Table of contents

The 27th International Conference on Phenomena in Ionized Gases (ICPIG) was held in the conference resort of NH Koningshof in Veldhoven, near Eindhoven, The Netherlands, 17–22 July 2005. ICPIG is an important biennial event at which academics and industrialists working in low-temperature plasma science can meet. The 27th ICPIG was organized under the sponsorship of the International Union of Pure and Applied Chemistry (IUPAP), the Royal Dutch Academy of Sciences (KNAW), the Research School Centre for Plasma Physics and Radiation Technology (CPS), the Dutch Organization for Fundamental Research on Matter (FOM), Stichting Physica, the Dutch organization for Scientific Research (NWO), Philips Lighting, and the Eindhoven University of Technology.

The scientific scope of this joint conference focused on both experimental and theoretical aspects of the physics of ionized gases as well as on industrial applications. It covered the following topics:

• Kinetics, thermodynamics and transport phenomena • Elementary processes • Low-pressure glows • Coronas, sparks, surface discharges and high-pressure glows • Arc discharges • High-frequency discharges • Ionospheric, magnetospheric and astrophysical plasmas • Plasma diagnostic methods • Plasma wall interaction, electrode and surface effects • Physical aspects of plasma chemistry, plasma processing of surfaces and thin film technology • The generation and dynamics of plasma flows • Non-ideal plasmas, clusters and dusty plasmas • Waves and instabilities, including shock waves • Nonlinear phenomena, self-organization and chaos • Particle and laser beam interaction with plasmas • Plasma sources of radiation • Numerical modelling • Plasmas for environmental issues • Highly ionized, low-pressure plasmas (plasma thrusters, ion sources and surface treatment) • High-pressure, non-thermal plasmas.

ICPIG was attended by close to 400 scientists from 41 countries. A selection of the invited papers is published in this special issue. The 401 contributed papers were presented in five poster sessions. The abstracts of all the oral and poster contributions were published in the CD of the conference proceedings.

I would like to thank all members of the Local Organizing Committee as well as the members of the International Scientific Committee of ICPIG for their indispensable contributions to the success of this joint meeting. We are particularly grateful to the Editor-in-Chief of Plasma Sources Science and Technology, Professor Noah Hershkowitz, for the opportunity to publish the invited papers in this special issue and so bring the 27th ICPIG to a wider audience.

An analytical description is presented for the excitation of electrostatic modes in partially ionized magnetized space and laboratory plasmas. The neutrals introduce some effects that are responsible for the creation of ions, which in a stationary plasma is balanced by a number of phenomena in which ions and electrons are lost. The behaviour of several modes is discussed, such as the ion acoustic, drift, ion cyclotron and the Farley–Buneman mode. The instability conditions are obtained, showing that inelastic collisions can either modify some of these modes or make them unstable. The formation of a global nonlinear structure in a spatially bounded laboratory plasma is discussed. The structure is experimentally observed, and an analytical model is presented showing that the charge exchange plays a decisive role in its formation.

Extreme ultra-violet (EUV) lithography is most likely to be used for the production of semiconductors from 2009. One of the potential showstoppers in the commercialization is the availability of a compact high power source. This paper discusses the latest developments on the different kinds of plasma-based EUV sources. The progress of laser-produced plasmas seems to be limited as they are roughly at the same level as in 2000. In the field of gas discharges and vacuum arcs, a huge improvement in output power and in lifetime has been made recently. Vacuum arcs, especially, seem to offer the possibility of meeting the specifications for high-volume manufacturing.

Time-resolved optical emission spectroscopy is applied to analyse pulsed dc magnetron plasmas during sputter deposition of Ti and TiO₂ thin films. The studies are focused on the temporal behaviour of emissions of selected atomic titanium, argon and oxygen during the 'on-time', when the magnetron plasma source is turned on, and during the 'reverse-time', when the magnetron plasma source is turned off. Single and double exponential decays of the optical emissions were found during the 'reverse-time'. During the 'on-time', we observed a rapid increase in some emissions with a more or less pronounced overshoot and a slow increase in other emissions which follow the temporal evolution of the conduction current. The various time constants can be related to the dynamics of the fast electrons, the Ar metastables, the Ti atoms (metallic mode) and the O atoms (oxide mode).

