Rüdiger Foest

Antimicrobial treatment of heat sensitive products by miniaturized atmospheric pressure plasma jets (APPJs)

The technological potential of non-thermal plasmas for the antimicrobial treatment of heat sensitive materials is well known. Despite a multitude of scientific activities with considerable progress within the last few years, the realization of industrial plasma-based decontamination or sterilization technology remains a great challenge. This may be due to the fact that an antimicrobial treatment process needs to consider all properties of the product to be treated as well as the requirements of the complete procedure, e.g. a reprocessing cycle of medical instruments. The aim of this work is to demonstrate the applicability of plasma-based processes for the antimicrobial treatment on selected heat sensitive products. The strategy is to use modular, selective and miniaturized plasma sources, which are driven at atmospheric pressure and adaptable to the products to be treated.

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.

Detection of ozone in a MHz argon plasma bullet jet

This study for the first time confirms the presence of plasma bullets in a MHz argon atmospheric pressure plasma jet. Bullet characteristics are investigated by phase-resolved optical emission measurements. Regarding the jets reactive component output, its ozone production rates are investigated by two independent diagnostic techniques yielding complementary results. The first method—UV-absorption spectroscopy in the Hartley band—determines space-resolved distribution of the ozone concentration in the jet effluent. The second method—quantum cascade laser-absorption spectroscopy in the mid-infrared spectral region—yields high sensitivity results of the average ozone concentration in a multipass cell, in which the effluent is directed. The results of both diagnostic techniques show excellent agreement.

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.

Study of an atmospheric pressure glow discharge (APG) for thin film deposition

Abstract We studied a homogeneous atmospheric pressure dielectric barrier discharge in helium with small admixtures ( −3 ) of hexamethyldisiloxane (HMDSO) vapor for the deposition of thin silicon organic films on a technical aluminum sheet metal. The powered (100 kHz) plane electrode (80×15 mm 2 ) is covered by a glass insulator layer. Power absorption, sustaining voltage (2 kV pp ), gap voltage (700 V), current density (∼20 mA cm −2 ), and total light emission are monitored to characterize the discharge in the gap (1–1.5 mm). The gas composition of the exhaust gas is studied by mass spectrometry. During discharge operation a decrease of the precursor concentration is observed, due to dissociation and thin film deposition. Typical deposition rates range from approximately 0.2 to 2.0 nm s −1 , as measured by substrate weighing. The films display water contact angles of 63±3°. A protection of the Al sheet metal against 0.1n-NaOH for 3 min is observed. FT–IR (dominant SiOSi band) and XPS (low C content) measurements both reveal the dominance of non-organic components in the film. The spatially averaged electron concentration (2×10 11 to 5×10 11 cm −3 ) is experimentally determined by heterodyne interferometry. Discharge properties and thin film deposition are discussed in relation to the ionization rate of the precursor molecules and the current density.

Controlling the NO production of an atmospheric pressure plasma jet

The production of NO radicals by an atmospheric pressure plasma jet has been investigated by means of absorption spectroscopy in the mid-infrared region (IR) and optical emission spectroscopy (OES) in the ultraviolet (UV) part of the spectrum. The plasma jet investigated here operates in argon with air admixtures up to 1%. The study shows that OES can be used to characterize the relative NO production at small air admixtures. The Production of NO radicals can be controlled by variation of air admixture. Important to note—especially for operation in ambient conditions—is that a small addition of water vapour strongly affects the production of NO radicals especially at higher air admixtures (greater than 0.2%).

Absolute production rate measurements of nitric oxide by an atmospheric pressure plasma jet (APPJ)

Tunable diode laser absorption spectroscopy (TDLAS) has been applied to measure the absolute production rate of NO molecules in the gas phase of an atmospheric pressure plasma jet (APPJ) operating at rf (13.56 MHz) in argon with small (up to 1%) admixtures of air. The resulting NO production rates were found to be in the range (0.1–80) × 10−3 sccm or (0.05–35) × 1018 molecules s−1 depending on the experimental conditions. Maximum rates were obtained at 0.2% air. For TDLAS measurements the APPJ was arranged inside an astigmatic multi-pass cell of Herriott type with 100 m absorption length. The insertion into a closed volume differs slightly from the normal, open operation with the jet propagating freely into air. Therefore, the measuring results are compared with optical emission of the open jet to verify equivalent experimental conditions. The dependence of the optical emission of NO (237 nm) on power and gas mixture has been measured. The similar shape of the dependence of absorption and emission signals gives evidence that the comparability of experimental conditions is sufficiently satisfied. It is concluded that the NO production rate of the APPJ in ambient air can be characterized using TDLAS and provides reliable results in spite of differing experimental conditions due to the set-up.

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.

Vacuum UV Radiation of a Plasma Jet Operated With Rare Gases at Atmospheric Pressure

The vacuum ultraviolet (VUV) emissions from 115 to 200 nm from the effluent of an RF (1.2 MHz) capillary jet fed with pure argon and binary mixtures of argon and xenon or krypton (up to 20%) are analyzed. The feed gas mixture is emanating into air at normal pressure. The Ar2 excimer second continuum, observed in the region of 120-135 nm, prevails in the pure Ar discharge. It decreases when small amounts (as low as 0.5%) of Xe or Kr are added. In that case, the resonant emission of Xe at 147 nm (or 124 nm for Kr, respectively) becomes dominant. The Xe2 second continuum at 172 nm appears for higher admixtures of Xe (10%). Furthermore, several N I emission lines, the O I resonance line, and H I line appear due to ambient air. Two absorption bands (120.6 and 124.6 nm) are present in the spectra. Their origin could be unequivocally associated to O2 and O3. The radiance is determined end-on at varying axial distance in absolute units for various mixtures of Ar/Xe and Ar/Kr and compared to pure Ar. Integration over the entire VUV wavelength region provides the integrated spectral distribution. Maximum values of 2.2 mW middotmm-2middotsr-1 are attained in pure Ar and at a distance of 4 mm from the outlet nozzle of the discharge. By adding diminutive admixtures of Kr or Xe, the intensity and spectral distribution is effectively changed.