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.
Dynamics and Electronegativity of Oxygen RF Plasmas
The capacitively coupled radio frequency plasma at 13.56 MHz in oxygen was systematically studied by 160 GHz Gaussian beam microwave interferometry at high temporal resolution (200 ns) and simultaneous laser photodetachment for electron and negative ion density analysis. Additionally, spatio‐temporally resolved electric probe measurements were performed for comparison with microwave interferometry. A high and low electronegative operation mode was found in the asymmetric rf discharge. In the high electronegative mode it was shown the significant role of the metastable excited oxygen molecules in electron attachment and detachment processes. In particular, a temporary electron density increase is observed in the early afterglow of a pulsed rf plasma. The transition between both modes is driven by the rf power and the self‐bias voltage, respectively. In connection with the phase resolved optical emission spectroscopy and the study of the electron heating mechanisms the transition into the low electronegative mode at higher rf power shows a relation to the alpha to gamma mode transition. Furthermore, electron density fluctuations are measured over a wide field of processing parameters, e.g. due to the attachment‐induced ionization instability. PIC‐MCC simulation and fluid model calculation of a symmetric oxygen rf discharge confirm the different electron heating mechanisms and the dominance of negative atomic oxygen ions (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Formation of discontinuities in multistage evolution associated with diffusion of a multicomponent weakly ionized hot-electron plasma
A qualitative difference in the way irregularities in two-component and multicomponent plasmas decay is demonstrated. In particular, it is shown that in a multicomponent plasma with hot electrons (electron temperature much greater than the ion temperature) ion density discontinuities can develop. The discontinuities can develop in a plasma with either positive or negative ions. Diffusive evolution in the nonlinear stage can occur in several stages; the final stage can be controlled by the low ion temperature. 10 refs., 3 figs.