A portable microwave plasma torch at atmospheric pressure by making use of magnetrons operated at 2.45 GHz and used in a home microwave oven has been developed. This electrodeless torch can be used in various areas including commercial, environmental and military applications. For example, perfluorocompounds (PFCs), which have long lifetimes and serious global warming implications, are widely used during plasma etching and plasma-assisted chamber cleaning processes in chemical vapour deposition systems. The microwave torch effectively eliminates PFCs. Efficient decomposition of toluene gas indicates the effectiveness of volatile organic compound eliminations from industrial emission and the elimination of airborne chemical and biological warfare agents. The microwave torch has been used to synthesize carbon nanotubes in an on-line system, thereby providing the opportunity of mass production of the nanotubes. There are other applications of the microwave plasma torch.

Experimental studies of pulsed low-pressure discharges reveal that the mean electron energy exhibits high constant values in the afterglow. Therefore, energy losses in (quasi-)elastic collisions with neutral atoms and through particle losses to the walls must be compensated by some electron re-heating mechanism. The spatio-temporal dynamics of the discharge and the electron re-heating are examined with a hybrid model. The electron energy distribution function is calculated using the non-local approximation. Coulomb collisions among electrons are treated as well as collision processes between electrons and atoms in their ground or metastable states. A hydrodynamic model is used to describe the spatially resolved ion transport. Poisson's equation is solved throughout the plasma volume allowing for a spatial resolution of the plasma boundary sheaths.

The re-heating of the electron gas from metastable atoms in the late afterglow is treated in detail. It is shown that superelastic collision processes from the upper to the lower metastable state give the dominant contribution. A further important contribution to the re-heating is established by the chemo-ionization process involving the dominant metastable state.

Calculations are presented for argon and helium. The simulations are compared with experimental results in argon and excellent agreement is found.

Using wavelength filtered two dimensional (2-D) optical imaging, the temporal and spatial evolution of selected excited species in a pulsed magnetron discharge has been studied. A titanium target was sputtered at a pulse frequency of 100 kHz, in an argon atmosphere, at an operating pressure of 0.27 Pa. The radial information of the emissivity was determined using the Abel inversion technique. The results show strong excitation of the observed species above the racetrack in the on-time, and the possible development of an ion–acoustic wave, initiated after the off–on transition. The on–off transition is accompanied by a burst of light from the plasma bulk consistent with the transient plasma potential reaching about +200 V. During this phase, we argue that there is a release of secondary electrons from the grounded substrate and walls due to ion bombardment, as well as an increased confinement of energetic plasma electrons. The characteristic decay times of the selected transitions at 750.4, 751.5, 810.4 and 811.5 nm (ArI), present within the bandpass width of our filters, is briefly discussed in terms of the production of fast electrons in the system.

The characteristics of discharge plasma generated by piezoelectric transformers (PTs) and the practical applications of PT-based plasma reactors are reviewed in this paper. PTs of Pb(Zr · Ti)O₃ generate high voltage by the piezoelectric effect, which can cause excitation and ionization of atoms and molecules, resulting in the generation of discharge plasma. When a narrow gap exists between the PT surface and a dielectric layer having a metal back electrode, dielectric barrier discharge (DBD) occurs at atmospheric pressure and above. The mechanical vibration of the PT and the resultant surface potential, which relate to electro-mechanical energy conversion by the piezoelectric effect, are investigated using a laser Doppler vibrometer and probe method, respectively. The characteristics of DBD are examined based on light patterns detected by the intensified charge coupled device camera, and the fundamental parameters of the filamentary discharge are determined. PT-generated DBD is shown to be applicable for ozone generation and as excimer lamps, and high-power operation is demonstrated using a parallel-driven hexad plasma reactor.

State-resolved cross sections for electron–H₂ and H–H₂ collision-induced processes have been calculated, using semiclassical and quasiclassical approaches, respectively. Corresponding results for the deuterium system are presented, introducing mass-scaling relations.

