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

Neon-nitrogen and argon-nitrogen d.c. diode discharges have been investigat ed using spatially resolved optical emission spectroscopy (OES) to enable s ampling inside the cathode sheath and plasma regions. The discharges were o perated at -2 kV cathode bias and at 6.0 Pa total pressure with cathode cur rent density and sheath thickness measurements evaluated for all gas mixtur es. 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. Pennin g ionisation of nitrogen by neon metastables is the suggested mechanism and the effect is maximised at low nitrogen partial pressures. For 10-50% nitr ogen in neon, the current density is higher than that achieved from neon or nitrogen alone or the equivalent argon-based mixtures. Moreover, informati on from cathode sheath thickness studies suggest that the dominant ionic sp ecies reaching the cathode is N+ rather than N-2(+) or Ne+ at low nitrogen partial pressures. From OES studies, the production of nitrogen atomic spec ies is attributed to N-2(+)-N-2(0) dissociative charge exchange collisions in the sheath and since this mechanism depends on the availability of N-2(), the effect is significant in argon-based mixtures only at high nitrogen partial pressures. The implications for plasma processing are also discusse d. (C) 2001 Elsevier Science B.V. All rights reserved.