Discharges In Electronegative Gases Research Articles

Simulation of the positive column of a glow discharge in an inert gas-chorine mixture with allowance for the dissociation of chlorine molecules

The model of the positive column (PC) of a moderate-pressure (5–30 Torr) glow discharge in a mixture of inert gases and chlorine under currents of up to 50 mA/cm2 extends the existing models of discharges in electronegative gases. The novelty of the model is the inclusion of radial nonuniformity in the gas temperature and dissociation of chlorine molecules into atoms, as well as allowance for heating of ions by the electric field and step ionization. As a result, the calculated radial profiles of charged particle concentrations are not only in qualitative agreement with earlier-developed theories of electronegative discharges, but also in quantitative agreement with experimental data. It is shown that allowance for dissociation of chlorine molecules and the effect of longitudinal field Ez on the ion temperatures is essential for this conformity in the model of the PC of a glow discharge in the mixture of inert gases and chlorine. However, radial field Er can be disregarded in estimating the ion temperatures. The smallness of the contribution of the ion current to the total current of the discharge, which had earlier only been postulated, is demonstrated. It is shown that diffusion-aided removal is the predominant mechanism of perishing of electrons and positive ions; for this reason, electron-ion recombination (as well as ion-ion recombination in the balance of positive ions) can be disregarded.

Adaptive mass alteration to model ion–ion recombination in a Particle-in-Cell simulation of silane radio-frequency discharges

In discharges in electronegative gases, negative ions are generated by (dissociative) attachment and lost by recombination with positive ions. Straightforward modeling of the recombination in a Particle-in-Cell simulation by removing a positive and negative super-particle gives rise to large numerical fluctuations. These fluctuations lead to non-physical electric fields and heating of the electrons. In this paper, a method is proposed that enables the simulation of the recombination of positive and negative ions in an accurate way, even at low densities. The recombination in a cell is done by changing the mass of the super-particles in this cell. Thus it is possible to recombine any desired amount of real particles even if it is less than one super-particle. Moreover, the number of super-particles in each cell is kept nearly constant by adding super-particles (if a cell contains too few particles) or by removing a super-particle while increasing the mass for each of the rest (if a cell contains too many). It is shown that this simulation of ion recombination results in a lower level of non-physical fluctuations. The lower level of computational noise is shown to result in a lower ionization rate and lower ion fluxes compared to PIC/MCC simulations without adaptive mass alteration.

NUMERICAL SIMULATION OF THE FORMATION AND PROPAGATION OF STREAMER

There has been considerable interest in non-thermal discharges over the past decade due to the increased number of industrial applications. The properties of discharges in electronegative gases are most frequently used for technological applications. For the improvement of performance in these applications, it is necessary to understand discharge dynamics experimentally and numerically. In this paper, a Monte Carlo simulation is carried out in sulfur hexafluoride (SF6) in uniform electric fields. The streamer propagation, electron, positive and negative ion distributions and space charge fields are studied in detail as time increases.

Fundamentals and applications of ion–ion plasmas

Ion–ion plasmas can form in the late afterglow of pulsed discharges or downstream of continuous wave discharges in electronegative gases. In ion–ion plasmas, negative ions replace electrons as the negative charge carriers. In the absence of electrons, ion–ion plasmas behave quite differently compared to conventional electron–ion plasmas. Application of a radio frequency bias to a substrate immersed in an ion–ion plasma can be used to extract alternately positive and negative ions, thereby minimizing charging on device features during micro-device fabrication. Ion–ion plasmas are also important in negative ion sources, dusty plasmas, and the D-layer of the earth's atmosphere.

Radio-frequency electronegative gas discharge behaviour in a parallel-plate reactor for material processing

In this paper, a second-order continuum model for radio-frequency glow discharge plasma between parallel-plate electrodes is presented. Simulations have been performed for discharges in argon and O2. The basic data needed for different collision processes are explicitly given with a high degree of accuracy. These data are rigorously calculated by using a multiterm Boltzmann equation solution for electrons and a Monte Carlo simulation technique for ions. The specificity of the present work is the use of a second-order fluid model solved by using a powerful finite-element scheme associated with basic data rigorously calculated. Detailed results for two types of discharge are presented and contrasted. The phenomena associated with the transition of the discharge into different operating regimes at relatively low and high pressures are demonstrated and discussed.

