Reflectance enhancement with sub-quarterwave multilayers of highly absorbing materials

A new theory of multilayers with enhanced normal reflectance has been developed based on the superposition of a few layers of various different radiation-absorbing materials. Every layer in the multilayer had a subquarterwave optical thickness. The theory was developed for materials with small refractive-index differences, although it is also valid in some cases for materials with large refractive-index differences. Reflectance enhancements were obtained in a very broad band and over a wide range of incidence angles. The theory is particularly suited to designing multilayers with enhanced reflectance in the extreme ultraviolet for wavelengths above 50 nm. In this spectral region the reflectance of single layers of all materials is relatively low, and standard multilayers are not possible because of the high absorption of materials.