Table of contents

The physical and chemical aspects of plasma–surface interaction in high-power impulse magnetron sputtering (HiPIMS) discharges are overviewed. The data obtained by various plasma diagnostic methods representing the important sputtering discharge regions, namely the cathode vicinity, plasma bulk, and substrate vicinity, are reported. After a detailed introduction to the problem and description of the plasma characterization methods suitable for pulsed magnetron discharge analysis, an overview of the recent plasma diagnostics achievements in both non-reactive and reactive HiPIMS discharges is presented. Finally, the conclusions and perspectives suggesting possible directions and research strategies for increasing our knowledge in this domain are given.

Experimental studies based on mass-selected ion beams are reviewed as a means to examine plasma–surface interactions for industrial plasma processing. Plasma etching and deposition processes widely used in the microelectronics industry exploit surface chemical reactions induced by plasmas. Such reactions typically result from a conglomeration of complex interactions of the surface material with incident free radicals and ions. While it is in general difficult to analyse individual surface reactions separately in a plasma system, ion and/or charge-neutral beam experiments allow one to analyse specific surface reactions involving only selected gaseous species. In this review, experiments on beam–surface interactions are discussed in detail and beam reaction data for silicon surfaces are presented as sample data.

This paper introduces an integral approach to the study of plasma–surface interactions during the catalytic growth of selected nanostructures (NSs). This approach involves basic understanding of the plasma-specific effects in NS nucleation and growth, theoretical modelling, numerical simulations, plasma diagnostics, and surface microanalysis. Using an example of plasma-assisted growth of surface-supported single-walled carbon nanotubes, we discuss how the combination of these techniques may help improve the outcomes of the growth process. A specific focus here is on the effects of nanoscale plasma–surface interactions on the NS growth and how the available techniques may be used, both in situ and ex situ to optimize the growth process and structural parameters of NSs.

Plasma catalysis holds great promise for environmental applications, provided that the process viability can be maximized in terms of energy efficiency and product selectivity. This requires a fundamental understanding of the various processes taking place and especially the mutual interactions between plasma and catalyst. In this review, we therefore first examine the various effects of the plasma on the catalyst and of the catalyst on the plasma that have been described in the literature. Most of these studies are purely experimental. The urgently needed fundamental understanding of the mechanisms underpinning plasma catalysis, however, may also be obtained through modelling and simulation. Therefore, we also provide here an overview of the modelling efforts that have been developed already, on both the atomistic and the macroscale, and we identify the data that can be obtained with these models to illustrate how modelling and simulation may contribute to this field. Last but not least, we also identify future modelling opportunities to obtain a more complete understanding of the various underlying plasma catalytic effects, which is needed to provide a comprehensive picture of plasma catalysis.

The first part of the review summarizes the problem of air pollution and related air-cleaning technologies. Volatile organic compounds in particular have various effects on health and their abatement is a key issue. Different ways to couple non-thermal plasmas with catalytic or adsorbing materials are listed. In particular, a comparison between in-plasma and post-plasma coupling is made. Studies dealing with plasma-induced heterogeneous reactivity are analysed, as well as the possible modifications of the catalyst surface under plasma exposure. As an alternative to the conventional and widely studied plasma–catalyst coupling, a sequential approach has been recently proposed whereby pollutants are first adsorbed onto the material, then oxidized by switching on the plasma. Such a sequential approach is reviewed in detail.

Discharge–surface interaction in liquids includes many phenomena which are reviewed in this work. This is used to examine results in the area of nanoparticle synthesis and to propose a general sketch of formation mechanisms.

The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten.

High power impulse magnetron sputtering (HiPIMS) plasmas generate energetic metal ions at the substrate as a major difference to conventional direct current magnetron sputtering (dcMS). The origin of these very energetic ions in HiPIMS is still an open issue, which is unravelled using two fast diagnostics: time-resolved mass spectrometry with a temporal resolution of 2 µs and phase resolved optical emission spectroscopy with a temporal resolution of 1 µs. A power scan from dcMS-like to HiPIMS plasmas was performed, with a 2 inch magnetron and a titanium target as sputter source and argon as working gas. Clear differences in the transport as well as the energetic properties of Ar⁺, Ar²⁺, Ti⁺ and Ti²⁺ were observed. For discharges with highest peak power densities a high energetic group of Ti⁺ and Ti²⁺ could be identified with energies of approximately 25 eV and of 50 eV, respectively. A cold group of ions was always present. It is found that hot ions are observed only when the plasma enters the spokes regime, which can be monitored by oscillations in the IV characteristics in the MHz range that are picked up by the used VI probes. These oscillations are correlated with the spokes phenomenon and are explained as an amplification of the Hall current inside the spokes as hot ionization zones. To explain the presence of energetic ions, we propose a double layer (DL) confining the hot plasma inside a spoke: if an atom becomes ionized inside the spokes region it is accelerated because of the DL to higher energies whereas its energy remains unchanged if it is ionized outside. In applying this DL model to our measurements the observed phenomena as well as several measurements from other groups can be explained. Only if spokes and a DL are present can the confined particles gain enough energy to leave the magnetic trap. We conclude from our findings that the spoke phenomenon represents the essence of HiPIMS plasmas, explaining their good performance for material synthesis applications.

