A focus calibration method is developed to determine the focus position of a grating alignment system. An illumination beam scanning module is utilized to generate a circular motion for the beam, which forms an angular modulation interference image on the reference mark. A theoretical model is presented to determine the focus by determining the alignment grating z-position, at which the alignment offset is independent of the incident beam tilt. The standard uncertainty of the focus calibration results is estimated to be better than 150 nm. This technique may improve the measurement performance for lithography systems and precision machine applications.
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