RF Discharge Research Articles

A Prototype of Electric Discharge Gas-Flow Oxygen–Iodine Laser: 2. Simulation of the Parameters of the Active Medium Formed in a Gas-Flow Slab RF Discharge in O2 : He : CF3I Mixtures

A calculated analysis of the possibility of achieving the generation in a gas-flow electric discharge oxygen–iodine laser when it is excited by a transverse slab RF discharge for various conditions of passing of the CF3I : O2 : He gas mixture components along the gas duct was carried out: in subsonic flow without cooling the channel walls and with cooling of various sections of the channel walls down to 100 K. To do this, reactions involving CF3I iodide and the photon balance equation were added to the model. It was assumed that the active medium was formed in two ways: (i) all components of the gas mixture simultaneously passed through the RF excitation zone; (ii) oxygen and helium passed through the RF discharge zone, and the iodide-containing component (CF3I : He mixture) was introduced through an injector located downstream gas flow from the discharge zone. Calculations have shown that it is possible, in principle to achieve the laser generation mode under conditions implemented on the experimental setup.

Similarity of capacitive radio-frequency discharges in nonlocal regimes

Similarity transformations are essential for correlating discharges at different scales, which are mostly utilized with local field or local energy approximations. In this work, we report the fully kinetic results from particle-in-cell/Monte Carlo collision simulations that unambiguously demonstrate the similarity of radio frequency (rf) discharges in nonlocal regimes where the electron energy relaxation length is much larger than the gap dimension. It is found that at a constant rf voltage amplitude, discharges will be similar if the gas pressure, inverse of gap distance, and rf driving frequency are all changed by the same scaling factor. The scaling relations of fundamental parameters are illustrated for rf discharges in the alpha-mode with secondary electron emission ignored, and the temporal electron kinetics are shown to have invariance in similar discharges. The results explicitly validate the scaling laws in nonlocal kinetic regimes, indicating promising application potentials of the similarity transformations across a wide range of kinetic regimes.

Deuterium Retention in a Nanostructured Tungsten Surface Layer Formed during High-Temperature Irradiation with Helium Plasma

The preliminary irradiation of a tungsten sample with low-energy helium ions (80 eV, flux of 1021 m−2 s−1) at a temperature of 1200–1250 K in a facility with an inductive RF discharge leads to the formation of a nanostructured tungsten surface layer, which is referred to as fuzz. After that, the sample is subjected to a set of annealing procedures and irradiations with $${\text{D}}_{3}^{ + }$$ ions with an energy of 2 keV (0.67 keV per D) at low fluences of 1019–1020 m−2. Deuterium retention at each stage is analyzed by in-situ thermal desorption spectroscopy. An increase in the helium concentration in the sample leads to a significant change of deuterium retention. At high helium concentrations, deuterium retention becomes low. Annealing in the temperature range of 1000–1400 K leads to helium desorption, modification of the surface layer and defects, and, as a consequence, an increase in the amplitude of the main deuterium desorption peak and a shift of the peak to higher temperatures. Annealing at a temperature of 1600 K leads to removal of the nanostructured fuzz from the tungsten surface and a decrease in the deuterium retention on account of an increase in the reflection coefficient from a smoother surface.

Топография поверхности и пропускание света кварцевыми окнами после экспозиции в высокочастотном разряде в дейтерии и смеси дейтерия с азотом

The effect of a cleaning discharge plasma on the KU-1 optical quartz surface topography and transmission in visible and near IR ranges (400-1000 nm) was studied. For the study, we used crossed DC and RF discharges in D2 and D2/N2 mixture, where the N2 fraction was ~25 mol %. The addition of nitrogen increased sputtering rate from 250 to 1200 nm / h while maintaining stoichiometry of the surface layers. The rms surface roughness decreased after the plasma exposure from 1.3 to 0.6 nm, while the quartz transmittance has not changed. The surface roughness analysis and calculation of diffusion scattered light at 400 nm transmitted through the quartz were performed using the power spectral density function of AFM.

