Time-of-Flight Mass Spectrometry of Positive Ions in Helicon-Wave Excited High-Density CF₄ and C₄F₈ Plasmas

The composition of positive ions was investigated using a time-of-flight mass spectrometer in high-density CF 4 and C 4 F 8 helicon plasmas. For low ( n e <1 ×10 12 cm -3 ) electron densities, the dominant positive ions were CF 3 + and CF + in the CF 4 and C 4 F 8 plasmas, respectively, while for high ( n e >8 ×10 12 cm -3 ) electron densities, the dominant positive ions were C + and F + in both the CF 4 and C 4 F 8 plasmas. However, the C + /F + ion current ratio was higher in the C 4 F 8 plasma than the CF 4 plasma. The reaction kinetics of positive ions is discussed based on the electron density dependence of the mass spectral intensities.