Phase defect characterization on an extreme-ultraviolet blank mask using microcoherent extreme-ultraviolet scatterometry microscope

Defect-free mask production is a critical issue in extreme-ultraviolet (EUV) lithography. On EUV masks, phase defects are buried by multilayer coating, which is a serious EUV-specific issue. These defects should be hidden or be compensated completely by the absorber pattern for the production of defect-free masks. A phase image of the phase defects at the EUV lithography exposure wavelength is essential to characterize the defects. For characterization of phase defects, the authors have developed the microcoherent EUV scatterometry microscope (micro-CSM). This system is lensless and is based on a coherent diffraction imaging method, which records diffraction images. The intensity and phase images of the defects are reconstructed through iterative calculations. The micro-CSM system has focusing optics of a Fresnel zone plate to observe small defect. The detection size limits of the phase defects were a width of 25 nm and a depth of 1.4 nm. Diffraction images from an asymmetric phase defect were related well to the defect shapes and volumes. The defect position was also inspected by mapping image that was measured using by step-and-repeat observation. The actinic defect signals were observed by the micro-CSM system.