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Quantitative evaluation of mask phase defects from through-focus EUV aerial images

We present an improved method of phase retrieval from through-focus image series with higher precision and reduced sensitivity to noise. The previous method, developed for EUV, actinic mask measurements, was based on the Gerchberg-Saxton algorithm and made use of two aerial images recorded in different focal planes. The new technique improves the reconstruction uncertainty and increases the convergence speed by integrating information contained in multiple images from a through focus series. Simulations characterize the new technique in terms of convergence speed, accuracy and stability in presence of photon noise. We have demonstrated the phase-reconstruction method on native, mask-blank phase defects and compared the results with phase predictions made from AFM data collected after the multilayer deposition. Measurements show that a defect's top-surface height profile is not a reliable predictor of phase change in all cases. The method and the current results can be applied to improve defect modeling and to enhance our understanding of the detectability and printability of native phase defects.