A new realm of plasma science has been entered recently with the demonstration of devices in which a nonequilibrium, low temperature plasma is produced within, and essentially confined to, a microcavity having a characteristic dimension (d) between 10 and 100 µm. This development has introduced glow microplasmas exhibiting unique properties with respect to, for example, specific power loading and operation at atmospheric pressure (and beyond). Of equal import is the reduction of d below 100 λ_D (where λ_D is the Debye length), and the associated weakening of charge quasineutrality as well as the growing importance of plasma-wall interactions are evident. The general properties of several microcavity plasma devices occupying this region of parameter space are briefly described here, but recent experimental results with arrays (as large as 30 × 30) of Al/Al₂O₃ devices are emphasized. Several potential applications of single microcavity plasma devices and planar arrays are also discussed.

In semiconductor processes, reactive plasma is the most important technology for etching, deposition and surface modification of thin films. Radicals have played key roles in plasma processing. In order to realize the high performance of semiconductor processing, important molecular radicals have been measured and their behaviour has been clarified using laser spectroscopic methods such as infrared diode laser absorption spectroscopy, laser induced fluorescence spectroscopy, cavity ring down spectroscopy and recently atomic radicals have also been measured using compact vacuum ultraviolet absorption spectroscopy. Quantitative understanding of kinetics of radicals in plasma will be necessary for nano-scaled semiconductor processing. Their progress is reviewed and the future prospects are presented.

Optical emission spectroscopy was used to investigate collisional and radiative processes in a high-pressure cylindrical dielectric barrier discharge excited by radio frequency power at 13.56 MHz in gas mixtures containing Ne and N₂. Emissions attributable to N₂, and N were analysed in three regions ranging from 70–120 nm, 120–180 nm, and 330–365 nm. The dependence of the intensities of the various emission features from the discharge plasma on the plasma operating parameters (pressure, power, gas mixture) are explained in terms of the energy transfer channels from Ne metastables and excimers to N₂. We compare the observations from the Ne/N₂ plasmas to those obtained earlier for Ar/N₂ plasmas.

Many outstanding problems in lightning physics are linked with a difference in macroscopic behaviour between positive and negative polarity. Such differences are referred to broadly as 'polarity asymmetry'. As specific examples, the positive and negative ends of lightning propagate at different speeds, with different degrees of steadiness, and with different radiated electromagnetic energy. Positive and negative flashes to ground transfer their charge in markedly different ways—negative flashes with multiple discrete strokes (often) and positive flashes with single strokes followed by continuing current. Positive ground flashes cause sprites and negative flashes do not (generally). Positive intracloud flashes send gamma radiation upward to space and negative intracloud flashes do not (generally). Speculative arguments are presented that all of these macroscopic asymmetries are rooted in the microscopic asymmetry in mobility for free electrons and positive ions.

Self-organization of direct current xenon microdischarges in cathode boundary layer configuration has been studied for pressures in the range 30–140 Torr and for currents in the range 50 µA–1 mA. Side-on and end-on observations of the discharge have provided information on the structure and spatial arrangement of the plasma filaments. The regularly spaced filaments, which appear in the normal glow mode when the current is lowered, have a length which is determined by the cathode fall. It varies, dependent on pressure and current, between 50 and 70 µm. The minimum diameter is approximately 80 µm, as determined from the radiative emission in the visible. The filaments are sources of extensive excimer emission. Measurements of the cathode fall length have allowed us to determine the secondary emission coefficient for the discharge in the normal glow mode and to estimate the cathode fall voltage at the transition from normal glow mode to filamentary mode. It was found that the cathode fall voltage at this transition decreases, indicating the onset of additional electron gain processes at the cathode. The regular arrangement of the filaments, self-organization, is assumed to be due to Coulomb interactions between the positively charged cathode fall channels and positive space charges on the surface of the surrounding dielectric spacer. Calculations based on these assumptions showed good agreement with experimentally observed filament patterns.

Streamers are a generic mode of electric breakdown of large gas volumes. They play a role in the initial stages of sparks and lightning, in technical corona reactors and in high altitude sprite discharges above thunderclouds. Streamers are characterized by a self-generated field enhancement at the head of the growing discharge channel. We briefly review recent streamer experiments and sprite observations. Then we sketch our recent work on computations of growing and branching streamers, we discuss concepts and solutions of analytical model reductions and we review different branching concepts and outline a hierarchy of model reductions.

