[en] Localized light emission patterns observed during on time of a high power impulse magnetron sputtering (HiPIMS) discharge on a planar magnetron, known as spokes or ionization zones, have been identified as a potential source of anomalous cross-B field diffusion. In this paper experimental evidence is presented that anomalous diffusion is triggered by the appearance of spokes. The Hall parameter ${\mathrm{\omega <!--\; ??\; -->}}_{\text{ce}}{\mathrm{\tau <!--\; ??\; -->}}_{\text{c}}$, product of the electron cyclotron frequency and the classical collision time, reduces from Bohm diffusion values ($\sim <!--\; ???\; -->16$ and higher) down to the value of 3 as spokes appear, indicating anomalous cross-B field transport. A combination of intensified charge coupled device imaging and electric probe measurements reveals that the ions from the spokes are instantaneously diffusing away from the target. The ion diffusion coefficients calculated from a sideways image of the spoke are six times higher than Bohm diffusion coefficients, which is consistent with the reduction of the Hall parameter. (letter)

[en] A time-resolved analysis of the emission of high power impulse magnetron sputtering (HiPIMS) plasmas reveals inhomogeneities in the form of rotating spokes. The shape of these spokes is very characteristic depending on the target material. The localized enhanced light emission has been correlated with the ion production. Based on these data, the peculiar shape of the emission profiles can be explained by the localized generation of secondary electrons, resulting in an enhanced outward diffusion. This general picture is able to explain the observed emission profile for different target materials including gas rarefaction and second ionization potential of the sputtered elements. (fast track communication)

[en] Reactive plasmas are highly valued for their ability to produce large amounts of reactive radicals and of energetic ions bombarding surrounding surfaces. The non-equilibrium electron driven plasma chemistry is utilized in many applications such as anisotropic etching or deposition of thin films of high-quality materials with unique properties. However, the non-equilibrium character and the high power densities make plasmas very complex and hard to understand. Mass spectrometry (MS) is a very versatile diagnostic method, which has, therefore, a prominent role in the characterization of reactive plasmas. It can access almost all plasma generated species: stable gas-phase products, reactive radicals, positive and negative ions or even internally excited species such as metastables. It can provide absolute densities of neutral particles or energy distribution functions of energetic ions. In particular, plasmas with a rich chemistry, such as hydrocarbon plasmas, could not be understood without MS. This review focuses on quadrupole MS with an electron impact ionization ion source as the most common MS technique applied in plasma analysis. Necessary information for the understanding of this diagnostic and its application and for the proper design and calibration procedure of an MS diagnostic system for quantitative plasma analysis is provided. Important differences between measurements of neutral particles and energetic ions and between the analysis of low pressure and atmospheric pressure plasmas are described and discussed in detail. Moreover, MS-measured ion energy distribution functions in different discharges are discussed and the ability of MS to analyse these distribution functions with time resolution of several microseconds is presented.

[en] The velocity distribution function of titanium neutrals in the target region of a high power impulse magnetron sputtering discharge was investigated by optical emission spectroscopy. A high-resolution plane grating spectrograph combined with a fast, gated, intensified CCD camera was used to study the shape of selected optical emission lines. Doppler broadening and shift were analyzed to gain information about the velocity distribution of sputtered titanium neutrals. The velocity distribution function was found to depend on the discharge power for target power densities up to 0.6 kW cm⁻². Above that value, the velocity distribution was constant. The collision processes of sputtered neutrals close to the target were found to be describable using a modified version of the Krook collisional operator. Using this interpretation, evidence for strong scattering of the titanium neutrals in the target region was found. This scattering can be explained by resonant charge exchange with previously scattered titanium ions. (paper)

[en] High power impulse magnetron sputtering (HIPIMS) is a novel deposition technology successfully implemented on full scale industrial machines. HIPIMS utilizes short pulses of high power delivered to the target in order to generate high amount of metal ions. The life-span of ions between the pulses and their energy distribution could strongly influence the properties and characteristics of the deposited coating. In modern industrial coating machines the sample rotates on a substrate holder and changes its position and distance with regard to the magnetron. Time resolved measurements of the ion energy distribution function (IEDF) at different distances from the magnetron have been performed to investigate the temporal evolution of ions at various distances from target. The measurements were performed using two pressures, 1 and 3 Pa to investigate the influence of working gas pressure on IEDF. Plasma sampling energy-resolved mass spectroscopy was used to measure the IEDF of Ti¹⁺, Ti²⁺, Ar¹⁺, and Ar²⁺ ions in HIPIMS plasma discharge with titanium (Ti) target in Ar atmosphere. The measurements were done over a full pulse period and the distance between the magnetron and the orifice of the mass spectrometer was changed from 25 to 215 mm.

[en] High power impulse magnetron sputtering (HIPIMS) discharges produce metal ions with energies up to 100 eV and the degree of metal ionization can be up to 90%. The ion energy distribution function (IEDF) of ions in HIPIMS of a chromium target in an argon atmosphere was measured using plasma sampling energy resolved mass spectrometry. The time resolved measurements show that in the pulse-on phase the IEDF of Cr¹⁺ comprises a single high energy group of ions, whereas at the end of the pulse, ions are thermalized due to collisions with Ar and the IEDF comprises both a low energy group of ions and a high energy group of ions. Finally, after 320 μs from the start of the pulse only a main low energy peak is present. The IEDF of Ar¹⁺ comprises an intense low energy peak and a high energy shoulder, the latter appearing simultaneously with the high energy Cr ions and probably created due to collisions with the hot metal ions. After the pulse only a low energy group of ions of Ar¹⁺ IEDF was detected. The temporal evolution of the total ion density extracted from Langmuir probe measurements is linked with metal ion-to-gas ion ratios giving an insight into the time evolution of deposition fluxes in the plasma. The influence of the time evolution of IEDF and plasma composition on the thin film growth is discussed.

