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The purpose of our study is to evaluate the benefit of contrast enhancement strategies on a logic metal layer at pitch 28 nm. We build up on three studies from imec and ASML [1][2][3]. We take as a reference a Negative Tone Development (NTD) Metal Oxide Resist (MOR) process used in combination with a binary TaBN mask absorber, without SRAF, exposed with an X/Y symmetric pupil on a 0.33 NA EUV scanner, the NXE:3400 from ASML [7]. The fading mitigation strategies leverage asymmetrical pupil (monopole), wavefront injection (Z6 aberration) and low-n attenuated Phase Shift Mask (PSM). We find very good agreement between our simulations on design clips, the theoretical expectations and the experimental data shared in the above mentioned papers on building blocks (L/S through pitch and dense tip-to-tip). Overall the three fading correction techniques are efficient to improve the printability of our use case in term of ILS. It also improves the best focus shift of L/S through pitch and between L/S and tip-to-tip. In conclusion, the most promising exposure strategy for the logic metal pitch 28 nm use case is the attenuated PSM. It provides the highest ILS, the narrower best focus range, the largest overlapping process window without any compromise on the illumination efficiency, i.e. using the full NXE:3400 throughput.
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