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

We greatly reduced the stress of Mo/Si multilayers deposited by ion beam sputtering from about -450 MPa to about +14 MPa by combining the methods of sub-multilayering and ion-beam polishing of Mo-layers. These methods require no heating processes, thereby eliminating the fear that heat will worse the figure and the optical properties of precise mirror substrates. The low-stress multilayers had much thinner interdiffusion layers and more abrupt interfaces than did conventional Mo/Si multilayers, and had similar reflectivities to those of the conventional Mo/Si multilayers. It is expected that the application of this low-stress multilayer to EUV mirrors will make it possible to compose EUV optics without worsening the optical properties caused by stress deformation of substrates.