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Ever since they have been studied, gas discharges have been classified by their visual appearance as well as by their current and voltage levels. Glow and arc discharges are the most prominent and well-known modes of discharges involving electrodes. In a first approximation, they are distinguished by their current and voltage levels, and current–voltage characteristics are a common way to display their relations. In this review, glow discharges are defined by their individual electron emission mechanism such as secondary electron emission by photons and primary ions, and arcs by their respective collective mechanism such as thermionic or explosive electron emission. Emitted electrons are accelerated in the cathode sheath and play an important role in sustaining the discharge plasma. In some cases, however, electron emission is not important for sustaining the plasma, and consequently we have neither a glow nor an arc discharge but a third type of discharge, the ohmic discharge. In part 1 of this review, these relationships are explained for quasi-stationary discharges, culminating with updated graphical presentations of I–V characteristics (Figs. 15 and 16). In part 2, further examples are reviewed to include time-dependent discharges, discharges with electron trapping (hollow cathode, E×B discharges) and active anode effects.

Deposition of epitaxial oxide semiconductor films using physical vapor deposition methods requires a detailed understanding of the role of energetic particles to control and optimize the film properties. In the present study, Ga 2 O 3 thin films are heteroepitaxially grown on Al 2 O 3 (0001) substrates using oxygen ion beam sputter deposition. The influence of the following relevant process parameters on the properties of the thin films is investigated: substrate temperature, oxygen background pressure, energy of primary ions, ion beam current, and sputtering geometry. The kinetic energy distributions of ions in the film-forming flux are measured using an energy-selective mass spectrometer, and the resulting films are characterized regarding crystalline structure, microstructure, surface roughness, mass density, and growth rate. The energetic impact of film-forming particles on the thin film structure is analyzed, and a noticeable decrease in crystalline quality is observed above the average energy of film-forming Ga + ions around 40 eV for the films grown at a substrate temperature of 725 ° C.

Pulsed filtered cathodic arc deposition involves formation of energetic multiply charged metal ions, which help to form dense, adherent, and macroparticle-free thin films. Ions possess not only significant kinetic energy, but also potential energy primarily due to their charge, which for cathodic arc plasmas is usually greater than one. While the effects of kinetic ion energy on the growing film are well investigated, the effects of the ions’ potential energy are less known. In the present work, we make a step toward decoupling the contributions of kinetic and potential energies of ions on thin film formation. The potential energy is changed by enhancing the ion charge states via using an external magnetic field at the plasma source. The kinetic energy is adjusted by biasing the arc source (“plasma bias”), which allows us to approximately compensate the differences in kinetic energy, while the substrate and ion energy detector remain at ground. However, application of an external magnetic field also leads to an enhancement of the ion flux and hence the desired complete decoupling of the potential and kinetic energy effects will require further steps. Charge-state-resolved energy distribution functions of ions are measured at the substrate position for different arc source configurations, and thin films are deposited using exactly those configurations. Detailed characterization of the deposited thin films is performed to reveal the correlations of changes in structure with kinetic and potential energies of multiply charged ions. It is observed that the cathode composition (Al:V ratio) strongly affects the formation of the thermodynamically stable wurtzite or the metastable cubic phase. The external magnetic field applied at the arc source is found to greatly alter the plasma and, therefore, to be the primary, easily accessible “tuning knob” to enhance film crystallinity. The effect of “atomic scale heating” provided by the ions’ kinetic and potential energies on the film crystallinity is investigated, and the possibility to deposit crystalline (V,Al)N films without substrate heating is demonstrated. This study shows an approach toward distinguishing the contributions stemming from kinetic and potential energies of ions on the film growth, however, further research is needed to assess and distinguish the additional effect of ion flux intensity (current).

Iron oxide nanoparticles with a mean size of approximately 5 nm were synthesized by irradiating micro-emulsions containing iron salts with energetic electrons. The properties of the nanoparticles were investigated using scanning electron microscopy, high-resolution transmission electron microscopy, selective area diffraction and vibrating sample magnetometry. It was found that formation of superparamagnetic nanoparticles begins at a dose of 50 kGy, though these particles show low crystallinity, and a higher portion is amorphous. With increasing doses, an increasing crystallinity and yield could be observed, which is reflected in an increasing saturation magnetization. The blocking temperature and effective anisotropy constant were determined via zero-field cooling and field cooling measurements. The particles tend to form clusters with a size of 34 nm to 73 nm. Magnetite/maghemite nanoparticles could be identified via selective area electron diffraction patterns. Additionally, goethite nanowires could be observed.

