Edge die focus-exposure monitoring and compensation to improve CD distributions

Abstract

As design rules shrink and process windows become smaller, it is increasingly important to monitor exposure tool focus and exposure in order to maximize device yield. Economic considerations are forcing us to consider nearly all methods to improve yield across the wafer. For example, it is not uncommon in the industry that chips around the edge of the wafer have lower yield or device speed. These effects are typically due to process and exposure tool errors at the edge of the wafer. In order to improve yield and chip performance, we must characterize and correct for changes in the effective focus and exposure at the edge. Monitoring focus and exposure on product wafers is the most effective means for correction, since product wafers provide the most realistic view of exposure tool interactions with the process. In this work, on-product monitoring and correction is based on optical measurement using a compact line end shortening (LES) target that provides a unique separation of exposure and focus on product wafers. Our ultimate objective is indirect CD control, with maximum yield and little or no impact on productivity.