Optimizations aspects for EUV low-k1 logic with the low-n mask

Experimental results demonstrated that the EUV low-n mask improves both LCDU and dose for dense features. For semi-isolated and isolated features best-focus shifts through pitch can occur with the low-n mask as well as with the Tabased mask. In this paper, we show how mask-3D induced phase offsets between diffraction orders lead to best focus shifts for dense and (semi-) isolated features. Subsequently we investigate what are the options to mitigate best focus shifts. First, when the pitch is large enough, sub-resolution assist features (SRAFs) can significantly improve contrast and reduce best-focus shifts. Second, optimizing the target and mask bias, is very effective for focus-shift mitigation. Especially the low-n mask shows a strong response to target and mask bias. Third, using bright field imaging instead of dark-field imaging generally also results in smaller best focus shifts through pitch. Using source-mask optimization (SMO) examples through pitch, we show how these mitigation strategies are effectively applied to through pitch use cases. We find NILS DOF improvements up to 65% with the low-n mask and optimized retargeting.