Measurement of pressure shift and broadening coefficients for Ne 3s[3/2]2→3p[5/2]3 transition in Ne and He using diode-laser absorption spectroscopy

Measurements of pressure broadening and shift coefficients for 640.2 nm Ne line were performed in an RF discharge without stabilization of neutral gas temperature. A diode laser with a short external cavity was used for metastable neon absorption spectroscopy in a natural mixture of neon isotopes. An appropriate fit function was constructed permitting deduction of Doppler and Lorentz components for the Voigt profile of the spectral line. To determine pressure broadening ξ and shift β coefficients for pure Ne and Ne:He mixture we used the information about Lorentz linewidth, position of the line center and gas temperature derived from spectral line shape fitted to a model line profile. The values of the collisional coefficients reduced to 300 K in units of 10-10 s-1 cm3, are ξNe-Ne = (1.9±0.2), ξNe-He = (3.4±0.2); βNe-Ne = -(0.7±0.1), βNe-He = (0.13±0.04).

Mechanisms of Resonant Power Input into a Magnetoactive RF Discharge Plasma

We consider the excitation and absorption of waves in a magnetoactive rf discharge plasma in the conditions when the generator frequency is lower than the electron cyclotron frequency. We consider the cases of unconfined and confined plasmas in the cylindrical geometry and different regimes of excitation of plasma waves with different dispersion relations and field polarizations. The power input to the plasma depends on the distribution of external-source currents initiating the discharge and on the density parameter characterizing the plasma density and transverse sizes of the system. In the case of a plasma cylinder with a free surface or a plasma cylinder in a large conducting casing, which is most interesting for applications, plasma contains only potential and nonpotential E-type oblique Langmuir waves as well as a strongly nonpotential surface wave. The latter wave is practically not excited by external currents flowing over the cylinder surface for real parameters of the system. For high values of the density parameter, the effective resistance of the plasma with inducting excitation of the discharge is predominant. For moderate and low values of this parameter, capacitive excitation of the wave by the current on the plasma cylinder surface is found to be most effective.

Experimental study on the performances of second-harmonic dispersion interferometers at 10.6 µm and 1064 nm for plasma density measurements.

Two common-path interferometers based on CO2 and Nd:Y3Al5O12 (Nd:YAG) lasers are benchmarked with a two-arm microwave interferometer on a hydrogen plasma produced by an RF discharge and injected into a large magnetic-confinement vessel. The ∼1019m-2 line-integrated electron density is clearly measured in agreement by the interferometers. The frequency spectrum of the measured data is analyzed on the 20 kHz range for all interferometers and up to 600 kHz for the Nd:YAG laser-based interferometer. Mechanical vibration measurements performed on the components of the two common-path interferometers result in a peak-to-peak displacement up to about one and twenty wavelengths for the CO2 and Nd:YAG laser-based interferometers, respectively. Such results set for the first time, to the best of our knowledge, a quantitative limit for the displacement that the two second-harmonic dispersion interferometers can sustain while still providing a high sensitivity for accurate plasma density measurements.

Dual frequency DBD: influence of the amplitude and the frequency of applied voltages on glow, Townsend and radiofrequency DBDs

To study dual frequency Townsend glow and radio frequency DBDs in an Ar/NH3 Penning mixture, different voltages are applied on each electrode of a plane/plane DBD. The one which has the higher amplitude and frequency, V H, determines the discharge regime. The other one, which has a lower amplitude and frequency, V L, is more similar to a bias voltage. V H frequency ranges from 50 kHz to 13.56 MHz and V L frequency from 1 kHz to 2 MHz. The amplitude of V L is always kept lower than the breakdown voltage. The discharge behavior is characterized by space and phase resolved optical emission spectroscopy and Fourier transform of the plasma induced light intensity. When the frequencies of two voltages are close to each other, V L higher than 150 V increases the discharge light intensity. In glow and Townsend discharges, this increase is attributed to the enhancement of the plasma density, due to the ions drift from the plasma bulk to the cathode, which produces a higher secondary electron emission. For the radiofrequency discharge, the enhancement of the light intensity is attributed to an enhancement of the positive space charge due to the higher voltage amplitude.When the frequencies of two voltages are very different, e.g. for V L between 1 and 100 kHz and a 5.5 MHz α-RF discharge, the behavior largely depends on V L amplitude. Above some low frequency (LF), the discharge tends to extinguish when V L amplitude is at its maximum. This is explained by a diminution of the plasma density resulting from the ion drift to the cathode due to V L. When V L is very high, it enhances the discharge intensity. This discharge amplification is associated with a transition from α to γ RF mode, which only occurs when the two voltages have the same polarity. Whatever the RF discharge mode, γ or α, the discharge is diffuse. To determine the interest of LF-RF dual frequency DBD for the PECVD, SiOx coatings, made with a single and a dual LF-RF frequency DBDs, are compared. It is concluded that the ɣ dual frequency removes OH functions from the material lattice and densifies the layer.

