Prof. Dr. Achim von Keudell

Experimental Physics
Ruhr-Universität Bochum

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  • Dedicated setup to isolate plasma catalysis mechanisms
    Stewig, C. and Urbanietz, T. and Chauvet, L. and Böke, M. and Von Keudell, A.
    Journal of Physics D: Applied Physics 54 (2021)
    Plasma catalysis, the combination of plasma and catalysis, is used to achieve efficient molecule conversion, supporting the flexibility of operating parameters and feed gases. By combining plasmas with conventional thermal catalysis, the temperature windows may be changed and the process may be made insensitive to catalyst poisoning. However, understanding plasma catalysis mechanisms is extremely difficult, due to the strong coupling between plasma, gas-phase chemistry and surface. A multitude of reaction pathways may be enhanced or reduced by the presence of a plasma that provides excited species as reaction partners. We developed a robust setup to analyse those processes, based on a parallel-plate atmospheric-pressure plasma jet that allows a plug flow design. The plasma chemistry is analysed by Fourier transform infrared absorption spectroscopy and mass spectrometry. The electrodes in contact with the plasma are temperature controlled and can easily be replaced to apply a catalyst on top of them. The basic characteristics of the setup are discussed and three examples for its application are given: (a) the analysis of methane oxidation using the plug flow scheme; (b) the plasma catalytic conversion of CO2, and (c) the plasma catalytic conversion of methane in methane–oxygen mixtures. © 2021 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/abd65b
  • Determining Chemical Reaction Systems in Plasma-Assisted Conversion of Methane Using Genetic Algorithms
    Reiser, D. and von Keudell, A. and Urbanietz, T.
    Plasma Chemistry and Plasma Processing 41 (2021)
    Even for processes with only a few gas species involved the detailed description of plasma-assisted conversion processes in gas mixtures requires a large amount of processes to be taken into account and a large number of neutral and charged particles must be considered. In addition, setting up the corresponding reaction kinetics model needs the knowledge of the rate coefficients and their temperature dependence for all possible reactions between those species. Reduced reaction networks offer a simplified and pragmatic way to obtain an overall reaction kinetics model, already useful for the analysis of experimental data even if not all details of chemistry can be covered. In this paper we present a derivation of a data driven reduced model for plasma-assisted conversion of methane in an helium environment. By consideration of a small number of elementary reactions, a simple model is set up. Experimental data are analyzed by a genetic algorithm that provides best-fit approximations for the open parameters of the model. In a further step non-relevant parameters of the model are identified and a further model reduction is achieved. The data driven analysis of methane conversion serves as an illustrative example of the proposed method. The parameters and reaction channels found are compared with known results from the literature. The method is described in detail. The main goal of this work is to present the potential of this data driven method for a simplified and pragmatic modeling in the increasingly important field of plasma-assisted catalytic processes. © 2021, The Author(s).
    view abstract10.1007/s11090-021-10159-6
  • Ignition and propagation of nanosecond pulsed plasmas in distilled water - Negative vs positive polarity applied to a pin electrode
    Grosse, K. and Falke, M. and Von Keudell, A.
    Journal of Applied Physics 129 (2021)
    Nanosecond plasmas in liquids are being used for water treatment, electrolysis, or biomedical applications. The exact nature of these very dynamic plasmas and, most importantly, their ignition physics are strongly debated. The ignition itself may be explained by two competing hypotheses: ignition in water may occur (i) via field effects at the tip of the electrode followed by tunneling of electrons in between water molecules causing field ionization or (ii) via gaseous processes of electron multiplication in nanovoids that are created from liquid ruptures due to the strong electric field gradients. Both hypotheses are supported by theory, but experimental data are very sparse due to the difficulty in monitoring the very fast processes in space and time. In this paper, we analyze nanosecond plasmas in water that are created by applying a positive and a negative polarity to a sharp tungsten electrode. The main diagnostics are fast camera measurements and fast emission spectroscopy. It is shown that plasma ignition is dominated by field effects at the electrode-liquid interface either as field ionization for positive polarity or as field emission for negative polarity. This leads to a hot tungsten surface at a temperature of 7000 K for positive polarity, whereas the surface temperature is much lower for negative polarity. At ignition, the electron density reaches 4 × 10 25 m - 3 for the positive and 2 × 10 25 m - 3 for the negative polarity. At the same time, the emission of the H α light for the positive polarity is four times higher than that for the negative polarity. During plasma propagation, the electron densities are almost identical of the order of 1- 2 × 10 25 m - 3 followed by a decay after the end of the pulse over 15 ns. It is concluded that plasma propagation is governed by field effects in a low density region that is created either by nanovoids or by density fluctuations in supercritical water surrounding the electrode that is created by the pressure and temperature at the moment of plasma ignition. © 2021 Author(s).
    view abstract10.1063/5.0045697
  • Link between plasma properties with morphological, structural and mechanical properties of thin Ti films deposited by high power impulse magnetron sputtering
    Moskovkin, P. and Maszl, C. and Schierholz, R. and Breilmann, W. and Petersen, J. and Pflug, A. and Muller, J. and Raza, M. and Konstantinidis, S. and von Keudell, A. and Lucas, S.
    Surface and Coatings Technology 418 (2021)
    The main focus of this work is to correlate the basic plasma properties with morphological, structural and mechanical properties of thin films to bridge the gap between the energy source, the plasma and materials. For this purpose, the deposition and growth of thin titanium films deposited by high power impulse magnetron sputtering (HiPIMS) at various discharge power densities, from 0.17 kW/cm2 to 3.5 kW/cm2 were studied, both experimentally and by kinetic Monte Carlo simulation. Simulations of film growth were performed with a three-dimensional kinetic Monte Carlo code (NASCAM) with ion fraction and species energy determined experimentally by mass spectroscopy. Our approach, which is not purely empirically driven, intends to reveal some insights of the mechanisms underlying the synthesis process, which determine the intrinsic material properties. In order to link HiPIMS plasma parameters and Ti film properties, we used different techniques to analyse Ti films. TEM, X-ray diffraction and AFM were used to evaluate the structural and morphological properties of the films, and nano indention was used to evaluate their mechanical properties. We observed that the orientation of micro-crystals, which constitute the films, changes when the discharge power density increases. At the same time, we show that the films nano hardness changes non-monotonically with the increase of the discharge power density; it decreases first, then increases. The surface roughness behaviour is also non-monotonic; first increasing, then decreasing with the further increase of the discharge power density. 3D modelling helped to reveal that these non-monotonic evolutions are due to a transition between thermally-driven to ballistically-driven Ti atom mobility. © 2021 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2021.127235
  • Oxygen Removal from a Hydrocarbon Containing Gas Stream by Plasma Catalysis
    Urbanietz, T. and Stewig, C. and Böke, M. and von Keudell, A.
    Plasma Chemistry and Plasma Processing 41 (2021)
    Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert O 2 in presence of hydrocarbons into CO or CO 2 on demand. The underlying reaction mechanisms of plasma conversion of O 2 in the presence of hydrocarbons are analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of O 2 and of CH 4. The plasma process is analysed with infrared absorption spectroscopy to monitor CH 4 as well as the reaction products CO, CO 2 and H 2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from O 2 by electron. Oxygen atoms are efficiently converted into CO, CO 2 and H 2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced O 2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of O 2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion efficiency is enhanced by 15–20% already at temperatures of 430 K. © 2021, The Author(s).
    view abstract10.1007/s11090-020-10151-6
  • Chemistry in nanosecond plasmas in water
    Chauvet, L. and Nenbangkaeo, C. and Grosse, K. and von Keudell, A.
    Plasma Processes and Polymers 17 (2020)
    Discharges in liquids are the basis of a range of applications in electrochemistry, wastewater treatment, or plasma medicine. One advantage of discharges in water is their ability to produce radicals and molecules directly inside liquid with a high conversion efficiency. In this study, H2O2 production in a 10 ns pulsed discharge in water is investigated. The dynamic of these discharges is based on plasma ignition directly inside liquid followed by the formation of a bubble that expands in time before it eventually collapses. This sequence can be well described by cavitation theory. H2O2 is produced using different plasma conditions varying the treatment time, the pulse frequency between 1 and 100 Hz, and the applied voltage in a range from 15–30 kV. The resulting H2O2 concentration is measured using absorption spectroscopy ex situ based on a colorimetry method. The results indicate that the main parameter controlling the H2O2 production constitutes the applied voltage. The measured concentrations are compared with a global chemistry model simulating the chemistry involved during a single pulse using pressures and temperatures from the cavitation model. In addition, a global chemical equilibrium model for H2O2 creation is evaluated as well. The models show a good agreement with the data. The energy efficiency for the production of H2O2 reaches values up to 2 g/kWh. © 2020 The Authors. Plasma Processes and Polymers published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/ppap.201900192
  • Corrigendum to: SiO 2 microstructure evolution during plasma deposition analyzed via ellipsometric porosimetry (Plasma Processes and Polymers, (2019), 16, 9, (1900015), 10.1002/ppap.201900015)
    Buschhaus, R. and von Keudell, A.
    Plasma Processes and Polymers 17 (2020)
    The paper by Rahel Buschhaus and Achim von Keudell, published online on 2 April 2019 and in issue 9, 2019, e1900015 (https://doi.org/10.1002/ppap.201900015), contains some errors the authors wish to correct. These errors were: (i)The substrate holder was not placed at the opposing end of the plasma line, instead the holder was surrounding the plasma line concentrically. The sample was placed on the substrate holder at a distance of 3.7 cm from the plasma line. (ii)The pressure in the MW process was 25 Pa instead of 100 Pa. This implies different values in Table 1 and a four times larger mean free path in the MW process. (iii)The SEI values in Table 1 are changed because the flow was improperly used in the calculation. The correct Table 1 is as follows: (Table presented.) These errors do not affect significantly the reasoning in the manuscript and the main conclusions remain valid. The authors apologize for any inconvenience caused. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ppap.202000036
  • Electron density, temperature and the potential structure of spokes in HiPIMS
    Held, J. and Maaß, P.A. and Gathen, V.S.-V.D. and Keudell, A.V.
    Plasma Sources Science and Technology 29 (2020)
    In high power impulse magnetron sputtering (HiPIMS) bright plasma spots are observed during the discharge pulses that rotate with velocities in the order of 10 km s-1 in front of the target surface. It has proven very difficult to perform any quantitative measurements on these so-called spokes, which emerge stochastically during the build-up of each plasma pulse. In this paper, we propose a new time shift averaging method to perform measurements integrating over many discharge pulses, but without phase averaging of the spoke location, thus preserving the information of the spoke structure. This method is then applied to perform Langmuir probe measurements, employing magnetized probe theory to determine the plasma parameters inside the magnetic trap region of the discharge. Spokes are found to have a higher plasma density, electron temperature and plasma potential than the surrounding plasma. The electron density slowly rises at the leading edge of the spoke to a maximum value of about 1 1020 m-3 and then drops sharply at the trailing edge to 4 1019 m-3. The electron temperature rises from 2.1 eV outside the spoke to 3.4 eV at the trailing end of the spoke. A reversal of the plasma potential from about -7 V outside the spoke to values just above 0 V in a spoke is observed, as has been proposed in the literature. © 2020 IOP Publishing Ltd.
    view abstract10.1088/1361-6595/ab5e46
  • Excitation and dissociation of CO2 heavily diluted in noble gas atmospheric pressure plasma
    Stewig, C. and Schüttler, S. and Urbanietz, T. and Böke, M. and Von Keudell, A.
