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Title: Plasma-Surface Reactions at a Spinning Wall

Abstract

We report a new method for studying surface reactions and kinetics at moderately high pressures (<10 Torr) in near real time. A cylindrical substrate in a reactor wall is rotated at up to 200,000 rpm, allowing the surface to be periodically exposed to a reactive environment and then analyzed by a triple-differentially pumped mass spectrometer in as little as 150 {mu}s thereafter. We used this method to study oxygen plasma reactions on anodized aluminum. When the substrate is spun with the plasma on, a large increase in O{sub 2} signal at m/e=32 is observed with increasing rotation frequency, due to O atoms that impinge and stick on the surface when it is in the plasma, and then recombine over the {approx}0.7 to 40 ms period probed by changing the rotation frequency. Simulations of O{sub 2} signal versus rotation frequency indicate a wide range of recombination rate constants, ascribed to a range of O-binding energies.

Authors:
; ;  [1]
  1. Department of Chemical Engineering University of Houston, Houston, Texas 77204 (United States)
Publication Date:
OSTI Identifier:
20775026
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 96; Journal Issue: 1; Other Information: DOI: 10.1103/PhysRevLett.96.018306; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ALUMINIUM; BINDING ENERGY; CHEMICAL ANALYSIS; MASS SPECTROMETERS; OXYGEN; PERIODICITY; PLASMA; PLASMA DIAGNOSTICS; PLASMA SIMULATION; PRESSURE RANGE KILO PA; REACTION KINETICS; RECOMBINATION; ROTATION; SIGNALS; SUBSTRATES; SURFACES; WALL EFFECTS

Citation Formats

Kurunczi, P.F., Guha, J., and Donnelly, V.M. Plasma-Surface Reactions at a Spinning Wall. United States: N. p., 2006. Web. doi:10.1103/PhysRevLett.96.018306.
Kurunczi, P.F., Guha, J., & Donnelly, V.M. Plasma-Surface Reactions at a Spinning Wall. United States. doi:10.1103/PhysRevLett.96.018306.
Kurunczi, P.F., Guha, J., and Donnelly, V.M. Fri . "Plasma-Surface Reactions at a Spinning Wall". United States. doi:10.1103/PhysRevLett.96.018306.
@article{osti_20775026,
title = {Plasma-Surface Reactions at a Spinning Wall},
author = {Kurunczi, P.F. and Guha, J. and Donnelly, V.M.},
abstractNote = {We report a new method for studying surface reactions and kinetics at moderately high pressures (<10 Torr) in near real time. A cylindrical substrate in a reactor wall is rotated at up to 200,000 rpm, allowing the surface to be periodically exposed to a reactive environment and then analyzed by a triple-differentially pumped mass spectrometer in as little as 150 {mu}s thereafter. We used this method to study oxygen plasma reactions on anodized aluminum. When the substrate is spun with the plasma on, a large increase in O{sub 2} signal at m/e=32 is observed with increasing rotation frequency, due to O atoms that impinge and stick on the surface when it is in the plasma, and then recombine over the {approx}0.7 to 40 ms period probed by changing the rotation frequency. Simulations of O{sub 2} signal versus rotation frequency indicate a wide range of recombination rate constants, ascribed to a range of O-binding energies.},
doi = {10.1103/PhysRevLett.96.018306},
journal = {Physical Review Letters},
number = 1,
volume = 96,
place = {United States},
year = {Fri Jan 13 00:00:00 EST 2006},
month = {Fri Jan 13 00:00:00 EST 2006}
}
  • This article reviews methods for studying reactions of atoms and small molecules on substrates and chamber walls that are immersed in a plasma, a relatively unexplored, yet very important area of plasma science and technology. Emphasis is placed on the ''spinning wall'' technique. With this method, a cylindrical section of the wall of the plasma reactor is rotated, and the surface is periodically exposed to the plasma and then to a differentially pumped mass spectrometer, to an Auger electron spectrometer, and, optionally, to a beam of additional reactants or surface coatings. Reactants impinging on the surface can stick and reactmore » over time scales that are comparable to the substrate rotation period, which can be varied from {approx}0.5 to 40 ms. Langmuir-Hinshelwood reaction probabilities can be derived from a measurement of the absolute desorption product yields as a function of the substrate rotation frequency. Auger electron spectroscopy allows the plasma-immersed surface to be monitored during plasma operation. This measurement is critical, since wall ''conditioning'' in the plasma changes the reaction probabilities. Mass spectrometer cracking patterns are used to identify simple desorption products such as Cl{sub 2}, O{sub 2}, ClO, and ClO{sub 2}. Desorption products also produce a measurable pressure rise in the second differentially pumped chamber that can be used to obtain absolute desorption yields. The surface can also be coated with films that can be deposited by sputtering a target in the plasma or by evaporating material from a Knudsen cell in the differentially pumped wall chamber. Here, the authors review this new spinning wall technique in detail, describing both experimental issues and data analysis methods and interpretations. The authors have used the spinning wall method to study the recombination of Cl and O on plasma-conditioned anodized aluminum and stainless steel surfaces. In oxygen or chlorine plasmas, these surfaces become coated with a layer containing Si, Al, and O, due to slow erosion of the reactor materials, in addition to Cl in chlorine plasmas. Similar, low recombination probabilities were found for Cl and O on anodized Al versus stainless steel surfaces, consistent with the similar chemical composition of the layer that forms on these surfaces after long exposure to the plasma. In chlorine plasmas, weakly adsorbed Cl{sub 2} was found to inhibit Cl recombination, hence the Cl recombination probability decreases with increasing Cl{sub 2}-to-Cl number density ratios in the plasma. In mixed Cl{sub 2}/O{sub 2} plasmas, Cl and O recombine to form Cl{sub 2} and O{sub 2} with probabilities that are similar to those in pure chlorine or oxygen plasmas, but in addition, ClO and ClO{sub 2} form on the surface and desorb from the wall. These and other results, including the catalytic enhancement of O recombination by monolayer amounts of Cu, are reviewed.« less
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  • The plasma synthesis of ammonia was studied at pressures of 1-5 torr and flow rates of up to 200 torr cm/sup 3/ min/sup -1/ using Pyrex and silver surfaces cooled to 77 K. The N conversion to ammonia was about 13% in experiments in which the afterglow was trapped on the Pyrex surface. By quenching the plasma rather than the afterglow, the percent N conversion could be doubled using the Pyrex surface and quadrupled using the silver surface. Increasing the hydrogen pressure and/or hydrogen discharge cleaning decreased the percent N conversion; nitrogen discharge conditioning had no significant effect. With increasingmore » nitrogen flow rate the percent N conversion decreased linearly in the quenched plasma reaction on the silver surface, suggesting nitriding and reduction by hydrogen to form ammonia. The exponential decrease of the percent N conversion in the quenched afterglow reaction of the Pyrex surface is explained by the formation and/or dissociation of adsorbed N/sub 2/ determining the ammonia yield at 77 K.« less
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