Inspection of contamination in nitrogen plasmas by monitoring the temporal evolution of the UV bands of NO-γ and of the fourth positive system of N2

In this study, contamination by oxygen species in nitrogen plasmas produced by the active screen system used for plasma nitriding has been investigated by optical emission spectroscopy in the spectral range of 200–900 nm. Temporal evolution of emission intensity of different species (N2, N2+, NO, OH, H, NH, and Fe) was monitored, as well as electrical characteristics (current and voltage) of the discharge produced by a pulsed unipolar power supply. In nitrogen plasma, it was found that the emission of oxygen-containing species (NO and OH) decreases with time, while Fe emission intensity increases. Such behavior is observed only when the discharge is initiated immediately after venting the reactor. Starting from the hypothesis of contamination of the reactor walls by water vapor, we propose an explanation based on the synergistic effect between the temperature and the reactive nitrogen created by the plasma. Such a long decay of NO emission was not observed in N2–H2 gas mixtures. After decontamination, the fourth positive system of N2 could be observed. Such a not commonly observed system can be used as a tool to control reactor cleanliness in pure nitrogen discharge for nitriding applications.