@phdthesis{Kuellig2013,
  author    = {Christian K{\"u}llig},
  title     = {Negative Ions and Operation Modes in Capacitively Coupled RF Oxygen Discharges},
  journal    = {Negative Ionen und Operationsmodi in kapazitiv gekoppelten RF Sauerstoffentladungen},
  url       = {https://nbn-resolving.org/urn:nbn:de:gbv:9-001585-6},
  year      = {2013},
  abstract  = {The electron and negative ion densities in an asymmetric capacitively coupled low-pressure RF plasma in oxygen were systematically studied and compared to the electropositive argon RF plasma during continuous and pulsed power input. This work presents the careful design and realization of a non-invasive 160.28 GHz Gaussian beam microwave interferometry (MWI) as an innovative diagnostic tool. MWI directly provides the line integrated electron density without any model assumption. The high microwave frequency enables one to accurately describe the microwave free space propagation by means of Gaussian beam theory. The microwave interferometer is simultaneously coupled with laser photodetachment to experimentally determine the negative ion density in the CCRF oxygen discharge. This is the first time that both diagnostics were combined in low-pressure capacitively coupled RF oxygen plasmas. This thesis first presents comprehensive measurements of the steady state line integrated electron density in dependence on RF power and pressure for an argon and oxygen plasma. For both gases the electron density increases with RF power. However, the line integrated electron density in oxygen is about a factor 3 to 10 smaller than in argon. The reduced electron density is accompanied by a high number of negative ions, which exceeded the electron density and resulted in a high electronegative mode. With increasing RF power, the plasma switches into a low electronegative mode. Consequently, the discharge operates in two different modes, which are distinguished by their degree of electronegativity. The transition between the high and low electronegative modes is step-like and it was concluded that one can here directly see the discharge switches from the \&alpha-mode to the \&gamma-mode. The \&gamma-mode (low electronegative mode, high RF power) is characterized by a strong increase of the electron density and a simultaneous decrease of the negative ion density. The increase may be connected to the production of secondary electrons by collision detachment of negative ions within the RF sheath (“pseudo-secondary electron”), in addition to the classical \&gamma process due to positive ion bombardment of the powered electrode. In comparison to the \&gamma-mode the \&alpha-mode (high electronegative mode, low RF power) reveals more negative ions than electrons. Furthermore, a simple 0d attachment-detachment model was applied to calculate the effective rate coefficients for dissociative electron attachment and collisional detachment from the experimentally determined values of steady state electron and negative ion density, as well as the detachment decay time constant. Hence, the attachment rate coefficient of the molecular ground and the excited metastable state in dependence on RF power were determined. Moreover, the density of metastable molecular oxygen was estimated to 10\% of the molecular ground state oxygen. The influence of each electronegative mode to the entire temporal behavior of the oxygen discharge was intensively investigated by pulsing the discharge. Here it was shown that for the low electronegative mode the afterglow behavior is similar to that of an electropositive argon plasma. In the high electronegative mode an electron density peak in the early afterglow was observed. It was concluded that the electron production originates from the collisional detachment of negative ions. The negative ion loss and the electron production in the early afterglow were modeled numerically with a 0d rate equation system. The model accurately describes the afterglow behavior of both electronegative modes and the additional electron density peak in the early of the high electronegative mode. For the high electronegative mode the molecular oxygen plays an important role as a detachment partner for the production of electrons in the early afterglow. Furthermore, the presence of the negative ions causes fluctuations of plasma parameters. 2d spatial and temporal fluctuations of the ion saturation current are measured during the instability. The temporal and phase resolved optical emission spectroscopy shows a strong change in emission pattern during the instability, which becomes more obvious for one RF cycle at characteristic instability phases. Here, the excitation patterns reveal significant changes in the electron heating mechanisms.},
  language  = {en}
}