key role in the plasma equilibrium for molecular plasmas in enclosed reactors. For simplicity, surface recombination is generally characterized by a constant surface reaction probability. This parameter is nearly always very poorly characterized, or taken as the adjustable variable to fit models to experimental data. Ab-initio theories are not currently able to reliably predict the rates of these processes, so they must be determined by in-situ measurements (either through the measurement of spatial mole-fraction gradients adjacent to the surface, or (more commonly) by time-resolved measurements in modulated plasmas).
The surface recombination of oxygen atoms on borosilicate glass tube walls was studied by time-resolved optical emission actinometry of partially modulated DC positive column discharges. The observed reaction probabilities (~10-3) were not constant, but can be explained by Eley-Rideal reaction of incident atoms with chemisorbed atoms with an activation energy provided by the kinetic energy of the incident atom. For full modulation, the atom decay in the afterglow (measured by Cavity Ringdown spectroscopy, CRDS) is non-exponential, starting fast then slowing down. This suggests a second, quadratic, term in the recombination probability which can be attributed to reaction with weakly-bound physisorbed atoms. The presence of small quantities (10-3) of air from vacuum leaks was found to strongly reduce surface recombination, explaining the large variability in oxygen atom density.
CRDS measurements in RF capacitively-coupled plasmas in an aluminium showed surprisingly small surface recombination coefficients (~10-4) at pressures above 2 Torr. At lower pressures the surface reaction probability increases dramatically with RF power, indicating a transition from an oxidized (unreactive) surface to a metallic surface when the ion flux and energy is higher.
The surface recombination of oxygen atoms on borosilicate glass tube walls was studied by time-resolved optical emission actinometry of partially modulated DC positive column discharges. The observed reaction probabilities (~10-3) were not constant, but can be explained by Eley-Rideal reaction of incident atoms with chemisorbed atoms with an activation energy provided by the kinetic energy of the incident atom. For full modulation, the atom decay in the afterglow (measured by Cavity Ringdown spectroscopy, CRDS) is non-exponential, starting fast then slowing down. This suggests a second, quadratic, term in the recombination probability which can be attributed to reaction with weakly-bound physisorbed atoms. The presence of small quantities (10-3) of air from vacuum leaks was found to strongly reduce surface recombination, explaining the large variability in oxygen atom density.
CRDS measurements in RF capacitively-coupled plasmas in an aluminium showed surprisingly small surface recombination coefficients (~10-4) at pressures above 2 Torr. At lower pressures the surface reaction probability increases dramatically with RF power, indicating a transition from an oxidized (unreactive) surface to a metallic surface when the ion flux and energy is higher.
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Presenters
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Jean-Paul Booth
LPP-CNRS
Authors
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Jean-Paul Booth
LPP-CNRS
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Olivier Guaitella
LPP, CNRS, École Polytechnique, Sorbonne Université, Université Paris-Saclay, IP-Paris, Palaiseau, France
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Shu Zhang
LPP-CNRS
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Dmitry Lopaev
Lomonosov Moscow State University
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Sergey Zyryanov
Lomonosov Moscow State University
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Andrey Volynets
LPP-CNRS