Ion beam shaping and figuring of X-ray sub-microfocusing mirrors

High-performance synchrotron X-ray focusing mirrors require aspheric surfaces with nanoradian slope accuracy, which are challenging to provide for many manufacturers. Conventional grinding and polishing techniques typically introduce tooling marks to the surface during the fabrication process, and these marks can impose significant challenges for the deterministic figuring processes used as the final step in mirror fabrication, ultimately limiting the slope errors of the mirrors. To overcome this limitation, we present an alternative approach by using ion beam shaping to form an elliptical sub-microfocusing mirror from a flat mirror, followed by ion beam figuring to improve the remaining errors. Two mirrors fabricated using this approach achieved tangential slope errors below 200 nrad root mean squared (rms), in contrast to a conventionally pre-shaped mirror with identical ellipse parameters that was limited to > 500 nrad rms after ion beam figuring due to residual tooling marks. In X-ray focusing tests, the ion beam shaped mirrors are able to realise a spot size below 250 nm (full width half maximum), compared to 404 nm for the pre-shaped mirror. This demonstrates the excellent potential of ion beam shaping as a method for rapid, efficient and high-quality X-ray mirror fabrication.