Damage resistant and low stress EUV multilayer mirrors

Applications of multilayer mirrors for extreme ultraviolet lithography (EUVL) require not only a high normal incidence reflectivity but also a long lifetime and a minimum residual stress. We focused our interests on a comparative study of two perspective Si-based multilayer systems: Mo/Si and Mo2C/Si. The mirrors were designed for normal incidence reflection at about 13 nm wavelength. The multilayer mirrors were deposited by dc magnetron sputtering. X-ray scattering, transmission electron microscopy, atomic force microscopy and mechanical stress measurements were used for the characterization of the multilayer structures. Maximum normal incidence reflectivities of 67.5 % @ 13 nm for Mo/Si multilayer mirrors and 66.3 % @ 12.8 nm for Mo2C/Si have been achieved. Investigating the thermal stability of the multilayers in the temperature range from 200 deg C to 700 deg C it was shown that the reflectivity of Mo/Si mirrors is drastically decreasing after annealing above 300 deg C, whereas the Mo2C/Si mul tilayers show a superior thermal stability up to 600 deg C. It was found that as-deposited Mo/Si and Mo2C/Si multilayer mirrors have a similar level of compressive stress of -520±20 MPa. The reduction of residual stress in Mo-Si and Mo2C/Si multilayer systems with post-deposition thermal treatment has been investigated. Using a slow thermal annealing (2 deg C/min), it is possible to reduce the stress from -520 MPa to nearly zero by heating the Mo-Si specimen up to * 310 deg C. However, it results in a reflectivity drop of ~ 3 % (absolute) at the wavelength of 13 nm. For the Mo2C/Si system, a stress reduction from -520 MPa to nearly zero is possible by a post annealing at ~ 290 deg C without a considerable drop in the reflective properties.