Deposition of epitaxial oxide semiconductor films using physical vapor deposition methods requires a detailed understanding of the role of energetic particles to control and optimize the film properties. In the present study, Ga 2 O 3 thin films are heteroepitaxially grown on Al 2 O 3 (0001) substrates using oxygen ion beam sputter deposition. The influence of the following relevant process parameters on the properties of the thin films is investigated: substrate temperature, oxygen background pressure, energy of primary ions, ion beam current, and sputtering geometry. The kinetic energy distributions of ions in the film-forming flux are measured using an energy-selective mass spectrometer, and the resulting films are characterized regarding crystalline structure, microstructure, surface roughness, mass density, and growth rate. The energetic impact of film-forming particles on the thin film structure is analyzed, and a noticeable decrease in crystalline quality is observed above the average energy of film-forming Ga + ions around 40 eV for the films grown at a substrate temperature of 725 ° C. [ABSTRACT FROM AUTHOR]

Gallium oxide thin films were grown by ion beam sputter deposition (IBSD) at room temperature on Si substrates with systematically varied process parameters: primary ion energy, primary ion species (O 2 + and Ar +), sputtering geometry (ion incidence angle α and polar emission angle β), and O 2 background pressure. No substrate heating was applied because the goal of these experiments was to investigate the impact of the energetic film-forming species on thin film properties. The films were characterized with regard to film thickness, growth rate, crystallinity, surface roughness, mass density, elemental composition and its depth profiles, and optical properties. All films were found to be amorphous with a surface roughness of less than 1 nm. The stoichiometry of the films improved with an increase in the energy of film-forming species. The mass density and the optical properties, including the index of refraction, are correlated and show a dependency on the kinetic energy of the film-forming species. The ranges of IBSD parameters, which are most promising for further improvement of the film quality, are discussed.

High-temperature superconductors such as REBa2Cu3O7 - δ (REBCO, RE = rare earth) enable high-current cables and high-field magnets. By removing the turn-to-turn insulation in a magnet application, recent experiments demonstrated that REBCO magnets can self-protect against catastrophic damage during a superconducting-to-normal transition (quench), i.e., when the stored magnetic energy rapidly converts to heat. The current can bypass the hot spot during a quench, thereby reducing the localized heat dissipation. The removal of the insulation between turns, however, leads to excessive eddy currents during current ramping, thereby forcing a much-prolonged magnet charging time. To address this issue, we investigate vanadium oxide (VOx) coatings as a temperature-dependent self-switching medium that automatically manages current sharing. VOx coatings (with 1.70 ≤ × ≤ to 2.07) were deposited by reactive cathodic arc deposition, initially on insulating glass to determine the electrical properties, and later on commercial REBCO tapes. The coatings are x-ray amorphous but with a short-range crystalline ordering according to Raman spectrometry. The resistivity of VOx decreased by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process is compatible with commercial REBCO tapes as evidenced by the negligible change in the critical current caused by the coating process. The results from current sharing experiments and circuit analysis suggest that the VOx coating can effectively self-regulate current sharing in REBCO magnets, suppress excessive eddy currents, and enable self-protection during quenches.

Cathodic arcs have been utilized in various applications including the deposition of thin films and coatings, ion implantation, and high current switching. Despite substantial progress in recent decades, the physical mechanisms responsible for the observed plasma properties are still a matter of dispute, particularly for multi-element cathodes, which can play an essential role in applications. The analysis of plasma properties is complicated by the generally occurring neutral background of metal atoms, which perturbs initial ion properties. By using a time-resolved method in combination with pulsed arcs and a comprehensive Nb-Al cathode model system, we investigate the influence of cathode composition on the plasma, while making the influence of neutrals visible for the observed time frame. The results visualize ion detections of 600 μs plasma pulses, extracted 0.27 m from the cathode, resolved in mass-per-charge, energy-per-charge and time. Ion properties are found to be strongly dependent on the cathode material in a way that cannot be deduced by simple linear extrapolation. Subsequently, current hypotheses in cathodic arc physics applying to multi-element cathodes, like the so-called 'velocity rule' or the 'cohesive energy rule', are tested for early and late stages of the pulse. Apart from their fundamental character, the findings could be useful in optimizing or designing plasma properties for applications, by actively utilizing effects on ion distributions caused by composite cathode materials and charge exchange with neutrals.

Ionization zones, also known as spokes, are plasma instabilities manifested as locations of intensified excitation and ionization over a sputtering magnetron's racetrack. Using a linear magnetron and a streak camera, we were able to observe and quantify spoke dynamics. The technique allows us to image the onset and changes for both direct current magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS). Spokes in dcMS exhibit substructures. Spokes in HiPIMS are not stable as they shift along the racetrack; rather, they tend to grow or diminish, and they may split and merge. Their evolution can be interpreted in the context of localized electric fields and associated electron heating.