Role of electronegative gas admixtures in streamer start, propagation and branching phenomena

A critical analysis of photoionization as the major process for seed electron production ahead of the cathode-directed streamer has been made. The accumulation of ions between pulses and fast electron detachment in an electric field could be an effective source of seed electrons for repetitively pulsed discharges in electronegative gases. Measurements and two-dimensional calculations in the hydrodynamic approximation of streamer parameters in air (anode current, propagation velocity and radiative channel diameter) can only agree if one assumes uniform background pre-ionization and not by taking into account only the photoionization process generally adopted for pulsed discharges in air. The results of analytical estimates and full three-dimensional simulations confirm that the seed charge distribution in the undisturbed gap has an effect on the streamer head formation and leads to the streamer branching phenomena.

Model of solitary current filament in a high pressure electronegative gas discharge

A model of solitary current filaments formation in a high pressure electronegative gas discharge is proposed. An explicit analytical solution of the balance equation for electrons including stepwise ionization, electron attachment and diffusional losses is obtained. The contracted distribution of electrons in the positive column is found to have a soliton form. The conditions favouring non-thermal plasma contraction are considered. Analytical results are compared successfully with simulation and experimental data.

Development of optical computerized tomography in capacitively coupled plasmas and inductively coupled plasmas for plasma etching

In this paper we review the results of optical emission spectroscopy (OES) in diagnostics of radio frequency (rf) discharges. The status of OES diagnostics of rf plasmas used primarily for plasma etching has been reviewed for the past 15 years. The results obtained at Keio University have been reviewed in greater detail. Time resolved and time-averaged computerized tomography (CT) of OES has been applied to obtain 2D and 3D profiles of emission for a large number of gases (Ar, H 2, SiH 4, Cl 2, CF 4, O 2, SF 6) and gas mixtures. Application of OES–CT has become essential in understanding the sustaining mechanisms in capacitively coupled plasmas (CCPs) and in inductively coupled plasmas (ICPs) operating in rf. Those plasmas are produced in a wide range of frequencies, or combinations of frequencies and pulsed conditions. In addition, a wide range of geometries has been covered, providing an insight into spatial profiles of excitation (i.e. production of radicals) and therefore the uniformity of plasma etching. In our laboratory the technique was first applied to magnetrons. In CCP spatially resolved emission profiles were obtained for complex geometries and furthermore the effects of high frequency, two frequencies and pulsed operation were studied. Control of dust particles could be implemented by the same technique. Absolute values as well as the time resolved data showed the importance of the double layers in front of the instantaneous anode in maintaining the rf discharges in electro-negative gases. In ICP, CT was used to study the azimuthal anisotropy and the axial dependence of emission. Transition from capacitively to inductively coupled mode of operation could be proved directly by OES–CT profiles. Time resolved data for emission close to the coil showed two peaks with 45° phase difference, one of which was unexpected. In addition the effect of electro-negative gas and pulsing of the rf field were studied. It has been concluded that OES supplemented by fast, detailed models may be used for efficient real time control of plasma etching devices of the next generation.

A comprehensive treatment of the positive column of discharges in electronegative gases

Restricted accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Franklin R.N 2001A comprehensive treatment of the positive column of discharges in electronegative gasesProc. R. Soc. Lond. A.457307–330http://doi.org/10.1098/rspa.2000.0668SectionRestricted accessResearch articleA comprehensive treatment of the positive column of discharges in electronegative gases R.N Franklin R.N Franklin Oxford Research Unit, The Open University, Foxcombe Hall, Boars Hill, Oxford OX1 5HR, UK Google Scholar Find this author on PubMed Search for more papers by this author R.N Franklin R.N Franklin Oxford Research Unit, The Open University, Foxcombe Hall, Boars Hill, Oxford OX1 5HR, UK Google Scholar Find this author on PubMed Search for more papers by this author Published:08 February 2001https://doi.org/10.1098/rspa.2000.0668AbstractFluid equations are used to describe the plasma which is the positive column of a gas discharge in electronegative gases. These equations are solved computationally over a wide range of the parameters needed to characterize the plasma when attachment, detachment, recombination, ionization and particle collisions are significant processes. Where possible, comparison is made with results obtained by the method of matched asymptotic approximations. The conditions under which discharges may exist are elucidated and it is shown that it is necessary to include ion diffusion, through the ion temperature, to get realistic distributions of the ions at higher pressures. The conditions under which such discharges are structured are obtained with the general requirement that the attachment rate must be less than the ionization rate for there to be a central ion‐ion plasma core. Equally, the nearly collisionless situation is described in detail in the fluid approximation. Previous ArticleNext Article VIEW FULL TEXT DOWNLOAD PDF FiguresRelatedReferencesDetailsCited by Coche P, Guerra V and Alves L (2016) Microwave air plasmas in capillaries at low pressure I. Self-consistent modeling, Journal of Physics D: Applied Physics, 10.1088/0022-3727/49/23/235207, 49:23, (235207), Online publication date: 15-Jun-2016. RICHTER A, TESTRICH H, WAGNER H, LOFFHAGEN D and WILKE C (2009) Dynamic behaviour of an oxygen dc discharge, Journal of Plasma Physics, 10.1017/S0022377808007502, 75:1, (71-84), Online publication date: 1-Feb-2009. Franklin R (2003) Electronegative plasmas diluted by rare gases, Journal of Physics D: Applied Physics, 10.1088/0022-3727/36/21/009, 36:21, (2655-2659), Online publication date: 7-Nov-2003. Franklin R (2003) Ambipolar diffusion is a misnomer, Journal of Physics D: Applied Physics, 10.1088/0022-3727/36/7/310, 36:7, (828-831), Online publication date: 7-Apr-2003. Franklin R (2002) Electronegative plasmas why are they so different?, Plasma Sources Science and Technology, 10.1088/0963-0252/11/3A/304, 11:3A, (A31-A37), Online publication date: 1-Aug-2002. Kono A (2002) Negative ions in processing plasmas and their effect on the plasma structure, Applied Surface Science, 10.1016/S0169-4332(02)00023-5, 192:1-4, (115-134), Online publication date: 1-May-2002. Franklin R (2001) A critique of models of electronegative plasmas, Plasma Sources Science and Technology, 10.1088/0963-0252/10/2/305, 10:2, (162-167), Online publication date: 1-May-2001. This Issue08 February 2001Volume 457Issue 2006 Article InformationDOI:https://doi.org/10.1098/rspa.2000.0668Published by:Royal SocietyPrint ISSN:1364-5021Online ISSN:1471-2946History: Published online08/02/2001Published in print08/02/2001 License: Citations and impact Keywordsrecombinationmatched asymptotic approximationsdetachmentelectronegative gasesplasmaion diffusion

