Low-stress molybdenum/silicon multilayer coatings for extreme ultraviolet lithography

Modified molybdenum/silicon (Mo/Si) multilayers were deposited by ion beam sputtering (IBS). We obtained low-stress multilayers by sub-multilayering each Mo-layer into a trilayer of Mo/Ru (ruthenium)/Mo, and by argon (Ar) ion beam polishing (IBP) after each Mo-layer deposition. Conventional Mo/Si multilayers have a compressive stress of about -450 MPa, while the low-stress multilayers we have developed have a tensile stress of +14 MPa, on an average. Low-stress multilayers have similar reflectances to those of conventional Mo/Si multilayers, and exhibit no significant temporal changes in the layer period and the stress for up to one year. The method used for stress reduction is not a heating process such as annealing, thus it does not cause irreversible deformation of the precisely-figured mirror substrates of optics. It is expected that the application of low-stress multilayers to mirrors will make it possible to compose optics without worsening the optical properties due to deformation of substrates by the stress of multilayer coatings.