Capacitive discharges have classically been operated in the electrostatic regime, for which the excitation wavelength λ is much greater than the electrode radius and the plasma skin depth δ is much greater than the electrode spacing. However, contemporary reactors are larger and excited at a higher frequency so that electromagnetic effects become significant. A self-consistent transmission line model valid in the entire range of λ and δ of practical interest is solved. The model is the electromagnetic generalization of the lumped-element circuit model classically used for capacitive discharges. We find that the plasma may either be sustained by the usual capacitive (E) field or by an inductive (H) field and that the discharge experiences E to H transitions as the voltage between the electrodes is raised. The transitions are global at low pressure and local at high pressure. In the latter case, the plasma parameters (e.g. the ion flux to the electrodes) are radially non-uniform, due to the non-uniformities of the rf voltage and current, leading to serious industrial issues.

A pulsed inductively-coupled radiofrequency plasma in oxygen is investigated by means of time-resolved microwave interferometry in a wide pressure range from 0.5 to 200 Pa. In the afterglow a peak of the electron density is observed. The effect is maximum for pressures around 50 Pa. The time-resolved measurements of the electron density are interpreted in the framework of a fluid model. This model points out the significance of negative ions. The overall electron density is comparatively small. Attachment and detachment processes nearly balance during the power-on phase. But when the power is switched off, the electron temperature drops very quickly. This means that the production of new negative ions is inhibited so that the negative ions are destroyed by collisions. These reactions quickly set free electrons in the afterglow and are the reason for the observed peak in the electron density after switching off the power.

Modelling approaches used for the description of the cathode region of dc glow discharges are reviewed, with the focus on hybrid models which combine the fluid description of positive ions and bulk electrons with the kinetic simulation of fast electrons. The reliability of the calculated discharge characteristics is analysed by testing the different assumptions of the models and the sensitivity of the calculated characteristics on the input data. The applicability of the particle-in-cell technique (complemented with Monte Carlo simulation of collision processes) for the simulation of dc glow discharges is also discussed.

The characteristics of the spatial distribution of electron cyclotron resonance (ECR) plasma in an ECR-plasma enhanced metalorganic chemical vapour deposition system with a divergent magnetic field are investigated by a single Langmuir probe. The results show that the spatial distribution in the resonance room has a significant density gradient in both radial and axial directions. The ECR plasma density attains its highest value, about 3.0 × 10¹¹ cm⁻³, at a position that moves away from the ECR point in the direction of the microwave window for about 2.5 cm. This peak is about ten times higher than the central plasma density in the downstream of the reactor chamber. Analysis of the spatial distribution in the reactor chamber shows that the ECR plasma in the upper region has poor radial and axial uniformity of plasma density and electron temperature under the influence of the magnetic field, whereas the plasma in the downstream region has fine radial uniformity. This excellent uniformity has extensive application in plasma processing. Furthermore, there is a maximum plasma density and a maximum electron temperature corresponding to a proper magnetic current. A change of magnetic current does not distort the characteristics of spatial distribution in the reactor chamber.

Many of today's continuous-wave plasma etch processes of semiconductor devices utilize chlorine or chlorine-based gases which require stable power delivery conditions. When the plasma process is intentionally (pulsed) or unintentionally driven into an unstable mode, the population of electronegative species become time-dependent. This paper examines the instability coupling between the external circuit and a chlorine plasma within an inductively coupled plasma tool. Standing-wave ratio measurements are used to map instabilities in a 2 mTorr pure chlorine discharge, as a function of power and the parallel capacitor (C_P) of the T network (matching box and antenna). At low power (<180 W) no instabilities are observed whether the system is matched or mismatched. Instabilities are only observed at input power levels from 180 to 280 W when the system is matched. At larger powers (>280 W) instabilities are observed when the system is mismatched with respect to the system input impedance (50 Ω) and the mismatch makes the load reactance negative. The asymmetry of the match is explained using an equivalent electrical model and power balance arguments. The methodology developed allows the identification of four different types of instability and indicates operating regimes which will be free of instabilities.