    Journal of Physics D: Applied Physics 53 (2020)
    The excitation and dissociation of CO2 admixed to argon and helium atmospheric pressure radio frequency plasmas is analyzed. The absorbed plasma power is determined by voltage and current probe measurements and the excitation and dissociation of CO2 and CO by transmission mode Fourier-transform infrared spectroscopy (FTIR). It is shown, that the vibrational temperatures of CO2 and CO are significantly higher in an argon compared to a helium plasma. The rotational temperatures remain in both cases close to room temperature. The conversion efficiency, expressed as a critical plasma power to reach almost complete depletion, is four times higher in the argon case. This is explained by the lower threshold for the generation of energetic particles (electrons or metastables) in argon as the main reactive collision partner, promoting excitation and dissociation of CO2, by the less efficient quenching of vibrational excited states of CO and CO2 by argon compared to helium and by a possible contribution of more energetic electrons in an argon plasma compared to helium. © 2020 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab634f
  • Nanosecond pulsed discharges in distilled water-Part II: Line emission and plasma propagation
    Von Keudell, A. and Grosse, K. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 29 (2020)
    Nanosecond plasmas in liquids can initiate chemical processes that are exploited in the fields of water treatment, electrolysis or biomedical applications. The understanding of these chemical processes relies on unraveling the dynamics of the variation of pressures, temperatures and species densities during the different stages of plasma ignition and plasma propagation as well as the conversion of the liquid into the plasma state and the gas phase. This is analyzed by monitoring the emission of nanosecond pulsed plasmas that are generated by high voltages of 20 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. The spectra are acquired with a temporal resolution of 2 ns and the emission pattern is modelled by a combination of black body radiation from the hot tungsten tip and the pronounced emission lines of the hydrogen Balmer series. The data indicate two contributions of the hydrogen line radiation that differ with respect to the degree of self-absorption. It is postulated that one contribution originates from a recombination region showing strong self absorption and one contribution from an ionization region showing very little self-absorption. The emission lines from the ionization region are evaluated assuming Stark broadening, that yielded electron densities up to 5 × 1025 m-3. The electron density evolution follows the same trend as the temporal evolution of the voltage applied to the tungsten tip. The propagation mechanism of the plasma is similar to that of a positive streamer in the gas phase, although in the liquid phase field effects such as electron transport by tunneling should play an important role. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/1361-6595/aba4b9
  • Nanosecond pulsed discharges in distilled water: I. Continuum radiation and plasma ignition
    Grosse, K. and Schulz-Von Der Gathen, V. and Von Keudell, A.
    Plasma Sources Science and Technology 29 (2020)
    Nanosecond plasmas in liquids are an important method to trigger the water chemistry for electrolysis or for biomedical applications in plasma medicine. The understanding of these chemical processes relies on knowing the variation of the temperatures in these dynamic plasmas. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages at 20 kV and pulse lengths of 15 ns applied to a tungsten tip with 50 μm diameter immersed in water. Plasma emission is analyzed by optical emission spectroscopy ranging from UV wavelengths of 250 nm to visible wavelengths of 850 nm at a high temporal resolution of 2 ns. The spectra are dominated by the black body continuum from the hot tungsten surface and line emissions from the hydrogen Balmer series. Typical temperatures from 6000 K up to 8000 K are reached for the tungsten surface corresponding to the boiling temperature of tungsten at varying tungsten vapor pressures. The analysis of the ignition process and the concurrent spectral features indicate that the plasma is initiated by field ionization of water molecules at the electrode surface. At the end of the pulse, field emission of electrons can occur. During the plasma pulse, it is postulated that the plasma contracts locally at the electrode surface forming a hot spot. This causes a characteristic contribution to the continuum emission at small wavelengths. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/1361-6595/aba487
  • Pattern Formation in High Power Impulse Magnetron Sputtering (HiPIMS) Plasmas
    Held, J. and von Keudell, A.
    Plasma Chemistry and Plasma Processing 40 (2020)
    High power impulse magnetron sputtering (HiPIMS) plasmas produce a very energetic growth flux for the synthesis of thin films with superior properties. High power densities in the range of a few kW / cm 2 are applied to a metal target electrode in short pulses with a length of 10–400μs and duty cycles of a few percent or less in an argon plasma gas. Fast camera and probe measurements revealed the formation of very characteristic plasma patterns that become visible as rotating localized ionization zones, so called spokes. The appearance of these spokes at high plasma powers is believed to be essential for the good performance of HiPIMS plasmas. The rotation direction of the spokes is in E→ × B→ direction at high plasma powers, but in retrograde E→ × B→ direction at low plasma powers. This characteristic behavior is explained by applying a simple drift wave model from literature and comparing the dispersion relation of those waves with measured data. The pronounced rotation reversal is explained by either a change in the governing density gradient in the plasma or by the change in the direction of the streaming ions during the transition from an argon dominated regime at low powers to a metal dominated regime at high powers. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s11090-019-10052-3
  • Study of the transition from self-organised to homogeneous plasma distribution in chromium HiPIMS discharge
    Šlapanská, M. and Hecimovic, A. and Gudmundsson, J.T. and Hnilica, J. and Breilmann, W. and Vašina, P. and Von Keudell, A.
    Journal of Physics D: Applied Physics 53 (2020)
    The self-organised plasma patterns, known as spokes or ionisation zones in magnetron sputtering discharges, were observed in a wide range of power densities, from low power direct current magnetron sputtering (dcMS) discharge to high power impulse magnetron sputtering (HiPIMS) discharge. For some target materials and non-reactive gases, it was observed that at very high power densities (>3 kW cm-2) the plasma exhibits a transition from a regime where spokes are observed to a homogeneous plasma regime. In this contribution, we present a comparison of plasma properties: plasma emission (optical emission spectroscopy) and flux of argon and chromium ions (mass spectrometry), measured both in the spoke regime and in the homogeneous plasma regime, aimed to expand the understanding of the plasma transition between the two modes. A simple biased flat probe was used to distinguish between the spoke regime and the homogeneous plasma regime. It was found that the flux of multiply charged ions (Ar2+, Cr2+, Cr3+, Cr4+) increases abruptly at the transition between the spoke regime and the homogeneous plasma regime. Similarly, the emission from Cr+ ions exhibits a strong increase of about 50% when the plasma torus becomes homogeneous. These observations are interpreted as an increase in electron temperature and a change in the electron heating mode, from a combination of secondary electron heating and Ohmic heating towards pure Ohmic heating. The transition to the homogeneous plasma regime and pure Ohmic heating is only observed in non-reactive HiPIMS discharges for target atoms with the second ionisation potential higher than the first ionisation potential of Ar (15.76 eV), and a self-sputter yield larger than 1. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab6a8c
  • Velocity distribution of metal ions in the target region of HiPIMS: The role of Coulomb collisions
    Held, J. and Thiemann-Monjé, S. and Von Keudell, A. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 29 (2020)
    High power impulse magnetron sputtering (HiPIMS) discharges have become an important tool for the deposition of thin, hard coatings. Such discharges are operated at a very low working gas pressure in the order of 1 Pa. Therefore, elastic collisions between ions and other heavy particles are often calculated to occur with low frequency, using the hard sphere approximation. However, inside the magnetic trap region of the discharge, a very dense plasma is created and Coulomb collisions become the dominant collision process for ions. In this article, we show that Coulomb collisions are a necessary part of a complete description of ion movement in the magnetic trap region of HiPIMS. To this end, the velocity distribution function (VDF) of chromium and titanium ions is measured using high-resolution optical emission spectroscopy. The VDF of those ions is then described using a simple simulation which employs a direct simulation Monte Carlo scheme. The simulation describes the self-relaxation of the VDF that is initially a Thompson distribution as being created during the sputtering process. Measurement positions inside the discharge are matched to the simulation results choosing an appropriate relaxation time. In this fashion, excellent agreement between simulation and measurement is obtained. We find, that the distribution quickly becomes mostly Maxwellian with a temperature of 9 eV for titanium ions and 4.5 eV in the case of chromium ions. Only the high energy tail of the VDF retains the shape of the initial Thompson distribution. The observed high temperature is explained with an energy redistribution from the highly energetic Thompson distribution into an partly-thermalized Maxwell-like distribution. Finally, the temperature resulting from this energy redistribution is calculated using a simple analytical model which shows good agreement with the measurements. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/1361-6595/abbf94
  • Corrigendum: Characterization of the effluent of a He/O2 micro-scaled atmospheric pressure plasma jet by quantitative molecular beam mass spectrometry (New Journal of Physics (2010) 12 (013021) DOI: 10.1088/1367-2630/12/1/013021)
    Willems, G. and Golda, J. and Ellerweg, D. and Benedikt, J. and Von Keudell, A. and Knake, N. and Schulz-Von Der Gathen, V.
    New Journal of Physics 21 (2019)
    In this corrigendum, we report and correct an error in the calibration procedure of the molecular beam mass spectrometry (MBMS) diagnostics used for measurement of atomic oxygen densities in the effluent of a microscaled atmospheric pressure plasma jet (μ-APPJ) in He/O2 gas mixture (Ellerweg et al 2010 New J. Phys. 12 013021). The difference in the tuning of the used electron impact ionization mass spectrometer at low electron energy used for O atom detection (Eel=18 eV) and for the measurement of the calibration gases CH4 and Ne (Eel=70 eV) has resulted in the underestimation of the O atom densities by a factor of 2.9. The O3 signal and the signal of its calibration gas (Ar) have been both measured at 70 eV electron energy and O3 density stays, therefore, unaffected. Updated figures with rescaled O densities corrected according to new MBMS measurements with updated mass spectrometry setup, as well as a detailed description of the calibration procedure, are provided here. The corrected O densities provide much better agreement with the results of the two-photon absorption laser induced fluorescence (TALIF) diagnostic and the corrected MBMS data, the TALIF data, or an average of both should be used in the future for the μ-APPJ source. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ab1dfc
  • Nanosecond plasmas in water: Ignition, cavitation and plasma parameters
    Grosse, K. and Held, J. and Kai, M. and Von Keudell, A.
    Plasma Sources Science and Technology 28 (2019)
    Nanosecond plasmas in liquids play an important role in the field of decontamination, electrolysis or plasma medicine. The understanding of these very dynamic plasmas requires information about the temporal variation of species densities and temperatures. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages (HVs) between 14 and 26 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. Ignition of the plasma causes the formation of a cavitation bubble that is monitored by shadowgraphy to measure the dynamic of the created bubble and the sound speed of the emitted acoustic waves surrounding this tungsten tip. The temporal evolution of the bubble size is compared with cavitation theory yielding good agreement for an initial bubble radius of 25 μm with an initial pressure of 5 ×108 Pa at a temperature of 1200 K for a HV of 20 kV. This yields an initial energy in the range of a few 10-5 J that varies with the applied HV. The dissipated energy by the plasma drives the adiabatic expansion of water vapor inside the bubble from its initial supercritical state to a low pressure, low temperature state at maximum bubble expansion reaching values of 103 Pa and 50 K, respectively. These predictions from cavitation theory are corroborated by optical emission spectroscopy. After igniting the nanosecond plasma, the electrical power oscillates in the feed line between HV pulser and plasma chamber with a ring down time of the order of 60 ns. These reflected pulses re-ignite a plasma inside the expanding bubble periodically. Broadband emission due to recombination and Bremsstrahlung becomes visible within the first 30 ns. At later times, line emission dominates. Stark broadening of the spectral lines of Hα (656 nm) and OI (777 nm) is evaluated to determine both the electron density and the electron temperature in these re-ignited plasmas. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6595/ab26fc
  • SiO2 microstructure evolution during plasma deposition analyzed via ellipsometric porosimetry
    Buschhaus, R. and von Keudell, A.
    Plasma Processes and Polymers 16 (2019)
    The evolution of (Formula presented.) microstructures, deposited from hexamethyldisiloxane (HMDSO) and oxygen gas mixtures by two different low pressure plasma sources, namely an inductively coupled plasma (ICP process) at 3 Pa and a microwave plasma (MW process) at 100 Pa, is evaluated and compared. The microstructure is monitored using ellipsometric porosimetry (EP) applying three different solvent molecules (water, ethanol, and toluene) to probe the different adsorption and absorption mechanisms as well as the pore sizes. Both plasma processes are adjusted so that an equivalent oxygen atom contribution to the growth flux is established and that an equivalent specific energy per molecule is dissipated in the process. The major difference is the partial pressure of the HMDSO precursor molecules, which is 0.04 Pa in the ICP process and 1 Pa in the MW process. The porosimetry analysis indicates that the (Formula presented.) films originating from the MW process are more porous than those from the ICP process. The pore sizes are typically in the range of 0.3 nm for films deposited from both plasma processes. This is explained by assuming that the gas phase polymerization in the MW process is much stronger due to the higher HMDSO partial pressure and, therefore, the (Formula presented.) films are deposited from larger HMDSO fragments in the MW process compared with smaller HMDSO fragments in the ICP process. This difference in the main growth species becomes visible in the different microstructures. Consequently, a plasma process using smaller precursor partial pressures seems to be optimal. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ppap.201900015
  • Validation of etching model of polypropylene layers exposed to argon plasmas
    Corbella, C. and Pranda, A. and Portal, S. and de los Arcos, T. and Grundmeier, G. and Oehrlein, G.S. and von Keudell, A.
