Active phase engineering of extreme ultraviolet masks for mitigating pitch-dependent image split

BackgroundPattern placement accuracy in high-numerical-aperture (NA) extreme ultraviolet (EUV) lithography is constrained by mask-induced image split in the aerial image, resulting from mask 3D effects under off-axis illumination (OAI). This image split stems from pole-dependent phase asymmetry between diffraction orders generated by the thick, reflective EUV mask stack, with increased severity for fine-pitch patterning in high-NA systems. The conventional approach of targeting zero phase delta (Δϕ) between the zeroth and first diffraction orders is suboptimal (could not suppress image split under realistic OAI conditions). Source averaging over finite illumination angles disrupts the symmetry assumed in simplified interference models, rendering Δϕ=0 ineffective for image alignment.AimThis study proposes and validates a design paradigm, termed active phase engineering, which treats Δϕ as an actively optimized parameter. Rigorous coupled-wave analysis for 0.33 and 0.55 NA systems demonstrates that the phase delta minimizing image split is generally nonzero and depends on pattern pitch.ApproachBy analyzing a broad absorber parameter space, we derive a quantitative design curve that directly relates the optimal Δϕ to the target half-pitch. For a representative 12 nm half-pitch, tuning the mask phase to this predicted target value suppresses image split, reduces the normalized image log-slope penalty, and yields a sharper aerial image.ResultsActively controlling mask-induced phase—rather than enforcing a zero-phase condition—preserves image fidelity and pattern placement accuracy in high-NA EUV lithography.ConclusionThe active phase engineering framework therefore provides a practical and quantitative guideline for next-generation EUV mask design across 0.33 to 0.55 NA systems.