Showing papers in "Plasma Sources Science and Technology in 2000"
TL;DR: In this paper, a review of the most commonly used methods for the generation of plasmas with special emphasis on non-thermal, low-temperature Plasmas for technological applications is presented.
Abstract: This paper reviews the most commonly used methods for the generation of plasmas with special emphasis on non-thermal, low-temperature plasmas for technological applications. We also discuss various technical realizations of plasma sources for selected applications. This paper is further limited to the discussion of plasma generation methods that employ electric fields. The various plasmas described include dc glow discharges, either operated continuously (CW) or pulsed, capacitively and inductively coupled rf discharges, helicon discharges, and microwave discharges. Various examples of technical realizations of plasmas in closed structures (cavities), in open structures (surfatron, planar plasma source), and in magnetic fields (electron cyclotron resonance sources) are discussed in detail. Finally, we mention dielectric barrier discharges as convenient sources of non-thermal plasmas at high pressures (up to atmospheric pressure) and beam-produced plasmas. It is the main objective of this paper to give an overview of the wide range of diverse plasma generation methods and plasma sources and highlight the broad spectrum of plasma properties which, in turn, lead to a wide range of diverse technological and technical applications.
683 citations
TL;DR: In this article, an introductory discussion of direct current gas discharges is presented, starting with basic ideas from kinetic theory, gas discharge plasmas are described in terms of phenomena observed in the laboratory.
Abstract: This is a tutorial article. An introductory discussion of direct current gas discharges is presented. Beginning with basic ideas from kinetic theory, gas discharge plasmas are described in terms of phenomena observed in the laboratory. Various models are introduced to account for electrical breakdown, plasma boundaries and the longitudinal and transverse structure of discharges.
320 citations
TL;DR: In this paper, the transition from a glow silent discharge to micro-discharges in nitrogen at atmospheric pressure was studied by emission spectroscopy and electrical measurements, and it was shown that the maximum power that can be used while maintaining a glow discharge depends on the nature of dielectric surface in contact with the gas.
Abstract: At atmospheric pressure, the electrical breakdown of a silent discharge can occur in many thin filaments (leading to micro-discharges) or in a single discharge canal covering the entire electrode surface (leading to a glow discharge). The aim of this paper is to contribute to a better understanding of the transition from a glow silent discharge to micro-discharges in nitrogen at atmospheric pressure. For this purpose, the two types of regime have been studied by emission spectroscopy and electrical measurements. The transition is always observed due to an increase of the power dissipated in the gas gap, but the maximum power that can be used while maintaining a glow discharge depends on the nature of the dielectric surface in contact with the gas. These results have been explained by the predominance of the density of metastable nitrogen molecules on the discharge regime. Due to the creation of seed electrons via Penning ionization, these metastable molecules can control the gas breakdown and so the discharge regime. Their density essentially depends on their quenching rate. The products etched from the surfaces in contact with the discharge appear to be the main source of the metastable molecules quenching. Therefore, the nature of the surface controls the nature of the quenching of the metastable molecules and the power dissipated in the discharge the quencher density.
293 citations
TL;DR: In this article, the Advanced Oxidation Process pulsed corona discharges have been utilized for the degradation of phenol in aqueous solution, and experiments have been performed using both an air and argon atmosphere.
Abstract: The Advanced Oxidation Process pulsed corona discharges have been utilized for the degradation of phenol in aqueous solution. The pulsed positive corona discharges are struck in the ambient gas phase over the solution. Experiments have been performed using both an air and argon atmosphere. Phenol conversion and the production of major oxidation products in the solution have been determined, using ion-exclusion chromatography with UV absorbance and conductivity detectors in series. The corona pulse energy has been measured from voltage and current waveforms using capacitive current correction. Oxidation products are polyhydroxybenzenes and carboxylic acids. Even though phenol conversion efficiencies by pulsed positive corona discharges in air and argon are similar, the degradation pathways are different.
132 citations
TL;DR: In this paper, a low-power, compact microwave-induced plasma source for applications in atomic emission spectrometry at atmospheric pressure using microstrip technology is described, where the gas channel of about 1 mm2 is integrated in a fused silica dielectric wafer.
