Andreas Ohl

Non-thermal atmospheric pressure discharges for surface modification

A series of different discharge configurations suitable for surface treatment at atmospheric pressure is discussed, including a non-thermal modular radio frequency (13.56, 27.12 or 40.78 MHz) jet plasma.The capacitively coupled configuration allows the operation with both rare gases (e.g. Ar) and reactive gases (N2, air, reactive admixtures of silicon-containing compounds). Several capillaries are arranged in an array to allow plasma assisted treatment of surfaces including non-flat geometries. Optical emission spectroscopy, mass spectrometry and measurements of the axial and radial temperature profiles are used to characterize the discharge.The surface energy of different polymer materials is significantly enhanced after plasma treatment. Many applications are possible, such as plasma activation of surfaces for adhesion control, surface cleaning, plasma enhanced CVD, plasma cleaning, plasma activation and biomedical applications.

Local deposition of SiOx plasma polymer films by a miniaturized atmospheric pressure plasma jet (APPJ)

An atmospheric plasma jet (APPJ, 27.17 MHz, Ar with 1% HMDSO) has been studied for the deposition of thin silicon-organic films. Jet geometries are attractive for local surface treatment or for conformal covering of 3D forms, e.g. inner walls of wells, trenches or cavities, because they are not confined by electrodes and their dimensions can be varied from several centimetres down to the sub-millimetre region. Deposition experiments have been performed on flat polymer and glass samples with a deposition rate of 0.25–23 nm s−1. The knowledge of the static deposition profile of the plasma source (footprint) is essential to allow for a controlled deposition with the source moving relative to the substrate. By adjusting the plasma parameters (RF power and gas flow) to the geometry (i.e. electrode configuration, tube diameter, relative tube position, substrate distance) the footprint can be shaped from a ring form reflecting the tube dimension to a parabolic profile. Next to the conventional stochastic mode of operation we observe a characteristic locked mode—reported here for the first time for an RF-APPJ which can improve the film deposition process distinctively. The experimental results of the local film distribution agree well with an analytical model of the deposition kinetics. The film properties have been evaluated (profilometry, XPS, FT-IR spectroscopy and SEM) for different deposition conditions and substrate distance. The FT-IR spectra demonstrate dominating SiO absorption bands, thus providing an indication for the prevailing (inorganic) SiOx character of the films. HMDSO molecules disintegrate to a sufficient degree as proved by the absence of CH2 absorption in the spectra. XPS measurements confirm the local dependence with a slightly increased organic character a few millimetres away from the maximum in the deposition profile. The substrate distance and the source direction both seem relevant and require consideration during coating of 3D objects.

Pulsed and cw microwave plasma excitation for surface functionalization in nitrogen-containing gases

Abstract Results are presented of polymer surface functionalization processes in pulsed and continuous wave (cw) microwave-excited plasmas in nitrogen-containing gases under admixture of hydrogen. A maximum selectivity of 100% for amino groups with respect to all nitrogen functional groups (NH 2 /N) was obtained in cw microwave (MW) plasmas either for very short treatment durations below 100 μs in pure NH 3 , or within approximately 10 s in hydrogen-rich nitrogen-containing plasmas. The amino and overall nitrogen surface densities, NH 2 /C and N/C, reach up to 3.5% and 35%, respectively. Post plasma processes of functionalized polymers are discussed in the light of monofunctionalization. Down to pulse duration of 1 ms, plasma decomposition rates of NH 3 , determined by infrared absorption spectroscopy, are found to scale linearly with the duty cycle. In this regime, the main effect of a duty cycle variation in pulsed NH 3 plasmas on surface functionalization can be interpreted to result from changes in the concentration of the dominant stable species in the gas phase, NH 3 , N 2 and H 2 , which are activated by subsequent plasma pulses. With increasing duty cycle, NH 3 decomposition to N 2 and 3H 2 more and more dominates over the supply of fresh NH 3 . The nitrogen-removing role of hydrogen in the plasma is discussed in detail, whereas the role of the numerous transient nitrogen-containing species remains to be studied in the future.

On the Applicability of Plasma Assisted Chemical Micropatterning to Different Polymeric Biomaterials

A plasma process sequence has been developed to prepare chemical micropatterns on polymeric biomaterial surfaces. These patterns induce a guided localized cell layover at microscopic dimension. Two subsequent plasma steps are applied. In the first functionalization step a microwave ammonia plasma introduces amino groups to obtain areas for very good cell adhesion; the second passivation step combines pattern generation and creation of cell repelling areas. This downstream microwave hydrogen plasma process removes functional groups and changes the linkages of polymer chains at the outermost surfaces. Similar results have been obtained on different polymers including polystyrene (PS), polyhydroxyethylmethacrylate (PHEMA), polyetheretherketone (PEEK), polyethyleneterephthalate (PET) and polyethylenenaphthalate (PEN). Such a rather universal chemical structuring process could widen the availability of biomaterials with specific surface preparations.

