Benefits of three-dimensional mask (M3D) effects in high-NA and hyper-NA EUV lithography

BackgroundThree-dimensional mask (M3D) effects in EUV lithography arise from complex electromagnetic scattering associated with the three-dimensional geometry of mask absorbers and multilayer stacks. Historically, M3D effects have been regarded as detrimental to process performance.AimThis study aims to offer an alternative perspective on M3D effects in EUV lithography. We demonstrate that M3D effects can be harnessed under certain conditions to improve imaging fidelity for high-NA and hyper-NA EUV lithography.ApproachWe performed fully rigorous source-mask optimizations (SMO) for different line-space (L/S) configurations to compare the achievable performance of M3D with Kirchhoff-type flat masks and to identify the best imaging solutions for both.ResultsM3D with L/S on dark-field masks can outperform Kirchhoff-type masks in terms of efficient use of EUV light, achievable contrast, and depth of focus (DoF). Realizing these benefits requires split-pupil exposure or other techniques to offset the contrast loss due to M3D-induced image fading. In-depth analyses of near-fields, diffraction, and aerial images explain the advantages of 3D dark-field masks: more efficient light coupling into diffraction orders within the projection NA and tighter, higher near-field intensity peaks due to light guiding in low-n absorbers. The benefits of M3D are less pronounced for bright-field masks and for horizontal L/S near the resolution limit of hyper-NA systems.ConclusionsM3D effects, when combined with rigorous SMO and split-pupil exposure, can enhance imaging in high-NA EUV lithography and hyper-NA regimes. These benefits are use-case-dependent and have a significant impact on OPC and SMO strategies in terms of biasing, assist sizes, and optimum source shapes.