Studies of atomic oxygen in O2+CF4 rf discharges by two-photon laser-induced fluorescence and optical emission spectroscopy

Abstract

We have used two-photon laser-induced fluorescence to obtain quantitative measurements of the concentration of ground state O atoms in O2+CF4 rf discharges. Absolute calibration was achieved by generating a known concentration of atomic oxygen by UV laser photolysis of O2. Trace amounts of Ar were added to serve as an inert reference gas for concurrent optical emission measurements, in which the plasma-induced optical emission intensities from O* and Ar* lines were recorded. Emission line shapes were measured using a Fabry–Perot interfermoter to gain information on the mechanisms for formation of excited oxygen atoms in the plasma. Two excitation mechanisms were found to be important: (1) electron impact excitation of ground state atoms, e+O → O*+e, and (2) dissociative excitation of O2, e+O2 → O*+O+e. Evidence for both excitation mechanisms was obtained for O* (8446 Å) emission, with atomic excitation being dominant, whereas dissociative excitation appeared to be the dominant mechanism for O* (7774 Å) emission. Argon actinometry for the determination of ground state oxygen was directly tested. Because of the contribution from dissociative excitation, a strict proportionality, O*/Ar*∝[O]/[Ar], was not satisfied where O* (Ar*) is the intensity of an atomic oxygen (argon) emission line, and [O] ([Ar]) is the oxygen (argon) atom concentration. However, within certain limitations, the O* (8446 Å)/Ar* emission intensity ratio gives the right qualitative trends for the O atom concentration.