Investigations of a Metastable Dependence on the Ionization of Sputtered Species in Neon Glow Discharges

Optical investigations of a low-pressure (0.3–4.0 Torr), low-current (1–4 mA), coaxial geometry glow discharge operating with neon as the fill gas are described. Studies were designed to experimentally illustrate the role of neon metastable atoms in the population of selected excited-state ion levels of copper atoms sputtered from a brass cathode. Methane was employed as a quenching agent to reduce the neon metastable population, and ion emission signals from a variety of copper ion transitions showed a decrease in intensity corresponding to the introduction of methane to the plasma. In addition, with variations in discharge pressure, a correlation between the number of neon metastables and the strength of the ion emission signals was observed. These results provide evidence that Penning ionization is an important mechanism for the ionization of sputtered atoms in neon glow discharges, similar to the results obtained for an argon system. Finally, a brief comparison of the neon and argon systems was made which showed the neon discharge gas to be more efficient at populating the monitored copper ion levels. This is most likely due to the higher energy of the neon metastables, which permits the direct population of these ion levels from the copper ground state.