    Plasma Processes and Polymers 16 (2019)
    Thin layers of polypropylene (PP) have been treated by argon low-temperature plasmas in an inductively coupled plasma setup. The etched thickness of PP was monitored in situ by means of single-wavelength ellipsometry. The ellipsometric model of the polymer surface exposed to plasma consists of a UV-modified layer, a dense amorphous carbon layer because of ion bombardment, and an effective medium approximation layer, which accounts for moderate surface roughness. The etching behavior has been compared to a model based on argon ion beam irradiation experiments. In this approach, surface processes are described in terms of etching yields and crosslinking probabilities as a function of incident fluxes and energies of Ar ions and UV photons. The ion beam model fits well with the plasma etching results. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ppap.201900019
  • Connection between target poisoning and current waveforms in reactive high-power impulse magnetron sputtering of chromium
    Layes, V. and Corbella, C. and Monjé, S. and Schulz-Von Der Gathen, V. and Von Keudell, A. and De Los Arcos, T.
    Plasma Sources Science and Technology 27 (2018)
    Global models of high-power impulse magnetron sputtering (HiPIMS) plasmas in the literature predict a unique connection between target current waveform and oxidation state of the target (metallic versus poisoned): in the metallic mode, the current waveform reaches a plateau due to metal atom recycling, in the poisoned mode a triangular current waveform is predicted driven by plasma gas recycling. This hypothesis of such a unique connection is tested by measuring the surface chemical composition of chromium magnetron targets directly during reactive high-power impulse magnetron sputtering (r-HiPIMS) by spatially resolved x-ray photoelectron spectroscopy (XPS). The sputtering setup was connected to the ultra-high vacuum XPS spectrometer so that the targets could be transferred between the two chambers without breaking the vacuum. The O2/Ar feed gas ratio, the input power and the pulse frequency of the HiPIMS plasmas were varied. The racetrack oxidation state was measured for different plasma parameters and correlated to the target current waveform shape. It was found that a shift of the target operation from the poisoned mode at low powers to the metallic mode at high powers when operating the discharge at 20 Hz pulse frequency occurs. The transition between these modes was directly correlated with analysis of the Cr2p core level peak on the complete target area. A unique correlation between the metallic and poisoned state of the target and the plateau and triangular current waveform was identified for very low powers and very high powers. In the intermediate power range, such a unique connection is absent. It is argued that the presence of already a small fraction of metal on the target may induce a plateau current waveform despite a significant oxidation of the target. This implies a finite contribution of metal sputtering during the pulse that dominates the recycling and leads to a plateau current waveform. Consequently, the shape of current waveforms cannot easily be connected to target poisoning, but a more detailed modeling of the recycling mechanisms is required. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6595/aad0e2
  • Decoupling of ion- and photon-activation mechanisms in polymer surfaces exposed to low-temperature plasmas
    Budde, M. and Corbella, C. and Große-Kreul, S. and de los Arcos, T. and Grundmeier, G. and von Keudell, A.
    Plasma Processes and Polymers (2018)
    The modification of polypropylene (PP) by an argon plasma is emulated in a particle beam experiment. An ion beam deflector, used to steer argon ions from an electron-cyclotron-resonance (ECR) plasma source towards the sample, suppresses the UV and VUV photons generated in the plasma volume. The modification of PP surface by 200 and 500eV ions is monitored by in situ Fourier transform infrared spectroscopy (FTIR). One observes a transition from an initial region of fast etching to a steady state without chemical modification and lower etching rate. This behavior is attributed to the progressive graphitization at the surface due to ion bombardment. An anti-synergism arises by adding UV photons because of cross-linking of the polymer at the subsurface region, which renders the etch rate much smaller compared to the etch rate by ion only impact. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ppap.201700230
  • Influence of spokes on the ionized metal flux fraction in chromium high power impulse magnetron sputtering
    Biskup, B. and Maszl, C. and Breilmann, W. and Held, J. and Böke, M. and Benedikt, J. and Von Keudell, A.
    Journal of Physics D: Applied Physics 51 (2018)
    High power impulse magnetron sputtering (HiPIMS) discharges are an excellent tool for deposition of thin films with superior properties. By adjusting the plasma parameters, an energetic metal and reactive species growth flux can be controlled. This control requires, however, a quantitative knowledge of the ion-to-neutral ratio in the growth flux and of the ion energy distribution function to optimize the deposited energy per incorporated atom in the film. This quantification is performed by combining two diagnostics, a quartz crystal microbalance (QCM) combined with an ion-repelling grid system (IReGS) to discriminate ions versus neutrals and a HIDEN EQP plasma monitor to measure the ion energy distribution function (IEDF). This approach yields the ionized metal flux fraction (IMFF) as the ionization degree in the growth flux. This is correlated to the plasma performance recorded by time resolved ICCD camera measurements, which allow to identify the formation of pronounced ionization zones, so called spokes, in the HiPIMS plasma. Thereby an automatic technique was developed to identify the spoke mode number. The data indicates two distinct regimes with respect to spoke formation that occur with increasing peak power, a stochastic regime with no spokes at low peak powers followed by a regime with distinct spokes at varying mode numbers at higher peak powers. The IMFF increases with increasing peak power reaching values of almost 80% at very high peak powers. The transition in between the two regimes coincides with a pronounced change in the IMFF. This change indicates that the formation of spokes apparently counteracts the return effect in HiPIMS. Based on the IMFF and the mean energy of the ions, the energy per deposited atom together with the overall energy flux onto the substrate is calculated. This allows us to determine an optimum for the peak power density around 0.5 kW cm-2 for chromium HiPIMS. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aaac15
  • Non-equilibrium excitation of CO2 in an atmospheric pressure helium plasma jet
    Urbanietz, T. and Böke, M. and Schulz-Von Der Gathen, V. and Von Keudell, A.
    Journal of Physics D: Applied Physics 51 (2018)
    The energy efficient excitation of CO2 in atmospheric pressure plasmas may be a method to generate solar fuels from renewable energies. This energy efficiency can be very high, if only specific states of the molecules in the plasma are populated creating a strong non-equilibrium. This requires a specific design of the plasma source, method of plasma excitation and choice of gases and admixtures. In this paper, non-equilibrium excitation and dissociation of CO2 in an atmospheric pressure helium RF plasma jet is analysed for varying absorbed plasma power and admixture levels of CO2. The concentrations of CO2 and of CO, as well as the vibrational and rotational temperatures of the possible degrees of freedom of the molecules are evaluated by Fourier transform infrared spectroscopy (FTIR). The molecular rotational vibrational spectra are modelled based on Maxwell-Boltzmann state populations using individual temperatures for each degree of freedom. A strong non-equilibrium excitation of CO2 and CO has been found. Whereas the rotational temperatures are 400 K or below, the vibrational temperature for CO reaches values up to 1600 K and that of the asymmetric vibration of CO2 of 700 K. The dependence of these excitation temperatures on plasma power and admixture level is rather weak. The mass balance, the energy and conversion efficiency are consistent with a very simple chemistry model that is dominated by CO2 dissociation via Penning collisions with helium metastables. A conversion efficiency up to 30% and an energy efficiency up to 10% is observed in the parameter range of the experiment. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aad4d3
  • Spokes in high power impulse magnetron sputtering plasmas
    Hecimovic, A. and Von Keudell, A.
    Journal of Physics D: Applied Physics 51 (2018)
    High-power impulse magnetron sputtering is a deposition technique where a metal magnetron target is sputtered in a high-density plasma to synthesise thin layers with superior properties on a substrate material. These plasmas are characterised by short pulses in the range of 50 μs to 200 μs and very high peak powers in the range of several kW cm-2 per target area. Understanding these dynamic plasmas is of upmost importance for the further development of this coating technique. Fast camera measurements have revealed the formation of localised ionisation zones in these plasmas, which propagate with a velocity of the order km s-1. In the case of a circular magnetron, these ionisation zones appear to move like a set of spokes, which has led to the expression spoke being commonly used to illustrate the pattern formation in these high-density plasmas. Analysing, understanding and theoretically describing the spoke phenomenon is still a matter of open debate, which is hampered by the inherent complexity of these plasmas. In this paper, we review the experimental observations of the spoke phenomenon and highlight several approaches for their theoretical explanation. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aadaa1
  • Velocity distribution of titanium neutrals in the target region of high power impulse magnetron sputtering discharges
    Held, J. and Hecimovic, A. and Von Keudell, A. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 27 (2018)
    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. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6595/aae236
  • Absolutely calibrated mass spectrometry measurement of reactive and stable plasma chemistry products in the effluent of a He/H2O atmospheric plasma
    Willems, G. and Benedikt, J. and Von Keudell, A.
    Journal of Physics D: Applied Physics 50 (2017)
    Mass spectrometry has been used to analyse the effluent of a micro-scaled atmospheric plasma jet operated in helium with a controlled concentration of water vapour. Absolute densities of H2O2 and OH have been measured as function of water vapour concentration and distance from the jet nozzle. The trend for both species densities are correlated and after an initial increase, the densities of H2O2 and OH saturate around 5000 ppm to 6000 ppm of water admixture. The largest densities for H2O2 (2.37 × 1014 cm-3) and OH (1.96 × 1014 cm-3) were measured at 7980 ppm water admixture and 2 mm distance from the jet. Densities of HO2 (1 × 1014 cm-3) and O2 (4 × 1014 cm-3) have been measured as well, although no trend could be observed. The direct electron impact ionisation cross-section of H2O2 at 70 eV electron energy was experimentally determined to be 1.02 × 1016 cm2. The measured densities and profiles have been compared to a 2D axially symmetric fluid model of species transport and recombination reactions. The effluent reaction chemistry is dominated by the hydroxyl radical, where the hydrogen atoms seem to play an important role as well. The analysis of neutral plasma chemistry products have been complemented by measurements of qualitative ion signals. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa77ca
  • Composite targets in HiPIMS plasmas: Correlation of in-vacuum XPS characterization and optical plasma diagnostics
    Layes, V. and Monje, S. and Corbella, C. and Schulz-Von der Gathen, V. and Von Keudell, A. and De Los Arcos, T.
    Journal of Applied Physics 121 (2017)
    In-vacuum characterization of magnetron targets after High Power Impulse Magnetron Sputtering (HiPIMS) has been performed by X-ray photoelectron spectroscopy (XPS). Al-Cr composite targets (circular, 50 mm diameter) mounted in two different geometries were investigated: an Al target with a small Cr disk embedded at the racetrack position and a Cr target with a small Al disk embedded at the racetrack position. The HiPIMS discharge and the target surface composition were characterized in parallel for low, intermediate, and high power conditions, thus covering both the Ar-dominated and the metal-dominated HiPIMS regimes. The HiPIMS plasma was investigated using optical emission spectroscopy and fast imaging using a CCD camera; the spatially resolved XPS surface characterization was performed after in-vacuum transfer of the magnetron target to the XPS chamber. This parallel evaluation showed that (i) target redeposition of sputtered species was markedly more effective for Cr atoms than for Al atoms; (ii) oxidation at the target racetrack was observed even though the discharge ran in pure Ar gas without O2 admixture, the oxidation depended on the discharge power and target composition; and (iii) a bright emission spot fixed on top of the inserted Cr disk appeared for high power conditions. © 2017 Author(s).
    view abstract10.1063/1.4977820
  • Correlative plasma-surface model for metastable Cr-Al-N: Frenkel pair formation and influence of the stress state on the elastic properties
    Music, D. and Banko, L. and Ruess, H. and Engels, M. and Hecimovic, A. and Grochla, D. and Rogalla, D. and Brögelmann, T. and Ludwig, Al. and Von Keudell, A. and Bobzin, K. and Schneider, J.M.
