Matthias Wolter

Christian-Albrechts-Universität zu Kiel, Kiel, Schleswig-Holstein, Germany

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Publications (27)

  • [Show abstract] [Hide abstract] ABSTRACT: Quantum cascade laser absorption spectroscopy was used to measure the absolute concentration of acetylene in situ during the nanoparticle growth in Ar + C2H2 RF plasmas. It is demonstrated that the nanoparticle growth exhibits a periodical behavior, with the growth cycle period strongly dependent on the initial acetylene concentration in the chamber. Being 300 s at 7.5% of acetylene in the gas mixture, the growth cycle period decreases with the acetylene concentration increasing; the growth eventually disappears when the acetylene concentration exceeds 32%. During the nanoparticle growth, the acetylene concentration is small and does not exceed 4.2% at radio frequency (RF) power of 4 W, and 0.5% at RF power of 20 W. An injection of a single acetylene pulse into the discharge also results in the nanoparticle nucleation and growth. The absorption spectroscopy technique was found to be very effective for the time-resolved measurement of the hydrocarbon content in nanoparticle-generating plasmas.
    Article · Jun 2011 · Journal of Applied Physics
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    J Beckers · T Ockenga · M Wolter · [...] · G M W Kroesen
    [Show abstract] [Hide abstract] ABSTRACT: We used microparticles under hypergravity conditions, induced by a centrifuge, in order to measure nonintrusively and spatially resolved the electric field strength as well as the particle charge in the collisional rf plasma sheath. The measured electric field strengths demonstrate good agreement with the literature, while the particle charge shows decreasing values towards the electrode. We demonstrate that it is indeed possible to measure these important quantities without changing or disturbing the plasma.
    Full-text Article · Mar 2011 · Physical Review Letters
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    H.R. Maurer · V. Schneider · M. Wolter · [...] · H. Kersten
    [Show abstract] [Hide abstract] ABSTRACT: An interesting aspect in the research of complex (dusty) plasmas is the experimental study of the interaction of microparticles with the surrounding plasma for diagnostic purposes. Local electric fields can be determined from the behavior of particles in the plasma, i.e. particles may serve as electrostatic probes. From particle trajectories, also the determination of momentum flux in beams is possible, and the particles serve as force probes. Recently, temperature sensitive features of particular phosphors were utilized for measuring the surface temperature of microparticles confined in the sheath of a rf plasma. The experiments were performed under variation of gas pressure and rf power of the process plasma and offer a promising approach for the improvement of process plasmas and the understanding of plasma-particle interaction (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Full-text Article · Mar 2011 · Beiträge aus der Plasmaphysik
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    S. Bornholdt · M. Wolter · H. Kersten
    [Show abstract] [Hide abstract] ABSTRACT: . In this paper, an experimental study is presented to characterize a commercially available atmospheric pressure plasma jet (APPJ) kINPen which can be used for local surface modification, e.g. changing the wettability as well as for thin film deposition with silicon-organic and metal-organic precursors to enhance scratch resistance or to lower the gas permeability. Characterization of the jet discharge has been carried out by three methods: (i) measurement of the energy influx from the jet plasma to a substrate by a calorimetric probe, (ii) spatial resolved investigation of the plasma beam by optical emission spectroscopy (OES) and (iii) observation of the plasma jet by video imaging. The deposited SiO x and AlO x films were analyzed by XPS measurements.
    Full-text Article · Dec 2010 · The European Physical Journal D
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    [Show abstract] [Hide abstract] ABSTRACT: Two different diagnostics for the determination of the energy influx in plasma processes were used to characterize an ion beam source and an asymmetric RF discharge. The related energy fluxes were measured in dependence on the ion energy and on the RF power, respectively. The first sensor, called HFM (Heat Flux Microsensor) is a thermopile which allows for direct energy flux measurements. With the second sensor, a calorimetric probe, the energy influx has been calculated from the temporal temperature evolution preliminarily registered. Although the working principle of both sensors is different, the obtained results are in good agreement. In the ion beam (<1.5 keV)) rather high energy influxes are achieved (up to 700 mW cm−2), whereas the values measured in the asymmetric RF discharge were lower than 50 mW cm−2 for discharge powers in the range 10–100 W. The performances and limitations of both sensors are compared and discussed.