Modelling discharges in electronegative gases

This paper builds on earlier work to give a consistent treatment of the positive column of discharges in electronegative gases covering the transition from collisionless to collisional. In particular it seeks to elucidate the conditions under which there is an ion-ion plasma core surrounded by an electron-ion plasma, and when there is not. The parameters which describe the processes of ionization, attachment, detachment and recombination are related to the central negative ion density relative to the electron density and, where appropriate, the size of the core. The use, by earlier workers, of the Boltzmann approximation to describe the negative ion distribution and to obtain ambipolar diffusion coefficients at higher pressures is shown not to be justified. This leads to the clarification of an inconsistency in the literature. Where possible, the work is related to other recent treatments of the same problem in order to begin to build a comprehensive picture of such discharges. The need to have results which combine both detachment and recombination as the negative ion loss processes is identified as outstanding. This, when rectified, should lead to a fully comprehensive treatment.

High-Frequency Magnetoplasmas in Electronegative Gases

Discharges in electronegative gases are routinely used for the sub-micron etching of thin films in the microelectronics industry. Because of the strong electronegative character of these gases, negative ions constitute a significant fraction of the charged particles content of the discharge. The presence of these negative charge carriers affects the whole behavior of the discharge and, in particular, its electron power balance. This article compares a few characteristics of a high-density plasma produced either in SF6 or in Cl 2 with those of an argon plasma under similar experimental conditions. We show that the plasma presents characteristics when the gas is electronegative that significantly differ from those observed in argon.

Modelling plasma discharges at high electronegativity

Macroscopic models for the equilibrium of a three-component electronegative gas discharge are developed. Assuming the electrons and the negative ions to be in Boltzmann equilibrium, a positive ion ambipolar diffusion equation is derived. Such a discharge can consist of an electronegative core and may have electropositive edge regions, but the electropositive regions become small for the highly electronegative plasma considered here. In the parameter range for which the negative ions are Boltzmann, the electron density in the core is nearly uniform, allowing the nonlinear diffusion equation to be solved in terms of elliptic integrals. If the loss of positive ions to the walls dominates the recombination loss, a simpler parabolic solution can be obtained. If recombination loss dominates the loss to the walls, the assumption that the negative ions are in Boltzmann equilibrium is not justified, requiring coupled differential equations for positive and negative ions. Three parameter ranges are distinguished corresponding to a range in which a parabolic approximation is appropriate, a range for which the recombination significantly modifies the ion profiles, but the electron profile is essentially flat, and a range where the electron density variation influences the solution. The more complete solution of the coupled ion equations with the electrons in Boltzmann equilibrium, but not at constant density, is numerically obtained and compared with the more approximate solutions. The theoretical considerations are illustrated using a plane parallel discharge with chlorine feedstock gas of p = 30, 300 and 2000 mTorr and , corresponding to the three parameter regimes. A heuristic model is constructed which gives reasonably accurate values of the plasma parameters in regimes for which the parabolic profile is not adequate.