High Power Impulse Magnetron Sputtering (HiPIMS) is a coating technology that combines magnetron sputtering with pulsed power concepts. By applying power in pulses of high amplitude and a relatively low duty cycle, large fractions of sputtered atoms and near-target gases are ionized. In contrast to conventional magnetron sputtering, HiPIMS is characterized by self-sputtering or repeated gas recycling for high and low sputter yield materials, respectively, and both for most intermediate materials. The dense plasma in front of the target has the dual function of sustaining the discharge and providing plasma-assistance to film growth, affecting the microstructure of growing films. Many technologically interesting thin films are compound films, which are composed of one or more metals and a reactive gas, most often oxygen or nitrogen. When reactive gas is added, non-trivial consequences arise for the system because the target may become “poisoned,” i.e., a compound layer forms on the target surface affecting the sputtering yield and the yield of secondary electron emission and thereby all other parameters. It is emphasized that the target state depends not only on the reactive gas' partial pressure (balanced via gas flow and pumping) but also on the ion flux to the target, which can be controlled by pulse parameters. This is a critical technological opportunity for reactive HiPIMS (R-HiPIMS). The scope of this tutorial is focused on plasma processes and mechanisms of operation and only briefly touches upon film properties. It introduces R-HiPIMS in a systematic, step-by-step approach by covering sputtering, magnetron sputtering, reactive magnetron sputtering, pulsed reactive magnetron sputtering, HiPIMS, and finally R-HiPIMS. The tutorial is concluded by considering variations of R-HiPIMS known as modulated pulsed power magnetron sputtering and deep-oscillation magnetron sputtering and combinations of R-HiPIMS with superimposed dc magnetron sputtering.

Boron at room temperature is insulating and therefore conventionally sputtered using radio-frequency power supplies including their power-matching networks. In this contribution, we show that through a suitable ignition assistance, via temporary application of a high voltage (∼600 V) to the substrate holder or auxiliary electrode, the magnetron discharge can be ignited using a conventional mid-frequency power supply without matching network. Once the discharge is ignited, the assisting voltage can be reduced to less than 50 V, and after the boron target surface is at elevated temperature, thereby exhibiting sufficient conductivity, the assisting voltage can be turned off. The deposition of boron and boron nitride films has been demonstrated with a deposition rate of approximately 400 nm/h for a power of 250 W.

The effect of energetic ion bombardment on the properties of tantalum thin films was investigated. To achieve such energetic ion bombardment during the process the Ta thin films were deposited by deep oscillation magnetron sputtering (DOMS), an ionized physical vapor deposition technique related to high power impulse magnetron sputtering. The peak power was between 49 and 130 kW and the substrate was silicon at room temperature and ground potential. The directionality and the energy of the depositing species was controlled by changing the ionization fraction of the Ta species arriving at the substrate at different peak powers. In this work, the surface morphology (AFM), microstructure (SEM), structure (XRD) and hardness and Young's modulus (nanoindentation) of the films were characterized. The ion energy distributions (IEDs) were measured using an electrostatic quadrupole ion energy and mass spectrometer (HIDEN EQP 300). The IEDs showed that the DOMS process applies a very energetic (up to 120 eV) ion bombardment on the growing tantalum films. Therefore, with such conditions it was possible to deposit pure α-Ta (of 2 μm of thickness) without the use of additional equipment, i.e., without substrate bias or substrate heating. Conditions are therefore significantly different than in previous works, offering a much simpler and cheaper solution to up-scale for industrial operation.

Using movable emissive and floating probes, we determined the plasma and floating potentials of an ionization zone (spoke) in a direct current magnetron sputtering discharge. Measurements were recorded in a space and time resolved manner, which allowed us to make a three-dimensional representation of the plasma potential. From this information we could derive the related electric field, space charge, and the related spatial distribution of electron heating. The data reveal the existence of strong electric fields parallel and perpendicular to the target surface. The largest E-fields result from a double layer structure at the leading edge of the ionization zone. We suggest that the double layer plays a crucial role in the energization of electrons since electrons can gain several 10 eV of energy when crossing the double layer. We find sustained coupling between the potential structure, electron heating, and excitation and ionization processes as electrons drift over the magnetron target. The brightest region of an ionization zone is present right after the potential jump, where drifting electrons arrive and where most local electron heating occurs. The ionization zone intensity decays as electrons continue to drift in the Ez × B direction, losing energy by inelastic collisions; electrons become energized again as they cross the potential jump. This results in the elongated, arrowhead-like shape of the ionization zone. The ionization zone moves in the -Ez × B direction from which the to-be-heated electrons arrive and into which the heating region expands; the zone motion is dictated by the force of the local electric field on the ions at the leading edge of the ionization zone. We hypothesize that electron heating caused by the potential jump and physical processes associated with the double layer also apply to magnetrons at higher discharge power, including high power impulse magnetron sputtering.

