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 SF(6). The rf discharge in SF(6) is shown to exist not only in weak-current (alpha-) and strong-current (gamma-) modes but also in a dissociative delta- mode. This delta-mode is characterized by a high degree of SF(6) dissociation, high plasma density, electron temperature and active discharge current, and it is intermediate between alpha- and. gamma-modes. The delta-mode appears due to a sharp increase in the dissociation rate of SF(6) molecules via electron impact starting after a certain threshold value of rf voltage. At the same time the threshold ionization energy of SF(x) (x = 1-5) radicals formed is below the ionization potential of SF(6) molecules. The double layer existing in the anode phase of the near-electrode sheath is shown to play an important role in sustaining the alpha-mode as well as the delta-mode but it is not a cause of the rf discharge transition from alpha- to delta-mode.