Balancing DOF margin and stochastic failure in high-NA EUV process

BackgroundHigh-numerical aperture (NA) extreme ultraviolet (EUV) lithography delivers superior resolution, but it severely restricts the depth of focus (DOF) window. To expand this margin, the detrimental impact of stray photons leaking into unexposed areas must be mitigated. Crucially, we confirmed that increasing the photo contrast to decrease stochastic failure conversely increases sensitivity to this leakage light, thereby degrading the DOF margin.AimWe aim to maximize the functional DOF window in the High-NA EUV process by understanding the effects of the photoresist material configurations and post-exposure bake (PEB) process conditions.ApproachBased on lithographic simulations, we employed a low-contrast resist environment that exhibits low sensitivity to optical leakage in defocused condition.ResultThe DOF window was expanded by elevating the PEB temperature, reducing the polymer molecular weight, decreasing the protecting group (PG) ratio, or applying a PG with high activation energy (Ea) for deprotection. These adjustments reduced profile degradation and pattern collapse by systematically lowering the photosensitivity of the resist matrix against optical leakage in High-NA process.ConclusionThese findings expose a fundamental problem of High-NA process. Widening the DOF margin requires an insensitive resist environment, which can inherently degrade pattern uniformity. Therefore, achieving a manufacturing window demands a multidimensional approach that precisely balances process margin with stochastic problem.