Jürgen Meichsner

Ionization by bulk heating of electrons in capacitive radio frequency atmospheric pressure microplasmas

Electron heating and ionization dynamics in capacitively coupled radio frequency (RF) atmospheric pressure microplasmas operated in helium are investigated by particle-in-cell simulations and semi-analytical modeling. A strong heating of electrons and ionization in the plasma bulk due to high bulk electric fields are observed at distinct times within the RF period. Based on the model the electric field is identified to be a drift field caused by a low electrical conductivity due to the high electron?neutral collision frequency at atmospheric pressure. Thus, the ionization is mainly caused by ohmic heating in this ??-mode?. The phase of strongest bulk electric field and ionization is affected by the driving voltage amplitude. At high amplitudes, the plasma density is high, so that the sheath impedance is comparable to the bulk resistance. Thus, voltage and current are about 45? out of phase and maximum ionization is observed during sheath expansion with local maxima at the sheath edges. At low driving voltages, the plasma density is low and the discharge becomes more resistive, resulting in a smaller phase shift of about 4?. Thus, maximum ionization occurs later within the RF period with a maximum at the discharge center. Significant analogies to electronegative low-pressure macroscopic discharges operated in the drift-ambipolar mode are found, where similar mechanisms induced by a high electronegativity instead of a high collision frequency have been identified.

Fourier transform infrared spectroscopy study of molecular structure formation in thin films during hexamethyldisiloxane decomposition in low pressure rf discharge

The growth of plasma deposited organic films in general is non-homogeneous, i.e. the films can consist of several layers: substrate–film interface and cross-linked bulk plasma polymer. The fourier transform infrared spectroscopy study shows evidence for a substrate–film interface layer that appears to be formed during a gas conversion in a discharge. The reflection–absorption spectroscopy and evanescent wave spectroscopy techniques have been used to analyse the evolution of molecular structure of the films growing in hexamethyldisiloxane (HMDSO) plasmas of a low pressure capacitive rf (13.56 MHz) discharge. The pulsed operation mode of the rf discharge was used in order to provide successive steps of the HMDSO plasma–chemical conversion into stable neutral products, which were monitored by mass spectrometry. The HMDSO conversion exerts influences on the film deposition resulting in a gradient in the molecular structure of the growing films. The comparison of the film growth on substrates at floating and rf self-bias potentials shows that ions control the deposition kinetics and influence the molecular structure of films.

A novel approach for negative ion analysis using 160 GHz microwave interferometry and laser photodetachment in oxygen cc-rf plasmas

Microwave interferometry at 160.28 GHz with Gaussian beam propagation (beam waist: 5 mm) and laser photodetachment were combined for the analysis of negative atomic oxygen ions in the bulk plasma of an asymmetric capacitively coupled 13.56 MHz discharge (cc-rf). The line-integrated negative oxygen ion density amounts to between 2.5 × 1014 and 1015 m−2 depending on the oxygen pressure and rf power. Furthermore, the measured decay of the detachment signal reveals two modes of rf oxygen plasma characterized by different electronegativities. High electronegativity, α > 2, is associated with a low decay time constant of only a few microseconds, whereas in oxygen plasmas with low electronegativity, α < 1, the relaxation of electron density needs much longer with typical decay time constants of up to about 100 µs. The transition between the two modes shows a step-like characteristic and was observed at a specific rf power depending on the oxygen pressure. In the case of high electronegativity the electron density relaxation can be described by a simple 0D-attachment–detachment model, taking into consideration a constant density for positive ions and neutral oxygen species. Using the appropriate rate coefficients from the literature and the experimentally determined effective rate coefficients of first order kinetics, the evaluation of the attachment and detachment rates indicates the significant role of O2(a 1Δg) in the formation and loss of negative atomic oxygen ions.

