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8 October 2014 Phase-enhanced defect sensitivity for EUV mask inspection
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In this paper, we present a complete study on mask blank and patterned mask inspection utilizing the Zernike phase contrast method. The Zernike phase contrast method provides in-focus inspection ability to study phase defects with enhanced defect sensitivity. However, the 90 degree phase shift in the pupil will significantly reduce the amplitude defect signal at focus. In order to detect both types of defects with a single scan, an optimized phase shift instead of 90 degree on the pupil plane is proposed to achieve an acceptable trade-off on their signal strengths. We can get a 70% of its maximum signal strength at focus for both amplitude and phase defects with a 47 degree phase shift. For SNR, the tradeoff between speckle noise and signal strength has to be considered. The SNR of phase and amplitude defects at focus can both reach 11 with 13 degree phase shift and 50% apodization. Moreover, the simulation results on patterned mask inspection of partially hidden phase defects with die-to-database inspection approach on the blank inspection tool show that the improvement of the Zernike phase method is more limited. A 40% enhancement of peak signal strength can be achieved with the Zernike phase contrast method when the defect is centered in the space, while the enhancement drops to less than 10% when it is beneath the line.
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Yow-Gwo Wang, Ryan Miyakawa, Weilun Chao, Kenneth Goldberg, Andy Neureuther, and Patrick Naulleau "Phase-enhanced defect sensitivity for EUV mask inspection", Proc. SPIE 9235, Photomask Technology 2014, 92350L (8 October 2014);

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