Tungsten patterning for 1:1 x-ray masks

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 Å Cr on 5000 Å W on 200 Å 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. Pure SF6 gives high etching rates but the fluorine radicals attach the W sidewall causing undercutting. Depositing polymer on the sidewall by adding CHF3 or CBrF3 to the SF6 reduces undercutting, but produces sloping profiles. The undercutting found with pure SF6 can be eliminated with vertical profiles by etching at low temperature or by adding N2 or Cl2 to the gas mixture to form low volatility reaction products with tungsten on the sidewall.