Wolter mirrors fabricated by high-precision Ni electroforming process have been applied as focusing optics for x-ray telescopes. The typical replication accuracy is on the order of 100nm. For higher resolution observations, the figure accuracy is required to be improved. Recently, we have been developing an efficient figure correction method using an Si layer on Wolter mirror. Film thickness of Si can be measured with accuracy of 1nm level by thickness measurement gauge. Si is removed under wet process so that the figure accuracy improves. In this study, we developed a fluid jet polishing system especially for removing Si layer on the inner surface of Wolter mirrors. Surface roughness remained unchanged at 0.3nm in RMS (root mean square) value before and after processing to a depth of 133nm. For demonstration, a sine curve with a length of 10mm and PV (peak to valley) of 160nm was processed on Si on a plane surface, resulting in a processing accuracy of 25nm in PV and 6.7nm in RMS.
Steeply curved free-form x-ray mirrors, represented by monolithic ellipsoidal or Wolter mirrors, are required for submicron focusing without chromatic aberrations in the soft x-ray region. Such mirrors require a different approach to surface machining and metrology than conventional mirrors with small sags, such as Kirkpatrick-Baez or plane mirrors. A few examples of the fabrication of Wolter mirrors have been reported, in which surface measurements using a surface profilometer with a contact probe were used for figure correction. However, owing to differences in measurement methods and fabrication difficulties, the fabrication performance has not been fully evaluated on a comparative basis. In this study, a high-precision plane mirror was fabricated using the same process as that used for steeply curved free-form mirrors. The figure measurement accuracy was comparatively evaluated, and the results suggested the possibility of achieving a figure accuracy of 5nm in PV and 0.7nm in RMS in steeply curved free-form mirror fabrication.
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