Low-pressure Inductive Discharge Research Articles

Numerical model of a gaseous inductive discharge in oxygen, taking into account the complete scheme of the vibrational kinetics of O2 molecules

In this work, the two-dimensional model of inductively coupled plasma discharge in oxygen was developed. The model includes hydrodynamic equations in drift-diffusion approximation for describing the kinetics of charged (electrons and ions) and neutral plasma particles, Maxwell’s equations for electromagnetic fields, and equations for the temperature and neutral gas flow as well as a detailed scheme of plasma-chemical reactions. The model was tested against theoretical and experimental data in a simple cylindrical chamber. The electron density and temperature distributions were obtained over a wide range of powers deposited in the plasma (100–500 W) and compared with literature data. Also, the complete scheme of the vibrational kinetics of O2 molecules was included in the model. The vibrational distribution function was calculated in low-pressure inductive discharge as well as spatial distributions of plasma parameters (density and temperature of electrons, excited components, neutral gas temperature, etc.) and flows of charged and active neutral particles at the reactor walls.

Modeling the effect of stochastic heating and surface chemistry in a pure CF4 inductively coupled plasma

The physics and chemistry of inductively coupled plasma generated in pure tetrafluoromethane are analyzed using a self-consistent two-dimensional plasma fluid model coupled with Maxwell's equations. The model also takes into account the stochastic electron heating typical for low-pressure inductive discharges. We show that this effect is important for the explanation of experimentally measured plasma parameters. We also analyze the influence of several model parameters on the plasma. These parameters include the mechanism of surface reactions, the sticking coefficient of fluorine atoms at the walls, the uncertainty in the electron impact dissociation reactions, and the background gas pressure.

Structure of RF fields in a twin-chamber radio-frequency inductive plasma source placed in a weak external magnetic field

An experimental study of the structure of waves excited in the low-pressure inductive radio-frequency discharge located in the magnetic field with inductance less than 70 Gs was carried out. A plasma source consisting of gas-discharge and process chambers was used in the work. The results of the wave structure study showed that a partially traveling wave was excited in the plasma source when a certain threshold value of the induction of an external magnetic field was exceeded and when a metal flange separating the gas discharge and processing chambers was used. It was also noted that the above-mentioned threshold value decreased with an increase in operating frequency. The amplitude and phase profile of the Bz component of the RF wave indicates the presence of waves reflected from the bottom flange and the local minimum of the electron concentration within the junction of the gas-discharge and process chambers.

Power Losses in RF Inductor of Ferrite-Free Closed-Loop Inductively-Coupled Low Pressure Mercury Lamps

RF inductor power losses of ferrite-free electrode-less low pressure mercury inductively-coupled discharges excited in closed-loop dielectric tube were studied. The modelling was made within the framework of low pressure inductive discharge transformer model for discharge lamps with tubes of 16, 25 and 38 mm inner diam. filled with the mixture of mercury vapour (7.5×10–3 mm Hg) and argon (0.1, 0.3 and 1.0 mm Hg) at RF frequencies of 1, 7; 3.4 and 5.1 MHz and plasma power of (25–500) W. Discharges were excited with the help of the induction coil of 3, 4 and 6 turns placed along the inner perimeter of the closed-loop tube. It was found that the dependence of coil power losses, Pcoil, on the discharge plasma power, Ppl, had the minimum while Pcoil decreased with RF frequency, tube diameter and coil number of turns. The modelling results were found in good qualitative agreement with the experimental data; quantitative discrepancies are believed to be due skin-effect and RF electric field radial inhomogeneity that were not included in discharge modelling.

Transmission characteristics of optical nanofibers in metastable xenon.

We study the transmission characteristics of subwavelength diameter silica optical nanofibers (ONFs) surrounded with xenon plasma produced by low-pressure inductive RF discharge. In contrast with related experiments using rubidium vapor, we find essentially no degradation of optical transmission through the ONFs as a function of time. We also observe a pronounced ONF transmission modulation effect that depends on the conditions of the xenon plasma.

RF power absorption by plasma of a low-pressure inductive discharge

This paper aims to analyze the mechanism of power absorption and to reveal, both experimentally and numerically, the basic factors determining the ability of plasma to absorb RF power. This is done by determining the plasma equivalent resistance value under different conditions in a low-pressure RF inductive discharge such as different antenna shape, working gas pressure, electron density, operating frequency and geometrical dimensions of the plasma source. Experimental and numerical results show that the plasma equivalent resistance changes non-monotonously with an increase in electron density, increases with an increase in neutral gas pressure, and that the maximum plasma equivalent resistance shifts toward higher electron densities when the operating frequency is increased.

