Enhancement of DOF through RET and mask manufacturability in high-NA EUV

The insertion of sub-resolution assist features (SRAF) in high NA EUV lithography is investigated in this study to evaluate its feasibility and impact on wafers. Improvements in DoF with respect to wafer performance are evaluated, and mask manufacturability is assessed by analyzing mask edge placement error (EPE). The feasibility of SRAF insertion, based on its size, is examined in both horizontal and vertical directions. The increase in DoF due to SRAF insertion is evaluated through various L/S patterns and an imec A14 logic metal design. Overlap process windows (oPW) and process variation bands (PV bands) are examined through wafer measurements. For mask manufacturability, EPE is calculated by comparing the actual mask with the post-OPC design on L/S and the logic metal mask. For horizontal patterns, it is found that SRAF is manufacturable down to 2.8nm at the wafer level, with SRAF printing starting from 4.2nm. However, for vertical patterns, SRAF insertion is challenging under current mask rules. DoF is improved and best focus (BF) shifts are mitigated in horizontal patterns by SRAF. The investigation of mask variability and its impact revealed a mask level EPE of 5nm for both main and SRAF features, with wafer simulations showing an impact equivalent to approximately 10% of the main feature, which is not negligible.