A comparative study of the radio frequency discharge in gas mixtures of helium with flourine, oxygen, nitrogen, and argon

Abstract

Mass spectrometric sampling of the positive ions from the rf discharge in an 80 mole % helium‐20 mole % flourine mixture showed the principal ions to be F⁺₂ and F⁺. Impurity ions observed in the specially constructed discharge vessel were O⁺₂ and CF⁺₃ accounting for 10 –15% of the total ions. Very high ion flux was obtained from the discharge in the helium‐flourine mixture, relative to discharge in pure gases. This effect was also found for mixtures of helium with oxygen, nitrogen, and argon. Floating double probe measurements showed that the average electron energy is higher in the helium mixtures than in the pure gases for comparable pressure and power input. Probe measurements also showed that the degree of ionization was higher in the helium mixtures, but the variations were not as large as the mass spectrometric measurements indicated. Some evidence was found to indicate that F⁻ modifies the sheath, but retardation measurements clearly show the charged particle loss from helium‐flourine plasma to be controlled by positive ion‐electron ambipolar diffusion.