    Journal of Applied Physics 121 (2017)
    Correlatively employing density functional theory and experiments congregated around high power pulsed magnetron sputtering, a plasma-surface model for metastable Cr0.8Al0.2N (space group Fm 3 m) is developed. This plasma-surface model relates plasma energetics with film composition, crystal structure, mass density, stress state, and elastic properties. It is predicted that N Frenkel pairs form during Cr0.8Al0.2N growth due to high-energy ion irradiation, yielding a mass density of 5.69 g cm-3 at room temperature and Young's modulus of 358-130 GPa in the temperature range of 50-700 K for the stress-free state and about 150 GPa larger values for the compressive stress of 4 GPa. Our measurements are consistent with the quantum mechanical predictions within 5% for the mass density and 3% for Young's modulus. The hypothesis of a stress-induced Young's modulus change may at least in part explain the spread in the reported elasticity data ranging from 250 to 420 GPa. © 2017 Author(s).
    view abstract10.1063/1.4985172
  • Electric potential screening on metal targets submitted to reactive sputtering
    Corbella, C. and Marcak, A. and Von Keudell, A. and De Los Arcos, T.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 35 (2017)
    A very thin oxide layer is formed on top of metal surfaces that are submitted to reactive magnetron sputtering in an oxygen atmosphere. Having a few atomic monolayers thickness (1-5 nm), this oxide top layer shows properties of an electric insulator that retards the flux of incident ions. Here, the authors show that this layer can be modeled as a parallel combination of capacitance and resistance. The basic sputtering processes on the oxide layer have been mimicked by means of particle beam experiments in an ultra-high-vacuum reactor. Hence, quantified beams of argon ions and oxygen molecules have been sent to aluminum, chromium, titanium, and tantalum targets. The formation and characteristics of the oxide top layer have been monitored in situ by means of an electrostatic collector and quartz crystal microbalance. The charge build-up at the oxide layer interfaces generates a screening potential of the order of 1-10 V, which shows linear correlation with the total current through the target. The secondary electron yields of the oxides show the expected behavior with ion energies (500-1500 eV), thereby showing that this parameter is not significantly distorted by the screening potential. Charging kinetics of the oxide layer is investigated by means of time-resolved current measurements during bombardment with square-wave modulated ion fluxes. Finally, the dependence of secondary electron emission with surface oxidation state and surface charging issues in pulsed plasmas are studied within the context of the Berg's model. © 2016 American Vacuum Society.
    view abstract10.1116/1.4972566
  • Fast charge exchange ions in high power impulse magnetron sputtering of titanium as probes for the electrical potential
    Breilmann, W. and Maszl, C. and von Keudell, A.
    Plasma Sources Science & Technology 26 (2017)
    High power impulse magnetron sputtering (HiPIMS) plasmas exhibit a high ionization fraction of the sputtered material and ions with high kinetic energies, which produce thin films with superior quality. These ion energy distribution functions (IEDF) contain energetic peaks, which are believed to be linked to a distinct electrical potential hump Delta Phi(ionization) (zone) inside rotating localized ionization zones, so called spokes, at target power densities above 1 kW cm(-2). Any direct measurement of this electrical potential structure is, however, very difficult due to the dynamic nature of the spokes and the very high local power density, which hampers the use of conventional emissive probes. Instead, we use a careful analysis of the IEDFs for singly and doubly charged titanium ions from a HiPIMS plasma at varying target power density. The energy peaks in the IEDFs measured at the substrate depend on the point of ionization and any charge exchange collisions on the path between ionization and impact at the substrate. Thereby, the IEDFs contain a convoluted information about the electrical potential structure inside the plasma. The analysis of these IEDFs reveal that higher ionization states originate at high target power densities from the central part of the plasma spoke, whereas singly charged ions originate from the perimeter of the plasma spoke. Consequently, we observe different absolute ion energies with the energy of Ti2+ being slightly higher than two times the energy of Ti+. Additional peaks are observed in the IEDFs of Ti+ originating from charge exchange reactions from Ti2+ and Ti3+ with titanium neutrals. Based on this analysis of the IEDFs, the structure of the electrical potential inside a spoke is inferred yielding Delta Phi(ionization zone) = 25 V above the plasma potential, irrespective of target power density.
    view abstract10.1088/1361-6595/aa56e5
  • Foundations of low-temperature plasma physics - An introduction
    Von Keudell, A. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 26 (2017)
    The use of plasmas as a reactive mixture of ions, electrons and neutrals is at the core of numerous technologies in industry, enabling applications in microelectronics, automotives, packaging, environment and medicine. Recently, even the use of plasmas in medical applications has made great progress. The dominant character of a plasma is often its non equilibrium nature with different temperatures for the individual species in a plasma, the ions, electrons and neutrals. This opens up a multitude of reaction pathways which are inaccessible to conventional methods in chemistry, for example. The understanding of plasmas requires expertise in plasma physics, plasma chemistry and in electrical engineering. This first paper in a series of foundation papers on low temperature plasma science is intended to provide the very basics of plasmas as a common starting point for the more in-depth discussion of particular plasma generation methods, plasma modeling and diagnostics in the other foundation papers. In this first paper of the series, the common terminology, definitions and main concepts are introduced. The covered aspects start with the basic definitions and include further plasma equilibria, particle collisions and transport, sheaths and discharge breakdowns. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6595/aa8d4c
  • Fundamental study of an industrial reactive HPPMS (Cr,Al)N process
    Bobzin, K. and Brögelmann, T. and Kruppe, N.C. and Engels, M. and Von Keudell, A. and Hecimovic, A. and Ludwig, Al. and Grochla, D. and Banko, L.
    Journal of Applied Physics 122 (2017)
    In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the plasma is nearly constant for the frequency variation. In contrast, the metal to gas ion flux ratio is changed from JM/JG = 0.51 to JM/JG = 0.10 for increasing frequency. Regarding the coating properties, a structure refinement as well as lower residual stresses, higher universal hardness, and a changing crystal orientation from (111) to (200) were observed at higher frequencies. By correlating the plasma and coating properties, it can be concluded that the change in the gas ion to metal ion flux ratio results in a competitive crystal growth of the film, which results in changing coating properties. © 2017 Author(s).
    view abstract10.1063/1.4990997
  • Influence of nitrogen admixture to argon on the ion energy distribution in reactive high power pulsed magnetron sputtering of chromium
    Breilmann, W. and Maszl, C. and Hecimovic, A. and Von Keudell, A.
    Journal of Physics D: Applied Physics 50 (2017)
    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. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa5bfc
  • Investigation of plasma spokes in reactive high power impulse magnetron sputtering discharge
    Hecimovic, A. and Corbella, C. and Maszl, C. and Breilmann, W. and Von Keudell, A.
    Journal of Applied Physics 121 (2017)
    Spokes, localised ionisation zones, are commonly observed in magnetron sputtering plasmas, appearing either with a triangular shape or with a diffuse shape, exhibiting self-organisation patterns. In this paper, we investigate the spoke properties (shape and emission) in a high power impulse magnetron sputtering (HiPIMS) discharge when reactive gas (N2 or O2) is added to the Ar gas, for three target materials; Al, Cr, and Ti. Peak discharge current and total pressure were kept constant, and the discharge voltage and mass flow ratios of Ar and the reactive gas were adjusted. The variation of the discharge voltage is used as an indication of a change of the secondary electron yield. The optical emission spectroscopy data demonstrate that by addition of reactive gas, the HiPIMS plasma exhibits a transition from a metal dominated plasma to the plasma dominated by Ar ions and, at high reactive gas partial pressures, to the plasma dominated by reactive gas ions. For all investigated materials, the spoke shape changed to the diffuse spoke shape in the poisoned mode. The change from the metal to the reactive gas dominated plasma and increase in the secondary electron production observed as the decrease of the discharge voltage corroborate our model of the spoke, where the diffuse spoke appears when the plasma is dominated by species capable of generating secondary electrons from the target. Behaviour of the discharge voltage and maximum plasma emission is strongly dependant on the target/reactive gas combination and does not fully match the behaviour observed in DC magnetron sputtering. © 2017 Author(s).
    view abstract10.1063/1.4978348
  • Probing the electron density in HiPIMS plasmas by target inserts
    Hecimovic, A. and Held, J. and Schulz-Von Der Gathen, V. and Breilmann, W. and Maszl, C. and Von Keudell, A.
    Journal of Physics D: Applied Physics 50 (2017)
    High power impulse magnetron sputtering (HiPIMS) is a versatile technology to deposit thin films with superior properties. During HiPIMS, the power is applied in short pulses of the order of 100 μs at power densities of kW to a magnetron target creating a torus shaped dynamic high density plasma. This plasma torus is not homogeneous, but individual ionization zones become visible, which rotate along the torus with velocities of 10 km . Up to now, however, any direct measurement of the electron density inside these rotating ionization zones is missing. Here, we probe the electron density by measuring the target current locally by using small inserts embedded in an aluminium target facing the plasma torus. By applying simple sheath theory, a plasma density of the order of at the sheath edge can be inferred. The plasma density increases with increasing target current. In addition, the dynamics of the local target current variation is consistent with the dynamics of the traveling ionization zone causing a modulation of the local current density by 25%. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa9914
  • Review Article: Unraveling synergistic effects in plasma-surface processes by means of beam experiments
    Von Keudell, A. and Corbella, C.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 35 (2017)
    The interaction of plasmas with surfaces is dominated by synergistic effects between incident ions and radicals. Film growth is accelerated by the ions, providing adsorption sites for incoming radicals. Chemical etching is accelerated by incident ions when chemical etching products are removed from the surface by ion sputtering. The latter is the essence of anisotropic etching in microelectronics, as elucidated by the seminal paper of Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)]. However, ion-radical-synergisms play also an important role in a multitude of other systems, which are described in this article: (1) hydrocarbon thin film growth from methyl radicals and hydrogen atoms; (2) hydrocarbon thin film etching by ions and reactive neutrals; (3) plasma inactivation of bacteria; (4) plasma treatment of polymers; and (5) oxidation mechanisms during reactive magnetron sputtering of metal targets. All these mechanisms are unraveled by using a particle beam experiment to mimic the plasma-surface interface with the advantage of being able to control the species fluxes independently. It clearly shows that the mechanisms in action that had been described by Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)] are ubiquitous. © 2017 Author(s).
    view abstract10.1116/1.4983275
  • Species transport on the target during high power impulse magnetron sputtering
    Layes, V. and Monje, S. and Corbella, C. and Trieschmann, J. and De Los Arcos, T. and Von Keudell, A.
    Applied Physics Letters 110 (2017)
    High Power Impulse Magnetron Sputtering (HiPIMS) is a prominent technique to deposit superior materials due to the very energetic growth flux. The origin of this energetic growth flux is believed to be an electric potential structure inside localized ionization zones, the so-called spokes, in the HiPIMS plasma, which rotate in the E × B direction along the racetrack. The measurement of this electric potential or of the electric fields surrounding this ionization zone is extremely challenging due to the very high local power density that obstructs any traditional probe diagnostics. Here, we use a marker technique on the magnetron target to analyze the lateral transport of a target material on a HiPIMS target. We show that the target material is predominantly transported in the E × B direction irrespective of the presence of spokes. However, only when spokes are present, we observe also an enhanced transport in the opposite E × B direction. This is explained by the large electric field at the trailing edges of spokes. © 2017 Author(s).
    view abstract10.1063/1.4976999
  • Control of High Power Pulsed Magnetron Discharge by Monitoring the Current Voltage Characteristics
    von Keudell, A. and Hecimovic, A. and Maszl, C.