The ion energies and fluxes in the high power impulse magnetron sputtering plasma from a Nb target were analysed angularly resolved along the tangential direction of the racetrack. A reactive oxygen-containing atmosphere was used as such discharge conditions are typically employed for the synthesis of thin films. Asymmetries in the flux distribution of the recorded ions as well as their energies and charge states were noticed when varying the angle between mass-energy analyser and target surface. More positively charged ions with higher count rates in the medium energy range of their distributions were detected in +EB than in -EB direction, thus confirming the notion that ionisation zones (also known as spokes or plasma bunches) are associated with moving potential humps. The motion of the recorded negatively charged high-energy oxygen ions was unaffected. NbOxthin films at different angles and positions were synthesised and analysed as to their structure and properties in order to correlate the observed plasma properties to the film growth conditions. The chemical composition and the film thickness varied with changing deposition angle, where the latter, similar to the ion fluxes, was higher in +EB than in -EB direction.

We present evidence for breathing modes in magnetron sputtering plasmas: periodic axial variations of plasma parameters with characteristic frequencies between 10 and 100 kHz. A set of azimuthally distributed probes shows synchronous oscillations of the floating potential. They appear most clearly when considering the intermediate current regime in which the direction of azimuthal spoke motion changes. Breathing oscillations were found to be superimposed on azimuthal spoke motion. Depending on pressure and current, one can also find a regime of chaotic fluctuations and one of stable discharges, the latter at high current. A pressure-current phase diagram for the different situations is proposed.

Ultrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green (532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth, surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu 2 O to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (< 1000 J/cm 2), analogous results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7. Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a reduction of the SEY maximum below unity at doses of 5×10-4 C/mm 2. Consequently, optimization of processing parameters for application in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between processing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of selected magnets of the Large Hadron Collider or of future colliders. [ABSTRACT FROM AUTHOR]

Past research has revealed the propagation of dense, asymmetric ionization zones in both high and low current magnetron discharges. Here we report about the direction reversal of ionization zone propagation as observed with fast cameras. At high currents, zones move in the E B direction with velocities of 103 to 104 m/s. However at lower currents, ionization zones are observed to move in the opposite, the -E B direction, with velocities ~; 103 m/s. It is proposed that the direction reversal is associated with the local balance of ionization and supply of neutrals in the ionization zone.

10.1063/5.0088430.1 [ABSTRACT FROM AUTHOR]

The magnetic field of a magnetron serves to increase the residence time of electrons in the ionization region and thereby enables the discharge to be sustained at low working gas pressures. This hinders the electrons to reach the anode which is necessary to close the electrical circuit. At high atom densities in the ionization region, and in the presence of an electric field, collisions of electrons with heavy species consecutively push electrons across the magnetic field lines, which is known as the classical cross-field transport mechanism. At low atom densities in the ionization region, collisions are rare and the classical cross-field transport mechanism is insufficient to carry the discharge current. This gives rise to plasma instabilities, called spokes, that locally provide pathways for electrons to escape from the near-target region and across the magnetic field lines. Here, we show experimentally, for the case of a high power impulse magnetron sputtering discharge with an aluminum target, how spokes gradually disappear with the increase in local gas density. We present an analytical model that shows that under these high gas density conditions, the classical electron transport mechanism is indeed strong enough to solely carry the discharge current. This highlights the importance of the local gas density in the ionization region for the intensity of spokes in a magnetron sputtering discharge and suggests ways for process optimization. [ABSTRACT FROM AUTHOR]

Cathode spots in a magnetically steered arc source were studied under low-pressure noble gas (Ar) and reactive gas (N2, O2) atmospheres. The plasma was observed using a streak camera coupled with a long-distance microscope to study the evolution of cathode spots with high temporal and spatial resolution. We find two well-known types of cathode spots: "type 1" for less bright spots eroding the compound layer on the cathode surface and bright "type 2" spots on (clean) metallic surfaces. Cathode spots are characterized by a sequence of microexplosions that give the impression of a moving spot, which, in the presence of a magnetic field, is generally in the retrograde direction. However, the apparent displacement can also go in the opposite, the Amperian direction, especially when nitrogen is present. In oxygen, spot ignition often happens in approximately the same location repeatedly. For type 2 spots, we detected an apparent motion mainly in the retrograde direction with distinct jumps to new locations. Via the effects of spot appearance, we note the competing effects of cathode cleaning by spot-induced material removal (erosion) and compound formation in the presence of reactive gas. The streak images were analyzed by fast Fourier transformation, and we found that the arc fluctuations are stochastic without specific frequencies. The colored random noise (CRN) index tends to be reduced in the presence of a compound layer, indicating an enhanced spot ignition probability. A reduced CRN index implies reduced feedback (influence) of previously active spots, which is most apparent in the presence of elevated oxygen pressure. [ABSTRACT FROM AUTHOR]