Abstract: A new low-power, compact microwave-induced plasma source for applications in atomic emission spectrometry at atmospheric pressure using microstrip technology is described. The gas channel of about 1 mm2 is integrated in a fused silica dielectric wafer. The microstrip transmission lines are fabricated by sputtering and electro-plating. For example, a unit operates at an input power of 15 W with an argon gas flow of about 500 ml min-1 at atmospheric pressure. Rotational (OH) and excitation (Fe) temperatures of 650 K and 8000 K, respectively, were measured at these conditions. The emitted radiation can be taken up by an optical fibre positioned in the plasma-gas channel thus enabling an axial observation and coupling to a miniaturized spectrometer. The first devices showed an operation time of at least several hundred hours. Further investigations will lead to even smaller dimensions and lower power consumption and open the way for integrated microwave plasma sources with low detection limits as integrable parts of miniaturized total analytical systems applications.
129 citations
TL;DR: Trace rare gases-optical emission spectroscopy (TRG-OES) and Langmuir probe analysis have been used to measure the electron temperature, Te, in a high-density inductively (transformer) coupled (TCP) 10 mTorr oxygen plasma as a function of the 13.56 MHz radio frequency (rf) power.
Abstract: Trace rare gases-optical emission spectroscopy (TRG-OES) and Langmuir probe analysis have been used to measure the electron temperature, Te , in a high-density inductively (transformer) coupled (TCP) 10 mTorr oxygen plasma as a function of the 13.56 MHz radio frequency (rf) power. The oxygen atomic densities were estimated by O-atom optical emission (8446 ?), and rare gas actinometry (Ar, 7504 ?). In the H-(inductive)-mode, Te increases from 2.6 to 3.4 eV for the low-energy electrons sampled by the Langmuir probe and from ~3.5 to 6.0 eV for the high-energy electrons sensed by TRG-OES as the rf power is increased from 120 to 1046 W. In the E-(capacitive)-mode, below 50 W, Te measured by TRG-OES increases with rf power from ~4 eV at very low power (~7 W) to ~6.1 eV at 45 W. Between the highest E-mode power (~50 W) and lowest H-mode power (~120 W), the Te measured by TRG-OES drops from 6.1 to 3.5 eV, while Te derived from Langmuir probe measurements drops only slightly from 3.0 to 2.6 eV. In the H-mode, the electron energy distribution function (EEDF) is bi-Maxwellian from ~120 to 1046 W. In the E-mode, the EEDF changes from nearly Maxwellian (possibly Druyvesteyn) at low rf powers (~7 W) to bi-Maxwellian at the higher E-mode powers (~45 W). O2 dissociation is low (~2%) at the maximum rf power density of 5.7 W cm-2 (1046 W), and this low value is attributed to the high rate of O-atom recombination on the mostly stainless-steel walls. A detailed accounting of the sources of O (8446 ?) emission revealed significant contributions from electron impact excitation from O(1 S) and dissociative excitation of O2 .
113 citations
TL;DR: A large-area plasma processing system (LAPPS) is under development at the Naval Research Laboratory as discussed by the authors, where the plasma is generated by a sheet electron beam with voltages and current densities of the order of kilovolts and tens of milliamps per cm2.
Abstract: A large-area plasma processing system (LAPPS) is under development at the Naval Research Laboratory. In the LAPPS, the plasma is generated by a sheet electron beam with voltages and current densities of the order of kilovolts and tens of milliamps per cm2. The plasma dimensions are a metre square by a few centimetres thick. The beam is guided by a magnetic field of 50-300?G. Since an electron beam of this type efficiently ionizes any gas, high electron densities of n~1012-1013?cm-3 are easily generated at 30-100?mTorr background pressure. In addition to large area and high electron density, the LAPPS has advantages for plasma processing. These include independent control of ion and free radical fluxes to the surface, very high uniformity, very low electron temperature (Te, {<}1?eV, but can be controllably increased to a desired value) and a geometry that is well suited for many applications. This paper sketches an initial theoretical overview of issues in the LAPPS and compares aspects of the theory to a preliminary experiment.
94 citations
TL;DR: The means, both physical and numerical, for speeding up particle-in-cell (PIC) simulations of RF discharges are demonstrated, including implicit movers, longer ion timesteps, lighter-mass ions, different weights for electrons and ions, and improved initial density profiles.
