21 May 1996 Efficient overlay optimization of stepper correctables
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A crucial aspect of overlay optimization is proper selection of stepper input corrections. Automated metrology systems provide the ability to rapidly amass extensive overlay data on lithography systems and processes. The data can then be used to provide feedback in the form of stepper input correction terms to improve overlay. A common approach is an analysis of the overlay data using conventional grid and lens models to determine apparent corrections to be applied to the stepper. However, the standard models do not necessarily account for all the variability in the measured data. Determination of optimal corrections is further complicated by cross-correlation of the stepper input correctable terms. In these cases, the simple application of the grid and lens modeled terms will not provide optimal results. The use of efficient experimental design techniques can reduce the large uncertainty involved in determining and applying these stepper input corrections. Using traditional experimental factorial and response surface design techniques, a descriptive model was developed for the six grid correction terms. The resulting empirical model was generated by using a six factor Box-Behnken experimental design. Multiple wafers were run at these conditions and overlay was measured using an automated metrology system. This empirical model was used to derive the optimal set of inputs to the stepper. This descriptive model is compared with input settings determined from a conventional grid model.
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Warren W. Flack, Warren W. Flack, Susan Avlakeotes, Susan Avlakeotes, David Chen, David Chen, Gary E. Flores, Gary E. Flores, "Efficient overlay optimization of stepper correctables", Proc. SPIE 2725, Metrology, Inspection, and Process Control for Microlithography X, (21 May 1996); doi: 10.1117/12.240101; https://doi.org/10.1117/12.240101

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