Negative ions and operation modes in capacitively coupled RF oxygen discharges 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 rst time that both diagnostics were combined in low pressure capacitively coupled RF oxygen plasmas. This thesis rst 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 ... Hochfrequenzplasma Plasmadynamik Interferometrie Photodetachment Sauerstoff Plasma Elektronegative Plasmen negative Ionen Microwave interferometry Electron density Laserphotodetachment