US Patent 10209411 Multilayer mirror

A multilayer mirror for reflecting extreme ultraviolet (EUV) radiation, the mirror has a substrate and a stack of layers formed on the substrate. The stack of layers comprises layers including a low index material and a high index material, the low index material having a lower real part of the refractive index than the high index material at a given operating wavelength λ. The mirror provides a first peak of reflectivity of 20% or more at a first wavelength λ1 in a first wavelength band extending from 6 nm to 7 nm and a second peak of reflectivity of 20% or more at a second wavelength λ2 in a second wavelength band extending from 12.5 nm to 15 nm.