Nanolayered EUV mirrors for the next generation lithography of integrated circuits

High reflectivity multilayer mirrors are enabling extreme ultraviolet lithography (EUVL), a leading candidate for next generation semiconductor lithography, to print features smaller than 32 nm. However, the specifications for the lifetime and stability of these multilayer coatings are very stringent and current solutions are not satisfactory. The lifetime of EUV projection optics is limited by the oxidation of the capping layers while the lifetime of the condenser optics is limited by erosion, contamination, and thermal stability. Ceramic materials offer a new solution space, and in this talk I will present some preliminary studies that investigate their performance as protective, capping layers as well as materials to enhance multilayer thermal stability. I will also demonstrate how the microstructure of the capping layers affects the physical properties of the multilayers, such as reflectivity. Finally, the microstructural changes within the multilayers, designed for high thermal stability operation, will be discussed. For example, the microstructure of these multilayers changes as a function of the annealing temperature. With an appropriate multilayer design, all of the desired properties can be achieved and maintained stable for long periods of time.