Investigation of the radially resolved oxygen dissociation degree and local mean electron energy in oxygen plasmas in contact with different surface materials
Abstract:Energy Resolved Actinometry is applied to simultaneously measure the radially resolved oxygen dissociation degree and local mean electron energy in a low-pressure capacitively coupled radio-frequency oxygen plasma with an argon tracer gas admixture. For this purpose, the excitation dynamics of three excited states, namely, Ar(2p1), O(3p3P), and O(3p5P), were determined from their optical emission at 750.46 nm, 777.4 nm, and 844.6 nm using Phase Resolved Optical Emission Spectroscopy (PROES). Both copper and si… Show more
“…Frequently, these cross sections are not part of the complete set. An example is the calculation of the electron impact rate coefficients required to interpret actinometry measurements [185,186].…”
Numerous applications have required the study of CO2 plasmas since the 1960s, from CO2 lasers to spacecraft heat shields. However, in recent years, intense research activities on the subject have restarted because of environmental problems associated with CO2 emissions. The present review provides a synthesis of the current state of knowledge on the physical chemistry of cold CO2 plasmas. In particular, the different modeling approaches implemented to address specific aspects of CO2 plasmas are presented. Throughout the paper, the importance of conducting joint experimental, theoretical and modeling studies to elucidate the complex couplings at play in CO2 plasmas is emphasized. Therefore, the experimental data that are likely to bring relevant constraints to the different modeling approaches are first reviewed. Second, the calculation of some key elementary processes obtained with semi-empirical, classical and quantum methods is presented. In order to describe the electron kinetics, the latest coherent sets of cross section satisfying the constraints of "electron swarm" analyses are introduced, and the need for self-consistent calculations for determining accurate electron energy distribution function (EEDF) is evidenced. The main findings of the latest zero-dimensional (0D) global models about the complex chemistry of CO2 and its dissociation products in different plasma discharges are then given, and full state-to-state (STS) models of only the vibrational-dissociation kinetics developed for studies of spacecraft shields are described. Finally, two important points for all applications using CO2 containing plasma are discussed: the role of surfaces in contact with the plasma, and the need for 2D/3D models to capture the main features of complex reactor geometries including effects induced by fluid dynamics on the plasma properties. In addition to bringing together the latest advances in the description of CO2 non-equilibrium plasmas, the results presented here also highlight the fundamental data that are still missing and the possible routes that still need to be investigated.
“…Frequently, these cross sections are not part of the complete set. An example is the calculation of the electron impact rate coefficients required to interpret actinometry measurements [185,186].…”
Numerous applications have required the study of CO2 plasmas since the 1960s, from CO2 lasers to spacecraft heat shields. However, in recent years, intense research activities on the subject have restarted because of environmental problems associated with CO2 emissions. The present review provides a synthesis of the current state of knowledge on the physical chemistry of cold CO2 plasmas. In particular, the different modeling approaches implemented to address specific aspects of CO2 plasmas are presented. Throughout the paper, the importance of conducting joint experimental, theoretical and modeling studies to elucidate the complex couplings at play in CO2 plasmas is emphasized. Therefore, the experimental data that are likely to bring relevant constraints to the different modeling approaches are first reviewed. Second, the calculation of some key elementary processes obtained with semi-empirical, classical and quantum methods is presented. In order to describe the electron kinetics, the latest coherent sets of cross section satisfying the constraints of "electron swarm" analyses are introduced, and the need for self-consistent calculations for determining accurate electron energy distribution function (EEDF) is evidenced. The main findings of the latest zero-dimensional (0D) global models about the complex chemistry of CO2 and its dissociation products in different plasma discharges are then given, and full state-to-state (STS) models of only the vibrational-dissociation kinetics developed for studies of spacecraft shields are described. Finally, two important points for all applications using CO2 containing plasma are discussed: the role of surfaces in contact with the plasma, and the need for 2D/3D models to capture the main features of complex reactor geometries including effects induced by fluid dynamics on the plasma properties. In addition to bringing together the latest advances in the description of CO2 non-equilibrium plasmas, the results presented here also highlight the fundamental data that are still missing and the possible routes that still need to be investigated.
“…, where k O de is calculated with equation (2) with the corresponding cross section. Rearranging the different terms and using argon as actinometer, the O atom density can be obtained from the expression [67]:…”
Oxygen atom densities were measured in situ in a CO 2 glow discharge, at pressures between 0.2 and 5 Torr . Two measurement techniques were compared, namely optical emission actinometry (using Ar as the actinometer) and High-Resolution Two-photon Absorption Laser Induced Fluorescence (HR-TALIF) normalised to Xe, and were found to give consistent results. The variation of the atomic oxygen density with gas pressure shows two different regimes with a transition around 1 Torr. Measurements of the O atom loss frequency under plasma exposure showed that this behaviour is caused by a change in the O atom loss mechanisms, which are dominated by surface processes in our experimental conditions. The corresponding recombination probabilities on Pyrex γ O are found to vary with the gas temperature near the wall for a constant surface temperature, similarly to what has recently been obtained in pure O 2 . However, the measured values are more than two times lower than γ O obtained in a O 2 plasma in similar conditions. The O atom densities are also compared to the dissociation fraction of CO 2 determined by infra-red absorption. The obtained CO and O densities show different behaviour as a function of the energy input. The simultaneous measurement of gas temperature, electric field, O, CO and CO 2 densities and O atoms loss frequency in the same conditions provides an ideal set of constraints for validating CO 2 plasma kinetic models.
“…The foremost advantage of this method is that it remains unaffected by deposition or etching conditions as long as the optical access is clean. To this end, different approaches such as the trace of a rare gas (TRG) OES 31,32 using the mixture of Ar/He/Ne/Xe to the process gases, optical 33 and energy 34 resolved actinometry, and rotational spectroscopy of N 2 35,36 have been used for the determination of T e , n e , T g , and F atom concentration in the plasma etching of SiO 2 at low-pressure discharges. However, during the actual plasma process, the experimental gas contains no mixed gas of Ar/He/Ne/Xe.…”
We consider corona model and local thermal equilibrium approximations of a real plasma to present measurements of electron temperature (Te) and density (ne), respectively, using optical emission spectroscopy (OES) method...
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