Development of fabrication process for steeply curved x-ray mirror using electroformed substrate

X-ray focusing mirrors with sagittal radii of curvature of several millimeters are fabricated by a high-precision electroforming process involving mandrel fabrication, room-temperature copper electrodeposition, and separation. Although a minimum focusing size of less than 200nm has been achieved, the ideal focusing capability has not been realized due to replication errors. This study proposes a new fabrication process for steeply curved x-ray mirrors using an electroformed substrate. First, an electroformed mirror is cut by wire electrical discharge machining along the central axis. Next, the replicated surface is coated with approximately 300nm of silicon, and the figure error is measured with a scanning-type coordinate measuring machine using a contact probe. Finally, the figure error on the silicon surface is corrected. Using an ellipsoidal mandrel with radii of curvature below 10mm, the electroforming, cutting, and silicon coating processes were carried out, and the figure error of the uncorrected substrate was evaluated. The peak-to-valley figure error in a 40 × 3mm² area was 198nm, which was smaller than the silicon layer thickness. This result suggests that subsequent figure correction is feasible.