Accurate mapping of dose variations on EUV scanners using dose-to-clear exposures and optical ellipsometry

In this work, we demonstrate the use of a scatterometry-based technique to accurately monitor the dose variations seen on an EUV scanner. By carefully setting up the exposure conditions and data analysis, we can separate scanner-driven dose effects, mask reflectivity changes and process variations into its individual components. These measurements have a very high repro and throughput allowing us to use this technique to both monitor and provide active feedback to improve overall EUV cluster stability. We have used this technique to achieve a day-to-day dose variation of +-0.5% over a 6-month period on the NXE:3400B at IMEC.