Ion kinetic-energy distributions and Balmer-alpha (Hα) excitation in Ar-H2 radio-frequency discharges

Excited neutrals and fast ions produced in a 13.56 MHz radio-frequency discharge in a 90% argon −10% hydrogen gas mixture were investigated, respectively, by spatially and temporally resolved optical emission spectroscopy, and by mass-resolved measurements of ion kinetic energy distributions at the grounded electrode. The electrical characteristics of the discharge were also measured and comparisons are made with results obtained for discharges in pure H2 under comparable conditions. Measurements of Balmer-alpha (Hα) emission show Doppler-broadened emission that is due to the excitation of fast atomic hydrogen neutrals formed from ion neutralization processes in the discharge. Temporally and spatially resolved emission profiles of the Hα radiation from the Ar-H2 mixture are presented for the ‘‘slow’’ component produced predominately by electron-impact dissociative excitation of H2, and for the ‘‘fast’’ component corresponding to energies much greater than can be derived from dissociative excitation. For the Ar-H2 mixture, the fast component is significantly enhanced relative to the slow component. The measured kinetic-energy distributions and fluxes of predominant ions in the Ar-H2 mixture, such as H3+, H2+, H+, and ArH+, suggest mechanisms for the formation of fast hydrogen atoms. The interpretation of results indicate that H+ and/or H3+, neutralized and backscattered by collision with the powered electrode, are the likely sources of fast hydrogen atoms that produce Doppler-shifted Hα emission in the discharge. There is also evidence at the highest pressures and voltages of ‘‘runaway’’ H+ ions formed near the powered electrode, and detected with kinetics energies exceeding 100 eV at the grounded electrode.