Investigation into nitrogen–inert gas interactions in d.c. diode glow discharges

Neon–nitrogen and argon–nitrogen d.c. diode discharges have been investigated using spatially resolved optical emission spectroscopy (OES) to enable sampling inside the cathode sheath and plasma regions. The discharges were operated at −2 kV cathode bias and at 6.0 Pa total pressure with cathode current density and sheath thickness measurements evaluated for all gas mixtures. The results show that a greater proportion of ions reaching the cathode are nitrogen when neon is used as the primary gas instead of argon. Penning ionisation of nitrogen by neon metastables is the suggested mechanism and the effect is maximised at low nitrogen partial pressures. For 10–50% nitrogen in neon, the current density is higher than that achieved from neon or nitrogen alone or the equivalent argon-based mixtures. Moreover, information from cathode sheath thickness studies suggest that the dominant ionic species reaching the cathode is N+ rather than N2+ or Ne+ at low nitrogen partial pressures. From OES studies, the production of nitrogen atomic species is attributed to N2+–N20 dissociative charge exchange collisions in the sheath and since this mechanism depends on the availability of N2+, the effect is significant in argon-based mixtures only at high nitrogen partial pressures. The implications for plasma processing are also discussed.