Your worst nightmare: inspection of aggressive OPC on 14nm masks with emphasis on defect sensitivity and wafer defect print predictability

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To prevent catastrophic failures during wafer manufacturing, mask manufacturers employ sophisticated reticle inspection systems to examine every image on every reticle to identify defects. These advanced systems inspect at resolutions typically 3x higher at the reticle-plane than advanced wafer scanners; thus enabling them to detect the small defects necessary to ensure reticle quality. The most thorough inspection is done using a reticle-to-database comparison that ensures the reticle pattern matches the design pattern. For high defect sensitivity, the database must be carefully modeled to exactly match the reticle pattern. Further, sub-resolution OPC shapes are often at the limit of the mask manufacturing process, which adds subtle variations on such shapes across the reticle. These modeling errors and process variations can cause high numbers of unwanted detections, thereby limiting inspection system defect detection sensitivity.[1] OPC designs are expected to become more aggressive for future generations and may stress the performance of current reticle inspection systems. To systematically assess the capability of various inspection approaches and identify needed areas for improvement, a new “Nightmare” test reticle has been designed by IBM. The test reticle contains various sizes and shapes of sub-resolution features that might appear on reticle generations from today’s 22nm to future 7nm. It also contains programmed defects to assess defect detection capability of current and future generation inspection systems. This paper will discuss the design of the “Nightmare” test reticle, and the inspection results of the current generation reticle inspection methods with emphasis on both inspectability and defect sensitivity. The subresolution features will be ranked according to importance for advanced OPC design. The reticle will also be evaluated using wafer print simulation so lithographic impact of features and defects can be measured and compared against inspection approaches and results.