    Contributions to Plasma Physics 56 (2016)
    Discharge current voltage (IV) curves are directly measured at the target of a high impulse power magnetron sputtering (HiPIMS) plasma for the target materials aluminium, chromium, titanium and copper. These discharge IV curves have been correlated with ICCD camera images of the plasma torus. A clear connection between the change in the discharge IV curve slopes at specific currents and the appearance of localized ionization zones, so-called spokes, in a HiPIMS plasma is identified. These spokes appear above typical target current densities of 2 A/cm(2). The slope of the discharge IV curves, at current densities when spokes are formed, depends on the mass of the target atoms with a higher plasma conductivity for higher mass target materials. This is explained by the momentum transfer from the sputter wind to the argon background gas, which leads to higher plasma densities for heavier target materials. The change in the VI curve slope can be used to identify the spokes regime for HiPIMS plasmas, as being mandatory for deposition of good quality materials by HiPIMS. Consequently, the discharge IV curve slope monitoring can be regarded an essential control approach of any industrial HiPIMS process, where discharge IV curves are much easier accessible compared to more complex diagnostics such as time and space resolved ICCD camera measurements. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ctpp.201600004
  • Methods of gas purification and effect on the ion composition in an RF atmospheric pressure plasma jet investigated by mass spectrometry
    Grosse-Kreul, S. and Hubner, S. and Schneider, S. and von Keudell, A. and Benedikt, J.
    Epj Techniques and Instrumentation 3 (2016)
    The analysis of the ion chemistry of atmospheric pressure plasmas is essential to evaluate ionic reaction pathways during plasma-surface or plasma-analyte interactions. In this contribution, the ion chemistry of a radio-frequency atmospheric pressure plasma jet (mu-APPJ) operated in helium is investigated by mass spectrometry (MS). It is found, that the ion composition is extremely sensitive to impurities such as N-2, O-2 and H2O. Without gas purification, protonated water cluster ions of the form H+(H2O)(n) are dominating downstream the positive ion mass spectrum. However, even after careful feed gas purification to the sub-ppm level using a molecular sieve trap and a liquid nitrogen trap as well as operation of the plasma in a controlled atmosphere, the positive ion mass spectrum is strongly influenced by residual trace gases. The observations support the idea that species with a low ionization energy serve as a major source of electrons in atmospheric pressure helium plasmas. Similarly, the neutral density of atomic nitrogen measured by MS in a He/N-2 mixture is varying up to a factor 3, demonstrating the significant influence of impurities on the neutral species chemistry as well.
    view abstract10.1140/epjti/s40485-016-0034-1
  • Revising secondary electron yields of ion-sputtered metal oxides
    Corbella, C. and Marcak, A. and de los Arcos, T. and von Keudell, A.
    Journal of Physics D-applied Physics 49 (2016)
    The emission of secondary electrons (SE) during sputtering of Al and Ti foils by argon ions in an oxygen background has been measured in a particle beam reactor equipped with a SE-collector. This experiment mimics the process of reactive magnetron sputtering. Quantified beams of argon ions with energies between 500 eV and 2000 eV were employed, while simultaneously molecular oxygen fluxes impinged on the surface and caused oxidation. The measured secondary electron emission coefficients (gamma) ranged from approximately 0.1 (for clean aluminium and titanium) to 1.2 and 0.6 (in the case of aluminium oxide and titanium oxide, respectively). The increase of gamma is compared to SE measurements based on the modelling of magnetron plasmas. Moreover, the energy distributions of the emitted SE have been measured by varying the retarding potential of the SE-collector, which allows the monitoring of the oxidation state from the position of the Auger peaks. The origin of the observed SE yields based on the emission of low-and high-energy electrons generated on the oxide surface is discussed.
    view abstract10.1088/0022-3727/49/16/16LT01
  • Sampling of ions at atmospheric pressure: ion transmission and ion energy studied by simulation and experiment
    Gross-Kreul, S. and Hubner, S. and Benedikt, J. and von Keudell, A.
    European Physical Journal D 70 (2016)
    Mass spectrometry of ions from atmospheric pressure plasmas is a challenging diagnostic method that has been applied to a large variety of cold plasma sources in the past. However, absolute densities can usually not be obtained, moreover, the process of sampling of ions and neutrals from such a plasma inherently influences the measured composition. These issues are studied in this contribution by a combination of experimental and numerical methods. Different numerical domains are sequentially coupled to calculate the ion transmission from the source to the mass analyzer. It is found that the energy of the sampled ions created by a radio-frequency microplasma operated in a He-N-2 mixture at atmospheric pressure is of the order of 0.1 eV and that it depends linearly on the ion mass in good agreement with the expectation for seeded particles accelerated in a supersonic expansion. Moreover, the measured ion energy distribution from an afterglow of an atmospheric pressure plasma can be reproduced on basis of the particle trajectories in the sampling system. Eventually, an estimation of the absolute flux of ions to the detector is deduced.
    view abstract10.1140/epjd/e2016-60601-4
  • Spoke rotation reversal in magnetron discharges of aluminium, chromium and titanium
    Hecimovic, A. and Maszl, C. and Schulz-von der Gathen, V. and Boke, M. and von Keudell, A.
    Plasma Sources Science & Technology 25 (2016)
    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 E x 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 E x 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 E x 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 E x B rotation.
    view abstract10.1088/0963-0252/25/3/035001
  • Elementary surface processes during reactive magnetron sputtering of chromium
    Monje, S. and Corbella, C. and von Keudell, A.
    Journal of Applied Physics 118 (2015)
    The elementary surface processes occurring on chromium targets exposed to reactive plasmas have been mimicked in beam experiments by using quantified fluxes of Ar ions (400-800 eV) and oxygen atoms and molecules. For this, quartz crystal microbalances were previously coated with Cr thin films by means of high-power pulsed magnetron sputtering. The measured growth and etching rates were fitted by flux balance equations, which provided sputter yields of around 0.05 for the compound phase and a sticking coefficient of O-2 of 0.38 on the bare Cr surface. Further fitted parameters were the oxygen implantation efficiency and the density of oxidation sites at the surface. The increase in site density with a factor 4 at early phases of reactive sputtering is identified as a relevant mechanism of Cr oxidation. This ion-enhanced oxygen uptake can be attributed to Cr surface roughening and knock-on implantation of oxygen atoms deeper into the target. This work, besides providing fundamental data to control oxidation state of Cr targets, shows that the extended Berg's model constitutes a robust set of rate equations suitable to describe reactive magnetron sputtering of metals. (C) 2015 AIP Publishing LLC.
    view abstract10.1063/1.4932150
  • Exploring the Structure of the Modified Top Layer of Polypropylene During Plasma Treatment
    Corbella, C. and Grosse-Kreul, S. and von Keudell, A.
    Plasma Processes and Polymers 12 (2015)
    Plasma modifications of polypropylene (PP) surfaces are analyzed by means of vacuum beam experiments. A plasma source provides Ar ion beams and a background of UV photons. Additionally, neutral oxygen beams are sent to perform reactive sputtering of PP. The etch rate and chemical state are monitored in real time by in situ Fourier transform infrared (FTIR) spectroscopy. At the onset of Ar bombardment, PP shows high sputter yields, which decrease down to a constant etch rate indicating the formation of a modified top layer. The stationary top layer is modeled as combination of a pristine fraction plus a crosslinked fraction of amorphous hydrocarbon. Photonand ion-dominated etch processes provide different cross-linking fractions, whereas the sputter efficiency is maximized at intermediate ion energies (200 eV).
    view abstract10.1002/ppap.201400188
  • Fundamentals and Applications of Reflection FTIR Spectroscopy for the Analysis of Plasma Processes at Materials Interfaces
    Grundmeier, G. and von Keudell, A. and de los Arcos, T.
    Plasma Processes and Polymers 12 (2015)
    Plasma processes are widely used for the deposition of thin films and/or the functionalization of material surfaces and interfaces ranging from inorganic to organic structures. The characterization of such plasma-modified surfaces is challenging and most efficiently performed by optical methods, such as FTIR-spectroscopy and related techniques. The present review aims at bridging the gap between optical spectroscopy fundamentals and the application of such experimental techniques in plasma surface science and engineering. The first part of the review covers the most relevant theoretical aspects of different reflection FTIR-spectroscopy approaches; the second part presents the different applications of these principles for the investigation of surface processes induced by plasma. The applications take into account interaction of plasma with metal surfaces, semiconductors, and polymeric materials.
    view abstract10.1002/ppap.201500087
  • Mass spectrometry of atmospheric pressure plasmas
    Grosse-Kreul, S. and Hubner, S. and Schneider, S. and Ellerweg, D. and von Keudell, A. and Matejcik, S. and Benedikt, J.
    Plasma Sources Science & Technology 24 (2015)
    Atmospheric pressure non-equilibrium plasmas (APPs) are effective source of radicals, metastables and a variety of ions and photons, ranging into the vacuum UV spectral region. A detailed study of these species is important to understand and tune desired effects during the interaction of APPs with solid or liquid materials in industrial or medical applications. In this contribution, the opportunities and challenges of mass spectrometry for detection of neutrals and ions from APPs, fundamental physical phenomena related to the sampling process and their impact on the measured densities of neutrals and fluxes of ions, will be discussed. It is shown that the measurement of stable neutrals and radicals requires a proper experimental design to reduce the beam-to-background ratio, to have little beam distortion during expansion into vacuum and to carefully set the electron energy in the ionizer to avoid radical formation through dissociative ionization. The measured ion composition depends sensitively on the degree of impurities present in the feed gas as well as on the setting of the ion optics used for extraction of ions from the expanding neutral-ion mixture. The determination of the ion energy is presented as a method to show that the analyzed ions are originating from the atmospheric pressure plasma.
    view abstract10.1088/0963-0252/24/4/044008
  • Note: Ion-induced secondary electron emission from oxidized metal surfaces measured in a particle beam reactor
    Marcak, A. and Corbella, C. and de los Arcos, T. and von Keudell, A.
    Review of Scientific Instruments 86 (2015)
    The secondary electron emission of metals induced by slow ions is characterized in a beam chamber by means of two coaxial semi-cylindrical electrodes with different apertures. The voltages of the outer electrode (screening), inner electrode (collector), and sample holder (target) were set independently in order to measure the effective yield of potential and kinetic electron emissions during ion bombardment. Aluminum samples were exposed to quantified beams of argon ions up to 2000 eV and to oxygen atoms and molecules in order to mimic the plasma-surface interactions on metallic targets during reactive sputtering. The variation of electron emission yield was correlated to the ion energy and to the oxidation state of Al surfaces. This system provides reliable measurements of the electron yields in real time and is of great utility to explore the fundamental surface processes during target poisoning occurring in reactive magnetron sputtering applications. (C) 2015 AIP Publishing LLC.
    view abstract10.1063/1.4932309
  • Spoke transitions in HiPIMS discharges
    Hecimovic, A. and Schulz-von der Gathen, V. and Boke, M. and von Keudell, A. and Winter, J.
    Plasma Sources Science & Technology 24 (2015)
    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 E x B direction of rotation.
    view abstract10.1088/0963-0252/24/4/045005
  • Adsorption and reactivity of nitrogen atoms on silica surface under plasma exposure
    Marinov, D. and Guaitella, O. and de los Arcos, T. and von Keudell, A. and Rousseau, A.
    Journal of Physics D-applied Physics 47 (2014)
    The kinetics of adsorption, desorption and recombination of nitrogen atoms on a silica surface is investigated. Stable nitrogen atoms are grafted to the inner surface of a fused silica discharge tube by a discharge in N-2 at 0.53 mbar. After the pre-treatment, the surface is analysed using x-ray photoelectron spectroscopy and an isotopic exchange technique. The latter consists of the exposure of the pre-treated surface with a discharge in the heavy nitrogen isotope N-30(2). Nitrogen isotopologues N-29(2) and N-28(2) produced on the surface are detected using a mass spectrometer and provide information about the coverage and reactivity of adsorbed N-14 atoms. It is found that during the pre-treatment, a silicon oxynitride (SiOxNy) layer is formed on the initially clean SiO2 surface. The coverage of N on the surface increases from 5 x 10(13) to 5 x 10(15) cm(-2) for a pre-treatment duration in the range of 10(-2) - 10(4) s. Atoms on the surface demonstrate a distribution of reactivity, which is attributed to a distribution of their binding energies and configurations on the surface. We demonstrate that stable chemisorbed N-ads are not the main recombination sites for N atoms on the surface contrary to previous studies. We conclude that recombination takes place mainly on weakly bonding active sites with the binding energy smaller than 1 eV.
    view abstract10.1088/0022-3727/47/47/475204
  • Combined In Situ XPS and UHV- Chemical Force Microscopy ( CFM) Studies of the Plasma Induced Surface Oxidation of Polypropylene
    Ozkaya, B. and Grosse-Kreul, S. and Corbella, C. and von Keudell, A. and Grundmeier, G.