Abstract: We demonstrate the means, both physical and numerical, for speeding up particle-in-cell (PIC) simulations of RF discharges. These include implicit movers, longer ion timesteps, lighter-mass ions, different weights for electrons and ions, and improved initial density profiles. By using these methods (singly or together) on Ar and O2 RF discharges we were able to achieve speedups of six to 30 times with single-processor machines. In electrostatic 1d3v PIC simulations of RF discharges, the field solve is typically less than 1% of the work load. Even for 2d3v PIC simulations, the field solve can be a small percentage of the work load, especially when fast Fourier transform methods are used to solve the field. Thus, we can obtain significant gains by just paralleling particle processing (e.g., pushing/accumulating) without paralleling the field solve. We applied this simple scheme to conduct 1d3v and 2d3v PIC simulations of Ar RF discharges on two- and four-CPU symmetric multiprocessor machines and on a distributed network of workstations. For a fixed number of grid points, the speedup for this parallel particle processing became more linear with increasing number of particles. The combination of single-processor methods and paralleling makes run times for PIC codes more competitive with other types of codes.
88 citations
86 citations
TL;DR: In this article, the structure of the plasma-sheath boundary in low-pressure plasmas dominated by negative ions is examined in detail, and the results of analytical and computational treatments are brought together and it is suggested that experiment may be the only way to determine the relative merits of the two models, as well as demonstrating their general validity.
Abstract: This paper brings together recent work in low-pressure plasmas dominated by negative ions, examining in detail the structure of the plasma-sheath boundary. The parallels between negative ion-electron plasmas and two-electron-species plasmas are drawn, as are those between probe-plasma and plasma-wall systems. Particular attention is given to the parameter region where, for the temperature ratio Te / Tn greater than a critical value and a range of values of the density ratio nn / ne , there is a structure where two `plasmas', one an ion-ion plasma and the other essentially an electron-ion plasma, are separated by a potential structure. It is shown that in neither the fluid model nor in the free-fall model is the structure a simple double layer. The results of analytical and computational treatments are brought together and it is suggested that experiment may be the only way to determine the relative merits of the two models, as well as demonstrating their general validity.
84 citations
TL;DR: In this paper, the authors measured the neutral drag force on the dust and the curvature of the sheath electric potential in which the dust moves and then calculated the charge of the dust particles suspended in the plasma sheath.
Abstract: Dust can be suspended in a plasma sheath under certain conditions. An analysis of the dust trajectory as it approaches its equilibrium suspension height can provide a description of the spatial variation of the potential in the sheath. We describe an experiment in which we track such trajectories and calculate their oscillation frequencies, equilibrium heights and damping constants. These three measured parameters are then interpreted in such a manner as to reveal the neutral drag force on the dust and the curvature of the sheath electric potential in which the dust moves. We then calculate the charge on the dust particles suspended in the plasma sheath. We also show that, to a high degree of accuracy, the sheath potential predicted by several numerical models in the literature as well as by our experimental results is parabolic, i.e. the sheath field is quite linear.
TL;DR: In this paper, a collisional-radiative model of argon was used for determining the electron temperature and density in a positive column of an argon dc discharge, and compared with those obtained by a Langmuir probe.
Abstract: The present paper describes a spectroscopic method for determining the electron temperature Te and density Ne in argon plasmas on the basis of a collisional-radiative model of argon. We measured Te and Ne in a positive column of an argon dc discharge by the method developed, and compared them with those obtained by a Langmuir probe. The results for Te obtained by the spectroscopic method agreed roughly with those by the probe. However, a limitation of our method for obtaining Ne was found.
TL;DR: In this paper, a broad range of plasma-deposited materials as well as deposition methods for thin-film growth are presented. But the authors focus on the elementary surface mechanisms of radicals and ions, and typical growth models for typical surface reactions are discussed.
Abstract: The growth of thin films from low-temperature plasmas plays an important role in many applications such as optical or wear-resistant layers or for the fabrication of electronic devices. Albeit of great importance, the underlying growth mechanisms responsible for film formation from low-temperature plasmas are not well known. The direct identification of a growth mechanism is often hampered by the huge complexity of the bulk plasma processes and the plasma-surface interaction. The distribution of impinging species is very diverse, and ions, radicals and neutrals are interacting simultaneously with the growing film surface. A macroscopic quantity such as the growth rate can be the result of possible synergisms and anti-synergisms among a large variety of growth precursors. Due to the broad range of plasma-deposited materials as well as deposition methods, the objective of this paper is not to review them all, but to present a basic overview on the elementary surface mechanisms of radicals and ions. On the basis of these surface reactions, typical growth models will be discussed. As an example, for surface processes during thin-film growth, the current deposition models for amorphous hydrogenated carbon and silicon films are presented.