Relatively high plasma density in low pressure inductive discharges

Electron energy probability functions (EEPFs) were measured in a low pressure argon inductive discharge. As radio frequency (RF) power increases, discharge mode is changed from E-mode (capacitively coupled) to H-mode (inductively coupled) and the EEPFs evolve from a bi-Maxwellian distribution to a Maxwellian distribution. It is found that the plasma densities at low RF powers (<30 W) are much higher than the density predicted from the slope of the densities at high powers. Because high portion of high energy electrons of the bi-Maxwellian distribution lowers the collisional energy loss and low electron temperature of low energy electrons reduces particle loss rate at low powers. Therefore, the energy loss of plasma decreases and electron densities become higher at low powers.

An investigation of electrophysical and thermophysical characteristics of low-frequency low-pressure inductive discharge of transformation type

The electrophysical and thermophysical characteristics of the low-frequency transformation-type induction discharge in argon in a wide parameter range (the discharge current density varies from 0.1 to 0.8 A/cm2 and the argon pressure is 15–6000 Pa) are studied. A self-consistent radial model of plasma of the low-frequency low-pressure transformation-type induction discharge in argon, which is based on the simultaneous solution of balance equations for the electron density and metastable argon atom density and balance equations for electron energy and gas temperature, is designed. The dependencies of the electric field strength on the argon pressure and discharge current are calculated. It is shown that the numerical results are in good agreement with the measurement results and describe their basic features, namely, the falling current-voltage discharge characteristic and the availability of the local minimum in the dependence of the electric field strength on the pressure. The radial distributions of the plasma parameters of the low-frequency induction transformation-type discharge (electron density, gas temperature, and electron temperature) are obtained. It is shown that the gas heating-up accounting leads to a substantial change in the plasma parameters of the inductively coupled discharge.

Features of the plasma sputtering of polycrystalline Pb1 − x Sn x S films

The processes of the sputtering and modification of surfaces of polycrystalline films of the ternary solid solution Pb1 − xSnxS (x = 0.9–1.0) in a high-density Ar plasma of high-frequency low-pressure inductive discharge are studied. Films with thicknesses of 1–4 μm are grown on glass substrates using the “hot-wall” method and consist of plate-like crystallites. It is established that the sputtering rate for lead-tin sulfide films does not exceed 2.0 nm/s, which is determined by the presence of oxygen-containing compounds on the surfaces. In the case of plate-like crystallites with nanodimensional thicknesses, the effect of smoothing of the developed surfaces of the polycrystalline Pb1 − xSnxS layers during plasma treatment is observed; this is important for fabricating multilayer device structures.

Collisionless electron heating by radio frequency bias in low gas pressure inductive discharge

We show experimental observations of collisionless electron heating by the combinations of the capacitive radio frequency (RF) bias power and the inductive power in low argon gas pressure RF biased inductively coupled plasma (ICP). With small RF bias powers in the ICP, the electron energy distribution (EED) evolved from bi-Maxwellian distribution to Maxwellian distribution by enhanced plasma bulk heating and the collisionless sheath heating was weak. In the capacitive RF bias dominant regime, however, high energy electrons by the RF bias were heated on the EEDs in the presence of the ICP. The collisionless heating mechanism of the high energy electrons transited from collisionless inductive heating to capacitive coupled collisionless heating by the electron bounce resonance in the RF biased ICP.

Two-dimensional simulation of inductive–capacitive transition instability in an electronegative plasma

Plasma instabilities are observed in low-pressure inductive discharges in the transition between low density capacitively driven and high density inductively driven discharges when attaching gases are used. A two-dimensional hybrid fluid-analytic simulation is used to determine the space- and time-varying densities of electrons, positive and negative ions, and neutral species, and electron and neutral gas temperatures. The simulation includes both the capacitive and inductive coupling of the source coils to the plasma and the neutral gas dissociation and heating. The plasma is described using the time-dependent fluid equations, along with an analytical sheath model. The simulation is applied to an experiment in Cl2, in which gaps in the electron and positive ion densities versus power curves were observed, with our numerical results indicating the existence of an inductive–capacitive transition instability, corresponding approximately to the observed gaps. The fluid calculation captures various features that are not included in previous global instability models. A method is developed to match the numerical results to the global model formalism, which predicts the existence of the unstable mode, as numerically found. The time and space variations can be used to improve the global model formalism.