Rotational properties of annulus dusty plasma in a strong magnetic field

AbstractThe collective dynamics of an annulus dusty plasma formed between a co‐centric conducting (non‐conducting) disk and ring configuration is studied in a strongly magnetized radiofrequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In the absence of the magnetic field, the dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in the anticlockwise direction with increasing magnetic field (B > 0.02 T), and the constant value of the angular frequency at various strengths of the magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B > 0.6 T) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B > 1 T) lowers the angular frequency. Low value of the angular frequency is expected by reducing the width of the annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of a magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of a magnetic field.

FEATURES OF SiO2 REACTIVE-ION ETCHING KINETICS IN CF4 + Ar + O2 AND C4F8 + Ar + O2 GAS MIXTURES

The effect of Ar/O2 mixing ratio on gas-phase characteristics and SiO2 etching kinetics in CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas was studied under conditions of 13.56 MHz inductive RF discharge. The constant processing parameters were fraction of fluorocarbon component in a feed gas (50%) total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). It was found that the full substitution of Ar for O2 in both gas systems results in non-monotonic (with a maximum at ~ 25% Ar + 25% O2) SiO2 etching rates as well as in monotonically increasing photoresist etching rate with higher absolute values for CF4-containing mixture. The steady-state densities of active species were determined using a combination of plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling. Corresponding results indicated that both gas systems are characterized by quite close parameters of electron and ion components while exhibit sufficient differences in the kinetics of neutral species, especially in the presence of O2. The latter produces opposite changes in F atom density as well as in effective probability of ion-assisted chemical reaction vs. Ar/O2 mixing ratio. Relationships between type of fluorocarbon component and heterogeneous process kinetics were analyzed through the set of gas-phase-related parameters (fluxes, flux-to-flux ratios) characterizing chemical etching pathways for SiO2 and formation/destruction balance for the fluorocarbon polymer film. It was suggested that the transition toward O2-rich plasma in the low-polymerizing CF4 + Ar + O2 plasma suppresses the effective probability for SiO2 + F reaction through decreasing efficiency for oxide bond breaking and desorption of etching products due to decreasing ion energy flux. Oppositely, an increase in O2 content in the high-polymerizing C4F8 + Ar + O2 mixture lifts up the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.

Maximizing optical production of metastable xenon.

The wide range of applications using metastable noble gas atoms has led to a number of different approaches for producing large metastable state densities. Here we investigate a recently proposed hybrid approach that combines RF discharge techniques with optical pumping from an auxiliary state in xenon. We study the effect of xenon pressure on establishing initial population in both the auxiliary state and metastable state via the RF discharge, and the role of the optical pumping beam power in transferring population between the states. We find experimental conditions that maximize the effects, and provide a robust platform for producing relatively large long-term metastable state densities.

Preparation of graphene-based nanomaterials by pulsed RF discharges on liquid organic compounds

A new pulsed RF plasma reactor has been used for the synthesis of graphene-based nanomaterials under atmopheric pressure conditions. This is an environmental-friendly system, not using catalyst and where no hazardous components are generated. Nanographenes formed upon plasma treatment of some hydrocarbons (cyclohexane, cyclohexene, heptane, hexane, pentane, and toluene), while N-containing organic compounds (cyclohexylamine, pyridine, dipropylamine, and triethylamine) resulted in N-doped graphenes. Synthesis took place in a very fast way, mediated by plasma reactive species.

