Enhanced thermal stability of extreme ultraviolet multilayers by balancing diffusion-induced structural changes

A multilayer design that compensates period thickness compaction at elevated temperatures is presented. The design is based on a reference multilayer that exhibits compaction upon thermal loading and includes an additional sub-structure, which expands upon thermal loading to compensate for the basic compaction. Using extreme ultraviolet reflecting multilayers as an example, the optimization of the ratio of the number of the expanding Mo/B4C periods to that of compacting B4C-barriered Mo/Si periods is demonstrated. Both the average periodicity and the centroid wavelength of the composite multilayer were preserved during annealing at 250 °C for 60 h.