Improved theoretical reflectivities of extreme ultraviolet mirrors

We show that the theoretical reflectivities of multilayered Mo/Be and Mo/Si extreme ultraviolet (EUV) mirrors tuned for the 11-14 nm spectral region can be enhanced significantly by incorporating additional materials within the stack. The reflectivity performance of these quarter-wave multilayers can be enhanced further by global optimization procedures by which the layer thicknesses are varied for optimum performance. By incorporating additional materials of differing complex refractive indices - e.g. Rh, Ru and Sr - in various regions of the stack we calculate peak reflectivity enhancements of up to similar to 5% for a single reflector compared to standard unoptimized stacks. For an EUV optical system with nine near-normal-incidence mirrors, the theoretical optical throughput may be increased by up to 100%. We also show that protective capping layers such as Rh and Ru, in addition to protecting the mirrors from environmental attack, may serve to improve the reflectivity characteristics.