A New Pre-alignment Approach Based on Four-Quadrant-Photo-Detector for IC Mask

In this paper, a new pre-alignment approach based on Four-Quadrant-Photo-Detector (FQPD) for IC mask is presented. The voltage outputs from FQPDs are the functions of alignment marks position offsets with respect to FQPDs. The functions are obtained with least squares error (LSE)-based polynomial fitting after the normalization of experimental data. As the acquired functions are not monotonic about their variables, the alignment marks position offset cannot be given by direct inverse operation on the obtained functions. However, the piecewise polynomial fitting gives the inverse function, with which the alignment marks position offset can be predicted according to the voltage outputs of FQPDs. On the basis of prediction, a pre-alignment control strategy is proposed. The feasibility and robustness of the pre-alignment approach is shown by experiments. Furthermore, the results demonstrate that the maximum error of masks position offset in the X- and Y- directions is less than 15m after coarse pre-alignment.