Low-pressure RF Discharge Research Articles

Collisionless magnetized sheath resonance heating induced by a transverse magnetic field in low-pressure capacitive rf discharges

Abstract The mechanism of resonance heating between the gyrating electrons and the oscillating sheath induced by a small transverse magnetic field in low-pressure capacitively coupled plasmas is investigated. The gyrating electrons will coherently collide with the expanding sheath if the electron gyro-frequency coincides with half the driving frequency. These electrons will gain substantial energy from collision-less heating, which strongly enhances the electron power absorption and the plasma density at a constant driving voltage. The electron kinetics is revealed at resonant conditions by particle simulations. Our numerical results demonstrate that the relation between the magnetic field and the driving frequency determines this resonance effect. Besides, it is found that the operating pressure, electrode gap, and driving voltage all strongly affect this electron resonance mechanism. The resonance effect is more pronounced at the conditions of low pressure, large gap, and high voltage.

Experimental study of a low-pressure hybrid RF discharge

Experimental study of the low-pressure hybrid RF discharge with both inductive and capacitive channels was carried out. The RF power unit consists of inductor (antenna) and capacitor plates connected in parallel to the same RF power source. A separating capacitor Csep is included into the circuit between the antenna ends and the lead connected to the discharge capacitor plate in order to prevent the closing of the capacitive circuit through direct current by inductor and to control the contribution of capacitive channel to discharge sustaining. It is shown that at low power of the RF power source, power coupling to the discharge mainly occurs through the capacitive channel. Increasing the power of the RF power source increases the power coupled in the inductive channel, electron density, and current flowing through the capacitive channel. This leads to increasing voltage drop on the separating capacitor and partial cutoff of the capacitive channel. At separating capacitance values below certain value (below 50 pF in the present experiments), the self-bias of the loaded plate of the discharge capacitor becomes positive indicating that the thickness of the electrode sheath of the loaded electrode decreases compared to thickness of the sheath of the grounded electrode. The thickness of the space-charge sheath of the grounded electrode decreases with increasing power coupled to the plasma. At separating capacitance below 50 pF, higher harmonics of the RF voltage and current are actively generated in the capacitive discharge channel. Increasing the separating capacitance leads to decreasing electron density, increasing effective electron temperature and more effective RF energy coupling to plasma due to increasing relative importance of the capacitive discharge channel.

Investigation of the influence of low-pressure RF discharge on the physical properties of camel skins

Investigation of the influence of low-pressure RF discharge on the physical properties of camel skins

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.

Ion energy and angular distributions in low-pressure capacitive oxygen RF discharges driven by tailored voltage waveforms

We investigate the energy and angular distributions of ions reaching the electrodes in low-pressure, capacitively coupled oxygen radio-frequency discharges. These distributions, as well as the possibility of the independent control of the ion flux and the ion energy are analysed for different types of excitation: single- and classical dual-frequency, as well as valley- and sawtooth-type waveforms. The studies are based on kinetic, particle-based simulations that reveal the physics of these discharges in great detail. The conditions cover weakly collisional to highly collisional domains of ion transport via the electrode sheaths. Analytical models are also applied to understand the features of the energy and angular distribution functions.

Mathematical simulation of a low-pressure capacitive RF discharge in an external radial magnetic field using the KARAT code

A high-frequency capacitive discharge is simulated in the geometry of a plasma accelerator with closed electron drift. It has been shown that, in such a discharge, as in a dc discharge, an azimuthal electron drift takes place and a potential drop is formed at the discharge channel exit, which leads to the emergence of an accelerated ion beam from the channel.

Mass spectrometry of positive ions in capacitively coupled low pressure RF discharges in oxygen with water impurities

A capacitively coupled RF oxygen discharge is studied by means of mass spectroscopy. Mass spectra of neutral and positive species are measured in the mid plane between the electrodes at different distances between plasma and mass-spectrometer orifice. In the case of positive ions, as expected, the largest flux originates from . However, a significant number of impurities are detected, especially for low input powers and larger distances. The most abundant positive ions (besides ) are , , and . In particular, for the case of hydrated hydronium ions (n = 1, 2) a surprisingly large flux (for low pressure plasma conditions) is detected. Another interesting fact concerns the ions. Despite the relatively high ammount of water impurities ions are present only in traces. The reaction mechanisms leading to the production of the observed ions, especially the hydrated hydronium ions are discussed.

