Design of multilayer extreme-ultraviolet mirrors for enhanced reflectivity

We show numerically that the reflectivity of multilayer extreme-UV (EUV) mi rrors tuned for the 11-14-nm spectral region, for which the two-component, Mo/Be and Mo/Si multilayer systems with constant layer thickness are common ly used, can be enhanced significantly when we incorporate additional mater ials within the stack. The reflectivity performance of the quarter-waveleng th multilayers can be enhanced further by global optimization procedures wi th which the layer thicknesses are varied far optimum performance. By incor porating additional materials of differing complex refractive indices-e.g., Rh, Ru, Sr, Pd, and RbCl-in various regions of the stack, we observed peak reflectivity enhancements of as much as similar to 5% for a single reflect or compared with standard unoptimized stacks. We show that, in an EUV optic al system with nine near-normal-incidence mirror surfaces, the optical thro ughput may be increased by a factor as great as 2. We also show that protec tive capping layers, in addition to protecting the mirrors from environment al attack, may serve to improve the reflectivity characteristics. (C) 2000 Optical Society of America.