X‐ray mirrors with sub‐nanometre figure errors obtained by differential deposition of thin WSi2 films

Differential deposition by DC magnetron sputtering was applied to correct for figure errors of X‐ray mirrors to be deployed on low‐emittance synchrotron beamlines. During the deposition process, the mirrors were moved in front of a beam‐defining aperture and the required velocity profile was calculated using a deconvolution algorithm. The surface figure was characterized using conventional off‐line visible‐light metrology instrumentation (long trace profiler and Fizeau interferometer) before and after the deposition. WSi2 was revealed to be a promising candidate material since it conserves the initial substrate surface roughness and limits the film stress to acceptable levels. On a 300 mm‐long flat Si mirror the average height errors were reduced by a factor of 20 down to 0.2 nm root mean square. This result shows the suitability of WSi2 for differential deposition. Potential promising applications include the upgrade of affordable, average‐quality substrates to the standards of modern synchrotron beamlines. The surface figure of an X‐ray mirror has been corrected by differential deposition of WSi2 films.