16 June 2003 Asymmetry and thickness effects in reflective EUV masks
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The use of a reflective mask in extreme ultraviolet (EUV) lithography requires illumination of the mask at a small but obliquely incident angle. This results in an inherent asymmetry in the diffraction pattern from the mask. This asymmetry results in a shift of the image at the wafer. The image shift depends on the mask structure and the exposure condition. The thicknesses of the patterned mask layers, specifically, the absorber and anti-reflection layer (ARL), are particularly important for the image formation. A rigorous approach to simulating these effects is required. In this paper, a fast frequency-domain method for simulating EUV masks with the underlying multilayer is used for the study of various mask variation effects on image placement as well as linewidth, for nested 35 nm and 25 nm lines. Results indicate that EUV radiation reflected from the mask before entering the multilayer cannot be neglected. Consequently, it is of utmost importance to maintain better than ±1 nm control of absorber thickness and ARL thickness over the entire mask, in order to minimize the CD impact of variations in reflectance swing.
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Frederick T. Chen, Frederick T. Chen, } "Asymmetry and thickness effects in reflective EUV masks", Proc. SPIE 5037, Emerging Lithographic Technologies VII, (16 June 2003); doi: 10.1117/12.483602; https://doi.org/10.1117/12.483602

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