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)
Current and Voltage Measurements in the Gaseous Electronics Conference RF Reference Cell
Measurements of the electrical characteristics of discharges in the Gaseous Electronics Conference Radio-Frequency Reference Cell are reviewed here. Topics include: common sources of error in the measurements; comparisons of current and voltage data among GEC cells; the effects of gas impurities, surface conditions and the external circuitry on the reproducibility of the electrical characteristics; and comparisons of current and voltage data with results of other measurements.