In the known height sensor referred lo above, at the detector the (MS) radiation beam is split into two branches. The light in the two branches is detected by photodiodes. The projection grating image shift is measured by the imbalance in the two branches (AC signal) divided by total intensity detected in the two branches (DC signal). In a first order approximation we have: Z_(MS)= gain_(MS)*AC_(MS)/DC_(MS), where gain_(MS) is calibrated. The normalization of the AC signal by Ihe DC signal (Zraw = AC/DC) makes the height sensor response independent of the surface reflectivity and it enables accurate calibration. However, the normalization makes the Zraw_(MS) signal periodic as a function of the height of the substrate. The substrate may be a wafer supported by a wafer stage (WS). Accurate calibration and accurate height measurements are done only in the center/correct period, this is when the full projection grating image (the MS radiation beam) is detected by the deteclion grating.
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