Methods and apparatus for processing a substrate are provided. For example, a method of processing a substrate comprises supplying oxygen (O2) into a processing volume of an etch chamber to react with a silicon-based hardmask layer atop a base layer of ruthenium to form a covering of an SiO-like material over the silicon-based hardmask layer and etching the base layer of ruthenium using at least one of O2 or chloride (Cl2) while supplying nitrogen (N2) to sputter some of the SiO-like material onto an exposed ruthenium sidewall created during etching.