An atmospheric pressure capacitive discharge source has been developed that operates at power densities over 100 W cm⁻³. The ground state nitrogen atom concentration was measured at the exit of the source by titration with NO, and it was found to reach a maximum of 3.0 ± 0.8 × 10¹⁷ cm⁻³ at 6.0 vol% N₂ in argon, 250 °C and 150 W cm⁻³. This is equivalent to 2.3 vol% of N atoms in the afterglow. At these conditions, the electron density and temperature are estimated to be 3.1 × 10¹² cm⁻³ and 1.2 eV. A plug-flow model of the plasma and afterglow was developed, and it was determined that the maximum N atom concentration achievable is limited by three body recombination.

A miniature electrothermal thruster has been proposed using azimuthally symmetric microwave-excited plasmas, and numerical investigations have been conducted for design consideration. The microthruster consists of a microplasma source and a micronozzle. The former, made of a dielectric chamber 1 mm in radius and 10 mm long covered with a grounded metal, produces high temperature plasmas in Ar at around atmospheric pressures. The latter converts such high thermal energy into directional kinetic energy through supersonic nozzle expansion to obtain the thrust required. The numerical model consists of three modules: a global model and an electromagnetic model for microplasma sources and a fluid model for micronozzle flows. Simulation was conducted separately for the plasma source and nozzle flow. The numerical results indicated that the microwave power absorbed in plasmas increases with increasing microwave frequency and relative permittivity of dielectrics, to achieve plasma density in the range 10¹⁹–10²² m⁻³, electron temperature in the order of 10⁴ K and heavy particle temperature in the range 10³–10⁴ K at a microwave input power of ≤ 10 W; in practice, surface waves tend to be established in the microplasma source at high frequencies and permittivities. A certain combination of frequency and permittivity was found to significantly enhance the power absorption, enabling the microplasma source to absorb almost all microwave input powers. Moreover, the micronozzle flow was found to be very lossy because of high viscosity in thick boundary layers, implying that shortening the nozzle length with increasing half-cone angles suppresses the effects of viscous loss and thus enhances the thrust performance. A thrust of 2.5–3.5 mN and a specific impulse of 130–180 s were obtained for a given microwave power range of interest, which is applicable to a station-keeping manoeuvre for microspacecraft less than 10 kg.

Cylindrical dc magnetron discharges in argon in post-cathode (direct) and hollow-cathode (inverted) configurations are studied experimentally and theoretically. The electron component is modelled based on the non-hydrodynamic solution of the Boltzmann kinetic equation in the radially inhomogeneous electric and axially directed uniform magnetic fields. Two different representations of the ionization operator involving discrete and continuous spectra of secondary electrons are studied. The form of the ionization operator is shown to affect the electron distribution functions and macroscopic properties. The absolute values and radial profiles of the electron density, excitation and ionization rates and charged particle fluxes are obtained and compared for the direct and inverted magnetron configurations.

A dielectric barrier discharge generated by flowing inert gas (helium) ionized by a high-voltage source through a cylindrical reactor working at atmospheric pressure has been studied and an electrical model characterizing this discharge is proposed. A sinusoidal voltage of up to 2 kV peak to peak with frequencies from 10 to 125 kHz has been applied to the discharge electrodes. The proposed model considers the geometry of the reactor and dielectric materials. From experimental and analytical results, a semi-empirical relation of the breakdown voltage is presented as a function of the operating frequency. The microdischarge regime is characterized by a dynamic equivalent capacitance.

The carbon dioxide (CO₂) formation mechanism and co-conversion of CO₂ with coal was investigated in the process of coal gasification in a steam medium at atmospheric pressure under arc plasma conditions in a tube-type setup. The arc plasma was diagnosed in situ by optical emission spectroscopy and the gas products were analysed by gas chromatography. CO₂ yields are correlated with the quantitative emission peak intensity of the active species in plasma when the operating parameter is changed. The results show that the greater the emission peak intensity of the CH radicals, C₂ radicals, OH radicals or O atoms, the smaller the CO₂ yield is, which means that the CO₂ formation process is inhibited by increasing the concentration of the mentioned active species under arc plasma conditions. On the basis of the diagnosis results, co-conversion of CO₂ and coal in a steam medium under plasma conditions was carried out in the same setup and the results show that CO₂ conversion reaches 88.6% while the concentration of CO + H₂ reaches 87.4%; at the same time, coal conversion is in the range 54.7–68.7%, which proves that co-conversion of CO₂ and coal in a steam medium under plasma conditions might be a prospective way to utilize CO₂ and the production of synthesis gas.