Gas-separating properties of membranes coated by HMDSO plasma polymer

Abstract Si-containing gas-separating membranes, produced by conventional chemical methods, demonstrate a high level of gas permeability. The plasma polymerization technique was used for membrane preparation from hexamethyldisiloxane (HMDSO). Plasma polymers were deposited under extremely different conditions. The gas-separating properties and surface morphology of the prepared membranes were investigated. The surface morphology depends strongly on the substrate surface: the plasma polymer on the porous substrate has a hemispherical macrostructure. The composite membrane with plasma polymer has permeability coefficients of the tested gases (O 2 , N 2 , He) comparable with those of the most permeable membrane produced by chemical methods. Such composite membranes demonstrate the higher selectivity for He/N 2 gas pair.

Novel insights into the development of barrier discharges by advanced volume and surface diagnostics

The comprehensive characterization of microdischarges (MDs) requires complementary diagnostics of volume and surface processes at the same discharge configuration under identical conditions. This contribution summarizes the results from optical, spectroscopic and electric investigations as well as the determination of surface charges and metastable nitrogen molecules in filamentary and diffuse barrier discharges. The feasibility of such an approach is demonstrated on selected examples.Fast optical and spectroscopic methods are reviewed for the example of a pulsed driven single filament dielectric barrier MD. It is demonstrated that the methods of streak recording and cross-correlation spectroscopy can complement one another for a comprehensive study of the MD development. Using these techniques it is shown that the so-called prephase is present also in sub-microsecond pulsed barrier discharges. The excitation starts directly with the voltage increase. In the case of diffuse barrier discharges in nitrogen, the combination of spectroscopic and electrical characterization, surface charge measurement by the Pockels effect, and the determination of nitrogen metastables N2(A) by laser-induced fluorescence provides detailed knowledge about the time-integrated surface charge which correlates with the discharge current for each half cycle, whereas the temporal maximum of the metastables of the order of few 1013?cm?3 is delayed in relation to the current maximum. The spatial (axial) maximum of the metastable density is located near the anode like the emission maximum from N2 second positive system at ??=?337?nm. Furthermore, the lifetime of surface charges beyond a typical discharge period has been investigated.

Aging effects of plasma polymerized ethylenediamine (PPEDA) thin films on cell-adhesive implant coatings.

Thin plasma polymer films from ethylenediamine were deposited on planar substrates placed on the powered electrode of a low pressure capacitively coupled 13.56 MHz discharge. The chemical composition of the plasma polymer films was analyzed by Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) as well as by X-ray photoelectron spectroscopy (XPS) after derivatization of the primary amino groups. The PPEDA films undergo an alteration during the storage in ambient air, particularly, due to reactions with oxygen. The molecular changes in PPEDA films were studied over a long-time period of 360 days. Simultaneously, the adhesion of human osteoblast-like cells MG-63 (ATCC) was investigated on PPEDA coated corundum blasted titanium alloy (Ti-6Al-4V), which is applied as implant material in orthopedic surgery. The cell adhesion was determined by flow cytometry and the cell shape was analyzed by scanning electron microscopy. Compared to uncoated reference samples a significantly enhanced cell adhesion and proliferation were measured for PPEDA coated samples, which have been maintained after long-time storage in ambient air and additional sterilization by γ-irradiation.

Evaluation of Osseointegration of Titanium Alloyed Implants Modified by Plasma Polymerization

By means of plasma polymerization, positively charged, nanometre-thin coatings can be applied to implant surfaces. The aim of the present study was to quantify the adhesion of human bone cells in vitro and to evaluate the bone ongrowth in vivo, on titanium surfaces modified by plasma polymer coatings. Different implant surface configurations were examined: titanium alloy (Ti6Al4V) coated with plasma-polymerized allylamine (PPAAm) and plasma-polymerized ethylenediamine (PPEDA) versus uncoated. Shear stress on human osteoblast-like MG-63 cells was investigated in vitro using a spinning disc device. Furthermore, bone-to-implant contact (BIC) was evaluated in vivo. Custom-made conical titanium implants were inserted at the medial tibia of female Sprague-Dawley rats. After a follow-up of six weeks, the BIC was determined by means of histomorphometry. The quantification of cell adhesion showed a significantly higher shear stress for MG-63 cells on PPAAm and PPEDA compared to uncoated Ti6Al4V. Uncoated titanium alloyed implants showed the lowest BIC (40.4%). Implants with PPAAm coating revealed a clear but not significant increase of the BIC (58.5%) and implants with PPEDA a significantly increased BIC (63.7%). In conclusion, plasma polymer coatings demonstrate enhanced cell adhesion and bone ongrowth compared to uncoated titanium surfaces.

