52.40.Hf Plasma-material interactions; boundary layer effects
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Die vorliegende Arbeit widmet sich der plasmachemischen Herstellung und physikalisch-chemischen Charakterisierung von dünnen organischen Schichten auf der Basis von Ethylenglykol (Präkursor). Die Oberflächen können die Adsorption von Proteinen minimieren und daher als neues biokompatibles Material getestet werden. Im Schwerpunkt der Arbeit liegt die Entwicklung eines plasmachemischen RF-Reaktors (genannt Nevada) und einer innovativen Beschichtungstechnologie TFPD (Temperature Forced Plasma Deposition) als Erweiterung der PECVD (Plasma Enhanced Chemical Vapour Deposition). Ein Gemisch aus Ethylenglykol und Argon wurde als Modellsystem untersucht. Die Plasmabedingungen und die Oberflächentemperatur wurden optimiert und an Phaseneigenschaften von Ethylenglykol angepasst. Die komplexe Polykondensation steht für die plasmagestützte Umwandlung der Kondensatschicht zu einem stabilen Poly(Ethylenglykol)-ähnlichen Plasmapolymer. Der Prozess wurde durch die simultane Temperaturkontrolle und in situ FTIR-Spektroskopie analysiert. Zur Untersuchung der erzeugten Proben wurden weitere ex situ Oberflächenanalysen wie XPS, AFM, TDS, MALDI, XRD und die optische Ellipsometrie verwendet. Durch die neue Methode TFPD entstehen extrem glatte, amorphe und wasserunlösliche Schichten mit einem Potenzial für die Biokompatibilität.
Infrared laser absorption spectroscopy (IRLAS) employing both tuneable diode and quantum cascade lasers (TDLs, QCLs) has been applied with both high sensitivity and high time resolution to plasma diagnostics and trace gas measurements.
TDLAS combined with a conventional White type multiple pass cell was used to detect up to 13 constituent molecular species in low pressure Ar/H2/N2/O2 and Ar/CH4/N2/O2 microwave discharges, among them the main products such as H2O, NH3, NO and CO, HCN respectively. The hydroxyl radical has been measured in the mid infrared (MIR) spectral range in-situ in both plasmas yielding number densities of between 1011 ... 1012 cm-3. Strong indications of surface dominated formation of either NH3 or N2O and NO were found in the H2-N2-O2 system. In methane containing plasmas a transition between deposition and etching conditions and generally an incomplete oxidation of the precursor were observed.
The application of QCLs for IRLAS under low pressure conditions employing the most common tuning approaches has been investigated in detail. A new method of analysing absorption features quantitatively when the rapid passage effect is present is proposed. If power saturation is negligible, integrating the undisturbed half of the line profile yields accurate number densities without calibrating the system. By means of a time resolved analysis of individual chirped QCL pulses the main reasons for increased effective laser line widths could be identified. Apart from the well-known frequency down chirp non-linear absorption phenomena and bandwidth limitations of the detection system may significantly degrade the performance and accuracy of inter pulse spectrometers. The minimum analogue bandwidth of the entire system should normally not fall below 250 MHz.
QCLAS using pulsed lasers has been used for highly time resolved measurements in reactive plasmas for the first time enabling a time resolution down to about 100 ns to be achieved. A temperature increase of typically less than 50 K has been established for pulsed DC discharges containing Ar/N2 and traces of NO. The main NO production and depletion reactions have been identified from a comparison of model calculations and time resolved measurements in plasma pulses of up to 100 ms. Considerable NO struction is observed after 5 ... 10 ms due to the impact of N atoms.
Finally, thermoelectrically cooled pulsed and continuous wave (cw) QCLs have been employed for high finesse cavity absorption spectroscopy in the MIR. Cavity ring down spectroscopy (CRDS) has been performed with pulsed QCLs and was found to be limited by the intrinsic frequency chirp of the laser suppressing an efficient intensity build-up inside the cavity. Consequently the accuracy and advantage of an absolute internal absorption calibration is not achievable. A room temperature cw QCL was used in a complementary cavity enhanced absorption spectroscopy (CEAS) configuration which was equipped with different cavities of up to ~ 1.3 m length. This spectrometer yielded path lengths of up to 4 km and a noise equivalent absorption down to 4 x 10-8 cm-1Hz-1/2. The corresponding molecular concentration detection limit (e.g. for CH4, N2O and C2H2 at 1303 cm-1/7.66 μm) was generally below 1 x 1010 cm-3 for 1 s integration times and one order of magnitude less for 30 s integration times. The main limiting factor for achieving even higher sensitivity is the residual mode noise of the cavity. Employing a ~ 0.5 m long cavity the achieved sensitivity was good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar.
(A paperback version is published by Logos under ISBN 978-3-8325-2345-9.)
Beams of ions and electrons are a source of free energy which can be transferred to waves via an instability. Beams exist in almost all plasma environments, but their instabilities are particularly important for the dynamics of space plasmas. In the absence of collisions, the instability drives waves to large amplitudes and forms nonlinear structures such as solitary waves. The electric fields in these waves can scatter particles in the background plasma, or disrupt currents. Both of these effects are important for the overall dynamics of the plasma. In this thesis, both electron and ion beam plasma instabilities have been investigated in the linear plasma device VINETA and using a Particle-in-Cell simulation. The electron beam instability has been demonstrated by previous authors to be a useful diagnostic for the plasma density. The spatial resolution of previous results was confirmed at a few millimetres, and a temporal resolution of 1ms was shown for the first time. An ion beam was generated with a double plasma discharge. Compared to space, this environment and indeed most laboratory plasmas have considerably higher collisionality and a limited spatial extent which introduces gradients in the plasma. Gradients perpendicular to the beam propagation direction are linked to a decrease of both the wavelength and amplitude of the instability. It was observed in both experiment and simulation that gradients in sheaths at the boundaries of the plasma not only affect the time averaged plasma parameters, but also excite instabilities. Fluctuations within the sheath spread the beam in velocity space, effectively increasing its temperature. Warmer beams require a higher drift velocity to excite an instability. This was also confirmed by experimental and numerical results. Collisions are shown to be the dominant damping force for the electron beam instability. For ions, collisions play an important role in the simulation, but appear to be overshadowed by Landau damping from impurities in the experiment. When boundary conditions are removed from the simulation, wave amplitudes increase and nonlinear effects become important. Saturation by particle trapping and coalescence of phase space holes is observed, which could eventually lead to the solitary waves as they are observed in space plasmas.
In this work, various aspects of fundamental physics and chemistry of molecular gas discharges are presented with emphasis on the interaction between species, activated by low-pressure plasmas, and surfaces. As already known, synergistic effects of multiple plasma-generated species are responsible for surface modification. However, due to the large number of internal parameters of a discharge and the complex plasma processes the identification of correlations between plasma characteristics and their effects on surfaces are complicated. Therefore, the aim of this thesis is to improve the understanding of several phenomena associated with plasma–surface interactions by measuring or calculating fundamental kinetic, transport or spectroscopic data needed to interpret measurements and hereby, to support some future applications of plasmas.