Depth of focus in high-NA EUV lithography: a simulation study

High-NA EUV lithography tools are projected to be deployed in HVM by 2025/2026, with the introduction of the N1.4 node. This should allow single-exposure EUVL at 30 nm pitch and below and reduce cost and complexity. One of the main challenges in process control will be the budgeting of depth of focus. With Rayleigh's equations in mind and considering a relatively thick photoresist needed for etching purposes, an overall focus window of less than 40 nm is expected for high-NA EUV exposures. This is concerning, considering that after almost two decades at roughly constant depth of focus (similar to 100 nm for 1.35 NA ArF immersion and 0.33 NA EUV), we may potentially be dealing with 2-3X lower usable focus range. On top of that - and differently from the DUV case - increasing NA in EUVL exacerbates the impact of complex 3D mask effects which, in a non-telecentric tool with an oblique incidence at the mask, negatively impact best focus shifts across mask pitch and feature CD. In this paper, we look at how to mitigate lines/spaces depth of focus loss across pitch by exploring source, mask pattern and mask stack optimization. Moreover, we compare the best strategy between bright/dark exposures, mask transmissivity, and phase-shifting combined with assist features, resist thickness, and feature orientation. Finally, we compare the optical performance with defect-aware process windows at 0.33 vs. 0.55 NA based upon stochastic resist calibration for generic positive-tone organic CAR, and negative-tone inorganic platforms.