The role of plasma–surface interactions during microcrystalline silicon thin film growth has been studied under low power–low pressure conditions, and a correlation between the plasma–surface interactions and material properties has been provided. The atomic hydrogen flux, inferred by optical emission spectroscopy measurements, has been investigated. The hydrogen-to-silicon growth flux resulted in a ratio of 25 : 140 for the amorphous-to-mixed phase transition, whereas it increased to 40 : 170 for the mixed phase-to-microcrystalline phase transition. The ion contribution to the plasma–surface interaction has also been investigated. For this purpose, a capacitive probe has been implemented locally for direct measurement of the ion flux. For the amorphous-to-microcrystalline phase transition the ion-to-silicon growth flux ratio has been determined to increase from 0.15 to 1, two orders of magnitude smaller than the atomic hydrogen-to-silicon growth flux ratio. The ion energy has been measured with a retarding field energy analyser, which allowed determination of the ion-energy distribution as a function of the silane flow rate. Average ion energies of 15–20 eV have been found for a decreasing silane flow rate, thereby indicating the limited energy transfer to the surface. The main effect caused by the ion arrival at the surface is a locally induced thermal spike enhancing the radical surface diffusion. No prominent detrimental effect, i.e. Si surface or bulk displacement requiring energies greater than 18 eV and 40 eV respectively, or sputtering occurring above 50 eV, has been observed.

Carrying out molecular dynamics (MD) simulations is a relevant way to understand growth phenomena at the atomic scale. Initial conditions are defined for reproducing deposition conditions of plasma sputtering experiments. Two case studies are developed to highlight the implementation of MD simulations in the context of plasma sputtering deposition: Zr_xCu_1−x metallic glass and AlCoCrCuFeNi high entropy alloy thin films deposited onto silicon. Effects of depositing atom kinetic energies and atomic composition are studied in order to predict the evolution of morphologies and atomic structure of MD grown thin films. Experimental and simulated x-ray diffraction patterns are compared.

The effect of surface reactions of O, O₃ and N radicals during the growth of silica-like (SiO_xC_yH_z) films on film properties is investigated. A SiO_xC_yH_z film is deposited from a He/Hexamethyldisiloxan (HMDSO) cold atmospheric plasma on a rotating substrate. The surface of this film is, during the growth, treated on the opposite site of the substrate by a second cold atmospheric plasma with helium and an addition of O₂ or N₂. A reactor with four separated cells and gas curtains between them is used to avoid cross-contamination of the ambient atmosphere in each cell. The changes in film composition after the deposition with and without a treatment by O, O₃ and N are investigated by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. Additionally, the effect of each species on the deposition rate is also presented and discussed.

In recent years, atmospheric pressure plasma-enhanced chemical vapour deposition has been identified as a convenient way to deposit good quality thin films. With this type of process, where the gas mixture is injected on one side of the electrodes, the chemical composition of the gas evolves with the gas residence time in the plasma. The consequence is a possible gradient in the chemical composition over the thickness of in-line coatings. The present work shows that the modulation of the plasma with a square signal significantly reduces this gradient while the drawback of low growth rate is avoided by increasing the discharge power. This study deals with plane/plane glow dielectric barrier discharges (DBDs) in an Ar/NH₃/SiH₄ gas mixture to make thin films. The 50 kHz discharge power of the glow DBD was varied by increasing voltage and modulating excitation. The impact on (i) the plasma development was observed through emission spectroscopy and (ii) the thin film coating through Fourier transform infrared measurements. It is shown that the modulation significantly decreases the time and the energy needed to achieve stable chemistry, enhances secondary chemistry and limits disturbance induced by impurities because of a slower decrease of SiH₄ concentration and thus a higher ratio of SiH₄/impurities, all very important points for in-line AP-PECVD development. When the growth rate is limited by diffusion, coating growth continues when the discharge is off, so long as there is a precursor gradient between the surface and the gas bulk. A higher discharge power steepens this gradient, which enhances diffusion from the bulk and thus growth rate.

High density polyethylene surfaces were exposed to the atmospheric post-discharge of a radiofrequency plasma torch supplied in helium and oxygen. Dynamic water contact angle measurements were performed to evaluate changes in surface hydrophilicity and angle resolved x-ray photoelectron spectroscopy was carried out to identify the functional groups responsible for wettability changes and to study their subsurface depth profiles, up to 9 nm in depth. The reactions leading to the formation of C–O, C = O and O–C = O groups were simulated by molecular dynamics. These simulations demonstrate that impinging oxygen atoms do not react immediately upon impact but rather remain at or close to the surface before eventually reacting. The simulations also explain the release of gaseous species in the ambient environment as well as the ejection of low molecular weight oxidized materials from the surface.

