Chemical Mechanical Polishing for Extreme Ultraviolet Lithography Mask Substrates

Extreme ultraviolet (EUV) lithography is a nextgeneration lithography technology that uses an exposure wavelength of 13.5 nm. Since all matter absorbs this wavelength, all the optical elements including the photomask need to be reflective. To enable reflection at 13.5 nm, a series of alternative Mo/Si bilayers are deposited on square mask substrates which are 6.35 mm thick. The substrates need to have an ultra-low thermal expansion coefficient, require sub-Angstrom surface roughness, sub-50 nm P-V flatness, and single digit defects larger than 1 nm in height/depth. Currently chemical mechanical polishing (CMP) to achieve such surfaces results in many pit and scratch type defects.