Differential deposition for figure correction of x-ray mirrors

Differential deposition techniques were applied to reduce the figure error of x-ray mirrors. Cr layers were sputtered on flat substrates that were moved with variable speed in front of a beam defining aperture. The required velocity profile was calculated using a deconvolution algorithm. The Cr thickness profiles were derived in two ways: directly, using x-ray reflectivity and indirectly, by measuring the surface figure before and after the deposition. After two iterations the mirror surface figure could be improved by almost one order of magnitude. This work will describe the experimental techniques and discuss the achieved accuracy. It will also address open questions such as layer stress, roughness evolution, and limitations of the available instrumentation.