Measurement of Excitation, Ionization, and Electron Temperatures and Positive Ion Concentrations in a 144 MHz Inductively Coupled Radiofrequency Plasma

Abstract

Diagnostic measurements of 144 MHz radiofrequency inductively coupled plasmas at pressures between 0.5 and 14 Torr have been made. Other variables studied included the gas type (Ar or Ne) and material in plasma (Ti or Tl). Parameters measured included excitation temperatures via the atomic Boltzmann plot and the two-line method, ionization temperatures via the Saha expression, electron temperatures via double electric floating probes, and ion densities via the same electric probes. Excitation temperatures increased as the pressure of Ar or Ne plasmas decreased and reached a maximum of ∼9000°K in the latter case and ∼6700°K in the former case; Tl in the Ar plasma resulted in a smaller rate of decrease of excitation temperature with increase of pressure of Ar. The ionization temperatures were lower than the excitation temperatures and were similar for both the Ar and Ne plasmas. Electron temperatures were about 10 times higher than the excitation temperatures indicating non-LTE behavior. Again, the electron temperatures in Ne were considerably higher than in Ar. With the presence of metals, the electron temperatures with a metal in the Ar plasma were higher than in the absence. Positive ion concentrations were also measured for the various plasmas and were found to be similar (∼1018 m−3) in both the Ar and Ne plasmas. The presence of metals caused significant increase in the positive ion concentrations. From the results obtained, the optimum Ar pressure for Tl electrodeless discharge lamps operated at 144 MHz would be between 2 and 4 Torr.