Euv multilayer mirror, optical system including a multilayer mirror and method of manufacturing a multilayer mirror

A method of manufacturing a multilayer mirror reflecting extreme ultraviolet (EUV) radiation from a first wavelength range in an EUV spectral region, comprising:

• providing a substrate;
• forming a stack of layers on the substrate,
• wherein the stack of layers comprises layers comprising a low index material and layers comprising a high index material, the low index material having a lower real part of the refractive index than does the high index material at a given operating wavelength λ in the first wavelength range, and
• forming a spectral purity filter on top of the stack of layers, wherein:
• the spectral purity filter is effective as an anti-reflection layer for ultraviolet (UV) radiation from a second wavelength range in a UV spectral region, to increase an EUV-UV-reflectivity ratio of the multilayer mirror in the first wavelength range, and
• forming the spectral purity filter includes forming a non-diffractive graded-index anti-reflection layer configured to reduce reflectivity in the second wavelength range,
• wherein the non-diffractive graded-index anti-reflection layer comprises a sub-wavelength structure,
• wherein the sub-wavelength structure is a periodic structure comprising an array of tapered structural elements forming a periodic surface relief structure, and
• wherein a period length of the periodic sub-wavelength structure is in a range from 25 nm to 100 nm.