Global model of instabilities in low-pressure inductive chlorine discharges

Experimental studies have shown that low-pressure inductive discharges with attaching gases are subject to instabilities near the transition between capacitive (E) and inductive (H) modes. A global model, consisting of two particle and one energy balance equations, has been previously proposed to describe the instability mechanism. This model, which agrees qualitatively well with experimental observations, leaves significant quantitative differences. In this paper, the model is revisited with Cl2 as the feedstock gas. A novel treatment of inductive power deposition is evaluated and chlorine chemistry is included. Old and new models are systematically compared. It is found that the alternative inductive coupling description slightly modifies the results. The effect of gas chemistry is even more pronounced. The instability window is smaller in pressure and larger in absorbed power, the frequency is higher and the amplitudes of oscillations are reduced. The feedstock gas is weakly dissociated (≈16%) and is the dominant positive ion, which is consistent with the moderate electron density during the instability cycle.

Plasma Diagnostics Using Computed Tomography Method

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In the last decade, the range of applications of various low-temperature plasma sources has steadily grown. New applications require development of advanced diagnostic methods to control plasma parameters. Computed tomography is a powerful method which can provide much useful information on the plasma structure and its evolution in time. From a mathematical point of view, tomographic reconstruction is an ill-posed problem, which should be solved by using the regularization (in the large sense of the word) method. In this paper, we discuss a “technology” of tomographic study of plasma objects. It has been shown that numerical testing is a necessary component of the tomographic technology of plasma sources, particularly in situations with limited number of views and with ordinary discontinuities or high-gradient source functions. We are focused on a comparative analysis of the tomographic technology in equilibrium and nonequilibrium plasma sources. The analysis has been made using some examples from our practical diagnostics. The analyzed objects were as follows: nonequilibrium plasma of high-frequency low-pressure inductive discharge, equilibrium arc plasma of high-intensity discharge lamp, and methane plasma evolution in plasma-chemical reactor. We have concluded that the tomographic reconstruction in equilibrium plasma sources is the standard technology at present. The tomographic technology of nonequilibrium plasma sources is a developing approach, which has a great potential at making a contribution to understanding the fundamental processes and phenomena going on in the plasma. </para>

Sterilization by pure oxygen plasma and by oxygen–hydrogen peroxide plasma: An efficacy study

Plasma is an innovative sterilization method characterized by a low toxicity to operators and patients, and also by its operation at temperatures close to room temperatures. The use of different parameters for this method of sterilization and the corresponding results were analyzed in this study. A low-pressure inductive discharge was used to study the plasma sterilization processes. Oxygen and a mixture of oxygen and hydrogen peroxide were used as plasma source gases. The efficacy of the processes using different combinations of parameters such as plasma-generation method, type of gas, pressure, gas flow rate, temperature, power, and exposure time was evaluated. Two phases were developed for the processes, one using pure oxygen and the other a mixture of gases. Bacillus subtilis var. niger ATCC 9372 ( Bacillus atrophaeus) spores inoculated on glass coverslips were used as biological indicators to evaluate the efficacy of the processes. All cycles were carried out in triplicate for different sublethal exposure times to calculate the D value by the enumeration method. The pour-plate technique was used to quantify the spores. D values of between 8 and 3 min were obtained. Best results were achieved at high power levels (350 and 400 W) using pure oxygen, showing that plasma sterilization is a promising alternative to other sterilization methods.

Properties of a low-pressure inductive RF discharge II: Mathematical simulations

Self-consistent numerical simulations of a low-pressure inductive RF discharge have been carried out. It is shown that, on the one hand, the plasma parameters are determined by the RF power absorbed in the plasma and, on the other, they themselves govern the power absorption. This results in a nonmonotonic dependence of the plasma parameters on the magnetic field, as well as in discharge disruptions, similar to those observed experimentally in such discharges. An inductive RF discharge with a capacitive component is simulated. The experimentally observed characteristic properties of the discharges are explained based on the regular features of the absorption of RF power in the plasma. Traditional inductive plasma sources (both without and with a magnetic field) are considered.

Properties of a low-pressure inductive RF discharge I: Experiment

Results are presented from experimental studies of low-pressure inductive RF discharges (including those with a capacitive component) employed in plasma technology. It is shown that both the RF power absorbed in the plasma and the electron density depend nonmonotonically on the external magnetic field. Discharge disruptions occurring at critical values of the magnetic field and the spatial redistribution and hysteresis of the plasma parameters were observed when varying the magnetic field and RF generator power. The parameters of the plasma of low-pressure (0.5–5 mTorr) inductive RF discharges were investigated, and the discharge properties related to the redistribution of the RF generator power between the plasma and the discharge external circuit were revealed. The experiments were performed with both conventional unmagnetized inductive plasma sources and plasma sources with a magnetic field.

