In the last 30 days

• Open/close Plasma generation and plasma sources

  H Conrads and M Schmidt 2000 Plasma Sources Sci. Technol. 9 441 Tag this article

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  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.

• Open/close Collisional kinetics of non-uniform electric field, low-pressure, direct-current discharges in H₂

  A V Phelps 2011 Plasma Sources Sci. Technol. 20 043001 Tag this article

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  A model of the collisional kinetics of energetic hydrogen atoms, molecules and ions in pure H ₂ discharges is used to predict H _α emission profiles and spatial distributions of emission from the cathode regions of low-pressure, weakly ionized discharges for comparison with a wide variety of experiments. Positive and negative ion-energy distributions are also predicted. The model developed for spatially uniform electric fields and current densities less than 10 ⁻³ A m ⁻² is extended to non-uniform electric fields, current densities of 10 ³ A m ⁻² and electric field to gas density ratios E/ N = 1.3 MTd at 0.002–5 Torr pressure. (1 Td = 10 ⁻²¹ V m ² and 1 Torr = 133 Pa.) The observed far-wing Doppler broadening and spatial distribution of the H _α emission is consistent with reactions among H ⁺, H ₂ ⁺, H ₃ ⁺ and H ⁻ ions, fast H atoms and fast H ₂ molecules, and with reflection, excitation and attachment to fast H atoms at surfaces. The H _α excitation and H ⁻ formation occur principally by collisions of fast H, fast H ₂ and H ⁺ with H ₂. Simplifications include using a one-dimensional geometry, a multi-beam transport model, and the average cathode-fall electric field. The H _α emission is linear with current density over eight orders of magnitude. The calculated ion-energy distributions agree satisfactorily with experiment for H ₂ ⁺ and H ₃ ⁺, but are only in qualitative agreement for H ⁺ and H ⁻. The experiments successfully modeled range from short-gap, parallel-plane glow discharges to beam-like, electrostatic-confinement discharges.

• Open/close Solving the Boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models

  G J M Hagelaar and L C Pitchford 2005 Plasma Sources Sci. Technol. 14 722 Tag this article

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  Fluid models of gas discharges require the input of transport coefficients and rate coefficients that depend on the electron energy distribution function. Such coefficients are usually calculated from collision cross-section data by solving the electron Boltzmann equation (BE). In this paper we present a new user-friendly BE solver developed especially for this purpose, freely available under the name BOLSIG+, which is more general and easier to use than most other BE solvers available. The solver provides steady-state solutions of the BE for electrons in a uniform electric field, using the classical two-term expansion, and is able to account for different growth models, quasi-stationary and oscillating fields, electron–neutral collisions and electron–electron collisions. We show that for the approximations we use, the BE takes the form of a convection-diffusion continuity-equation with a non-local source term in energy space. To solve this equation we use an exponential scheme commonly used for convection-diffusion problems. The calculated electron transport coefficients and rate coefficients are defined so as to ensure maximum consistency with the fluid equations. We discuss how these coefficients are best used in fluid models and illustrate the influence of some essential parameters and approximations.

• Open/close Modeling and experimental study of molecular nitrogen dissociation in an Ar–N₂ ICP discharge

  Namjun Kang et al 2011 Plasma Sources Sci. Technol. 20 045015 Tag this article

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  The dissociation of nitrogen molecules in an Ar–N ₂ inductively coupled plasma (ICP) discharge is studied both experimentally and theoretically. To measure the absolute N atom density and emission intensity of Ar and N ₂ excited levels, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy and optical emission spectroscopy are used. We observe an increase in N atom density with increasing pressure whereas the N atom density decreases for pressures higher than 100 mTorr in a pure nitrogen discharge. On adding argon to the mixture, we observe that the dissociation rate is enhanced when going from a pure nitrogen discharge to an argon mixed discharge. To calculate the plasma parameters, a global (volume-averaged) model is developed. The variation of the electron temperature and the particle densities are calculated by solving the particle and energy balance equations. The model calculations are compared with the measurement results and the production and loss rates of each species are described under each discharge condition. From the model calculation, the dissociation of N ₂ molecules in the Ar–N ₂ mixed discharge occurs mainly by electron impact dissociation at low pressures, while at high pressures the dissociative recombination is enhanced by charge transfer between Ar ⁺ and N ₂( X) as well as metastable–metastable pooling dissociation due to the high density. In addition, the surface sticking coefficient of nitrogen atoms in a planar ICP discharge (including glass and stainless steel walls) is deduced from TALIF measurements and is estimated to be 0.02 under our set-up conditions.

