Modes of rf capacitive discharge in low-pressure sulfur hexafluoride

This paper presents the results of an experimental study of rf capacitive discharge in low-pressure SF6. The rf discharge in SF6 is shown to exist not only in weak-current (α-) and strong-current (γ-) modes but also in a dissociative δ-mode. This δ-mode is characterized by a high degree of SF6 dissociation, high plasma density, electron temperature and active discharge current, and it is intermediate between α- and γ-modes. The δ-mode appears due to a sharp increase in the dissociation rate of SF6 molecules via electron impact starting after a certain threshold value of rf voltage. At the same time the threshold ionization energy of SFx (x = 1–5) radicals formed is below the ionization potential of SF6 molecules. The double layer existing in the anode phase of the near-electrode sheath is shown to play an important role in sustaining the α- mode as well as the δ-mode but it is not a cause of the rf discharge transition from α- to δ-mode.