Optical proximity stability control of ArF immersion clusters

Once a process is set-up in an integrated circuit (IC) manufacturer's fabrication environment, any drift in the proximity fingerprint of the cluster will negatively impact the yield. In complement to the dose, focus and overlay control of the cluster, it is therefore also of ever growing importance to monitor and maintain the proximity stability (or CD through pitch behavior) of each cluster. In this paper, we report on an experimental proximity stability study of an ASML XT:1900i cluster for a 32 nm poly process from four different angles. First, we demonstrate the proximity stability over time by weekly wafer exposure and CD through pitch measurements. Second, we investigate proximity stability from tool-to-tool. In a third approach, the stability over the exposure field (intra-field through-pitch CD uniformity) is investigated. Finally, we verify that proximity is maintained through the lot when applying lens heating correction. Monitoring and maintaining the scanner's optical proximity through time, through the lot, over the field, and from toolto- tool, involves extensive CD metrology through pitch. In this work, we demonstrate that fast and precise CD through pitch data acquisition can be obtained by scatterometry (ASML YieldStarTM S-100), which significantly reduces the metrology load. The results of this study not only demonstrate the excellent optical proximity stability on a XT:1900i exposure cluster for a 32 nm poly process, but also show how scatterometry enables thorough optical proximity control in a fabrication environment.