Comparison of probe measurements and emission spectroscopy in a radiofrequency discharge

Langmuir probe measurements were performed in connection with optical emission spectroscopy to study the alpha to gamma transition in a symmetric radiofrequency (f=13.56 MHz) discharge between parallel plates in helium. The measurements were carried out over a wide range of pressures (30-300 Pa) and discharge voltages (20-400 V). The correlation between these two diagnostic methods is investigated. The alpha to gamma transition can be seen in the sudden change both in the measured electron-energy distribution function and in the line intensities with increasing discharge voltage. The observed dramatic lowering of the electron temperature in the gamma mode (the secondary-electron-dominated mode) results in extreme differences in the optical emission between singlet and triplet helium lines due to spin-changing collisions of electrons and metastable atoms. It is deduced, using two different electrode materials (aluminium and stainless steel), that secondary electrons and metastable atoms play the dominant role in the formation of the two discharge modes. Furthermore, the probe measurement data are shown to be in very good agreement with those recently reported by Godyak et al. (1992).