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.