TL;DR: In this article, the authors investigated the surface and volume losses of ground-state atomic nitrogen in a parallel plate rf reactor using a pulsed N2 discharge and two-photon laser induced fluorescence detection of ground state atomic nitrogen.
Abstract: The loss of atomic nitrogen due to surface and volume reactions in a parallel plate rf reactor was investigated using a pulsed N2 discharge and two-photon laser induced fluorescence detection of ground-state atomic nitrogen. Stainless-steel and aluminium electrode surfaces as well as silicon and boron nitride substrates were investigated for their reactivity with atomic nitrogen within the pulsed discharge environment at 1-5 Torr N2. Aluminium was found to have a surface loss rate of three to five times less than that of stainless-steel, while boron nitride had the lowest N atom recombination rate of the materials studied. The N atom recombination probability coefficient was found to have an inverse pressure dependence for each of the materials, with values ranging from 0.5 to 0.02%. The volume loss rate of N atoms was also quantified due to minute O2 impurities introduced into the pulsed rf N2 discharge.
TL;DR: In this paper, the authors investigated the breakdown in low-pressure helium gas both experimentally and by computer simulations at low breakdown voltages (VBR 1000 V) and found that the experimental and simulation results show a good agreement (differences are within 20%), while at higher voltages the simulations and experiments agree qualitatively.
Abstract: We investigated the breakdown in low-pressure helium gas both experimentally and by computer simulations At low breakdown voltages (VBR 1000 V) the experimental and simulation results show a good agreement (differences are within 20%), while at higher voltages the simulations and experiments agree qualitatively Our simulations indicate that several processes contribute to the particular shape of the Paschen curve in helium at low pressures These processes are: (1) the dependence of the (ion-induced) secondary electron emission yield on the ion energy, (2) the appearance of ion impact ionization of the gas at high electric fields and (3) the secondary electron emission from the cathode due to fast neutral atoms
TL;DR: In this article, the operating characteristics of a linear geometry Hall plasma source scaled to operate in the 50 to 100 Watt power range are described, and two thruster acceleration channels are implemented-one of alumina and one of boron nitride.
Abstract: The operating characteristics of a linear geometry Hall plasma source scaled to operate in the 50 to 100 Watt power range are described. Two thruster acceleration channels are implemented-one of alumina and one of boron nitride. Differences in operation with the two channel materials are attributable to differences in the secondary electron emission properties. In either case, however, operation is achieved despite the lack of a closed electron current drift in the Hall direction, suggesting that there is an anomalous axial electron mobility, due to either plasma fluctuations or collisions with the channel wall. Strong low frequency oscillations in the discharge current, associated with the depletion of propellant within the discharge, are seen to appear and vary with changes in the applied magnetic field strength. The frequency of this oscillatory mode is higher than that seen in larger (and higher power) discharges, due to the decreased residence time of the propellant within the channel. Linear geometry Hall thrusters permit simpler magnetic circuit configurations and enable stacking of multiple thrusters to provide modular arrays.
TL;DR: In this article, a one-dimensional fluid model of a pulsed (square-wave power modulated) chlorine discharge was developed in order to study the spatiotemporal evolution of species densities and electron temperatures for various pressures, powers, pulsing frequencies and duty ratios.
Abstract: A one-dimensional fluid model of a pulsed (square-wave power modulated) chlorine discharge was developed in order to study the spatiotemporal evolution of species densities and electron temperatures for various pressures, powers, pulsing frequencies and duty ratios. Simulation results show spontaneous separation of the plasma into an ion-ion core and an electron-ion edge during the power `on' (active glow) fraction of the cycle. A transition from an electron-dominated plasma to an ion-ion plasma occurs during the power `off' (afterglow) fraction of the cycle, under the conditions examined. The formation of an ion-ion plasma is favoured at lower power levels, higher pressures, and lower duty ratios. A minimum afterglow time is required for an ion-ion plasma to form and the negative ions to reach the walls. Increasing the afterglow period increases the fraction of time an ion-ion plasma is sustained in the reactor. The evolution of negative ion density profiles is especially complex due to the formation of self-sharpening fronts during power `on' and subsequent back-propagation of the fronts during the power `off' stage of the pulse. When possible, simulation results are compared to reported experimental data. In general, good agreement is obtained, except that the measured dependence of electron density on pulse period and duty ratio is more complex than predicted.
