Quantitative phase imaging of a small phase structure on an extreme-ultraviolet mask by coherent diffraction imaging

Extreme-ultraviolet (EUV) lithography poses a number of challenges. One of which is the production of a defect-free mask. To observe the phase defects on an EUV mask in a quantitative phase image, we have developed a micro-coherent EUV scatterometry microscope at the NewSUBARU synchrotron facility. This microscope focused coherent EUV on a 140-nm-diameter defect and recorded the diffraction. The intensity and phase images of the defect are reconstructed by the coherent diffraction imaging method of ptychography. Phase defects with a 30 nm spatial resolution were reconstructed. This quantitative phase imaging method should help improve EUV masks.