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Flare-induced CD variation correction using transmittance controlled mask

As the design rule shrinks, intra-field CD control becomes more difficult. Flare induced by lens contamination is one of CD variation sources across the exposed field and its distributions are different from tool to tool. To use the exposure tool with the contaminated lens, CD correction method is to be specified to improve the wafer CD uniformity. In this paper, the local flare values are measured using dose-to-clear method and CD measurement method in order to confirm the exposure tool condition. Then we design a mask whose transmittance is controlled locally for CD uniformity enhancement. The mask has several phase-out holes in the quartz side. By distributing the holes with respect to the local area flare, we can make the intensity distribution opposite to the lens local flare.