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Lot-to-lot ADI CD data are generally used to tighten the variation of exposure energy of an exposure tool through an APC feedback system. With decreasing device size, the process window of an exposure tool becomes smaller and smaller. Therefore, whether the ADI CD can reveal the real behavior of a scanner or not becomes more and more a critical question, especially for the polysilicon gate layer. CD-SEM has generally been chosen as the metrology tool for this purpose. Because of the limitations of top-down CD-SEMs, an APC system could be easily misled by improper ADI CD data if the CD data were measured on a T-topped photo resist. ArF resist shrinkage and line edge roughness are also traditional causes for improper CD feedback if the user did not operate the CDSEM carefully. Another candidate for this APC application is spectroscopic-ellipsometry-based scatterometry technology, commonly referred to as SpectraCD. In recent studies, SpectraCD was proven to be able to reveal profile variation with excellent stability. The feasibility of improving a CDSEM-based APC system by a SpectraCD-based system in a high-volume manufacturing fab is therefore worthy of study.
This study starts from an analysis of the historical data for the polysilicon ADI CD of a 130 nm product. Two different sets of CD measured from the two different metrology tools were analyzed. In the fab, CDSEM was the metrology tool chosen for the APC feedback. The CD data measured by SpectraCD over a 2 month timeframe were plotted as a CD trend chart of the specific exposure tool. There are several trend-ups and trend-downs observed, even though the overall CD range is small. After a series of analyses, the exposure tool has been proven to be quite stable and the CD data measured by SpectraCD also reveal the real behavior of the exposure tool correctly. The scanner is shown to have been misled by improper CD feedback. In comparison with CDSEM, the linearity of the correlation between ADI and AEI CDs, which represents the consistence of etch bias, can also be improved from 0.4 to 0.8 by SpectraCD. The root causes are still under investigation, but one suspected reason is related to resist profile. All the analysis results will be reported in this paper. The data provided sufficient motivation for switching the APC feedback system of the fab from a CDSEM-based system to a SpectraCD-based system. The results of the new APC system will also be discussed.
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