    Plasma Processes and Polymers 11 (2014)
    Modification of the surface chemistry and correlated adhesive properties of polypropylene (PP) by means of an electron cyclotron resonance (ECR) oxygen plasma source is studied based on an in situ ultra-high-vacuum (UHV)-analytical approach. To determine the plasma induced chemical changes without exposure to atmosphere, X-ray excited valence band (VB) spectroscopy and core level X-ray photoelectron spectroscopy (XPS) are performed. Adhesive properties are characterized by means of UHV chemical force microscopy (UHV-CFM). Correlation of XPS and UHV-CFM data indicate that interactions between a SiO2-tip and the modified PP surface is dominated by hydrogen bonds between surface silanol groups on the tip and induced oxidized species on PP surface. Such interactions are maximized in the initial phase of surface oxidation.
    view abstract10.1002/ppap.201300105
  • Fast Time Resolved Techniques as Key to the Understanding of Energy and Particle Transport in HPPMS-Plasmas
    Maszl, C. and Breilmann, W. and Berscheid, L. and Benedikt, J. and von Keudell, A.
    Ieee Transactions on Plasma Science 42 (2014)
    High-power pulsed magnetron sputtering (HPPMS) plasmas are pulsed discharges, where the plasma composition as well as the fluxes and energies of ions are changing during the pulse. The time resolved energy distribution for Ar1+ ions was measured and phase resolved optical emission spectroscopy for the Ar I line at 760 nm was done to get more insight in the transport properties of the plasma forming noble gas. These measurements were performed during HPPMS of titanium with argon at 0.5 Pa. The peak power density during the 50-mu s pulses was 1.8 kW/cm(2). In this contribution, we demonstrate how time resolved mass spectrometry and Intensified Charge-Coupled Device, Intensified CCD cameras can be used to shed more light on energy and particle transport in HPPMS-plasmas.
    view abstract10.1109/TPS.2014.2330700
  • Origin of the energetic ions at the substrate generated during high power pulsed magnetron sputtering of titanium
    Maszl, C. and Breilmann, W. and Benedikt, J. and von Keudell, A.
    Journal of Physics D-applied Physics 47 (2014)
    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 mu s and phase resolved optical emission spectroscopy with a temporal resolution of 1 mu s. 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.
    view abstract10.1088/0022-3727/47/22/224002
  • The effect of surface reactions of O, O-3 and N on film properties during the growth of silica-like films
    Rugner, K. and Reuter, R. and von Keudell, A. and Benedikt, J.
    Journal of Physics D-applied Physics 47 (2014)
    The effect of surface reactions of O, O-3 and N radicals during the growth of silica-like (SiOxCyHz) films on film properties is investigated. A SiOxCyHz 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 O-2 or N-2. 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, O-3 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.
    view abstract10.1088/0022-3727/47/22/224005
  • Various Shapes of Plasma Spokes Observed in HiPIMS
    Hecimovic, A. and von Keudell, A. and Schulz-von der Gathen, V. and Winter, J.
    Ieee Transactions on Plasma Science 42 (2014)
    A time-resolved analysis of the emission of power impulse magnetron sputtering plasmas reveals inhomogeneities in the form of rotating spokes. The shape of these spokes depends on the target material in a characteristic manner; the peculiar shape of the emission profiles is explained by the localization of the sputtering process as being governed by Ar gas rarefaction and the local dynamics of secondary electrons generation. This general picture is able to explain the observed emission patterns for different target materials as presented.
    view abstract10.1109/TPS.2014.2330454
  • Bimodal substrate biasing to control gamma-Al2O3 deposition during reactive magnetron sputtering
    Prenzel, M. and Kortmann, A. and Stein, A. and von Keudell, A. and Nahif, F. and Schneider, J. M.
    Journal of Applied Physics 114 (2013)
    Al2O3 thin films have been deposited at substrate temperatures between 500 degrees C and 600 degrees C by reactive magnetron sputtering using an additional arbitrary substrate bias to tailor the energy distribution of the incident ions. The films were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The film structure being amorphous, nanocrystalline, or crystalline was correlated with characteristic ion energy distributions. The evolving crystalline structure is connected with different levels of displacements per atom (dpa) in the growing film as being derived from TRIM simulations. The boundary between the formation of crystalline films and amorphous or nanocrystalline films was at 0.8 dpa for a substrate temperature of 500 degrees C. This threshold shifts to 0.6 dpa for films grown at 550 degrees C. (C) 2013 AIP Publishing LLC.
    view abstract10.1063/1.4819227
  • Chemical and Physical Sputtering of Polyethylene Terephthalate (PET)
    Grosse-Kreul, S. and Corbella, C. and von Keudell, A.
    Plasma Processes and Polymers 10 (2013)
    The polymer polyethylene terephthalate (PET) has been exposed to quantified beams of argon ions and oxygen atoms and molecules. The etch rate (ER) and the surface composition of PET thin films have been analyzed by real time in situ Fourier transform infrared spectroscopy (FTIR). After the onset of the exposure of PET to the ion beam, the ER decreases rapidly by one order of magnitude irrespective of the ion energy. This slowing down of the ER is caused by cross-linking of the polymer surface. The steady state etch yields are generally orders of magnitude higher than predicted by computer calculations. The addition of oxygen to the particle flux is only changing the surface composition. At low ion energies, chemical sputtering dominates causing very high sputter yields. In addition, no threshold ion energy is observed. [GRAPHICS] .
    view abstract10.1002/ppap.201200094
  • Development of the sputtering yields of ArF photoresist after the onset of argon ion bombardment
    Takeuchi, T. and Corbella, C. and Grosse-Kreul, S. and von Keudell, A. and Ishikawa, K. and Kondo, H. and Takeda, K. and Sekine, M. and Hori, M.
    Journal of Applied Physics 113 (2013)
    Modification of an advanced ArF excimer lithographic photoresist by 400 eV Ar ion irradiation was observed in situ in real time using both infrared spectroscopy and a quartz microbalance sensor. The photoresist sputtering yields had a characteristic behavior; the sputtering yields were higher than unity at the beginning, until an ion dose of 2 x 10(16) ions cm(-2). Thereafter, the yields decreased immediately to almost zero and remained constant with the yield at zero until a dose of approximately 4 x 10(16) ions cm(-2) was reached. At larger doses, the yields increased again and reached a steady-state value of approximately 0.6. This development of the sputtering yield after the onset of ion bombardment is explained by an ion-induced modification of the photoresist that includes preferential sputtering of individual groups, argon ion implantation and the generation of voids. All these effects must be taken into account to assess line-edge-roughness on a photoresist subjected to highly energetic ion irradiation. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772996]
    view abstract10.1063/1.4772996
  • Dynamic of the growth flux at the substrate during high-power pulsed magnetron sputtering (HiPIMS) of titanium
    Breilmann, W. and Maszl, C. and Benedikt, J. and von Keudell, A.
    Journal of Physics D-applied Physics 46 (2013)
    The temporal distribution of the incident fluxes of argon and titanium ions on the substrate during an argon HiPIMS pulse to sputter titanium with pulse lengths between 50 to 400 mu s and peak powers up to 6 kW are measured by energy-resolved ion mass spectrometry with a temporal resolution of 2 mu s. The data are correlated with time-resolved growth rates and with phase-resolved optical emission spectra. Four ion contributions impinging on the substrate at different times and energies are identified: (i) an initial argon ion burst after ignition, (ii) a titanium and argon ion flux in phase with the plasma current due to ionized neutrals in front of the target, (iii) a small energetic burst of ions after plasma shut off, and (iv) cold ions impinging on the substrate in the late afterglow showing a pronounced maximum in current. The last contribution originates from ions generated during the plasma current maximum at 50 mu s after ignition in the magnetic trap in front of the target. They require long transport times of a few 100 mu s to reach the substrate. All energy distributions can be very well fitted with a shifted Maxwellian indicating an efficient thermalization of the energetic species on their travel from target to substrate. The energy of titanium is higher than that of argon, because they originate from energetic neutrals of the sputter process. The determination of the temporal sequence of species, energies and fluxes in HiPIMS may lead to design rules for the targeted generation of these discharges and for synchronized biasing concepts to further improve the capabilities of high-power impulse magnetron sputtering (HiPIMS) processes.
    view abstract10.1088/0022-3727/46/48/485204
  • Formation of crystalline gamma-Al2O3 induced by variable substrate biasing during reactive magnetron sputtering
    Prenzel, M. and Kortmann, A. and von Keudell, A. and Nahif, F. and Schneider, J. M. and Shihab, M. and Brinkmann, R. P.
    Journal of Physics D-applied Physics 46 (2013)
    Reactive magnetron sputtering is a widely used technique to deposit various materials such as oxides and nitrides with a superior control of morphology and stoichiometry. The adjustment of the film properties at a given substrate temperature is believed to be affected by the average energy < E > per incorporated atom during film growth, which is controlled by the ion-to-neutral ratio in the film forming growth flux and the energy of the incident ions. This concept is tested for alumina growth in an rf-magnetron discharge by keeping < E >, the average energy of the incident ions E-ions, and the ion-to-neutral flux ratio constant, but varying only the energy distribution of the incident ions (ion energy distribution-IED). The influence of the IED on film growth is monitored by observing the transition of the films between x-ray amorphous Al2O3 to gamma-Al2O3. The results reveal that the substrate temperature necessary for the transition to gamma-crystalline films can be lowered by almost 100 degrees C, when the maximum energy of the incident ions is kept at 100 eV, while maintaining the energy per incorporated atom at 11 eV. This result is compared with TRIM calculations for the collision cascades of impacting ions.
    view abstract10.1088/0022-3727/46/8/084004
  • Insight into the Reaction Scheme of SiO2 Film Deposition at Atmospheric Pressure
    Rugner, K. and Reuter, R. and Ellerweg, D. and de los Arcos, T. and von Keudell, A. and Benedikt, J.
    Plasma Processes and Polymers 10 (2013)
    Characterisation of an atmospheric pressure microplasma jet in combination with simulations have been used to determine reaction mechanism of SiO2-like film formation and reaction rate constants for several gas phase reactions in the He/hexamethyldisiloxane (HMDSO)(/O-2) plasma chemistry. Using a variable-length quartz tube, the gas residence time in the plasma effluent could be well controlled without changing plasma properties. A possible reaction scheme has been developed. Deposition rates, deposited profiles, carbon content of the films and the depletion of HMDSO could be reproduced by the simulation. The simulation indicates that HMDSO in He(/O-2) plasma dissociates preferentially into (CH3)(3)SiO and Si(CH3)(3), where the former radical serves as a main growth precursor.
    view abstract10.1002/ppap.201300059
  • Ion-induced oxidation of aluminum during reactive magnetron sputtering
    Kreiter, O. and Grosse-Kreul, S. and Corbella, C. and von Keudell, A.
    Journal of Applied Physics 113 (2013)
    Particle beam experiments were conducted in an ultra-high-vacuum vessel to mimic target poisoning during reactive magnetron sputtering of aluminum. Aluminum targets were exposed to quantified beams of argon ions, oxygen atoms and molecules, and aluminum vapour. The growth and etch rates were measured in situ by means of an Al-coated quartz crystal microbalance. The chemical state of the target surface was monitored in-situ by real-time Fourier transform infrared spectroscopy. The surface processes were modelled through a set of balance equations providing sputter yields and sticking coefficients. The results indicate that the oxygen uptake of the aluminum surface is enhanced by a factor 1 to 2 by knock-on implantation and that the deposition of aluminum is not affected by the oxidation state of the surface. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799052]
    view abstract10.1063/1.4799052
  • Nonlinear evolution of surface morphology under shadowing
    Manz, P. and Fedorczak, N. and Dittmar, T. and Baloniak, T. and von Keudell, A.