The formation of NO₂ molecules on a Pyrex surface, as a result of NO oxidation by adsorbed O atoms on the wall, is experimentally demonstrated and quantified. The measurements reveal that the characteristic times of heterogeneous NO₂ production and NO gas phase decay change from ∼60 to ∼1500 s as the gas phase concentration of NO introduced in a tube pretreated with an oxygen radiofrequency discharge increases from 10¹³ to 10¹⁵ cm⁻³. Moreover, a clear variation of the characteristic loss frequency of NO molecules when small amounts of NO are successively injected in the tube is detected, between ∼7 × 10⁻² and ∼5 × 10⁻³ s⁻¹. The complex surface kinetics observed is studied and interpreted with the help of a mesoscopic surface model accounting for Eley–Rideal NO oxidation and slow NO₂ adsorption, confirming the existence of adsorption sites possessing a distribution of reactivity on the surface.

A molecular beam mass spectrometer has been calibrated and used to measure the air entrainment, nitric oxide and ozone concentrations in the effluent of a cold atmospheric pressure argon RF driven plasma jet. The approaches for calibrating the mass spectrometer for different species are described in detail. Gas phase densities of ozone and nitric oxide up to 7.5 ppm and 4 ppm, respectively, have been measured in the far effluent of the argon plasma jet. The difference in air entrainment when the plasma is undisturbed or is close to a well, which is the case for e.g. in vitro plasma–cell interaction studies, is shown. In addition, an exponential decay of the positive ion flux as a function of distance in the effluent is obtained. Furthermore, the effect of plasma power, duty cycle and air and O₂ admixtures introduced into the argon flow on the NO and O₃ production is presented, including the possibility of independent control of the NO and O₃ flux from the jet.

Non-thermal plasmas of atmospheric pressure air direct current corona discharges were investigated for potential applications in dental medicine. The objective of this ex vivo study was to apply cold plasmas for the decontamination of Streptococci biofilm grown on extracted human teeth, and to estimate their antimicrobial efficiency and the plasma's impact on the enamel and dentine of the treated tooth surfaces. The results show that both positive streamer and negative Trichel pulse coronas can reduce bacterial population in the biofilm by up to 3 logs in a 10 min exposure time. This bactericidal effect can be reached faster (within 5 min) by electrostatic spraying of water through the discharge onto the treated tooth surface. Examination of the tooth surface after plasma exposure by infrared spectroscopy and scanning electron microscopy did not show any significant alteration in the tooth material composition or the tooth surface structures.

This paper reports the influence of gas plasma flux on endotoxin lipid A film deactivation. To study the effect of the flux magnitude of reactive species, a modified low-pressure inductively coupled plasma (ICP) with O radical flux ∼10¹⁶ cm⁻² s⁻¹ was used. After ICP exposures, it was observed that while the Fourier transform infrared absorbance of fatty chains responsible for the toxicity drops by 80% through the film, no obvious film endotoxin deactivation is seen. This is in contrast to that previously observed under low flux exposure conducted in a vacuum beam system: near-surface only loss of fatty chains led to significant film deactivation. Secondary ion mass spectrometry characterization of changes at the film surface did not appear to correlate with the degree of deactivation. Lipid A films need to be nearly completely removed in order to detect significant deactivation under high flux conditions. Additional high reactive species flux experiments were conducted using an atmospheric pressure helium plasma jet and a UV/ozone device. Exposure of lipid A films to reactive species with these devices showed similar deactivation behaviour. The causes for the difference between low and high flux exposures may be due to the nature of near-surface structural modifications as a function of the rate of film removal.

A positive discharge in water is generated by applying a 30 ns high-voltage (HV) pulse on a micrometre scale electrode. The applied voltage ranges from 6 to 15 kV and a fast plasma propagating mode is launched with a velocity of up to 60 km s⁻¹. Time-resolved shadowgraphy and spectroscopy are performed to monitor the time evolution of the discharge structure and of the plasma emission spectra. By analysing the dynamics of the shock front velocity and the lateral expansion of the plasma channel, it is possible to estimate the pressure at the ignition of the plasma by two independent methods: very good agreement is found at 6 kV giving initial pressures of 0.4 GPa and 0.3 GPa, respectively. At 15 kV, only the shock front velocity method is applicable under our experimental conditions, giving an estimate of the initial pressure of 5.8 GPa. Such high initial pressures show that, under a nanosecond HV pulse, the plasma is ignited directly in the dense phase. Emission spectra show a strong continuum emission as well as a broad Balmer α line with a strong red shift, with an estimate of the initial plasma density of 1.3 × 10²⁶ m⁻³. The relaxation of discharge pressure and plasma density is studied under a series of six successive pulses.