16 July 2002 Application of feedforward reticle: offset for overlay APC in a high-part-count fab
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Numerous reports have shown consistent evidence that automated run-to-run feedback control of overlay correctable coefficients will provide clear benefits when applied to fabs with steady-stream process flows that are associated with low part count WIP profiles. When the same methods are deployed in unsteady-flow, higher part count operations the results have been mixed. Within these high part count operations, some process flow streams show improvement while others do not. Attempts at optimizing the feedback loop have failed to achieve desirable result for all process streams - some process streams would benefit while others would lose ground. In this study, we will show how a structural change in the run-to-run control algorithm provided a breakthrough in both performance and understanding of the underlying system dynamics. The first step was to recognize the fundamental difference between reticle-sourced overlay errors versus tool and process sourced errors. The recognized difference was that the reticle-sourced errors were highly stable over long periods of time, thus enabling deconvolution of reticle effects from the higher frequency tool and process effects. The second step was to recognize that the frequent reticle changes that occur in a high part count fab could be modeled as a feedforward disturbance rather than as discriminates in defining and dividing process streams.
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David A. Crow, David A. Crow, Etienne L. Joubert, Etienne L. Joubert, } "Application of feedforward reticle: offset for overlay APC in a high-part-count fab", Proc. SPIE 4689, Metrology, Inspection, and Process Control for Microlithography XVI, (16 July 2002); doi: 10.1117/12.473443; https://doi.org/10.1117/12.473443

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