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Highlighting stochastic manifestations in 193 nm immersion lithography with contour-based metrology metrics

Background: Stochastic effects stated as the ultimate limit of EUV lithography are widely studied by resist suppliers, mask and metrology tool fabricants. Aim: Although the phenomenon is less explicitly visible, we want to highlight its presence in DUV through variabilities in critical dimension (CD), in shape and in position. These manifestations are not yield killers but may become reliability killers, if combined with some local overlay. Approach: Contour extractions are done from CD-SEM images to enable in-depth computational analysis of the data, to characterize local variabilities, alternatively to massive metrology solutions. This augmented computational metrology multiplies the volume of accessible CD data without requiring any supplementary tool purchasing, as they are already included within conventionally stored SEM images. Results: Contour extractions allowed to detect 5σ deviations from normal law on CD distribution from 1000 images. A set of complementary metrics quantities shape and size uniformities, as well as pattern displacement. It leads to a narrower process window where stochastic effects are minimized. Conclusion: Contour-based metrology offers complementary metrics, useful to characterize or center a process, increasing reliability by doing an extensive analysis of a fair number of CD-SEM images. Are these metrics capable of detecting early signs of process instability?