An investigation of differential deposition for figure corrections in full-shell grazing-incidence X-ray optics

We are investigating differential deposition as a way of correcting small figure errors inside full-shell grazing-incidence X-ray optics. The optics in our study are fabricated using the electroformed-nickel-replication technique, and the figure errors arise from fabrication errors in the mandrel, from which the shells are replicated, as well as errors induced during the electroforming and mounting process. Combining, these give sub-micron-scale figure deviations which limit the angular resolution of the optics to 10–15 arcsec.Sub-micron figure errors can be corrected by selectively depositing (physical vapor deposition) material inside the shell. The requirements for this filler material are that it must not degrade the ultra-smooth surface finish necessary for efficient X-ray reflection (∼5 A rms), and must not be highly stressed. In addition, a technique must be found to produce well controlled and defined beams within highly constrained geometries, as some of our mirror shells are less than 3 cm in diameter.We report on our efforts to date to implement this technique.