    Full-text Article · Nov 2010 · Journal of Physics D Applied Physics
  • Matthias Wolter · Morten Hundt · Holger Kersten
    [Show abstract] [Hide abstract] ABSTRACT: For thin film deposition or plasma etching often organic precursors are used in the process plasma and related transient species are formed. In general it is not possible to measure the converted quantity of these precursors directly. In the present work we have used a special laser absorption spectroscopy to investigate characteristic molecular lines in the plasma to determine the concentration of stable organic molecules. Quantum cascade laser absorption spectroscopy (QCLAS) is a rather new technique for the precise measurement of absolute molecule concentrations. QCL’s can be operated at room temperature. They emit light within the mid infrared and have similar spectroscopic characteristics to Tunable Diode Lasers (TDL). The commercially available system Q-MACS (Quantum Cascade Laser Measurement and Control System) offers a solid platform for the measurement of absolute molecule concentrations in plasmas and gas mixtures. The used Q-MACS is due to its laser characteristics particularly well suitable for determination of the concentrations of acetylene and methane. Molecular concentrations of methane were measured in hexamethyldisiloxane (HMDSO) containing plasmas, too. The methane concentration was found to depend on rf power and HMDSO flow.
    Article · Oct 2010 · Vacuum
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    M. Wolter · I. Levchenko · H. Kersten · [...] · K. Ostrikov
    [Show abstract] [Hide abstract] ABSTRACT: The possibility to discriminate between the relative importance of the fluxes of energy and matter in plasma-surface interaction is demonstrated by the energy flux measurements in low-temperature plasmas ignited by the radio frequency discharge (power and pressure ranges 50–250 W and 8–11.5 Pa) in Ar, Ar+H2, and Ar+H2+CH4 gas mixtures typically used in nanoscale synthesis and processing of silicon- and carbon-based nanostructures. It is shown that by varying the gas composition and pressure, the discharge power, and the surface bias one can effectively control the surface temperature and the matter supply rates. The experimental findings are explained in terms of the plasma-specific reactions in the plasma bulk and on the surface.
    Full-text Article · Sep 2010 · Journal of Applied Physics
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    M. Quitzau · M. Wolter · V. Zaporojtchenko · [...] · F. Faupel
    [Show abstract] [Hide abstract] ABSTRACT: Hexamethyldisiloxane (HMDSO) films were deposited on polyethylene (PE, (C2H4)n) powder by hollow cathode glow discharge. The reactive species in different HMDSO/Ar plasmas were studied by optical emission spectroscopy (OES). Increasing the HMDSO fraction in the gas mixture additional compounds like CHx, OH, SiC and SiO can be identified. After deposition the formed silicon and carbon containing groups (C–O, C=O, SiC and SiO) on the PE powder surface have been analyzed by X-ray photoemission spectroscopy (XPS). Changes in wettability depending on the HMDSO fraction were investigated by contact angle measurements (CAM). The free surface energy of the PE powder decreases with increasing HMDSO fraction in the process gas and encapsulation of the powder particles occurs. An aging effect of the plasma treated PE surface was observed depending on the process gas composition. The higher the HMDSO fraction the less is the aging effect of the plasma treated PE surface.
    Full-text Article · Jun 2010 · The European Physical Journal D
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    Oleg Baranov · Maxim Romanov · Matthias Wolter · [...] · Kostya (Ken) Ostrikov
    [Show abstract] [Hide abstract] ABSTRACT: Current-voltage characteristics of the planar magnetron are studied experimentally and by numerical simulation. Based on the measured current-voltage characteristics, a model of the planar magnetron discharge is developed with the background gas pressure and magnetic field used as parameters. The discharge pressure was varied in a range of 0.7–1.7 Pa, the magnetic field of the magnetron was of 0.033–0.12 T near the cathode surface, the discharge current was from 1 to 25 A, and the magnetic field lines were tangential to the substrate surface in the region of the magnetron discharge ignition. The discharge model describes the motion of energetic secondary electrons that gain energy by passing the cathode sheath across the magnetic field, and the power required to sustain the plasma generation in the bulk. The plasma electrons, in turn, are accelerated in the electric field and ionize effectively the background gas species. The model is based on the assumption about the prevailing Bohm mechanism of electron conductivity across the magnetic field. A criterion of the self-sustained discharge ignition is used to establish the dependence of the discharge voltage on the discharge current. The dependence of the background gas density on the current is also observed from the experiment. The model is consistent with the experimental results.
