Yurii USPENSKII, Denis BURENKOV, Tadashi HATANO¹, and Masaki YAMAMOTO¹

Tamm Theory Department, Lebedev Physical Institute, Leninski prosp. 53, Moscow 119991, Russia
¹Research Center for Soft X-ray Microscopy, IMRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

(Received May 30, 2006; Accepted November 18, 2006)

A small bandwidth of periodic multilayers at wavelengths 2.4–4.4 nm presents problems for the spectral matching of mirrors. This leads to low throughput of a Schwarzschild microscope and its sensitivity to technological errors in layer thickness. We consider two cures for these difficulties: aperiodic coatings and a lateral gradient of layer thickness. Our design of aperiodic multilayers maximizes the throughput of a microscope made at a fixed level of technology. The method includes a new merit function, fast procedure for its minimization, linkage between multilayers and an equation for a lateral gradient of layer thickness. Computation is performed for Sc/Cr coatings at 398 eV. It shows that the aperiodicity makes optics stable to technological errors, while the lateral gradient increases throughput, but enhances sensitivity to the errors. The best results are obtained for aperiodic mirrors with a lateral gradient of thickness, which assure both high microscope throughput and stability to the errors.

Key words: water-window region, Schwarzschild microscope, aperiodic multilayers, lateral gradient, spectral matching, optimal design