Two magnification steps EUV microscopy with a Schwarzschild objective and an adapted zone plate lens

Due to the short wavelength microscopy with extreme ultraviolet (EUV) light is optimally suited for detecting defects e. g. on mask blanks for EUV lithography. In this work the use of a zone plate lens as a second magnification step in EUV microscopy with a multilayer coated Schwarzschild objective is suggested. The zone plate has to be adapted to the optical system and to have a magnification high enough to match the resolution of the Schwarzschild objective to the detector pixel size. The resulting zone plate should have only a few tens of zones and about 1 mu m resolution which reduces fabrication demands. Furthermore, this combination enables a scan and zoom procedure where first the measurements are carried out just with a Schwarzschild objective allowing only a small magnification but larger object field. Then, in areas of interest, the second magnification step is switched on by inserting a zone plate in front of the detector and refocusing the sample. The paper addresses regulations for the zone plate design, simulations of the whole optical system and corresponding demonstration experiments on test structures.