Study of late argon–nitrogen post-discharge interactions with oxidized iron surfaces

Abstract Nitrogen-containing post-discharges are widely used for enhancement of polymer printability and adhesion and wettability properties. It has also applications in the field of remote plasma enhanced chemical vapour deposition of thin nitride films or nitriding. Interactions occurring between an argon–N2 post-discharge and oxidized iron surfaces are studied. Measurements are based on optical emission spectroscopy measurements. Two regimes are considered. First, the analysis of transient effects sheds light on the cleaning effects which modify permanently the topmost surface of oxidized iron substrates. They are evidenced by XPS measurements. Two steps occur. During the first one, a reaction between nitrogen atoms and carbon of ketones or carbonyl-like groups probably due to acetone residue, identified by XPS analyses, gives CN radicals. The CN(B2Σ,v)→CN(X2Σ,v′) transition is identified by optical emission spectroscopy. This emission vanishes progressively as carbon is consumed and the second step starts at the end of the violet emission where the loss probability γN of nitrogen atoms increases. The increase of the recombination rate is attributed to abstraction reactions of adsorbed elements by nitrogen atoms to form nonemissive species. For example, hydroxide groups are removed by the post-discharge. The surface modifications induced by this treatment are reproducible. This was verified by comparison between the behaviours of the native oxide and a 0.5-μm-thick layer grown by controlled post-discharge oxidation. When the steady-state regime is reached, the loss probability γN on the treated surfaces is determined in a range of temperatures (i.e., 300–473 K) where the nitrogen bulk-diffusion is assumed to be negligible. Nitrogen atoms recombine according to an Eley–Rideal mechanism and γN=(8.2±0.5)×10−2 exp(−11400±500/RT).