Moiré Fringes-Based Wafer-to-Mask Gap Measurement in Lithography

This paper proposes a Moiré fringe-based lithography alignment system for accurately measuring the gap variations between the wafer and mask. By placing slightly different differential grating marks on the mask, Moiré fringe images are generated through the interference of diffraction levels of the marks under laser irradiation. To improve measurement accuracy, the 2nd-order two-dimensional desirable sidelobe convolution window (2D-DSCW) algorithm is incorporated to suppress the adverse effects of spectral leakage on phase extraction. Simulations demonstrate that for grating periods of 4 μm and 4.4 μm and a wavelength of 633 nm, the proposed system yields precise measurements for gaps ranging from −76 nm to 76 nm, consistent with the theoretical measurement range. The proposed system demonstrates significant potential for high-precision lithography alignment applications.