An engineer responsible for resist processing had to address the following problem. His resist supplier provided materials with batch-to-batch photospeed controlled to ± 3%. For his process, this corresponded to linewidth changes of ± 8 nm about the target value of 250 nm. The engineer wanted to determine the photospeed of a new batch of resist. This information would be used to adjust exposure doses in order to continue to pattern production wafers with linewidths at their target values. The engineer wanted to expose the minimum number of wafers in order to guarantee that the linewidths would change, on average, by no more than ± 1.5 nm after the facility changed to the new batch of resist.
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