Gas-phase reactions of SF₅, SF₂, and SOF with O(³ P): Their significance in plasma processing

Abstract

Reactions of both SF₅ and SF₂ with O(³ P) and molecular oxygen have been studied at 295 K in a gas flow reactor sampled by a mass spectrometer. For reactions with O(³ P), rate coefficients of (2.0±0.5)×10⁻¹¹ cm³ s⁻¹ and (10.8±2.0)×10⁻¹¹ cm³ s⁻¹ were obtained for SF₅ and SF₂ respectively. The rate coefficients for reactions with O₂ are orders of magnitude lower, with an estimated upper limit of 5×10⁻¹⁶ cm³ s⁻¹ for both SF₅ and SF₂. Reaction of SF₂ with O(³ P) leads to the production of SOF which then reacts with O(³ P) with a rate coefficient of (7.9±2.0)×10⁻¹¹ cm³ s⁻¹. Both SO and SO₂ are products in the reaction sequence initiated by reaction between SF₂ and O(³ P). Although considerable uncertainty exists for the heat of formation of SOF, it appears that SO arises only from reaction between SOF and O atoms which is also the source of SO₂. These results are discussed in terms of a reaction scheme proposed earlier to explain processes occurring during the plasma etching of Si in SF₆/O₂ plasmas. A comparison between the results obtained here and those reported earlier for reactions of both CF₃ and CF₂ with O and O₂ shows that there is a marked similarity in the free radical chemistry which occurs in SF₆/O₂ and CF₄/O₂ plasmas.