Performance of a rf neutralizer operating with noble gases and iodine

Neutralization of the extracted ion beam is a mandatory task for any ion thruster based space mission. The required lifetimes are in the order of ten thousand hours. This requirement is fulfilled by hollow cathodes operating with xenon, but has not been demonstrated for operation with iodine yet. Furthermore, the power consumption as well as the demand on mass flow should be kept as low as possible. Since the halogen iodine seems to be a viable alternative to xenon for operating ion thrusters, a suitable neutralizer concept that can operate with the corrosive gas for the time periods stated above is required. We propose to use a neutralizer based on an inductively coupled rf discharge as alternative to hollow cathodes. We studied the performance of a prototypical neutralizer operating with iodine as well as the inert gases xenon and krypton and compared the experimental studies with the results of global modeling.

Frequency-selective Q-switched repetitively pulsed slab RF-discharge carbon monoxide laser

Q-switched cryogenically cooled slab RF discharge CO laser operating in multi-line mode, single-line mode with a diffraction grating, and also with frequency selection by special spectral filters situated both outside and inside the laser cavity was experimentally studied. Characteristics of CO lasing on individual ro-vibrational spectral lines produced by this laser with various cavities under the same active medium and pump parameters were compared. Application of output couplers with a relatively narrow spectral band of high transmittance compared to the non-selective CO laser range of operation, but with high reflection in the rest of the range, was effective for obtaining narrow-band multiline lasing. For selection of the long-wave part of Q-switched CO laser spectrum, which was needed for frequency conversion into the long-wave mid-IR when mixing with CO2 laser emission, application of a broadband spectral filter with a very low reflection coefficient in the spectral range where lasing was necessary to be suppressed (sort of a step function) proved to be efficient being installed outside the laser cavity.

Three-dimensional dusty plasma in a strong magnetic field: Observation of rotating dust tori

This paper reports on the dynamics of a 3-dimensional dusty plasma in a strong magnetic field. An electrostatic potential well created by a conducting or non-conducting ring in the rf discharge confines the charged dust particles. In the absence of the magnetic field, dust grains exhibit a thermal motion about their equilibrium position. As the magnetic field crosses a threshold value (B > 0.02 T), the edge particles start to rotate and form a vortex in the vertical plane. At the same time, the central region particles either exhibit thermal motion or E→×B→ motion in the horizontal plane. At B > 0.15 T, the central region dust grains start to rotate in the opposite direction resulting in a pair of counter-rotating vortices in the vertical plane. The characteristics of the vortex pair change with increasing the strength of the magnetic field (B ∼ 0.8 T). At B > 0.8 T, the dust grains exhibit very complex motion in the rotating torus. The angular frequency variation of rotating particles indicates a differential or sheared dust rotation in a vortex. The angular frequency increases with increasing the magnetic field from 0.05 T to 0.8 T. The ion drag force and dust charge gradient along with the E-field are considered as possible energy sources for driving the edge vortex flow and central region vortex motion, respectively. The directions of rotation also confirm the different energy sources responsible for the vortex motion.

Experimental cell with a Fabry-Pérot resonator tuned in situ for magnetic resonance studies of matrix-isolated radicals at temperatures below 1 K.

We describe the design and construction of an experimental cell for the study of free radicals in macroscopically thick films of solidified molecular and rare gases by 128 GHz Electron Spin Resonance (ESR) at temperatures below 1 K. The ESR resonator has an open Fabry-Pérot design, and its frequency can be tuned in situ by adjusting the spacing between the mirrors. The tuning mechanism consists of a piezo positioner and a stainless-steel edge-welded bellows, which can change the resonator frequency by at least 6 GHz. The films of solidified gases can be deposited either directly from a room temperature reservoir or by recondensing from a specially arranged chamber. The free radicals can be created in the solid films by dissociating matrix species by running an rf discharge in a helium vapor. We suggest that such a sample cell design can also be used for a broad range of low-temperature ESR experiments where sample cooling needs to be enhanced by the presence of superfluid helium.

