Adjustment Strategy for Inclination Moiré Fringes in Lithography by Spatial Frequency Decomposition

The recently proposed four-quadrant grating Moiré fringe alignment scheme was demonstrated to have the potential of achieving nanometer alignment for proximity lithography. The mask-wafer misalignment was accurately detected by comparing the phase between two sets of Moiré fringes averaged along with the fringe direction, making the inclination adjustment of Moiré fringes highly imperative. The inclination closely related with the included angle between mask (alignment mark) and wafer (alignment mark) largely influences the alignment accuracy. To solve this issue, we present an adjustment strategy based on the spatial frequency decomposition of two sets of differential Moiré fringes, by which the included angle can be accurately adjusted with an accuracy of better than 10^-5 rad. The subsequent discussion indicates that the position of imaging system can be responsible for the inclination but actually it would not affect the mask-wafer alignment, so that our proposed adjustment strategy can be operated simply and feasibly, which is validated by the theoretical analysis and experimental results.