Nonlocal conductivity effects in low-pressure cylindrical inductive discharges

The study is on nonlocal conductivity and stochastic heating in low-pressure inductive discharges with cylindrical coils. The rf current density derived by solving the Boltzmann equation in cylindrical coordinates, with accounting for both the high-frequency heating field and the dc field in the discharge, is coupled with the wave equation resulting into the spatial distribution-under the conditions of anomalous skin-of the electromagnetic field components, of the rf current density and the power deposition in the free-fall regime of maintenance of hydrogen discharges.

Review of relaxation oscillations in plasma processing discharges

Relaxation oscillations due to plasma instabilities at frequencies ranging from a few Hz to tens of kHz have been observed in various types of plasma processing discharges. Relaxation oscillations have been observed in electropositive capacitive discharges between a powered anode and a metallic chamber whose periphery is grounded through a slot with dielectric spacers. The oscillations of time-varying optical emission from the main discharge chamber show, for example, a high-frequency (~ 40?kHz) relaxation oscillation at 13.33Pa, with an absorbed power being nearly the peripheral breakdown power, and a low-frequency (~ 3 Hz) oscillation, with an even higher absorbed power. The high-frequency oscillation is found to ignite plasma in the slot, but usually not in the peripheral chamber. The kilohertz oscillations are modelled using an electromagnetic model of the slot impedance, coupled to a circuit analysis of the system including the matching network. The model results are in general agreement with the experimental observations, and indicate a variety of behaviours dependent on the matching conditions. In low-pressure inductive discharges, oscillations appear in the transition between low-density capacitively driven and high-density inductively driven discharges when attaching gases such as SF6 and Ar/SF6 mixtures are used. Oscillations of charged particles, plasma potential, and light, at frequencies ranging from a few Hz to tens of kHz, are seen for gas pressures between 0.133 Pa and 13.33 Pa and discharge powers in a range of 75?1200 W. The region of instability increases as the plasma becomes more electronegative, and the frequency of plasma oscillation increases as the power, pressure, and gas flow rate increase. A volume-averaged (global) model of the kilohertz instability has been developed; the results obtained from the model agree well with the experimental observations.

BIFURCATION AND CHAOTIC TRANSITION TO RELAXATION OSCILLATIONS IN AN INDUCTIVELY DRIVEN ELECTRONEGATIVE PLASMA

Regular and chaotic relaxation oscillations in charged particle densities, light and floating potential are seen in low-pressure inductive discharges, in the transition between lower power capacitive operation and higher power inductive operation, if the plasma is electronegative, i.e. contains negative ions. As pressure or power is varied to cross a threshold, either from stable capacitive or stable inductive operation, the instability goes through a series of oscillatory, sometimes chaotic, states to large scale relaxation oscillations between higher and lower densities. A volume-averaged model, to describe the instability, contains time varying densities of charged species and temperature, indicating that the separation of the time scales of electron and negative ion motion is the driver of the complicated dynamics. The particle and energy balance equations are integrated to produce the dynamical behavior. A phase plane description of negative ions versus electrons is used to gain understanding of the instability. The theory shows that, depending on specific choices of parameters, as power is increased, the oscillations, born at Hopf bifurcation, can grow to a large amplitude relaxation oscillations, sometimes traversing chaotically between basins of attraction. The model qualitatively agrees with experimental observations which show the same range of behaviors, but with quantitative differences to be explained.

Stable and unstable behavior of inductively coupled electronegative discharges

Plasma instabilities have been observed in low-pressure inductive discharges, in the transition between low density capacitively driven and high density inductively driven discharges when attaching gases such as SF6 and Ar/SF6 mixtures are used. Oscillations of charged particles, plasma potential, and light, with frequencies from a few hertz to tens of kilohertz, are seen for gas pressures between 1 and 100 mTorr and discharge powers in the range of 75–1200 W. The region of instability increases as the plasma becomes more electronegative and the frequency of plasma oscillations increases as the power, pressure, and gas flow rate increase. Time-resolved measurements of electron temperature, and time averaged ion energy distribution at the wall have been made. A volume-averaged (global) model of the instability has been developed, for a discharge containing time varying densities of electrons, positive ions, and negative ions, and time invariant excited states and neutral densities. The particle and energy balance equations are integrated to produce the dynamical behavior. The model agrees well with experimental observations, and also shows a significant influence of the matching network. Values of plasma parameters and of chemical composition are measured to set the values used in the model, as required to obtain the good agreement with experiment. Capacitive coupling plays a crucial role in the instability process. A variable electrostatic (Faraday) shield has been used to control the capacitive coupling from the excitation coil to the plasma. The plasma instability disappears when the shielded area exceeds 65% of the total area of the coil. The model gives a slightly higher value of 85% for instability suppression with the same discharge conditions (Ar/SF6 1:1, 5 mTorr).