Ion beam profiling of aspherical X-ray mirrors

We present a fast and robust ion beam etching technique to profile initially flat surfaces into aspherical ones. The surfacing method is based on the displacement at variable speed of a double-blade system placed between a broad ion beam source and a mirror, thus allowing a spatial modulation of the ion dose along the mirror surface. The fabrication process was validated using various metrology techniques to assess both mirror figure and finish. The method was successfully applied to produce two strongly elliptically shaped mirror prototypes (source–mirror distance 150 m and demagnification 3000). The residual slope errors were found to be 18 μrad over a mirror length of 40 mm while the root mean squared roughness remains below 0.2 nm for spatial periods less than 130 μm.