A multilayer mirror (M) reflecting extreme ultraviolet (EUV) radiation from a first wave-length range in a EUV spectral region comprises a substrate (SUB) and a stack of layers (SL) on the substrate, the stack of layers comprising layers comprising 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 in the first wavelength range. The stack of layers further comprises a spectral purity filter on top of the stack of layers, the spectral purity filter effective as an anti-reflection layer for ultraviolet (UV) radiation from a second wavelength range in a UV spectral region to increase a EUV-UV-reflectivity ratio of the multilayer mirror. The spectral purity filter (SPF) comprises a non-diffracting graded-index anti-reflection layer (GI-AR) effective to reduce reflectivity in the second wavelength range.
|Patent number||WO2015EP52767 20150210|
|Publication status||Submitted - 18 Aug 2016|