    Physical Review E 87 (2013)
    Fluorocarbon thin-film deposition is studied, which shows an anomalous high dynamic growth exponent and therefore does not fit in any universal class of fractal surface growth models. A detailed analysis of the nonlinear behavior of the surface morphology evolution is carried out, quantifying several features of the shadowing instability. A synergy effect with the Kardar-Parisi-Zhang nonlinearity, which couple the large scales induced by shadowing with intermediate scales, may explain the anomalous high growth exponent. DOI: 10.1103/PhysRevE.87.042404
    view abstract10.1103/PhysRevE.87.042404
  • Particle beam experiments for the analysis of reactive sputtering processes in metals and polymer surfaces
    Corbella, C. and Grosse-Kreul, S. and Kreiter, O. and de los Arcos, T. and Benedikt, J. and von Keudell, A.
    Review of Scientific Instruments 84 (2013)
    A beam experiment is presented to study heterogeneous reactions relevant to plasma-surface interactions in reactive sputtering applications. Atom and ion sources are focused onto the sample to expose it to quantified beams of oxygen, nitrogen, hydrogen, noble gas ions, and metal vapor. The heterogeneous surface processes are monitored in situ by means of a quartz crystal microbalance and Fourier transform infrared spectroscopy. Two examples illustrate the capabilities of the particle beam setup: oxidation and nitriding of aluminum as a model of target poisoning during reactive magnetron sputtering, and plasma pre-treatment of polymers (PET, PP). (C) 2013 AIP Publishing LLC.
    view abstract10.1063/1.4826066
  • Surface Modification of Polypropylene (PP) by Argon Ions and UV Photons
    Grosse-Kreul, S. and Corbella, C. and von Keudell, A. and Ozkaya, B. and Grundmeier, G.
    Plasma Processes and Polymers 10 (2013)
    The surface modification of polypropylene (PP) by monoenergetic argon ions and UV photons is evaluated in a particle beam experiment. Thereby, the polymer pre-treatment in a plasma process can be mimicked. The etching and chemical modification of the spin-coated PP thin films is monitored in real-time by in situ Fourier transform infrared spectroscopy (FTIR). It is shown that the initial exposure to the plasma ion source causes a modification of the film surface, which slows down the initially high etch rate. The separately measured UV-induced damage is more severe compared to the oxygen-containing polymer polyethylene terephthalate (PET).
    view abstract10.1002/ppap.201300078
  • Target implantation and redeposition processes during high-power impulse magnetron sputtering of aluminum
    Will, A. and de los Arcos, T. and Corbella, C. and Hecimovic, A. and Machura, P. D. and Winter, J. and von Keudell, A.
    Journal of Physics D-applied Physics 46 (2013)
    The processes of argon retention by the target and redeposition of target material were investigated by x-ray photoelectron spectroscopy as a function of radial position for different plasma conditions in high-power impulse magnetron sputtering of aluminum targets. Significant differences in Ar radial concentration profiles were observed for different discharge conditions. Inside the racetrack area, Ar ion flux-dominated implantation is compensated by radiation-enhanced diffusion loss terms. Outside the racetrack, the role of ion implantation is diminished, and Ar retention by the target may stem from a balance between gettering by redeposited Al and ion-induced Ar desorption.
    view abstract10.1088/0022-3727/46/8/084009
  • Time-resolved measurement of film growth during high-power pulsed magnetron sputtering (HIPIMS) of titanium
    Mitschker, F. and Prenzel, M. and Benedikt, J. and Maszl, C. and von Keudell, A.
    Journal of Physics D-applied Physics 46 (2013)
    The growth rate during high-power pulsed magnetron sputtering (HIPIMS) of titanium is measured with a temporal resolution of up to 25 mu s using a rotating shutter concept. According to that concept a 200 mu m slit is rotated in front of the substrate synchronous with the HIPIMS pulses. Thereby, the growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry, the temporal variation of the growth flux per pulse is deduced. The time-resolved growth rates are measured for 0.25, 0.5 and 1 Pa with pulse lengths of 50, 200 and 400 mu s for an average power of 100 W. We can clearly identify, the individual phases of a HIPIMS pulse consisting of ignition, current rise, gas rarefaction, plateau/self-sputtering, and afterglow as described in the literature. In addition, the maximum film growth is only reached after gas rarefaction, indicating a dynamic change in local transport properties. After the end of the HIPIMS pulse, the growth rate decays following two time constants of 100 mu s and of similar to ms, respectively. The first is consistent with the decay of the ion flux in the afterglow; the second with a decay of reactive neutrals. The absolute comparison of growth rates indicates that a reduction of the efficiency to 30% for very short pulses is typical for a true HIPIMS plasma.
    view abstract10.1088/0022-3727/46/15/155204
  • Time-resolved measurement of film growth during reactive high power pulsed magnetron sputtering (HIPIMS) of titanium nitride
    Mitschker, F. and Prenzel, M. and Benedikt, J. and Maszl, C. and von Keudell, A.
    Journal of Physics D-applied Physics 46 (2013)
    The growth rate during reactive high power pulsed magnetron sputtering (HIPIMS) of titanium nitride is an inherent time-dependent process. By using a rotating shutter setup it is possible to gain an insight into its variation with a temporal resolution of up to 25 mu s. In this apparatus a 200 mu m slit is rotated in front of the substrate synchronous with the HIPIMS pulses. This ensures that the incoming growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry and x-ray photoelectron spectroscopy, the temporal variation of the titanium and nitrogen growth flux per pulse is deduced. The analysis reveals that film growth occurs mainly during an HIPIMS pulse, with the growth rate following the HIPIMS phases ignition, current rise, gas rarefaction, plateau and afterglow. The growth fluxes of titanium and nitrogen follow slightly different behaviours with titanium dominating at the beginning of the HIPIMS pulse and nitrogen at the end of the pulse. This is explained by the gas rarefaction effect resulting in a dense initial metal plasma and metal films which are subsequently nitrified.
    view abstract10.1088/0022-3727/46/49/495201
  • Embedded argon as a tool for sampling local structure in thin plasma deposited aluminum oxide films
    Prenzel, M. and de los Arcos, T. and Kortmann, A. and Winter, J. and von Keudell, A.
    Journal of Applied Physics 112 (2012)
    Al2O3 thin films, either amorphous or of varying degrees of crystallinity, were deposited by two-frequency radio-frequency magnetron sputtering. Film crystallinity was investigated by Fourier transform infrared spectroscopy and X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was employed to determine the amount of Ar naturally trapped within the films during the deposition process. A clear correlation was found between the existence of crystalline phases, as determined by XRD, and a shift towards lower binding energy positions of the Ar2p core levels of embedded gas. The shift is due to differences in the local Al2O3 matrix (amorphous or crystalline) of the embedded gas, thus, providing an XPS fingerprint that can be used to qualitatively determine the presence or absence of crystalline phases in very thin films. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767383]
    view abstract10.1063/1.4767383
  • Functional plasma polymers deposited in capacitively and inductively coupled plasmas
    Hegemann, D. and Korner, E. and Chen, S. and Benedikt, J. and von Keudell, A.
    Applied Physics Letters 100 (2012)
    Capacitively and inductively coupled plasmas were investigated in order to deposit functional plasma polymers. Considering plasma chemical and surface processes, comparable films can be obtained with both plasma sources yielding distinctly higher deposition rates for ICP. While the gas phase processes scaled with the energy input into the plasma, the surface processes were controlled by the energy dissipated during film growth (ion bombardment). (C) 2012 American Institute of Physics. [doi: 10.1063/1.3681382]
    view abstract10.1063/1.3681382
  • Quadrupole mass spectrometry of reactive plasmas
    Benedikt, J. and Hecimovic, A. and Ellerweg, D. and von Keudell, A.
    Journal of Physics D-applied Physics 45 (2012)
    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.
    view abstract10.1088/0022-3727/45/40/403001
  • The Role of Oxygen and Surface Reactions in the Deposition of Silicon Oxide like Films from HMDSO at Atmospheric Pressure
    Reuter, R. and Rugner, K. and Ellerweg, D. and de los Arcos, T. and von Keudell, A. and Benedikt, J.
    Plasma Processes and Polymers 9 (2012)
    The deposition of thin SiO2-like films by means of atmospheric pressure microplasma jets with admixture of hexamethyldisiloxane (HMDSO) and oxygen and the role of surface reactions in film growth are investigated. Two types of microplasma jets, one with a planar electrodes and operated in helium gas and the other one with a coaxial geometry operated in argon, are used to study the deposition process. The growth rate of the film and the carbon-content in the film are measured as a function of the O2 and HMDSO admixture in the planar jet and are compared to mass spectrometry measurements of the consumption of HMDSO. Additionally, the localized nature of the jetsubstrate interaction is utilized to study surface reactions by applying two jets on a rotating substrate. The addition of oxygen into the gas mixture increases HMDSO depletion and the growth rate and results in the deposition of carbon free films. The surface reaction is responsible for the carbon removal from the growing film. Moreover, carbon free films can be deposited even without addition of oxygen, when coaxial jet operated with argon is used for the surface treatment. We hypothesize that ions or excited species (metastables) could be responsible for the observed effect.
    view abstract10.1002/ppap.201100146
  • Time-resolved measurement of film growth during high-power pulsed magnetron sputtering (HPPMS) of titanium: the rotating shutter concept
    Mitschker, F. and Prenzel, M. and Benedikt, J. and von Keudell, A.
    Journal of Physics D-applied Physics 45 (2012)
    The growth rate during high-power pulsed magnetron sputtering (HPPMS) of titanium is measured with a temporal resolution of up to 54 mu s using a rotating shutter concept. According to that concept a 200 mu m slit is rotated in front of the substrate synchronous with the HPPMS pulses. Thereby, the growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry, the temporal variation of the growth flux per pulse is deduced. The analysis reveals that film growth occurs mainly during a HPPMS pulse, with the growth rate slowly increasing during the pulse and decaying afterwards with a decay time of 100 mu s. The maximum of film deposition shifts to earlier times in the pulse with increasing peak power.
    view abstract10.1088/0022-3727/45/40/402001
  • Unexpected O and O-3 production in the effluent of He/O-2 microplasma jets emanating into ambient air
    Ellerweg, D. and von Keudell, A. and Benedikt, J.
    Plasma Sources Science & Technology 21 (2012)
    Microplasma jets are commonly used to treat samples in ambient air. The effect of admixing air into the effluent may severely affect the composition of the emerging species. Here, the effluent of a He/O-2 microplasma jet has been analyzed in a helium and in an air atmosphere by molecular beam mass spectrometry. First, the composition of the effluent in air was recorded as a function of the distance to determine how fast air admixes into the effluent. Then, the spatial distribution of atomic oxygen and ozone in the effluent was recorded in ambient air and compared with measurements in a helium atmosphere. Additionally, a fluid model of the gas flow with reaction kinetics of reactive oxygen species in the effluent was constructed. In ambient air, the O density declines only slightly faster with distance compared with a helium atmosphere. In contrast, the O-3 density in ambient air increases significantly faster with distance compared with a helium atmosphere. This unexpected behavior cannot be explained by simple recombination reactions of O atoms with O-2 molecules. A reaction scheme involving the reaction of plasma-produced excited O-2* species of unknown identity with ground state O-2 molecules is proposed as a possible explanation for these observations.
    view abstract10.1088/0963-0252/21/3/034019
  • Surface reactions as carbon removal mechanism in deposition of silicon dioxide films at atmospheric pressure
    Reuter, R. and Ellerweg, D. and von Keudell, A. and Benedikt, J.
