Mitigating best focus shift in high-NA EUV lithography via optical constant screening

초록

EUV lithography is a key technology for advanced semiconductor manufacturing, currently being developed for high-NA systems to enable future 3 nm nodes. However, the conventional TaBN mask exhibits severe best focus shift (BF shift) caused by its thick absorber layer. Therefore, the development of next-generation EUV mask absorber materials to replace TaBN is required. In this study, we screened the range of optical constants to identify materials capable of mitigating BF shift, defined as variation in best focus with respect to pitch ratio. Simulations were performed using the KLA PROLITH 2023a simulation tool, which is based on rigorous coupled-wave analysis. For evaluation, a high-NA system was used under conditions including a 5.355° chief-ray angle, central obscuration, and anamorphic optics. The evaluation targeted 12 nm and 10 nm line/space (L/S) patterns in both vertical and horizontal directions, with absorber thicknesses ranging from 30 nm to 40 nm. TaBN was used as a reference for comparison of BF shift. Both vertical and horizontal patterns exhibited BF shift across all optical constant regions, and BF shift was mitigated compared to TaBN in the low-n, high-k, and near n≈1.0 regions. Furthermore, it was confirmed that the regions in which BF shift is mitigated by 30%, 50%, and 70% decrease as the pattern size becomes smaller. Additionally, depth of focus (DoF) evaluation was conducted for TaBN and materials within the regions where BF shift is mitigated by 30%, 50% and 70%. As a result, it was confirmed that the overlapping DoF margin across different pitches increases as BF shift is further mitigated, with values of 23.0 nm, 27.1 nm, 31.9 nm, and 35.2 nm, respectively. Based on these results, it is offering a pathway to extend process windows and enable reliable scaling for future high-NA EUV lithography.

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