A vertical stack including a dielectric hard mask layer and a titanium nitride layer is formed over an interconnect-level dielectric material layer such as an organosilicate glass layer. The titanium nitride layer may be partially or fully converted into a titanium oxynitride layer, which is subsequently patterned with a first pattern. Alternately, the titanium nitride layer, with or without a titanium oxynitride layer thereupon, may be patterned with a line pattern, and physically exposed surface portions of the titanium nitride layer may be converted into titanium oxynitride. Titanium oxynitride provides etch resistance during transfer of a combined first and second pattern, but can be readily removed by a wet etch without causing surface damages to copper surfaces. A chamfer may be formed in the interconnect-level dielectric material layer by an anisotropic etch that employs any remnant portion of titanium nitride as an etch mask.