Modes of rf capacitive discharge in low-pressure sulfur hexafluoride

Abstract

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 (α-) and strong-current (γ -) modes but also in a dissociative δ-mode. This δ-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 α-and γ -modes. The δ-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 α-mode as well as the δ-mode but it is not a cause of the rf discharge transition from α-to δ-mode.

Control of plasma properties in capacitively coupled oxygen discharges via the electrical asymmetry effect

“…As it has been observed in oxygen discharges [78][79][80] as well as in other electronegative plasmas [81][82][83][84], a double layer may appear in electronegative discharges under certain conditions. The formation of a double layer leads to a peak in the electron density localized in front of the electrode sheaths around the time of sheath collapse.…”

Rf discharge dissociative mode in NF3and SiH4

“…Therefore in the weak-current α-mode the dissociation degree of NF 3 molecules is not high and it grows uniformly with the rf voltage increasing due to the plasma density growth. However, as was shown with probe technique in [49] for SF 6 , with the growth of the rf voltage the energy and concentration of high energy electrons from the 'tail' of the electron energy probability function (EEPF) in the quasineutral plasma also grow. At low electron temperature just fast electrons from the EEPF 'tail' colliding with molecules may induce their dissociation.…”

“…Similar data are also absent in the papers of other authors. However we know that in a strongly electro-negative gas SF 6 T e ≈ 6-8 eV [49,61]. The electron temperature in NF 3 would have also be high.…”

“…As is known [41][42][43][44][45][46][47][48][49], the rf capacitive discharge may burn in two different modes: weak-current (α-) and strongcurrent (γ -) ones. The weak-current mode is characterized by low conductance of near-electrode sheaths, the ionization being accomplished by electrons gaining energy in the rf electric field within the plasma volume [41][42][43].…”

“…In paper [49] it was shown that rf discharge in SF 6 might burn not only in the weak-current α-and strong-current γ -modes but also in the so-called dissociative δ-mode. This δ-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 the α-and the γ -modes.…”