Mirrors for nanofocusing x-ray beams

Nanofocused x-ray beams provide a powerful tool for studying materials at the submicron level. One approach to achieve such a small-sized focused beam is to use a pair of mirrors, in the Kirkpatrick Baez (KB) arrangement. Each mirror focuses the beam in one direction and has an elliptical profile. To focus hard x-rays generated from high-brilliance third-generation synchrotron sources to nanometer spots, it is necessary that the mirror figure errors, defined as the deviation of the surface from the ideal ellipse, be in the submicroradian range, or about an order of magnitude smaller than those presently in use. In this paper, the advances in deterministic figuring of extreme ultraviolet (EUV) lithography optics is discussed and is proposed as a technique for developing elliptical mirrors. Such elliptical mirrors with slope errors under 0.2 murad rms are expected to provide nanometer-focused beams at third-generation synchrotron radiation facilities. The minimum focal size will then be governed by diffraction limit rather than optics quality. An alternative approach to developing an elliptically surface is to use flat or spherical mirrors of the same surface quality, and bend or differentially coat them to attain elliptical profiles. These additional steps would be unnecessary if an elliptical profile is directly polished into the substrate. In any case, the substrate is finally coated with a thin layer (< 400 Angstrom) of a high-atomic-number metal to increase the total external reflection critical angle and thereby increase the aperture.