Manufacturing and characterization of substrates for imaging multilayer X-ray optics
Résumé
Thanks to the successes of recent years, the technology of depositing multilayer mirrors (MLM) has made it possible to create a normal incidence optics for extreme ultraviolet and soft X-ray (XEUV) wavelength ranges (λ~2-60 nm). This potentially allows one to achieve nanometer scale resolution in microscopy and lithography, and ultra-high angular resolution of XEUV telescopes. The main factor limiting progress in this direction is the complexity of manufacturing substrates with the required quality. For one or two orders, shorter wavelengths in respect to the optical range proportionally increased the requirements for the roughness and the accuracy of the shape of the substrates for multilayer mirrors, reducing them to sub-nanometer and even sub-angstrom levels. The solution of the problem of creating diffraction quality optics required a revision of the possibilities of traditional methods of manufacturing and characterizing the surfaces in the entire range of spatial frequencies of roughnesses affecting the imaging and, if necessary, the development of new methods both for measurements and fabrications. A feature of the imaging optics for XEUV is the fact that in most cases it is aspherical. This is due to the need to minimize the number of elements in the optical scheme due to the relatively large power loss of radiation for each reflection. For the characterization of the substrate surface the conventionally used methods are an interferometry with a diffraction reference wave, white light interferometry and atomic force microscopy (AFM). The main method of correcting local errors and "weak" aspherization of the surface shape of substrates is the ion beam processing of the surface, the so-called ion- beam figuring (IBF). Despite the established approaches to the characterization of the surfaces, only the PDI can be attributed to the "first-principle" measurement method.
The report describes the equipment and methods by which the above-mentioned metrological problems are solved at the Institute fof Physics of Microstructures of the Russian Academy of Sciences. In conclusion, as a demonstration of the possibilities of the methods, the result of the ion beam correction of the shape of a spherical substrate with a numerical aperture NA=0.3 will be shown.