Radial profile of the electron energy distribution function in RF capacitive gas-discharge plasma

This paper reports experimental results on low-pressure argon capacitive RF discharge (parallel-plate capacitively-coupled plasma - CCP) under different conditions, namely, gas pressure in the range 3 -r- 30 Pa and RF power in the range 10 - 100 W. The IV characteristics measured were processed by two different second-derivative probe techniques for determination of the plasma parameters and the electron energy distribution function. The radial profiles of the main plasma parameters are presented.

Experimental investigation of hydrogen peroxide RF plasmas

This work reports a detailed experimental study of the plasma properties in low pressure RF discharges in hydrogen peroxide and a comparison with argon under the same operating conditions. H2O2 plasmas have been proposed for sterilization purposes. Electrical properties of the discharge were shown to be similar, as for the RF and DC voltages of the driving electrode. Bulk plasma volume remains stable, concentrated in an almost cylindrical region between the two facing electrodes. It was found that the electron temperature is almost uniform across the plasma and independent of the power level. This is higher than in argon discharges: Te = 4.6 ± 0.9 eV versus Te = 3.3 ± 1.1 eV. The plasma density increases almost linearly with the power level and a substantial negative ion component has been ruled out in hydrogen peroxide. Dissociation in the plasma gas phase was revealed by atomic hydrogen and hydroxyl radical emission in the discharge spectra. Emission from hydroxyl and atomic oxygen demonstrates that oxidizing radicals are produced by hydrogen peroxide discharges, revealing its usefulness for plasma processing other than sterilization, for instance to increase polymer film surface energy. On the other hand, argon could be considered as a candidate for the sterilization purposes due to the intense production of UV radiation.

Characteristics of Plasma Treated Electrospun Polycaprolactone (PCL) Nanofiber Scaffold for Bone Tissue Engineering.

Polycaprolactone (PCL) nanofibers (PCL-NF) with uniform fibrous structure were fabricated by electrospinning. However, PCL-NF has hydrophobic surface, lacks functional groups and hence it is not a good substrate for cell adhesion. To improve the cell adhesion, PCL-NF surfaces were modified by low pressure RF discharge plasma treatment using monomer such as acrylic acid or oxygen gas. The plasma treated PCL-NFs improved the wettability and cell proliferation.

Broadening of the frequency spectrum of waves forming a low-pressure helicon discharge

It is found experimentally that the spectrum of waves forming a low-pressure RF discharge in a strong magnetic field broadens toward lower frequencies. Such an asymmetric broadening is explained by the scattering of the pump wave by the electron flows escaping from the discharge. A similar effect can take place during the formation of the spectrum of artificial RF radiation in the Earth’s ionosphere.

Lead selenide nanowire growth by vapor-liquid-solid mechanism under mask during plasma processing

Data on the formation of lead selenide (PbSe) nanowires under a stencil mask during the processing of epitaxial PbSe films in high-density inductively coupled plasma (ICP) of low-pressure argon RF discharge are presented. The nanowires were studied by high-resolution scanning electron microscopy and energy-dispersive X-ray spectroscopy. A physical model is proposed that explains the local formation of PbSe nanowires in terms of their catalytic growth according to the vapor-liquid-solid mechanism.

Wave propagation and noncollisional heating in neutral loop and helicon discharges

Heating mechanisms in two types of magnetized low pressure rf (13.56 MHz) discharges are investigated: a helicon discharge and a neutral loop discharge. Radial B-dot probe measurements demonstrate that the neutral loop discharge is sustained by helicon waves as well. Axial B-dot probe measurements reveal standing wave and beat patterns depending on the dc magnetic field strength and plasma density. In modes showing a strong wave damping, the plasma refractive index attains values around 100, leading to electron-wave interactions. In strongly damped modes, the radial plasma density profiles are mainly determined by power absorption of the propagating helicon wave, whereas in weakly damped modes, inductive coupling dominates. Furthermore, an azimuthal diamagnetic drift is identified. Measurements of the helicon wave phase demonstrate that initial plane wave fronts are bent during their axial propagation due to the inhomogeneous density profile. A developed analytical standing wave model including Landau damping reproduces very well the damping of the axial helicon wave field. This comparison underlines the theory whereupon Landau damping of electrons traveling along the field lines at speeds close to the helicon phase velocity is the main damping mechanism in both discharges.

