Ultra-low roughness magneto-rheological finishing for EUV mask substrates
2013
EUV mask substrates, made of titania-doped fused silica, ideally require sub-Angstrom surface roughness, sub-30 nm
flatness, and no bumps/pits larger than 1 nm in height/depth. To achieve the above specifications, substrates must
undergo iterative global and local polishing processes. Magnetorheological finishing (MRF) is a local polishing
technique which can accurately and deterministically correct substrate figure, but typically results in a higher surface
roughness than the current requirements for EUV substrates. We describe a new super-fine MRF® polishing fluid
whichis able to meet both flatness and roughness specifications for EUV mask blanks. This eases the burden on the
subsequent global polishing process by decreasing the polishing time, and hence the defectivity and extent of figure
distortion.
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