Reduction in x-ray lithography shot noise exposure limit by dissolution phenomena

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Abstract
Statistical variations in the local energy deposition in resist can result in defect generation and unacceptable linewidth variation. The lower limit of exposure dose at which these variations limit pattern integrity is termed the shot noise limit. Several analyses of the shot noise limit have been published. We find that refinements in the underlying assumptions on which these analyses are based have led to a substantial reduction in the shot noise limit. The refined assumptions include optimization of the resist absorption, use of a binary exposure‐dissolution model with a sharp, percolation threshold, consideration of multipath dissolution effects, and separation of bias and linewidth variation in algebraic models. The infrequent occurrence of large defects is shown to dominate over the 3σ linewidth variation. While typically 30 exposure events per defect volume is still required, a factor of 30 reduction in the shot noise exposure limit is established primarily by showing that defect sizes up to (1)/(3) of the linewidth are tolerable. For lithography at 0.37‐μm‐feature size, consistent with 64 Mbit dynamic random access memories, the refined shot noise limit is below 1 mJ/cm2 for 1.0 μm‐resist coating thickness. Thus the shot noise limit does not appear to be a fundamental road block to the use of high sensitivity resists with nonsynchrotron x‐ray sources.

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