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As extreme ultraviolet lithography (EUVL) technology progresses to support more advanced nodes, mask designs are becoming more complex and difficult for mask makers to produce with the required fidelity. Resolution enhancement techniques (RETs) including mask optical proximity correction (OPC) models and the addition of sub-resolution assist features (SRAFs) are now critical technology enablers. Early adopters of EUVL benefited from the resolution improvement provided by the shorter wavelength. To extend the benefits of EUV to smaller technologies and broader applications, SRAFs are valuable tools to maximize the process window for a variety of critical layers which require both bright and dark field imaging. Regardless of mask tone, because of the short wavelength of EUV radiation, very small SRAFs are required, thus patterning SRAFs for EUVL poses significant mask process challenges. E beam exposure and resist characteristics are key factors for important patterning attributes including minimum resolvable size, line edge roughness (LER), corner rounding (CR), and line end shortening (LES). Patterning dark SRAFs for bright field masks involves an additional challenge of overcoming mechanical forces on these small structures that can cause feature toppling or collapse. In this study, we demonstrate a mask process capable of patterning dark SRAFs on mask at line widths below 25nm (4X). We will discuss patterning solutions including innovations in both the physical mask manufacturing processes as well as mask data preparation techniques, such as mask process correction (MPC). We will utilize advanced characterization techniques to evaluate SRAF pattern fidelity on the mask and its predicted impact on wafer printing performance. A process capable of resolving sufficiently small SRAFs enables an on-wafer analysis to validate whether the predicted lithographic benefits can be demonstrated. Finally, we will review the outlook of continued shrinking mask resolution requirements from both a design value and process implementation perspective. The coming challenges for EUV mask makers include supporting not only ever increasing design complexity, but also the introduction of new mask materials, including phase shift masks (PSMs) as part of the overall RET solution.
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