TL;DR: In this article, the electrical and plasma properties of a large-area inductive plasma source are investigated, where an antenna-plasma system is modelled as a lossy transmission line, and a transformer model is applied to study the electrical properties of the system.
Abstract: The electrical and plasma properties of a large-area inductive plasma source are investigated. The source is a 71 cm × 61 cm × 20 cm metal chamber providing a processing area of 37 cm × 47 cm for large-size wafers and glass substrates for flat panel displays. The exciting antenna is embedded inside the processing chamber, and the rf power is inductively coupled from the antenna to the plasma through thin (1.7 mm thick) quartz tubes. A tuning network is used to launch a travelling wave or a wave with a desired standing-wave ratio. The antenna-plasma system is modelled as a lossy transmission line, and a transformer model is applied to study the electrical properties of the system. The model is used to evaluate the conditions required to launch a travelling wave, and these conditions are verified experimentally. The plasma density distributions under various operating conditions are measured with Langmuir probes and compared to a global model for inductive discharges. Our experiments show that high plasma densities are produced over a large area, and that the density profiles are strongly influenced by the antenna configuration and standing-wave effects.
TL;DR: In this article, three scattering diagnostic techniques for low-temperature plasmas suitable for plasma processing applications using light scattering techniques are discussed, which can be applied to a variety of industrially used thermal and non-thermal plasma processing techniques.
Abstract: Diagnostics of low-temperature plasmas suitable for plasma processing applications using light scattering techniques is a research field of growing importance The three scattering diagnostic techniques discussed in this paper may be applied to a variety of industrially used thermal and nonthermal plasma processing techniques These methods are nonintrusive with high temporal and spatial resolution and could help to analyse the absolute composition of a plasma, where as many species as possible in different excitation and ionization states should be included They furthermore deliver information about the energy of particles under investigation, their temperature, density and fluxes The scattering theory in random media is summarized very briefly and this approach is applied to Rayleigh and Thomson scattering The difference between incoherent and coherent scattering is figured out A short overview over the process of coherent anti-Stokes Raman scattering (CARS) and an introduction into experimental techniques is given, which are required to detect Rayleigh, Thomson and CARS signals from a plasma Finally, applications of these three diagnostic techniques to miscellaneous plasma experiments are shown
TL;DR: In this paper, the authors investigated the X-radiation emission from a low-energy Mather-type plasma focus operated with argon and found that the argon line radiation yield was highest at 1.5 mbar and the emitted energy in 4π geometry, with a system efficiency of 0.0015%.
Abstract: X-radiation emission from a low-energy Mather-type plasma focus operated with argon is investigated. Attention is paid to finding the pressure range for the highest argon K-series line emission. The argon line radiation yield is highest at 1.5 mbar and the emitted energy in 4π geometry is estimated to be about 30 mJ, with a system efficiency of 0.0015%. The emission at an energy exceeding 3 keV is found to be highest at a 0.5 mbar filling pressure, giving a total yield of 0.7 J in 4π geometry, which corresponds to a device efficiency of about 0.028%. This emission is mainly due to the interaction of energetic electrons with the anode.
TL;DR: In this article, the equilibrium of electronegative discharges is studied in the plane-parallel approximation over a wide range of pressures and electron densities, encompassing a number of regimes that have previously been modeled analytically.
Abstract: The equilibrium of electronegative discharges is studied in the plane-parallel approximation over a wide range of pressures and electron densities, encompassing a number of regimes that have previously been modeled analytically. The transitions between the various regimes (models) have been determined in the input parameter space. It is shown that for a given feedstock gas, these transitions can be found in terms of the two input parameters p p and n e 0 p , where p is the pressure, n e 0 the electron density, and p the system half-length. Here n e 0 is used as a convenient input related to the power, and the conversion from electron power to n e 0 is given. The input parameter space is partitioned by whether ion flux to the wall or positive-negative ion recombination is the dominant positive ion loss mechanism. For each of the principal regimes, scaling laws are developed for the most important plasma parameters in terms of the input parameters.
TL;DR: In this paper, the suppression of the electrostatic coupling by terminating the coil (inductance L ) with a capacitor (capacitance C ) resonantly takes place when the termination reactance (1/C ) coincides with a half of the coil reactance, in comparison with the conventional internal antenna inductively coupled plasma without the capacitance termination.
