Overlay and focus stability control for 28-nm nodes on immersion scanners

For the 28 nm node lithographic production steps, the process window for both overlay and CD are becoming increasingly tight. The overlay stability of lithography tools must be at a level of 1-2 nm within the product cycle time, while focus needs to be stable within 5 nm. Well-matched tools are crucial to improve the flexibility of tool usage and the pressure for higher tool availability is allowing less time for periodic maintenance and tool recovery. Here, we describe the way of working and results obtained with a long-term stability control application, containing a scanner performance control system with a correction feedback loop deploying scatterometry. In this study the overlay performance for immersion scanners was stabilized and the point-to-point difference to a reference is maintained at less than 4 nm. The capability of tool recovery handling after interventions is demonstrated. Results of overlay matching between machines are shown. The tool stability for focus was controlled in a range of less than 5 nm while improving the total focus uniformity.