Abstract
Microwave induced plasmas are applied in many fabrication processes such as the deposition of SiO2 for the production of optical fibers and the deposition of Si to make solar cells. To control these deposition processes a good understanding of the plasma kinetics is required. Experimental investigation and modelling are two complementary strategies for the investigation of plasmas. In this thesis, the surfatron source is chosen as a model system for the validation of different diagnostic techniques and models. One of its main advantages is its flexible operation range and stability when operated in complex gases.
Various experimental techniques, such as Thomson scattering, laser (collisional) induced fluorescence and optical emission spectrometry, are used to investigate the plasma properties as a function of external parameters (e.g. the pressure and the gas composition). For pure argon, the results are compared with two-dimensional simulations of the plasma source, a time dependent global plasma model and a collisional radiative model. The validity and limitations of the various
techniques and models are discussed case by case. The radial electron density and temperature profiles are measured by Thomson scattering for various pressures. Radial contraction is observed for pure argon plasmas above 20 mbar. Molecular ions are found to be partially responsible for that trend but other effects such as Maxwell deviations cannot be neglected either. Deviations from maxwellian equilibrium are investigated as well along the plasma column by a combination of experimental and modelling methods. It is found that the tail of the electron energy distribution function is increasingly depleted for lower ionization degrees. Electron and heavy particles kinetics are an important part of plasma dynamics and need to be included in any plasma model. In combination with different diagnostic techniques, plasma sources can be used for the determination of rate coefficients as well. Electron transfer rate coefficients between the metastable and
resonant 4s states of argon are measured using laser diode absorption spectroscopy and pulsed dye laser perturbation of the plasma in steady state. Pulsed laser collisional induced fluorescence (LCIF) can be used as well for studying electron and heavy particles kinetics. The method is applied, for the first time, to a pure argon plasma. The results are compared with a time-dependent collisional
radiative model (CRM) simulating the LCIF experiment. The argon CRM is found to give relatively good results for steady state but significant discrepancies are found in the case of fast relaxation processes. Based on the experimental results, a path for improvements of the CRM is given. LCIF is used to probe quenching mechanisms in the case of an argon-hydrogen plasma. Significant quenching by atomic hydrogen of argon 4p states is demonstrated. Power modulation is a commonly used method to tune time-averaged plasma properties. The ignition and decay of the plasma are studied using different experimental techniques. The results are compared with a zero-dimensional plasma model and a relatively good agreement is found. It is shown that power interruption has a very small influence on the plasma properties when normalized to time averaged power density. The reasons for this behavior are finally discussed. The criterion to avoid perturbation of the plasma during scattering measurements is revised in the case of high pressure, low electron densities plasmas. It is shown that electron-atom inverse bremsstrahlung contributes predominantly to
laser heating of the electron gas for low ionization degree plasmas. In resumé, advanced (time-resolved) diagnostic techniques and modelling tools are combined to obtain a complete picture of the plasma dynamics. The surfatron source is shown to be a flexible and relatively simple tool for the investigation of plasma chemistry. Although argon plasmas are usually considered to be rather simple, if not trivial, the extensive comparison between experimental results and state-of-the-art modelling shows that significant improvements can still be achieved in our understanding of (microwave) argon plasmas.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 19 Mar 2013 |
Place of Publication | Eindhoven |
Publisher | |
Print ISBNs | 978-90-386-3349-7 |
DOIs | |
Publication status | Published - 2013 |