Abstract: In a conventional inductive rf discharge, the electrostatic coupling from the coil to the plasma causes a serous problem of sputtering of any dielectric materials in the vicinity of the coil. This paper reports the suppression of the electrostatic coupling by terminating the coil (inductance L ) with a capacitor (capacitance C ). The suppression resonantly takes place when the termination reactance (1/ C ) coincides with a half of the coil reactance ( L ). In this condition, the plasma density is observed to increase by ~50% at the same input power, in comparison with the conventional internal antenna inductively coupled plasma without the capacitance termination. The electrical transmission-line properties of the coil well account for the termination capacitance dependence of the coil voltages, currents, plasma potential oscillation, plasma density and dc self-bias voltage.
TL;DR: In this paper, the axial distributions of negative oxygen ions in an asymmetric, capacitively coupled oxygen radiofrequency discharge (of the gaseous electronics conference reference cell type) are investigated.
Abstract: Axial distributions of negative oxygen ions in an asymmetric, capacitively coupled oxygen radiofrequency discharge (of the gaseous electronics conference reference cell type) are investigated. The O- ion densities are measured by detecting the electrons released after photo-detachment with a Langmuir probe. At small power and a pressure of approximately 10 Pa the axial distributions of both the O- ions and the electrons are flat. At higher power (characterized by voltage amplitudes in excess of 200 V) both quantities exhibit maxima in the central region between the electrodes. These maxima are displaced with increasing pressure (>50 Pa) towards the driven electrode. The absolute concentration of O- is insensitive to admixtures of argon up to 50%. This can be understood, if one assumes that the dominant loss channel for the negative ions involves metastable O2(a 1Δg) molecules. This assumption is confirmed by measurements taken with the discharge operating in a pulsed mode.
TL;DR: In this paper, the authors compare the predictions of the simple hybrid model and those from a more detailed PIC-MCC (particle-in-cell Monte Carlo collisions) simulations, and show that the good qualitative agreement between hybrid and PIC models confirms the interpretation of the lowfrequency oscillations deduced from their previous work.
Abstract: Stationary Plasma Thrusters (SPTs) are ion sources used for satellite propulsion. They operate at low gas density and the electrons are confined by a magnetic field perpendicular to the discharge axis. Experimental observations and recent simulations (hybrid models) based on a simple description of electron transport have shown that SPTs are subject to strong current oscillations due to ionization instabilities. In this paper we present comparisons between the predictions of the simple hybrid model and those from a more detailed PIC-MCC (particle-in-cell Monte Carlo collisions) simulations. The good qualitative agreement between hybrid and PIC models confirms the interpretation of the low-frequency oscillations deduced from our previous work.
TL;DR: In this paper, the particle in cell/Monte Carlo collision method (PIC/MCC) is explained with practical suggestions for numerical implementation, and a one-dimensional, self-consistent model for a parallel-plate radiofrequency discharge in pure hydrogen is presented.
Abstract: The fundamentals of the Monte Carlo method for electron and ion transport in plasmas are explained with emphasis on the coupling of the particle transport with the space charge fields and chemical kinetics. In the first three sections the basics of the null-collision Monte Carlo methods and the particle in cell/Monte Carlo collision method (PIC/MCC) are explained with practical suggestions for numerical implementation. The fourth section shows that the Monte Carlo method can be derived from the linear Boltzmann equation. In the fifth section, we show how the charged particle transport can be self-consistently coupled to the chemical kinetics of the gas phase, in particular to the vibrational kinetics of molecules. In the penultimate section a one-dimensional, self-consistent model for a parallel-plate radio-frequency discharge in pure hydrogen is presented.
TL;DR: In this paper, a global model of a radiofrequency inductively coupled H2 plasma discharge in the Deuterium Negative Ion Source Experiment (DENISE) has been developed using the numerical code ''Global Model Solver'' (GMS).
Abstract: A global model of a radiofrequency (rf) inductively coupled H2 plasma discharge in the Deuterium Negative Ion Source Experiment (DENISE) has been developed using the numerical code `Global Model Solver' (GMS). The volume-averaged energy and particle balance equations, along with the quasi-neutrality condition, are numerically solved to determine the averaged densities of all the species included in the model and the electron temperature. The effects of the multicusp magnetic field and of the asymmetry of the source chamber are considered in the model. The values of the volume-averaged electron density and the average electron temperature obtained are compared to experimental measurements in the pressure and input power ranges of interest and a reasonably good agreement is found.
