Scatterometry-based on-product focus measurement and monitoring

Immersion lithography is being extended to below 20-nm and the lithography performance requirements need to be tightened further to enable this shrink. With depth of focus control requirements of the scanner going down to 60 nm and below, high quality metrology data is needed to quantify the disturbance of the exposed image. In this paper we present a metrology measurement method to allow accurate and robust measurements of the scanner focus state, through sampling of regular production wafers. Using a unique combination of specifically designed metrology targets and dedicated scatterometer measurements, accurate, precise and robust focus measurement capability is demonstrated. The application towards improved scanner focus control will be explained, demonstrating a measured 18% improvement of production wafer focus uniformity.