[en] Reactive high power impulse magnetron sputtering (HiPIMS) of metals is of paramount importance for the deposition of various oxides, nitrides and carbides. The addition of a reactive gas such as nitrogen to an argon HiPIMS plasma with a metal target allows the formation of the corresponding metal nitride on the substrate. The addition of a reactive gas introduces new dynamics into the plasma process, such as hysteresis, target poisoning and the rarefaction of two different plasma gases. We investigate the dynamics for the deposition of chromium nitride by a reactive HiPIMS plasma using energy- and time-resolved ion mass spectrometry, fast camera measurements and temporal and spatially resolved optical emission spectroscopy. It is shown that the addition of nitrogen to the argon plasma gas significantly changes the appearance of the localized ionization zones, the so-called spokes, in HiPIMS plasmas. In addition, a very strong modulation of the metal ion flux within each HiPIMS pulse is observed, with the metal ion flux being strongly suppressed and the nitrogen molecular ion flux being strongly enhanced in the high current phase of the pulse. This behavior is explained by a stronger return effect of the sputtered metal ions in the dense plasma above the racetrack. This is best observed in a pure nitrogen plasma, because the ionization zones are mostly confined, implying a very high local plasma density and consequently also an efficient scattering process. (paper)

[en] The rotation of localised ionisation zones, i.e. spokes, in magnetron discharge are frequently observed. The spokes are investigated by measuring floating potential oscillations with 12 flat probes placed azimuthally around a planar circular magnetron. The 12-probe setup provides sufficient temporal and spatial resolution to observe the properties of various spokes, such as rotation direction, mode number and angular velocity. The spokes are investigated as a function of discharge current, ranging from 10 mA (current density 0.5 mA cm⁻²) to 140 A (7 A cm⁻²). In the range from 10 mA to 600 mA the plasma was sustained in DC mode, and in the range from 1 A to 140 A the plasma was pulsed in high-power impulse magnetron sputtering mode. The presence of spokes throughout the complete discharge current range indicates that the spokes are an intrinsic property of a magnetron sputtering plasma discharge. The spokes may disappear at discharge currents above 80 A for Cr, as the plasma becomes homogeneously distributed over the racetrack. Up to discharge currents of several amperes (the exact value depends on the target material), the spokes rotate in a retrograde $\mathbf{E}\times <!--\; ??\; -->\mathbf{B}$ direction with angular velocity in the range of 0.2–4 km s⁻¹. Beyond a discharge current of several amperes, the spokes rotate in a $\mathbf{E}\times <!--\; ??\; -->\mathbf{B}$ direction with angular velocity in the range of 5–15 km s⁻¹. The spoke rotation reversal is explained by a transition from Ar-dominated to metal-dominated sputtering that shifts the plasma emission zone closer to the target. The spoke itself corresponds to a region of high electron density and therefore to a hump in the electrical potential. The electric field around the spoke dominates the spoke rotation direction. At low power, the plasma is further away from the target and it is dominated by the electric field to the anode, thus retrograde $\mathbf{E}\times <!--\; ??\; -->\mathbf{B}$ rotation. At high power, the plasma is closer to the target and it is dominated by the electric field pointing to the target, thus $\mathbf{E}\times <!--\; ??\; -->\mathbf{B}$ rotation. (paper)

[en] Spokes, localised light emission patterns appearing during the HiPIMS discharge, are investigated using optical emission diagnostic and electrical probes. Both, temporal and spatial aspects of the spoke phenomenon have been investigated. The evolution from stochastic to self-organised patterns consisting of a finite number of spokes along the racetrack (= spoke mode number), the transition between two spoke modes, and the stability of a spoke mode is presented. For a constant discharge current, spokes steadily rotate above the magnetron racetrack. The transition between two spoke modes is described by a model which links the spoke mode number decrease with increasing discharge current. It is based on the interaction between the current in the spoke, the Ar rarefaction and Ar replenishment times that prevail over electrostatic repulsion and charged particle diffusion. Two spokes merge by an acceleration of the trailing spoke with respect to the $\mathbf{E}\times <!--\; ??\; -->\mathbf{B}$ direction of rotation. (paper)

[en] Microwave plasmas are a promising technology for energy-efficient CO₂ valorization via conversion of CO₂ into CO and O₂ using renewable energies. A 2.45 GHz microwave plasma torch with swirling CO₂ gas flow is studied in a large pressure (20–1000 mbar) and flow (1–100 L min⁻¹) range. Two different modes of the plasma torch, depending on the operating pressure and microwave input power, are described: at pressures below 120 mbar the plasma fills most of the plasma torch volume whereas at pressures of about 120 mbar an abrupt contraction of the plasma in the center of the resonator is observed along with an increase of the gas temperature from 3000 K to 6000 K. The CO outflow is generally found to be proportional to the plasma effective power and exhibits no significant dependence on the actual CO₂ flow injected into the reactor but only on the input power at certain pressure. Thermal dissociation calculations show that, even at the lowest pressures of this study, the observed conversion and energy efficiency are compatible with a thermal dissociation mechanism. (paper)