Electron Density Oscillations in CC-RF Oxygen Plasma Investigated by Gaussian Beam Microwave Interferometry
Gaussian beam microwave interferometry (160 GHz, beam waist of 5 mm) is applied to study the electron density in capacitively coupled radio-frequency (RF) oxygen plasma. The microwave interferometry provides immediately the line-integrated electron density without model assumptions. In the considered range of RF power and total pressure, the investigations have shown that the oxygen plasma is dominated by electron density oscillations due to the attachment-induced ionization instability. The frequency of the mostly nonharmonic oscillations ranges between 0.3 and 3 kHz with peak-to-peak line-integrated electron density variations up to 3.5 × 1015 m-2.
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.
Investigation of the disturbance of a Langmuir probe and its influence on measurement results
Measurements of plasma parameters by Langmuir probes are influenced by the probe system itself. This article considers the deviation of plasma parameters determined by probe measurements from those in the undisturbed plasma. For this reason the particle density of the excited atoms of a neon plasma in the very vicinity of the probe is compared with the particle density in the undisturbed case. The densities of the excited metastable 1s5 level were measured by laser absorption spectroscopy. By solving the plasma balance equations the deviation of the electron density and the deviation of the electric field could be deduced.