Influence of frequency, pressure, and mixture ratio of electronegative gas on electrical characteristics of rf discharges in N₂-SF₆ mixtures

Electrical characteristics of rf discharges in SF₆ and in its mixtures with N₂ were experimentally investigated. In addition space- and time-resolved emission spectroscopy was used to gain a better understanding of kinetics of processes leading to various observed characteristics. A complicated dependence of minimum sustaining voltage on frequency was observed with a peak at 3 MHz. It was explained as the result of transition from the conditions where discharge is sustained by ionization in the bulk and the double-layer region to the conditions where secondary electron yield makes a large contribution. Another possible explanation is the one invoking transition from the conditions where at high-frequency double layer is formed by electron modulation to the condition where a double layer is formed by positive and negative ions. In voltage-current dependencies at 13 MHz two distinct regions were observed similar to the α to γ transitions observed for electropositive gases. Ionization by secondary electrons, however, is not supported by spatiotemporal emission measurements, thus explanation may be sought in different processes such as development of double layers and increased field in the bulk. The current-to-voltage phase is much smaller than in electropositive gases, thus the discharge appears more resistive. For a range of operating conditions the phase even becomes inductive due to negative ion inertia.