Improving focus performance at litho using diffraction-based focus metrology, novel calibration methods, interface and control loop

摘要

In advanced optical lithography the requirements of focus control continues to tighten. Usable depth of focus (DoF) is already quite low due to typical sources of focus errors, such as topography, wafer warpage and the thickness of photoresist. And now the usable DoF is further decreased by hotspots (design and imaging hotspots). All these have put extra challenges to improve focus metrology, scanner focus stability calibrations and on-product correction mechanisms. Asymmetric focus targets are developed to address robustness in focus measurements using diffraction-based focus (DBF and μDBF) metrology. A new layout specific calibration methodology is introduced for baseline focus setup and control in order to improve scanner focus uniformity and stability using the measurements of the above mentioned asymmetric targets. A similar metrology is also used for on product focus measurements. Moreover, a few novel alternative methods are also investigated for on-product focus measurements. Data shows good correlation between DBF and process on record (POR) method using traditional FEM. The new focus calibration demonstrated robustness, stability and speed. This technical publication will report the data from all the above activities including results from various product layers.