    Applied Physics Letters 98 (2011)
    The deposition of thin SiO(x)C(y)H(z) or SiO(x)H(y) films by means of an atmospheric pressure microplasma jet with helium/hexamethyldisiloxane (HMDSO)/O(2) mixtures and the surface reactions involving oxygen have been studied. It is shown, that the carbon content in the film can be controlled by choosing the right O(2)/HMDSO ratio in the gas mixture. The microplasma jet geometry and localization of the deposition at a spot of few square millimeters allows studying the role of oxygen in the deposition process. This is done by alternating application of He/HMDSO plasma and He/O(2) plasma to the same deposition area, here achieved by a treatment of a rotating substrate by two jets with above mentioned gas mixtures. It is shown that carbon-free SiOxHy film can be deposited in this way and that surface reaction with oxygen is the main loss mechanism of carbon from the film. (C) 2011 American Institute of Physics. [doi:10.1063/1.3565965]
    view abstract10.1063/1.3565965
  • X-ray photoelectron spectroscopy on implanted argon as a tool to follow local structural changes in thin films
    Lahrood, A. R. and de los Arcos, T. and Prenzel, M. and von Keudell, A. and Winter, J.
    Thin Solid Films 520 (2011)
    Argon ions were implanted in metallic, semiconducting or insulating substrates, and investigated with X-ray photoelectron spectroscopy. Analysis of the Ar2p core level of argon showed clear differences in binding energy position and width as function of the matrix material, implantation energy, and post-annealing treatment. Although argon is not expected to form chemical bonds with the host matrix, the electronic shells within the gas atom can react to their environment according to different effects. It is shown that the precise determination and correct interpretation of the binding energy levels of the embedded gas atoms provides information about the local environment of the matrix such as amorphization of the crystalline structure, defect healing or gas bubble formation. (C) 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2011.07.040
  • A Physicist's Perspective on "Views on Macroscopic Kinetics of Plasma Polymerisation"
    von Keudell, A. and Benedikt, J.
    Plasma Processes and Polymers 7 (2010)
    view abstract10.1002/ppap.201000011
  • Calibration of a miniaturized retarding field analyzer for low-temperature plasmas: geometrical transparency and collisional effects
    Baloniak, T. and Reuter, R. and Flotgen, C. and von Keudell, A.
    Journal of Physics D-applied Physics 43 (2010)
    Retarding field analyzers (RFAs) are important diagnostics to measure fluxes and energies of ions impinging onto the wall of a plasma reactor. Any quantitative use of the data requires a proper calibration, which is here performed for a miniaturized RFA. The calibration accounts for the transparencies of the RFA grids as well as for collisions inside the RFA. An analytical model is derived which covers both geometrical and collisional effects. The model is calibrated and experimentally verified using a Langmuir probe. We find that the transparency of an RFA is a random variable which depends on the individual alignment of the RFA grids. Collisions inside the RFA limit the ion current transfer through the RFA at higher pressures. A simple method is presented which allows one to remove these artefacts from the RFA data and to obtain quantitative ion velocity distributions.
    view abstract10.1088/0022-3727/43/5/055203
  • Characterization of the effluent of a He/O-2 microscale atmospheric pressure plasma jet by quantitative molecular beam mass spectrometry
    Ellerweg, D. and Benedikt, J. and von Keudell, A. and Knake, N. and Schulz-von der Gathen, V.
    New Journal of Physics 12 (2010)
    The effluent of a microscale atmospheric pressure plasma jet (mu-APPJ) operated in helium with a small admixture of molecular oxygen (< 1.6%) has been analyzed by means of two independent diagnostics, quantitative molecular beam mass spectrometry (MBMS) and two-photon absorption laser-induced fluorescence spectroscopy (TALIF). The atomic oxygen density, the ozone density and the depletion of molecular oxygen have been measured by MBMS and the atomic oxygen density has been validated by TALIF. Absolute atomic oxygen densities in the effluent up to 4.7x10(15) cm(-3) could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4x10(15) cm(-3) and an O-2 depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O-2 admixture of 0.6%, whereas the ozone density continues to increase toward higher O-2 admixtures. With increasing distance from the jet, the atomic oxygen density decreases but is still detectable at a distance of 30 mm. The ozone density increases with distance, saturating at a distance of 40 mm. By applying higher powers to the mu-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.
    view abstract10.1088/1367-2630/12/1/013021
  • Elimination of Biological Contaminations from Surfaces by Plasma Discharges: Chemical Sputtering
    Rauscher, H. and Kylian, O. and Benedikt, J. and von Keudell, A. and Rossi, F.
    Chemphyschem 11 (2010)
    Plasma treatment of surfaces as a sterilisation or decontamination method is a promising approach to overcome limitations of conventional techniques. The precise characterisation of the employed plasma discharges, the application of sensitive surface diagnostic methods and targeted experiments to separate the effects of different agents, have led to rapid progress in the understanding of different relevant elementary processes. This contribution provides an overview of the most relevant and recent results, which reveal the importance of chemical sputtering as one of the most important processes for the elimination of biological residuals. Selected studies on the interaction of plasmas with bacteria, proteins and polypeptides are highlighted, and investigations employing beams of atoms and ions confirming the prominent role of chemical sputtering are presented. With this knowledge, it is possible to optimize the plasma treatment for decontamination/sterilisation purposes in terms of discharge composition, density of active species and UV radiation intensity.
    view abstract10.1002/cphc.200900757
  • Fundamental aspects of substrate biasing: ion velocity distributions and nonlinear effects
    Baloniak, T. and Reuter, R. and von Keudell, A.
    Journal of Physics D-applied Physics 43 (2010)
    Ion bombardment of the substrate is a significant parameter in plasma processing such as dry etching or thin film deposition. The ion bombardment is described by ion velocity distribution functions (IVDFs), which were here measured quantitatively at a sinusoidally and non-sinusoidally biased electrode. The electrode voltage was monitored and controlled in the frequency domain using fast Fourier transformation. IVDF measurements were performed by a floating retarding field analyzer. A full modulation of the IVDF by arbitrary bias waveforms is only achieved if sufficiently high sheath voltages are used. If the applied sheath voltages become too low, the IVDFs are only partly determined by the RF bias waveforms and the system response becomes nonlinear. An analytical sheath model is derived from the experimental data, which accounts for arbitrary bias waveforms as well as for collisional and nonlinear effects in the sheath. It is shown that a combined DC and RF biasing of the electrode is required to gain full control over the ion bombardment of the substrate.
    view abstract10.1088/0022-3727/43/33/335201
  • Inactivation of bacteria and biomolecules by low-pressure plasma discharges
    Von Keudell, A. and Awakowicz, P. and Benedikt, J. and Raballand, V. and Yanguas-Gil, A. and Opretzka, J. and Flötgen, C. and Reuter, R. and Byelykh, L. and Halfmann, H. and Stapelmann, K. and Denis, B. and Wunderlich, J. and Muranyi, P. and Rossi, F. and Kylián, O. and Hasiwa, N. and Ruiz, A. and Rauscher, H. and Sirghi, L. and Comoy, E. and Dehen, C. and Challier, L. and Deslys, J.P.
    Plasma Processes and Polymers 7 (2010)
    (Figure Presented) The inactivation of bacteria and biomolecules using plasma discharges were investigated within the European project BIODECON. The goal of the project was to identify and isolate inactivation mechanisms by combining dedicated beam experiments with especially designed plasma reactors. The plasma reactors are based on a fully computer-controlled, low-pressure inductively-coupled plasma (ICP). Four of these reactors were built and distributed among the consortium, thereby ensuring comparability of the results between the teams. Based on this combined effort, the role of UV light, of chemical sputtering (i.e. the combined impact of neutrals and ions), and of thermal effects on bacteria such as Bacillus atrophaeus, Aspergillus niger, as well as on biomolecules such as LPS, Lipid A, BSA and prions have been evaluated. The particle fluxes emerging from the plasmas are quantified by using mass spectrometry, Langmuir probe measurements, retarding field measurements and optical emission spectroscopy. The effects of the plasma on the biological systems are evaluated using atomic force microscopy, ellipsometry, electrophoresis, specially-designed western blot tests, and animal models. A quantitative analysis of the plasma discharges and the thorough study of their effect on biological systems led to the identification of the different mechanisms operating during the decontamination process. Our results confirm the role of UV in the 200-2 50 nm range for the inactivation of microorganisms and a large variability of results observed between different strains of the same species. Moreover, we also demonstrate the role of chemical sputtering corresponding to the synergism between ion bombardment of a surface with the simultaneous reaction of active species such as O, O2 or H. Finally, we show that plasma processes can be efficient against different micro-organisms, bacteria and fungi, pyrogens, model proteins and prions. The effect of matrices is described, and consequences for any future industrial implementation are discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/ppap.200900121
  • Ion-enhanced oxidation of aluminum as a fundamental surface process during target poisoning in reactive magnetron sputtering
    Kuschel, T. and von Keudell, A.
    Journal of Applied Physics 107 (2010)
    Plasma deposition of aluminum oxide by reactive magnetron sputtering (RMS) using an aluminum target and argon and oxygen as working gases is an important technological process. The undesired oxidation of the target itself, however, causes the so-called target poisoning, which leads to strong hysteresis effects during RMS operation. The oxidation occurs by chemisorption of oxygen atoms and molecules with a simultaneous ion bombardment being present. This heterogenous surface reaction is studied in a quantified particle beam experiment employing beams of oxygen molecules and argon ions impinging onto an aluminum-coated quartz microbalance. The oxidation and/or sputtering rates are measured with this microbalance and the resulting oxide layers are analyzed by x-ray photoelectron spectroscopy. The sticking coefficient of oxygen molecules is determined to 0.015 in the zero coverage limit. The sputtering yields of pure aluminum by argon ions are determined to 0.4, 0.62, and 0.8 at 200, 300, and 400 eV. The variation in the effective sticking coefficient and sputtering yield during the combined impact of argon ions and oxygen molecules is modeled with a set of rate equations. A good agreement is achieved if one postulates an ion-induced surface activation process, which facilitates oxygen chemisorption. This process may be identified with knock-on implantation of surface-bonded oxygen, with an electric-field-driven in-diffusion of oxygen or with an ion-enhanced surface activation process. Based on these fundamental processes, a robust set of balance equations is proposed to describe target poisoning effects in RMS. (C) 2010 American Institute of Physics. [doi:10.1063/1.3415531]
    view abstract10.1063/1.3415531
  • Phase resolved optical emission spectroscopy of coaxial microplasma jet operated with He and Ar
    Benedikt, J. and Hofmann, S. and Knake, N. and Bottner, H. and Reuter, R. and von Keudell, A. and Schulz-von der Gathen, V.
    European Physical Journal D 60 (2010)
    Atmospheric pressure plasma jet sources are currently in the focus of many researchers for their promising applications in medical industry (e.g. treatment of living tissues), surface modification or material etching or synthesis. Here we report on the study of excitation mechanisms of a coaxial microplasma jet with a hollow capillary as an inner electrode and a ceramic tube with metal ring as outer electrode. This microplasma jet is operated in He and Ar gas and it is investigated by means of electrical measurements, optical emission spectroscopy and space and phase resolved wavelength integrated optical spectroscopy. Measurements of a microscale atmospheric pressure plasma jet with parallel metal electrodes operated in He are shown for comparison as well. Four different modes are distinguished with He as plasma forming gas. The alpha discharge in annular space between the electrodes, observed at low applied voltages, is very similar to the discharge in the jet with parallel electrodes. As the voltage increases a gamma discharge appears, first localized at the tip of the capillary. As the voltage increases further the gamma discharge appears in the annular space as well. A hollow cathode plasma is observed at any voltage used on the symmetry axis of the jet. Only one mode of plasma operation is observed in argon gas with distinctively different behavior. We hypothesize that it is comparable to a single microdischarge of a filamentary dielectric barrier discharge.
    view abstract10.1140/epjd/e2010-00246-9
  • high power impulse magnetron sputtering

  • microplasmas

  • plasma applications

  • spectroscopy

  • sputtering

  • surfaces

  • x-ray photoelectron spectroscopy

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