Fig. 1. Optical grating image on CMOS pixel array

Fig. 2. (a) Preservation and (b) removal of optical grating image by digital grating

Fig. 3. Light intensity curves I₁ and I₂

Fig. 4. Normalized I curves

Fig. 5. Focusing and leveling sensor based on digital grating

Fig. 6. Displacement out of sync between the wafer surface pattern imaging and the optical grating imaging

Fig. 7. Schematic diagram of the optical grating image on the CMOS pixel array

Fig. 8. Schematic diagram of grating image segmentation

Fig. 9. The accumulated light intensity in one digital grating period before calibration

Fig. 10. The accumulated light intensity in one digital grating period with patterns on the surface of the water

Fig. 11. The accumulated light intensity after calibration

Fig. 12. Statistics of the distribution interval of M accumulated light intensity values

Fig. 13. Corrected accumulated light intensity curve (in one digital grating period)

Fig. 14. Flowchart of the image correction method

Fig. 15. Light intensity curves， alignment points and their enlarged views before and after correction

Fig. 16. The corresponding optical grating image when 4 patterns appear on the wafer surface

Fig. 17. The light intensity I curves for different wafer surfaces during the Z-axis from 0 μm to 40 μm, where the black curves are the light intensity curves when the surface is clean, the red curves are for the pattern existing on the surface, and the blue ones are the corrected light intensity curves (a,b,c,d represent circular pattern, lines, scratches, and dust respectively)

Fig. 18. Mean square error of fitting corresponding intensity I curves for different wafer surfaces during Z direction stepping. (a) Circular pattern, (b) lines, (c) scratch, (d) dust

Fig. 19. Testing platform of focusing and leveling sensor based on digital grating

Fig. 20. Optical grating images before and after Z-axis stepping

Fig. 21. Displacement values measured by the digital grating during stepping

Fig. 22. The movement value of the alignment point

Fig. 23. The error before and after correction of the algorithm