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This paper discusses the effect of varied humidity on lithographic performance during photoresist exposure. In the presence of water, exposure causes the sensitizer to be converted into acid; without water, a less soluble ester forms. The effect of humidity on the resist chemistry was measured using infrared spectroscopy. To completely convert the sensitizer to acid, humidities exceeding 40% were needed. At lower humidities, significant amounts of ester, rather than acid, were formed. The effect of humidity during exposure was also measured using lithographic techniques. As humidity decreased from 40 to 20%, the dissolution rate decreased by up to 65%, dose-to-clear (E 0 ) increased by up to 25%, the dose-to-print increased by up to 15%, and contrast decreased by up to 10%. The magnitude of these effects was resistdependent. The time of equilibration of water into and out of the resist film was less than 30 s.

Astigmatism in the modem scanning electron microscope (SEM) is a leading cause oftool drift and poor precision. A straightforward and productive way of assessing astigmatism in an SEM involves determining the best focus settings for neighboring sets ofparallel edges ofdifferent orientations, typically: vertical, horizontal, and diagonal. Clearly if best focus is the same for all targets, then the working astigmatism ofthe instrument is zero. When these readings are unequal, the degree ofastigmatism is proportional to the deviation of individual readings from the mean. This then provides a quantitative way to monitor the degree of astigmatism. Alternatively, this provides the basis for a rapid algorithm for correcting astigmatism with a minimum ofbeam writing. This paper presents methods for objectively monitoring and correcting astigmatism in any SEM with automatic focus. This work also discusses the optimal choice of focus and linewidth measurement targets to use in automatic statistical process control practices for best sensitivity to astigmatism.