• Open/close Pulsating fireballs with high-frequency sheath–plasma instabilities

  R L Stenzel et al 2011 Plasma Sources Sci. Technol. 20 045017 Tag this article

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  High-frequency instabilities are observed in connection with unstable fireballs. Fireballs are discharge phenomena near positively biased electrodes in discharge plasmas. They are bounded by a double layer whose potential is of order of the ionization potential. Fireballs become unstable when plasma losses and plasma production are not in balance, resulting in periodic fireball pulses. High-frequency instabilities in the range of the electron plasma frequency have been observed. These occur between fireball pulses, hence are not due to electron beam–plasma instabilities since there are no beams without double layers. The instability has been identified as a sheath–plasma instability. Electron inertia creates a phase shift between high-frequency current and electric fields which destabilizes the sheath–plasma resonance. High-frequency signals are observed in the current to the electrode and on probes near the sheath of the electrode. Waveforms and spectra are presented, showing bursty emissions, phase shifts, frequency jumps, beat phenomena between two sheaths, and nonlinear effects such as amplitude clipping. These reveal many interesting properties of sheaths with periodic ionization phenomena.

• Open/close Internal current measurements in high power impulse magnetron sputtering

  Daniel Lundin et al 2011 Plasma Sources Sci. Technol. 20 045003 Tag this article

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  The transport of charged particles in a high power impulse magnetron sputtering (HiPIMS) discharge is of considerable interest when optimizing this promising deposition technique with respect to deposition rate and control of the ion acceleration. In this study the internal current densities J (azimuthal direction) and J _z (axial direction) have therefore been spatially and temporally resolved in the bulk plasma region above a cylindrical magnetron using Rogowski coils. From the measurements a phenomenological model has been constructed describing the evolution of the current density in this pulsed plasma. The core of the model is based on three different types of current systems, which characterize the operating transport mechanisms, such as current transport along and across magnetic field lines. There is a gradual change between these current systems during the initiation, build-up and steady state of a HiPIMS plasma. Furthermore, the data also show that there are spatial and temporal variations of the key transport parameter J / J _z , governing the cross- B resistivity and also the energy of the charged particles. The previously reported faster-than-Bohm cross- B electron transport is verified here, but not for all locations. These results on the plasma dynamics are essential input when modeling the axial electric field, governing the back-attraction of ionized sputtered material, and might furthermore provide a link between the different resistivities reported in HiPIMS, pulsed-DC, and DC magnetron discharges.

• Open/close Characterization of an Ar/O₂ magnetron plasma by a multi-species Monte Carlo model

  E Bultinck and A Bogaerts 2011 Plasma Sources Sci. Technol. 20 045013 Tag this article

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  A combined Monte Carlo (MC)/analytical surface model is developed to study the plasma processes occurring during the reactive sputter deposition of TiO _x thin films. This model describes the important plasma species with a MC approach (i.e. electrons, Ar ⁺ ions, ions, fast Ar atoms and sputtered Ti atoms). The deposition of the TiO _x film is treated by an analytical surface model.

  The implementation of our so-called multi-species MC model is presented, and some typical calculation results are shown, such as densities, fluxes, energies and collision rates. The advantages and disadvantages of the multi-species MC model are illustrated by a comparison with a particle-in-cell/Monte Carlo collisions (PIC/MCC) model. Disadvantages include the fact that certain input values and assumptions are needed. However, when these are accounted for, the results are in good agreement with the PIC/MCC simulations, and the calculation time has drastically decreased, which enables us to simulate large and complicated reactor geometries.

