Toward the preparation of optical surfaces preserving the coherence of hard x-ray beams

The properties of optical components are described that are required by synchrotron x-ray scattering techniques needing to fully exploit the coherence properties of third-generation sources. For example, phase imaging and microfocusing applications require reflecting surfaces with errors much smaller than a nanometer up to spatial frequencies in the mm(-1) range while keeping roughness on the sub-Angstrom level. The two major challenges being addressed in this paper are the accuracy of the metrology and the technology of the machining based on the metrology for deterministic corrections. The low emittance and the stability of the synchrotron source combined with x-ray position sensitive detectors are suitable to measure the reference wavefront with picometer accuracy. The deformation of this wavefront by a reflecting mirror leads to surface error maps whose accuracy corresponds to the conditions for coherence preservation. These maps are used as an input for an ion-beam milling machine correcting these height errors on grazing incidence mirrors. First results are presented and limits of these techniques including possibilities of multilayer corrections are discussed.