Influence of oxygen content on structure and properties of multi-element AlCrSiON oxynitride thin films

Multi-element oxynitrides of type Al-Cr-Si-O-N were prepared using r.f. magnetron sputtering from Al80Cr2.5Si17.5 (at.%) target composition and O2/(O2 + N2) gas flow ratio between 0 and 100%. Two series of samples varying from pure nitrides to pure oxides and deposited at 400 °C and 650 °C were investigated by (WDS EPMA) + SIMS, XRD, SEM, and nanoindentation measurements. Chemical analysis revealed that the incorporation of oxygen into the films increases much faster than the fraction of oxygen in the gas flow so that the oxide phases can be formed from the gas ratio of about O2/(O2 + N2) = 20%. Addition of oxygen gradually alters the crystallinity of nitride phases due to the incorporation of interstitial atoms and the formation of metal vacancies, but nitride lattices seem to survive up to the overall oxygen incorporation of about 40% into the film. Up on supplying more oxygen flow, the amorphisation of films continues by absorbing more oxygen atoms up to the range of O2/(O2 + N2) ≈ 80% to 90%. Beyond that limit, the formation of crystalline α-Al2O3 was observed, probably stimulated by the presence of α-Cr2O3 and SiO2 in the films. Transition from nitride to oxide has strong consequences on film hardness, which changes from 30-33 GPa for nitrides to 12-13 GPa for amorphous layers, and increases again to 20-25 GPa for pure oxides.