Contrast measurement of reflection masks fabricated from Cr and Ta absorbers for extreme ultraviolet lithography

We chose Cr and Ta metals as new absorbing materials for extreme ultraviole t lithography (EUVL) reflection masks. The transmittances of EUV light for these metals were calculated to be nearly equal to those for W metal at the wavelength of 13.5 nm. Reflection masks were successfully fabricated from Ta metal films by dry-etching and from Cr metal films by the lift-off techn ique. The mask contrast was measured using an EUV reflectometer system with a synchrotron radiation source. For the Cr absorber mask, measured peak re flectivities at the reflecting and the absorbing part of the processed patt erns were 59% and 4.9%, respectively, and the contrast was about 12. The pe ak reflectivities at the reflecting and absorbing parts of the processed pa tterns for the Ta absorber mask were 50% and 0.48%, respectively, and the c ontrast was about 105. The masks, prepared about one year before the reflec tivity measurements and stored in air, exhibited no significant degradation of reflectivity or surface quality. However, a 4% reduction in reflectivit y was observed after two years' storage in air, even though the top layer o f the Mo/Si multilayer for the masks was Si, which has been reported to pre vent oxidation.