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Impact of scattering bar in the presence of lens aberrations

This paper presents a systematic theoretical and simulation study on how scattering bar could impact lithographic performance in the presence of lens aberrations. In particular, the effects of bar size, bar placement and pitch at conventional and annular illuminations are investigated. For the study, a simple 1-D two-bar structure is used. The effects of odd and even aberration terms are studied assuming the presence of either primary coma or primary astigmatism only. Simulations using a set of 37 Zernike coefficients from a state-of-the-art DUV step and scan are also carried out. Pattern asymmetry of the two-bar pattern is used to quantify the effects of odd aberration terms; the root-mean-square value of CD difference through focus of two orthogonal lines is used to quantify the effects of even aberration terms. Results show that scattering bar has a significant impact on the effects of lens aberrations. The magnitude and polarity of this influence depends on the bar size, bar placement, pitch and illumination conditions. Pattern asymmetry under annular illumination is particular sensitive to bar size and bar placement; CD difference through focus under conventional illumination at a range of pitch values decrease significantly with proper bar placement. The trends observed are similar even when a full set of Zernike coefficients are used. A thorough and more complete understanding of how scattering bar impact lens aberration effect for different mask structures and at different illumination conditions is thus needed for low-k1 imaging.