A subtractive process to form subhalf micron, vertical‐walled patterns in half‐micron thick tungsten on x‐ray masks has been developed. Electron‐beam lithography was used to form resist patterns on a structure consisting of 300 A Cr on 5000 A W on 200 A Cr on an approximately 1 μm thick poly‐silicon or silicon nitride membrane. The Cr masking and etch‐stop layers above and below the W layer are required because the resist and membrane materials etch rapidly in fluorine based W etching plasmas. Chromium was chosen for these layers because it has a high selectivity in the W etch (≊40:1), is compatible with the W deposition process, and can be patterned in an O2–Cl2 plasma which does not etch W or the membrane materials. Helium backside cooling at a pressure from 1 to 5 Torr controls membrane temperature during all plasma processing steps. Pure CBrF3 or CHF3 etch W slowly while simultaneously depositing polymer which produces sloping profiles where the base of the feature is wider than the initial mask width...