Models for reticle performance and comparison of direct measurement

It’s commonly reported that a difference exists between directly measured reticle feature dimensions and those produced in the final lithographic image. Quantifying this mask error function (MEF) and the sources of the perturbation has been the topic of many papers of the past several years. Past studies have been content to evaluate these functions by statistical averaging thereby neglecting the potential influence of process and exposure contributions. The material presented here represents the findings of an extensive study of reticle-process interactions. Phase I of the evaluation consisted of focus and dose exposures of the reticle and subsequent modeling of the full-profile response. This analysis provided extensive information on the optimum-printed feature profiles while removing the contribution of across-field focus variations. The reticle was directly characterized using both conventional SEM and a new Nanometrics OCD Scatterometer technique. The full-field modeled response surface of the directly measured feature characteristics are then used to calculate the across-field MEF and provide an improved estimate of the true response of the feature to exposure. Phase II of the analysis turns its attention to characterization of the full-wafer process response. Both the modeled and directly measured reticle surfaces were removed from Scatterometry measured full-wafer exposures. Normal process variations consisting of photoresist and ARC thickness volatility are next used to show the response of the printed feature. Finally a summary of the relative contribution of each process perturbation to the feature profile error budget is discussed.