Miniaturized non-thermal atmospheric pressure plasma jet—characterization of self-organized regimes

The study reports for the first time on self–organization effects in a radio frequency (RF) plasma generated with a miniaturized non-thermal atmospheric pressure plasma jet. The source is configured as a capacitively coupled RF jet (27.2 MHz) with two outer ring electrodes around a quartz capillary (d = 4.0 mm) between which a gas mixture flows at typical rates of 0.05—5 slm. The application background of this source is the deposition of thin films with a PECVD process. Therefore, thin film producing agents can be added in small quantities downstream the active discharge region. Commonly, the time-resolved observation of the discharge development reveals that the discharge consists of distinct discharge filaments that appear stochastically and evolve alongside the wall of the capillary. This stochastic mode can be easily found under most situations. However, under special conditions, a quasi-laminar flow is established and a controlled number of equidistant filaments develop which form fixed discrete rotating patterns (locked mode). In this paper, a systematic study is performed using Ar as process gas to define the range of existence of the locked mode. The temporal discharge behaviour is studied by performing a low frequency analysis on the optical emission of the plasma. RF power, gas flow rate and electrode distance are interpreted as scaling parameters that are responsible for the self-organization in the non-thermal atmospheric pressure plasma jet. The appearance of the different discharge regimes is described on a phenomenological basis and the collective behavior of the discharge filaments is explained based on the thermal interference of the discharge channels with the gas flow inside the capillary.

Hydrophobic coatings deposited with an atmospheric pressure microplasma jet

Successful plasma polymerization of a fluorocarbon compound (c-C4F8) using an atmospheric pressure plasma jet is described. The source is operated with argon as working gas at a flow rate of 6 slm and 10–100 sccm admixtures of c-C4F8. Deposition is limited to a discharge regime with strong localization and was observed for conductive substrates only (Al and Si). The deposition process is characterized by a high local growth rate (40 nm s−1) and produces films which show a Teflon-like chemical structure and hydrophobicity. The coatings are characterized using x-ray photoelectron spectroscopy, profilometry and scanning electron microscopy. Changing the ambient atmosphere from protective N2 to normal air only reduces the deposition rate but does not change the chemistry of the film.Based on the results of parameter variations and the electrical relations of the jet setup, the special form of the deposition regime of the jet is discussed and considered to be a γ-mode discharge dependent on the choice of substrate material.

On plasma ion beam formation in the Advanced Plasma Source

The Advanced Plasma Source (APS) is employed for plasma ion-assisted deposition (PIAD) of optical coatings. The APS is a hot cathode dc glow discharge which emits a plasma ion beam to the deposition chamber at high vacuum (p 2 × 10−4 mbar). It is established as an industrial tool but to date no detailed information is available on plasma parameters in the process chamber. As a consequence, the details of the generation of the plasma ion beam and the reasons for variations of the properties of the deposited films are barely understood. In this paper the results obtained from Langmuir probe and retarding field energy analyzer diagnostics operated in the plasma plume of the APS are presented, where the source was operated with argon. With increasing distance to the source exit the electron density (ne) is found to drop by two orders of magnitude and the effective electron temperature (Te,eff) drops by a factor of five. The parameters close to the source region read ne 1011 cm−3 and Te,eff 10 eV. The electron distribution function exhibits a concave shape and can be described in the framework of the non-local approximation. It is revealed that an energetic ion population leaves the source region and a cold ion population in the plume is build up by charge exchange collisions with the background neutral gas. Based on the experimental data a scaling law for ion beam power is deduced, which links the control parameters of the source to the plasma parameters in the process chamber.

Modelling and Simulation of the Advanced Plasma Source

Plasma ion assisted-deposition (PIAD) is a combination of conventional thermal evaporation deposition and plasma-beam surface modification; it serves as a well-established technology for the creation of high quality coatings on mirrors, lenses, and other optical devices. It is closely related to ion-assisted deposition to the extent that electrons preserve quasineutrality of the ion beam. This paper investigates the Advanced Plasma Source (APS), a plasma beam source employed for PIAD. A field enhanced glow discharge generates a radially expanding plasma flow with an ion energy of about 80-120 eV. Charge exchange collisions with the neutral background gas (pressure 0.1 Pa and below) produce a cold secondary plasma, which expands as well. A model is developed which describes the primary ions by a simplified Boltzmann equation, the secondary ions by the equations of continuity and momentum balance, and the electrons by the condition of Boltzmann equilibrium. Additionally, quasineutrality is assumed. The mode...

Plasma and optical thin film technologies

The PluTO project is aimed at combining thin-film and plasma technologies. Accordingly, the consortium comprises experts in optical coating (Laser Zentrum Hannover, Fraunhofer IOF) and such in plasma technology (INP Greifswald, Ruhr University of Bochum RUB). The process plasmas available, especially the sheath layers, will be thoroughly characterized by means of special probes, so that the types, numbers and energies of the particles participating in the coating formation processes can be determined comprehensively in every detail for the first time. The data thus obtained will provide a basis for a numerical modelling of layer growth at atomic scale (Bremen Center for Computational Materials Science BCCMS). The results are expected to deepen the understanding of the physical mechanisms responsible for the influence of plasma action on the layer properties. In parallel, suitable tools for process monitoring will be identified and made available. Some first results have already been achieved which prove the viability of the approach.

Influence of supersonic ions and nonlocal electron kinetics on the sheath voltage in an expanding plasma

We present the investigation of the sheath potential in an expanding plasma. The properties of the expanding plasma are measured by means of a Langmuir probe. The obtained data is used to calculate the sheath potential and the electron distribution function. We show that the sheath voltage is typically about 40% lower than in a case that neglects supersonic ions and assumes a Maxwellian electron distribution. We explain the magnitude of the measured sheath potential by balancing the ion flux density calculated with an analytical model for the expanding plasma and the electron flux density calculated with the electron distribution function.