EUV microscopy for defect inspection by dark-field mapping and zone plate zooming

A laboratory scale EUV microscope is presented to be used, e.g., in future inspection of EUV masks and mask blanks in extreme ultraviolet lithography. The system can be operated in bright and dark field mode. For defect inspection purpose the dark field mode is preferred, with increased contrast and sensitivity of the system to small structures. The characteristics of the used Schwarzschild objective as imaging component such as large object field and moderate magnification become advantageous for high process speeds, whereas the detector pixel size (13 μm) does not give the spatial resolution in principle possible with the imaging optics. The presence of a defect causes a spot (with otherwise dark background) to appear on the detector. As necessary it can be zoomed in with the help of a second magnification step. For this purpose we suggest to employ a zone plate adapted to the system. The method's feasibility is demonstrated by means of experiments on test structures and the apparatus is characterized with regard to design parameters for commercial systems.