Structure of RF Parallel-Plate Discharges

The intensity distribution along the discharge axis in a parallel-plate RF-glow discharge was measured in the pressure range of 80 mtorr to 1 torr in nitrogen, argon, and mixtures including attaching gases. Time-averaged spectroscopic measurements showed that the large intensity variations are caused by variations in electron density, at least in the space between electrodes and intensity maxima. In this space, the electron temperature was found to be constant; it decreases towards the center of the discharge. With decreasing pressure, in gases with lower collision frequency and at lower RF frequencies, the intensity peaks move away from the electrodes. Adding an attaching gas made the intensity distribution more uniform. A dc bias voltage on one electrode does not alter the distribution until very high voltages are applied. Time-resolved measurements indicate intensity peaks initially moving away from the electrode negative at a particular half-cycle of the RF frequency with intensities corresponding to the averaged distributions. The peaks are attenuated beyond the maximum, and their velocity increases towards the center and can become negative in time. A simple model including time- and spatially varying oscillations shows the same behavior.