Capping layers for extreme-ultraviolet multilayer interference coatings

The reflectivity of extreme-ultraviolet thin-film multilayer (ML) interference coatings, terminated by a native oxide or other capping layer (CL), is critically dependent on the thickness of the final deposited layer of the top period. We show in this numerical study that, for a molybdenum-silicon ML, a high reflectivity loss may be incurred if the final Si layer is not of optimum thickness. For maximum reflectivity the thickness of the final Si layer must be controlled such that the node of the standing wave lies within the absorptive CL.The final Si layer may be replaced, at the expense of reflectivity, by SiC and capped with another inert material for improved protection of the ML.