Crystal and liquid structures in strongly nonideal dusty plasmas under laboratory and microgravity conditions

This review presents the results obtained in a cycle of interrelated experimental works that have a priority character in the study of dusty plasma crystals and liquids under laboratory and microgravity conditions (on board the Mir Orbital Complex and the International Space Station) over wide ranges of temperatures and pressures: in a DC glow-discharge plasma and a low-pressure RF discharge plasma at room and cryogenic temperatures, in a nuclear-excited plasma and an UV-induced plasma, and in a thermal plasma at atmospheric pressure.

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.

Power dissipation analysis in N2O RF discharges using Monte Carlo modelling

In this paper, a microscopic approach for the calculation of partial and total power dissipation from energy losses by collisions is considered and applied in the case of N2O low pressure RF discharges. This approach is based on a Monte Carlo technique in a particle model permitting sampling of the energy deposited by different inelastic electron–N2O collisions. The calculated power densities presented in this paper are in good agreement with the experimental results and those obtained by the classical macroscopic formula based on spatio-temporal integration of the product of current density and electrical field. This microscopic approach presents, however, a major advantage in comparison with the classical method (which only offers the possibility to calculate the global power dissipation) by making possible the calculation of all the power density terms, thereby permitting one to examine the relative contribution of each collision process in the power dissipation. Its application to N2O electronegative discharges, at 503 K gas temperature, several RF voltages and two different gas pressures shows how the power is dissipated through electron–gas processes. The power density variation is found to be proportional to the electron density variation brought about by the changes in attachment (i.e. e + N2O → N2 + O−), detachment (i.e. NO− + N2O → NO + N2O + e) and ionization (i.e. e + N2 O → N2O+ + 2e) processes. The role of each of these processes is fully studied with our particle model in order to explain the dissipated power variation.

A software for optical emission spectroscopy-problem formulation and application to plasma diagnostics

The aim of the presented work was to develop a method, which would make possible to identify spectral lines in complicated optical emission spectra. This is an important task for many optical diagnostic methods. It was found out, that the most useful technique combines a manual identification of the lines by the user, according to the developed database of atomic and molecular lines, and an enhanced support of the user by various assistant mechanisms. Therefore, in this work a software for displaying, identification and analysis of the optical emission spectra was developed. Besides the identification, program enables a basic handling of the spectra, corrections of the wavelengths and intensities of the spectra and calculations of some plasma parameters (e. g. calculation of rotational, vibrational and electron temperatures) or other important quantities (e. g. calculation of integrated intensity). The developed software was applied to the study of low pressure RF discharge in neon.

Sterilization of Medical Equipment Using Radicals Produced by Oxygen/Water Vapor RF Plasma

The sterilization of medical equipment was performed using low-pressure RF discharge plasma. Oxygen radicals as oxidizing materials for sterilization were produced from oxygen or water vapor. The generation of atomic oxygen and OH radicals was confirmed using the light emission spectra of the plasma. The pressure in the discharge region was varied periodically during inactivation in order to produce radicals effectively and to penetrate the produced radicals into tiny gaps. Medium sheets for microbial detection indicated that the decimal reduction value of active bacillus was 5 and 8 min using oxygen plasma and water vapor plasma with ne=109 cm-3, respectively. Biological indicators clarified that the sterilization of bacilli spores was successful for a treatment time of 90 min.

Time-resolved measurements of the ion energy distribution function in a pulsed discharge using a double gating technique

This paper presents an improved technique for obtaining the time-resolved ion energy distribution functions (IEDFs) in a pulsed plasma using a commercial quadrupole mass energy analyser. In this technique, both the extractor and the detector of the analyser are gated at selected times relative to the pulse. The combination of the gating pulses allows the time-resolved IEDFs to be determined without a time delay inherent in the ion transit time through the spectrometer. The technique is applied to a low pressure RF discharge in Ar. The time-resolved IEDFs are obtained for two different modulation frequencies with 10 µs time resolution: (i) 1 kHz of modulation frequency, 50% duty (when the afterglow plasma exists within the entire ‘off’ period) and (ii) at 200 Hz modulation frequency, 10% duty (when the remnant plasma completely collapses before the beginning of the following ‘on’ pulse). The peak values in the ion energy are found to be constant in a steady state of the ‘on’ time (∼42 eV). The highest ion energy at 200 Hz is 90 eV, but only 80 eV at 1 kHz. At the beginning of the ‘off’ time they fall sharply to 10–15 eV and then slowly decrease in time to 4–6 eV. The normalized integral of the counts under the IEDFs and the IEDF peak energies are compared and agree well with the plasma densities and the plasma potential respectively obtained at the same time points by the single Langmuir probe.