Recent data on ionizing radiation in discharges in dry and moist air are analysed. It is shown that an account of both quenching of radiating states and absorption of ionizing radiation by water molecules allows one to explain these data in the framework of a commonly accepted model that is widely used for streamer simulations.

The spatiotemporal evolution of the electron energy distribution function (EEDF) and of plasma parameters such as the electron density, the electron temperature and the plasma and floating potentials has been investigated using spatially and temporally resolved single Langmuir probe measurements in dc and mid-frequency, short-pulse magnetron discharges with a repetition frequency of 10 kHz and a duty cycle of 10%. In the pulsed discharge of the short duty cycle, a peak electron temperature higher than 10 eV was observed near the cathode fall region during the early phase of the pulse-on, which is about three times higher than the steady-state value of the electron temperature in the dc discharge. The temporal evolution of the measured EEDFs showed the initial efficient electron heating during the early phase of the pulse-on and the subsequent relaxation of electron energy by the inelastic collisions and the diffusive loss. The high-energy electrons generated during the pulse-on phase diffused the downstream region toward the grounded substrate, resulting in a bi-Maxwellian EEDF consisting of the background low-energy electrons and the high-energy electrons. The results of the spatially and temporally resolved probe measurements will be presented and the enhanced efficiency of the electron heating in the short-pulse discharge will be explained on the basis of the global model of a pulsed discharge.

A new plasma torch device which combines arc and microwave discharges to enhance the size and enthalpy of the plasma torch is described. A cylindrical-shaped plasma torch module is integrated into a tapered rectangular cavity to form a microwave adaptor at one end, which couples the microwave power injected into the cavity from the other end to the arc plasma generated by the torch module. A theoretical study of the microwave coupling from the cavity to the plasma torch, as the load, is presented. The numerical results indicate that the microwave power coupling efficiency exceeds 80%. Operational tests of the device indicate that the microwave power is coupled to the plasma torch and that the arc discharge power is increased. The addition of microwave energy enhances the height, volume and enthalpy of the plasma torch when the torch operates at a low airflow rate, and even when the flow speed is supersonic, a noticeable microwave effect on the plasma torch is observed. In addition, the present design allows the torch to be operated as both a fuel injector and igniter. Ignition of ethylene fuel injected through the centre of a tungsten carbide tube acting as the central electrode is demonstrated.

A capacitive discharge connected through a dielectric or metal slot to a peripheral grounded region is a configuration of both theoretical and practical interest. The configuration is used in commercial dual frequency capacitive discharges, where a dielectric slot surrounding the substrate separates the main plasma from the peripheral grounded pumping region. Ignition of the peripheral plasma produces effects such as poor matching and relaxation oscillations that are detrimental to processing performance. Discharge models are developed for diffusion and plasma maintenance in the slot, and plasma maintenance in the periphery. The theoretical predictions of ignition conditions as a function of voltage and pressure are compared with experimental results for a driving frequency of 27.12 MHz and a gap spacing of 0.635 cm connecting the two regions, giving good agreement.

In this paper attention is focused on the possibility of obtaining a high production of negative hydrogen ions H⁻ within the volume of a nonequilibrium reflex discharge plasma for gas pressure levels from 0.1 to 10 Pa. To obtain the best values for both the negative hydrogen ion density n₋ and the negative ion fraction n₋/n_e the operating conditions are optimized as a function of pressure, discharge power, and magnetic field. The experiment shows that the reflex discharge steady-state plasma is uniform for a dissipated power P = 400 W and has the best efficiency within the pressure range from 0.5 to 2.0 Pa, where the negative ion density is almost 10¹⁷ m⁻³, and the negative ion fraction n₋/n_e is almost 2.5%.