PLASMA PARAMETERS AND SiO2 ETCHING KINETICS IN C4F8 + Ar + O2 GAS MIXTURE

The effect of Ar/O2 mixing ratio on plasma parameters, steady-state densities of active species and SiO2 etching kinetics in the three-component C4F8+Ar+O2 gas mixture was studied under typical conditions of reactive ion etching process (inductive 13.56 MHz RF discharge, total gas pressure of 6 mTorr, input power of 700 W and bias power of 200 W). The investigation combined etching rate measurements, plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling in order to determine steady-state densities and fluxes of plasma active species. It was found that the full substitution of Ar for O2 at constant fraction of fluorocarbon gas (in fact, the transition from 50% C4F8 + 50% Ar to 50% C4F8 + 50% O2 gas system): 1) results in weakly non-monotonic (with a maximum) SiO2 etching rate with close values for both O2-free and Ar-free plasmas; 2) causes the monotonic decrease in both F atom flux and ion energy flux; and 3) suppresses the formation of the fluorocarbon polymer film on the etched surface through its oxidative destruction pathway. The model-based analysis of SiO2 etching kinetics allowed one to conclude that an increase in effective probability for SiO2 + F reaction contradicts with the behavior ion energy flux as well as demonstrate the agreement with the change in gas-phase parameters characterizing the fluorocarbon film thickness. Therefore, an increase in O2 content in a feed gas influences the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.

Concerning the Effect of Type of Fluorocarbon Gas on the Output Characteristics of the Reactive-Ion Etching Process

The comparative research of the parameters, steady-state composition, and the effects of heterogeneous interaction in the plasma of fluorocarbon gases CxHyFz with various z/x relations in conditions of an induction RF (13.56 MHz) discharge is carried out. The binary systems CxHyFz + Ar based on CF4 (z/x = 4), CHF3 (z/x = 3), and C4F8 (z/x = 2) are used as the subjects of research. Through the methods of diagnostics and simulation of plasma we found (a) the reasons of the differences in the physical parameters (temperature and density of electrons, energy of ion bombardment) of the plasma in the studied systems; (b) the special characteristics of the kinetics of the plasma-chemical processes determining the steady state concentrations of fluorine atoms and polymer-forming radicals; and (c) the special characteristics of the kinetics of the heterogeneous process forming the output parameters (rate, selectivity, anisotropy) of the reactive ion etching of Si and SiO2.

Annealing effects on a-SiC:H and a-SiCN:H films deposited by plasma CVD methods

Hydrogenated amorphous silicon carbide (a-SiC:H) and carbonitride (a-SiCN:H) films were deposited in RF and mid-frequency (MF) discharges and electron cyclotron resonance (ECR) plasma. Annealing was performed in high vacuum up to 1000 °C and the temperature-dependent gas effusion from the films was investigated in situ by mass spectrometry. Changes of mechanical film properties, i.e. thickness, stress and Young's modulus, were determined ex situ by profilometry and nanoindentation, respectively. Using elastic recoil detection analysis, Rutherford backscattering and X-ray photoelectron spectroscopy, effects of annealing on elemental composition and chemical bonding were investigated. Annealing the a-SiC:H films significantly increased their Young's modulus and decreased their thickness by up to 25% in the case of ECR plasma deposition. The a-SiCN:H films deposited by MF and RF discharge practically retained their Young's modulus and the film thickness shrinkage was below 3%. At intermediate temperatures, these films showed a small thickness increase. For ECR-deposited a-SiCN:H, the expansion was larger and accompanied by a distinct reduction of Young's modulus. Hydrogen effusion and plastic flow under both compressive and tensile stress have a pronounced impact on the mechanical film properties. Annealing effects on film structure and properties are discussed and fundamental processes are derived.

Superhydrophobic carbonous surfaces production by PECVD methods

The paper presents the results of a complex study of superhydrophobic surfaces obtained by PECVD methods in RF discharge plasma with gas mixtures (Ar/CH4 and Ar/C2H2) and by plasma jet (Ar/CH4). In both experiments hydrophobic surfaces were obtained by deposition of carbon nanoparticles on glass and silicon substrates by plasma of Ar (93%) + CH4 (7%) and Ar (93%) + C2H2 (7%) gas mixtures. To compare surfaces with high hydrophobicity produced by the above two methods their contact angles were studied. The dependence of changes in the contact angle on the storage time of the samples under normal conditions show that samples obtained in Ar/CH4 plasma and plasma jet at atmospheric pressure are much better preserved.