Phase-shifting masks were developed for extreme ultraviolet (EUV) lithography to enlarge the process window, and some researchers developed mask phase defect compensation methods adopting absorber pattern modification. To evaluate these small phase structures, a phase-imaging microscope is required. For actinic phase imaging, we have developed a coherent EUV scatterometry microscope (CSM) based on a coherent diffraction imaging method. The image-forming optics are replaced by an inverse computation, where the frequency space phase data are retrieved. Therefore, the aerial image phase data are also reconstructed. The CSM thus observes the intensity and phase image. We improved the reconstruction algorithm by which the illumination probe was simultaneously reconstructed; phase images of a crossed line pattern, an 88 nm line-and-space pattern, and a phase defect were reconstructed quantitatively. The CSM will be helpful for phase-shift mask development and phase defect compensation.
Articles you may be interested inPhase defect characterization on an extreme-ultraviolet blank mask using microcoherent extreme-ultraviolet scatterometry microscopeIn extreme-ultraviolet (EUV) lithography, defect-free mask production is a critical issue for highvolume manufacturing. For mask inspection and metrology, we have developed a coherent EUV scatterometry microscope (CSM). It is a simple lensless system. An aerial image of the mask pattern is reconstructed with iterative calculation based on coherent diffraction imaging. Periodic patterns, aperiodic patterns, and phase structures were reconstructed well by the CSM. A defect in a line-and-space pattern was detected as a diffraction signal. The aerial image of the defect is also reconstructed. This paper demonstrates the capability of the CSM to observe complex diffraction amplitudes directly from the pattern and the defect.
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