Current pulses in dc glow discharges in electronegative gas mixtures

Regular oscillations in the current are predicted in low-pressure, planar discharges under certain conditions in electronegative gas mixtures in which the attachment rate coefficient is large at low values of E/P, the ratio of the electric-field strength to the gas pressure. The frequency of the oscillations is about 10 kHz, and depending on the conditions of pressure, gap spacing, and applied voltage, the current wave form varies from a near-sinusoidal shape to regularly repeating and well-separated spikes with a peak current density on the order of or less than 1 mA/cm2. The instability which gives rise to these oscillations is due to attachment, and the oscillations result from alternate phases of space charge buildup and decay. Thus, the current oscillations predicted here in planar discharges are analogous to Trichel pulses, periodic current spikes which are observed in negative point-plane corona discharges in electronegative gases.

Conduction de liquides aromatiques en géométrie pointe-plan

The electrical conduction of purified aromatic liquids in the tip-plane electrode geometry was studied for high electric fields. The current voltage curves, the current pulses were measured for dc applied voltage in toluene, benzyltoluene, and in the saturated hydrocarbon ndecane. In n-decane, current pulses are similar to corona discharges in electronegative gases and to pulses previously studied in liquid cyclohexane. In aromatic liquids, a discontinuity in the current-voltage curve was also observed for an electric field of 4.5±0.5 MV.cm -1 whereas current pulses were detected for higher fields. This behavior is attributed to the ionization and electronic attachment of aromatic molecules

On Electron Concentration Distribution along the Radius of the Positive Column of a Glow Discharge in Electronegative Gases

AbstractIt is shown in this paper that the form of the electron concentration distribution along the radius of the positive column in an oxygen discharge depends on the relative concentration of negative ions α = n‐/ne in the plasma.It is found that in discharges in electronegative gases everywhere along the radius of the positive column, where α > 10, the electron concentration is equal to its value on the tube axis (plane electron distribution along the radius of the positive column).It is shown that an increase of the discharge current leads to the decrease of α and to the change of the distribution of profile ne from plane to parabolic. However, weak laminar oxygen pumping along the positive column leads to more plane an electron concentration distribution because an increase of the negative ions concentration takes place. It is induced by the decrease of oxygen atoms density on account of their carrying out by the flow.

Hydrogen atom sources for electrically pulsed hydrogen halide lasers

With the aim of optimizing the performance of electrical discharge pulsed high-pressure hydrogen halide lasers, we have investigated the effects of over forty chemicals on these systems. We have examined those compounds for which the chain carrying step, X + HR \underrightarrow{(1)} RX + H (X = Br, Cl, or F) promised to be more exothermic than the corresponding step with the usual fuels (i.e., R = H or a simple hydrocarbon radical). In the case of the less reactive halogens, production of hydrogen atoms in a nonchain reaction, directly initiated by the discharge, may well compete or dominate reaction (1) on the time scale of the lasing pulse. Because of this and the low electron temperature associated with high pressure discharges in electronegative gases, we have examined chemicals that contain a weakly bound hydrogen or a weakly bound, but highly reactive radical that could react in a second step to yield hydrogen atoms. These could then react in the lasing step, H + XR' → HX + R', to give vibrationally excited HX. Chemicals used included H 2 S, H 2 O 2 , highly chlorinated alkanes, alkines, and aldehydes, hydrazine, silane, and several substituted derivatives thereof. Helium and argon which improve discharge characteristics were also studied. The reaction was initiated by a 30-kV electric discharge from an array of needle electrodes, each resistively ballasted. The halogen sources used were the usual Br 2 , Cl 2 , and SF 6 . Results such as the enhancement of HBr lasing in a H 2 + Br 2 mixture upon the addition of O 2 but not upon the addition of N 2 can only be explained in terms of the ensuing chemical reactions and will be discussed.

Recombination Coefficients for Positive and Negative Ions

The dielectric constant of an ionized medium is decreased not merely by the presence of electrons but also by that of other free carriers, the positive and negative ions. In a highly electronegative gas discharge, the effect of the ions cannot be neglected and sometimes their contributions may swamp that of the electrons. Under such conditions, measurements of the recombination coefficient between positive and negative ions have been made in the afterglow following pulsed radio-frequency discharges in iodine and bromine vapours.. The recombination coefficient αi was found to be 1.47 × 10-8 cm3 sec-1 for iodine molecular ions and 1.85 × 10-8 cm3 sec-1 for bromine at room temperature. α1 was constant over an extended pressure range and was found to be temperature dependent.