160 GHz Gaussian beam microwave interferometry in low-density rf plasmas

160 GHz Gaussian beam microwave interferometry is realized for electron density analysis in low pressure rf plasmas. Measurement of electron densities lower than 1016 m−3 with corresponding phase shift less than 0.3° demands high stability of the interferometer frequency and minimum disturbance due to external interfering voltages and mechanical vibrations of the optical components. The interferometer consists of a frequency stabilized (phase lock loop) heterodyne system operating at a frequency of fMWI = 160.28 GHz and wavelength of λMWI = 1.87 mm, respectively. A quasi-optical setup is used, considering specially designed horn antennas and elliptical mirrors as well as components which have to comply with the aperture limit in relation to the Gaussian microwave beam and its optimal coupling and focusing into the plasma center. A spatial and temporal resolution of about 10 mm (beam waist 5 mm) and 0.2 µs is achieved, respectively. In cc-rf plasma the lowest measurable phase shift is in the order of 0.01°, which corresponds to a line-integrated electron density of about 5 × 1013 m−2 or an electron density of 5 × 1014 m−3 averaged over the electrode diameter. Results are presented and discussed concerning line-integrated electron density in an asymmetric argon cc-rf plasma in dependence on rf power and total pressure.

Radio-frequency discharges in oxygen: II. Spatio-temporally resolved optical emission pattern

Axially and temporally resolved optical emission structures were investigated in the rf sheath region of a parallel plate capacitively coupled rf discharge (13.56 MHz) in pure oxygen and tetrafluoromethane. The rf discharge was driven at total pressures of between 10 and 100 Pa, gas flow rate of 3 sccm and rf power in the range 5–100 W. In particular, the emission of the atomic oxygen at 844.6 nm (3p3P → 3s3S0) and the atomic carbon at 193 nm (3s1P0 → 2p1D) were imaged with a lens onto the entrance slit of a spectrometer and detected by a fast ICCD-camera. The spatio-temporally resolved analysis of the emission intensity during the rf cycle (73.75 ns) provides two significant excitation processes inside the rf sheath: the electron impact excitation at the sheath edge, and heavy particle impact excitation in front of the powered electrode. In oxygen plasma the emission of atomic oxygen was found in both regions whereas in tetrafluoromethane the emission of atomic carbon was observed only in front of the powered electrode. The experimental results reveal characteristic dependence of the emission pattern in front of the powered electrode on plasma process parameters (self-bias voltage, pressure) and allow an estimation of the excitation threshold energy and effective cross section of energetic heavy particle loss.

Transient and stable species kinetics in pulsed cc-rf CF4/H2 plasmas and their relation to surface processes

Fluorocarbon plasmas are widely used in applications and as model systems for fundamental investigations of complex plasmas. In recent years pulsing of the rf discharge has been used as an additional parameter for process control, because many plasma parameters, e.g. densities and temperatures, become time dependent when the rf power is modulated. In this work tunable diode laser absorption spectroscopy in the mid-IR (IR-TDLAS) was applied to measure time-resolved densities of the transient species CF and CF2 and that of the stable product C2F4 in pulsed CF4/H2 asymmetrical capacitively coupled radio-frequency plasmas at 13.56 MHz. Simultaneously, the thickness of amorphous thin fluorocarbon films (a-C:F) on the powered electrode was determined by means of in situ ellipsometry. Therefore, it was possible to study the correlation between gas phase species and thin film formation. The decay curves of the CF and CF2 densities in the off-phase of the pulsed rf plasma were fitted with a combination of first and second order processes involving the loss processes of these radicals in the gas phase and at the surfaces. Particularly, in the plasma off-phase, the loss of CF2 radicals forming C2F4 was found to be dominant in the CF2 kinetics, but of minor importance for C2F4 production. Plasma process parameters such as total pressure, gas composition, power and power modulation were varied to investigate the interaction between gas phase species and surfaces.