Wideband multilayer gratings for the 17-25 nm spectral region

An approach to designing wideband blazed multilayer gratings is introduced and applied to gratings operating at 17-25 nm. We demonstrate single-order operation of broadband multilayer gratings, despite their very wide spectral and angular bandpass, when only one diffraction wave is excited and the diffraction efficiency reaches the reflectivity of a conventional depth-graded multilayer mirror, eliminating overlapping of different-order diffraction waves. The selection principles for the geometrical parameters of gratings are discussed. We formulate a "law of similarity" for wideband gratings that allows us to design gratings with different geometrical parameters but practically the same spectral dependence of the diffraction efficiency.