Development of electroforming process for soft x-ray ellipsoidal mirror

An x-ray ellipsoidal mirror is an ideal tool for focusing soft x-rays. Because nanometer-level shape accuracy is required in the internal surface of a mirror having a small diameter, it is difficult to fabricate the mirror by processing the surface directly. We developed a fabrication process for soft x-ray ellipsoidal mirrors in which the surface of a high-precision quartz mandrel with the inverted shape of the designed mirror is replicated by nickel sulfamate electroforming. In this study, an ellipsoidal mirror of 40-mm length was fabricated and the shape accuracy of the replicated surface was evaluated by a measurement method using a contact probe. The root mean square (RMS) of the replication error in the entire measured surface was 27.2 nm. When the evaluated area was half the replicated surface near the middle of the mirror, the RMS of the replication error was 14.7 nm. Wave-optical simulation suggested that it is possible to focus soft x-rays to a spot with a diameter of 400 nm.