    Full-text Article · May 2010 · Physics of Plasmas
  • Article · May 2010
  • [Show abstract] [Hide abstract] ABSTRACT: The possibility of independent control of the surface fluxes of energy and hydrogen-containing radicals, thus enabling selective control of the nanostructure heating and passivation, is demonstrated. In situ energy flux measurements reveal that even a small addition of H2 to low-pressure Ar plasmas leads to a dramatic increase in the energy deposition through H recombination on the surface. The heat release is quenched by a sequential addition of a hydrocarbon precursor while the surface passivation remains effective. Such selective control offers an effective mechanism for deterministic control of the growth shape, crystallinity, and density of nanostructures in plasma-aided nanofabrication.
    Article · Mar 2010 · Applied Physics Letters
  • J Beckers · T Ockenga · M Wolter · [...] · H Kersten
    Article · Jan 2010
  • [Show abstract] [Hide abstract] ABSTRACT: The electric field profile in the plasma sheath of an argon rf plasma has been determined by measuring the equilibrium height and the resonance frequency of plasma-confined microparticles. In order to determine the electric field structure at any position in the plasma sheath without the discharge being changed or disturbed an additional, non-electric, force is introduced which does not alter the plasma conditions, but which does allow for manipulation of the particle position through the sheath: (hyper-)gravity, induced by a centrifuge. Consequently, the electric field and the particle charge can be determined as function of the position in the sheath, using one and the same particle for measurements at several positions throughout the sheath. Particle charges between 6000 and 7000 times the electron charge are determined. Closer to the electrode, an increase of the particle charge is observed. Over the largest part of the sheath the electric field is linear, while close to the sheath edge its behaviour appears to be non-linear. Absolute values of the electric field at the electrode (-25.000V/m) are consistent with literature [1].
    Article · Jan 2010 · IEEE International Conference on Plasma Science
  • Holger Kersten · Matthias Wolter
    [Show abstract] [Hide abstract] ABSTRACT: Formation, trapping, and modification of powder particles in laboratory received growing interest during the last decade in research and technology with novel and unique properties. Applications of complex (dusty) plasmas are numerous, most of them emerging in modern material science. For the optimizations of these processes, a detailed knowledge of plasma–particle interaction is needed. On the other hand, due to the interaction between small (test) particles and the surrounding plasma information on the electric field in front of surfaces and the energy fluxes in the plasma can be obtained. Dust particles can be used as a diagnostic tool, for example, by observing position and motion of the particles in dependence on the discharge parameters. In a certain sense, one can state that “complex (dusty) plasmas” are a rapidly expanding field of research at the border between plasma physics, material processing, and diagnostics, for example, for synthesis and modification of powder particles.
    Chapter · Dec 2009
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    Wolter M · Bornholdt S · Häckel M · Kersten H
    [Show abstract] [Hide abstract] ABSTRACT: Purpose: Polymers are commonly used as packing materials as well as for optical and microelectronic applications. For these purposes different requirements like impermeability for different gases, scratching firmness and electrical conductivity are demanded. Since, polymers usually do not exhibit these attributes a surface modification is necessary.Design/methodology/approach: This paper describes possibilities for coating of polymers with a cold atmospheric pressure plasma jet (APPJ). Due to the rather low temperature of the process the plasma jet is suitable for the treatment of temperature-sensitive materials with melting points below 150°C. For coating of polymers the organic precursor Hexamethyldisiloxane (HMDSO) has been used to deposit silicon oxide layers on surface.Findings: Spatial distributions of reactive species have been measured by optical emission spectroscopy (OES) in the range between 280 and 1100 nm during the plasma process. The energy influx to the substrate was determined by thermal probe measurements. For the affirmation of the chemical composition of the surface X-ray photon spectroscopy (XPS) has been performed.Practical implications: It could be confirmed that SiOx thin film deposition on polymeric substrate using commercially available APPJ with no internal precursor feeding is possible.Originality/value: The examinations of atmospheric pressure plasma jet for treatment of polymers.
