As lithography tools continue their progress in both NA and wavelength in pursuit of Moore’s law, we have reached the point where the number of features printed in a single pass can now easily surpass 1 trillion. Statistically, then, one should not be surprised to see some members of such a population exhibit fluctuations as great as 7σ. But what do these fluctuations look like? We consider the problem in terms of variations in the effective local resist sensitivity caused by feature-to-feature differences in absorbed photons and resist component counts. We model such variations as a normal distribution, rather than the CDs themselves. As the CD vs. Dose curve is generally nonlinear over large ranges, the normal distribution of the local effective sensitivity then maps to a non-normal distribution in CD. For the case of individual vias printed near the resolution limit, this results in many more such undersized or completely closed vias than one would expect from a normal distribution of the CDs. We show examples of this behavior from both EUV exposures in the fab, and ebeam exposures in the lab. For the latter, results from a simple resist with a somewhat low quencher loading produce approximately the CD variation one would expect by modeling the quencher as distributed normally in the resist.
Robert L. Bristol and Marie E. Krysak, "Lithographic stochastics: Beyond 3sigma," Proc. SPIE 10143, Extreme Ultraviolet (EUV) Lithography VIII, 101430Z (Presented at SPIE Advanced Lithography: March 01, 2017; Published: 24 March 2017); https://doi.org/10.1117/12.2264046.
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