TL;DR: In this paper, a detailed study of the optical spectroscopy of two different RF plasma sources used for the growth of GaN by molecular beam epitaxy is presented. But the results of this study were limited to the case of two sources.
Abstract: We have made a detailed study of the optical spectroscopy of two different RF plasma sources used for the growth of GaN by molecular beam epitaxy. Our studies show that for both sources the predominant species present in the cavity are nitrogen atoms. The strongest optical emission occurs at 869 nm. We have also studied, in detail, the factors which influence the ion content of the flux. Two key parameters are the temperature of the wall of the cavity and the size of the holes in the aperture plate from which the species emerge into the vacuum. We have identified conditions under which the ion content can be made negligibly small and show that this results in films with improved optical properties.
TL;DR: In this article, the authors studied the effect of the electric field on the formation of a sheath in slightly ionized plasmas and showed that no lower bound of the ion drift speed vi exists to form a sheathe under collisional conditions.
Abstract: Sheath formation is studied for slightly ionized plasmas, i.e. for many typical low-pressure discharges. A sheath adjacent to an insulating wall is a space charge region where the electron flux to the wall is reduced to the corresponding ion flux. Numerical and analytical results show that no lower bound of the ion drift speed vi exists to form a sheath under collisional conditions. The product of the ambipolar ion speed vB and the square root of the ratio of the number density of the ions to that of the electrons occurs as a characteristic ion drift speed vc for the sheath formation. In the interval 0 vc both the electric field and the collisions support the sheath formation. The effect of the electric field alone can form the sheath. When the electron density is relatively small under collision-dominated conditions the space charge density can become relatively large in the neighbourhood of the point where vi = vB, and then the difference between vc and vB is significant. It is also possible that such a point does not exist. At low collisionality, in the centre of the plasma and close to the wall vi vc can be satisfied. When the collisionality is high vi vB occurs close to the wall. The sheath formation can be dominated by collisions in a larger range of parameters than recognized to date. The application of the Bohm criterion is inappropriate when the collisionality is high and the electron density is relative low. Multi-component plasmas are discussed briefly. The concept of a sheath edge is examined critically.
TL;DR: In this paper, the Boltzmann equation for free electrons in a low-temperature plasma is analyzed and it is shown how this equation can approximately be solved for electrons under a HF field of frequency ω (including the particular case of a dc field, which corresponds to the limit ω = 0).
Abstract: In the first part of this paper we briefly review some basic concepts of kinetic theory. The concept of the velocity distribution function is first introduced and its meaning is discussed. Then, the Boltzmann equation is presented on physical grounds and it is shown that the fluid equations are its moments. In the second part, the Boltzmann equation for free electrons in a low-temperature plasma is analysed. It is shown how this equation can approximately be solved for electrons under a HF field of frequency ω (including the particular case of a dc field, which corresponds to the limit ω = 0) by using a first-order double expansion in spherical harmonics in velocity space and a Fourier series in time. The electron transport parameters, particle balance and energy balance are analysed from a general point of view. Finally, an application to argon and nitrogen is given. The effects of changes in the field frequency on the electron energy distribution function, transport parameters and power balance are discussed. The importance of the coupling between the electron and the vibrational kinetics in N2 is emphasized.
TL;DR: In this article, the stable column mode (SCMPM) is optimized with respect to the compression ratio for high density (up to 1027 m-3) and temperature ( up to 1 keV) of high energy density deuterium plasmas.
Abstract: Deuterium discharges in the SPEED2 plasma focus doped with heavy gases (e.g. neon, argon) produce two pinch modes, the micropinch mode (MPM) or the stable column mode (SCM), with a transition regime where the initial SCM is followed by the MPM. Micropinches are local radiative collapses initiated by instabilities (m = 0 type) of low-energy-density pinch plasmas. These instabilities and the successive micropinches can be suppressed by kinetic deuterons produced during dynamical compression of high-energy-density deuterium plasma sheaths. Depending on the relaxation of this fast deuteron component the pinch column can be stabilized for several tens of nanoseconds. The SCM optimized with respect to the compression ratio is a powerful linear radiation source of high density (up to 1027 m-3) and temperature (up to 1 keV).