    Full-text Article · Dec 2009
  • [Show abstract] [Hide abstract] ABSTRACT: Summary form only given: For diagnostic purposes micrometer-sized particles can be used as floating electrostatic probes. Once injected into a complex rf plasma, these particles will become negatively charged and can be trapped in the plasma sheath due to an equilibrium of several forces working on them, e.g. the electrostatic force, gravity, drag forces and thermophoresis. Measuring for example the position of the particles in the plasma sheath and the interparticle distance while varying plasma parameters (power, pressure, temperature, gas etc.) gives important information about plasma properties like the ion flux and the sheath potential. We experimentally investigated the behavior of micrometer sized particles inserted and trapped in a rf plasma under varying gravity conditions in a centrifuge. Here we present first results of those measurements. The experiments were carried out in a Perspex box containing a parallel plate capacitively coupled rf argon plasma at pressures between 20 and 115 Pa. The typical forward power applied to the bottom electrode was ~10 Watt. The squared electrodes are separated 5 cm from each other and both contain centered holes in order to trap the particles in the created potential well. This also gains possibilities to observe particle behavior from below. The monodisperse particles which are made of melamineformaldehyde and have sizes ranging from 5 up to 12 ?m are illuminated by an expanded 532 nm laser beam. The height of the particles on which the forces are in equilibrium is measured from pictures collected with an onboard CCD camera. This whole setup is mounted on a centrifuge originally developed to study high pressure metal halide lamps under hyper gravity conditions. Results show that under these condition particles can be trapped in the plasma sheath when the gravitational force is 2.6g or less. When larger acceleration forces are applied the particles are lost from the discharge. Due to the increased apparent gravity of the- particles in the centrifuge the height of the cloud above the powered bottom electrode decrease with ~2 mm when the acceleration force is increased from 1g up to 2.6g.
    Article · Jun 2009 · IEEE International Conference on Plasma Science
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    Meike Quitzau · Matthias Wolter · Holger Kersten
    [Show abstract] [Hide abstract] ABSTRACT: Polyethylen (PE) is widely used in the production of foils, insulators, packaging materials, plastic bottles etc. Untreated PE is hydrophobic due to its unpolar surface. Therefore, it is hard to print or glue PE and the surface has to be modified before converting. In the present experiments a hollow cathode glow discharge is used as plasma source which is mounted in a spiral conveyor in order to ensure a combines transport of PE powder particles. With this set‐up a homogeneous surface treatment of the powder is possible while passing the glow discharge. The plasma treatment causes a remarkable enhancement of the hydrophilicity of the PE powder which can be verified by contact angle measurements and X‐ray photoelectron spectroscopy.
    Full-text Article · Jun 2009 · Plasma Processes and Polymers
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    Matthias Wolter · Marc Stahl · Holger Kersten
    [Show abstract] [Hide abstract] ABSTRACT: The total energy flux for an RF plasma (13.56 MHz) has been measured by means of a simple thermal probe. The procedure is based on the measurement of time dependent changes of the probe temperature during the plasma process. A substrate dummy which is thermally isolated and inserted into the plasma at substrate position served as thermal probe which can be moved in vertical and horizontal directions in order to measure the different energy fluxes and their distribution in the reactor vessel. The knowledge of the spatial distribution is important, for example, for coating or sputtering processes. Different contributions to the total energy flux are identified by different orientations of the thermal probe, e.g., if the probe is facing the RF electrode the energy flux is much higher than in the opposite direction. This difference can be addressed to an additional energetic contribution due to secondary electron emission from the powered RF electrode.
    Full-text Article · Jun 2009 · Plasma Processes and Polymers
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    Matthias Wolter · Moritz Haass · Taalke Ockenga · [...] · Holger Kersten
    [Show abstract] [Hide abstract] ABSTRACT: In basic plasma physics and plasma technology, micro-particles (dust grains) play an important role for the investigation and understanding of the behavior of different plasma species. It is well known that micrometer-sized particles can find an equilibrium position if all acting forces to the particles are balanced. This stable point is a few millimeters above the powered electrode in the plasma sheath. In our experimental set-up, we observed the position and the motion of microscopic test particles with a typical radius between 4.78 and 13.3 µm. In addition to the experiments, we also investigate the particle behavior by a numerical model. By comparison between the numerical model and the experimental observation, internal plasma parameters can be obtained.
    Full-text Article · Jun 2009 · Plasma Processes and Polymers
  • J Beckers · M Wolter · WW Stoffels · GMW Kroesen
    Article · Jan 2009 · Journal of Ambient Intelligence and Humanized Computing