The etch characteristics of Ru thin films patterned with TiN hard masks in O-2/Ar, CH4/Ar and O-2/CH4/Ar gas mixtures were studied using inductively coupled plasma reactive ion etching. When the Ru films were etched in O-2/Ar gas, the etch rates of the Ru films and TiN hard masks decreased but the etch selectivity of the Ru films to the hard mask increased with increasing O-2 concentration. The etch profiles of Ru films at various O-2 concentrations were very similar, showing high etch slopes of >80 degrees with uneven sidewalls. In the case of CH4/Ar gas, as the CH4 concentration increased, the etch rates of the Ru films and TiN hard masks and the etch selectivity of Ru films decreased. The etch profile of Ru films etched in 20% CH4/Ar showed the etch slope of approximately 60 degrees and the etch slopes became worse with increasing CH4 concentration. The addition of O-2 to the CH4/Ar gas resulted in a significant improvement in the etch profile, but both etch rates decreased slightly. Optical emission spectroscopy of O-2/CH4/Ar plasma showed that O species could oxidize the Ru films to RuOx, and that inhibition layers containing C or CHx were formed, which protected the pattern sidewalls. X-ray photoelectron spectroscopy also identified the formation of RuOx and C or CHx compounds on the film surface. These findings indicated that Ru etching in O-2/CH4/Ar gas chemistry could provide a good etch profile without redeposition or etch residues. (C) 2016 Elsevier B.V. All rights reserved.