Effect of the nature of the working gas on the dc magnetron sputter deposition of chromium nitride and oxi-nitride thin films on steel substrates

Chromium nitride and oxi-nitride films were deposited on low carbon steel substrates by d.c. reactive magnetron sputtering with Ar as the sputtering gas and N-2 or NH3 as the reactive gas (the two gases entering the reactor via two separate inlets in order to avoid cathode poisoning). The effects of different experimental parameters, such as the power density, pressure in the reactor and substrate bias, on the film composition and structure were studied. Stoichiometric or near-stoichiometric CrN coatings were easily obtained in the amorphous or crystalline form depending on the bias voltage applied to the substrate. In contrast, oxi-nitride films were more difficult to obtain because of the high reactivity of oxygen compared with nitrogen. This difficulty could only be overcome by using a very low oxygen flow rate which was provided by the water vapour adsorbed on the reactor walls when the reactor was brought to atmospheric pressure by the introduction of air. Under these conditions, oxi-nitrides of reproducible composition were obtained. The film composition was investigated by X-ray emission spectrometry (X-ray fluorescence spectrometry (XRFS), low energy electron induced X-ray spectrometry (LEEIXS)), X-ray photoelectron spectrometry (XPS) and X-ray diffraction (XRD).