  To illustrate this, the effect of larger target–substrate distances on the film properties is investigated. It is shown that a stoichiometric film is deposited at all investigated target–substrate distances (24, 40, 60 and 80 mm). Moreover, a larger target–substrate distance promotes film uniformity, but the deposition rate is much lower.

• Open/close Review of the methods to form hydrogen peroxide in electrical discharge plasma with liquid water

  Bruce R Locke and Kai-Yuan Shih 2011 Plasma Sources Sci. Technol. 20 034006 Tag this article

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  This paper presents a review of the literature dealing with the formation of hydrogen peroxide from plasma processes. Energy yields for hydrogen peroxide generation by plasma from water span approximately three orders of magnitude from 4 × 10 ⁻² to 80 g kWh ⁻¹. A wide range of plasma processes from rf to pulsed, ac, and dc discharges directly in the liquid phase have similar energy yields and may thus be limited by radical quenching processes at the plasma–liquid interface. Reactor modification using discharges in bubbles and discharges over the liquid phase can provide modest improvements in energy yield over direct discharge in the liquid, but the interpretation is complicated by additional chemical reactions of gas phase components such as ozone and nitrogen oxides. The highest efficiency plasma process utilizes liquid water droplets that may enhance efficiency by sequestering hydrogen peroxide in the liquid and by suppressing decomposition reactions by radicals from the gas and at the interface. Kinetic simulations of water vapor reported in the literature suggest that plasma generation of hydrogen peroxide should approach 45% of the thermodynamics limit, and this fact coupled with experimental studies demonstrating improvements with the presence of the condensed liquid phase suggest that further improvements in energy yield may be possible. Plasma generation of hydrogen peroxide directly from water compares favorably with a number of other methods including electron beam, ultrasound, electrochemical and photochemical methods, and other chemical processes.

• Open/close Non-equilibrium plasma in liquid water: dynamics of generation and quenching

  Andrey Starikovskiy et al 2011 Plasma Sources Sci. Technol. 20 024003 Tag this article

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  In most cases, the electric breakdown of liquids is initiated by the application of high electric field on the electrode, followed by rapid propagation and branching of plasma channels. Typically plasmas are only considered to exist through the ionization of gases and typical production of plasmas in liquids generates bubbles through heating or via cavitation and sustains the plasmas within those bubbles. The question arises: is it possible to ionize the liquid without cracking and void formation?

  To answer this question we used a pulsed power system with 32–220 kV pulse amplitude, 0.5–12 ns pulse duration, 150 ps rise time. The discharge cell had a point-to-plate geometry with a tip diameter of 100 µm. These parameters allowed us to observe non-equilibrium plasma generation. The measurements were performed with the help of a 4Picos ICCD camera. It was found that the discharge in liquid water forms on a picosecond time scale. The increase of emission intensity and plasma formation took 200–300 ps. The diameter of the excited region near the tip of the high-voltage electrode was ~1 mm. After this initial stage emission rapidly decreased and the plasma region became almost invisible after 500 ps. The absence of emission during the rest of the pulse is explained by a decrease of the electrical field on the boundary of the conductive zone.

  Thus we have demonstrated the possibility of formation of non-equilibrium plasma in the liquid phase and investigated the dynamics of excitation and quenching of non-equilibrium plasma in liquid water.

• Open/close Carbon nanotube growth by PECVD: a review

  M Meyyappan et al 2003 Plasma Sources Sci. Technol. 12 205 Tag this article

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  Carbon nanotubes (CNTs), due to their unique electronic and extraordinary mechanical properties, have been receiving much attention for a wide variety of applications. Recently, plasma enhanced chemical vapour deposition (PECVD) has emerged as a key growth technique to produce vertically-aligned nanotubes. This paper reviews various plasma sources currently used in CNT growth, catalyst preparation and growth results. Since the technology is in its early stages, there is a general lack of understanding of growth mechanisms, the role of the plasma itself, and the identity of key species responsible for growth. This review is aimed at the low temperature plasma research community that has successfully addressed such issues, through plasma and surface diagnostics and modelling, in semiconductor processing and diamond thin film growth.