Electrochemically Enhanced Wet Cleaning of Ru Capping Thin Film for EUV Lithography Reflector

We report the effective wet cleaning of a Ru thin-film surface for an extreme ultraviolet (EUV) lithography reflector by combining the passivation of the Ru layer with a surfactant [tetramethylammonium hydroxide (TMAH)] and by tuning the electrochemical interaction of the Ru surface in a propylene carbonate (PC) solution. Adding 1% TMAH to PC showed effective cleaning performance, which reduced the accumulation of carbon amounts and surface chemisorbed O species, while it does not etch or roughen Ru surfaces. It was observed that the TMAH solution effectively removes the residual photoresist on Ru thin films. The cleaning mechanism of TMAH is regarded to be a combined effect between the chemical Ru surface passivation of 1% TMAH and the electrostatic interaction between Ru surfaces and ions in the PC solution. This cleaning scheme may be extended to facilitate the elimination of carbonaceous species on the thin film or nanoparticle surfaces.