Surface Roughness of Stainless Steel Bender Mirrors for Focusing Soft X-rays

We have used polished stainless steel as a mirror substrate to provide focusing of soft x-rays in grazing incidence reflection. The substrate is bent to an elliptical shape with large curvature and high stresses in the substrate require a strong elastic material. Conventional material choices of silicon or of glass will not withstand the stress required. The use of steel allows the substrates to be polished and installed flat, using screws in tapped holes. The ultra-high-vacuum bender mechanism is motorized and computer controlled. These mirrors are used to deliver focused beams of soft x-rays onto the surface of a sample for experiments at the Advanced Light Source (ALS). They provide an illumination field that can be as small as the mirror demagnification allows, for localized study, and can be enlarged, under computer control, for survey measurements over areas of the surface up to several millimeters. The critical issue of the quality of the steel surface, polished and coated with gold, which limits the minimum achievable focused spot size is discussed in detail. Comparison is made to a polished, gold coated, electroless nickel surface, which provides a smoother finish. Surface measurements are presented as power spectral densities, as a function of spatial frequency. The surface height distributions measured with an interferometric microscope, and complemented by atomic force microscope measurements, are used to compute power spectral densities and then to evaluate the surface roughness. The effects of roughness in reducing the specular reflectivity are verified by soft x-ray measurements