Visual servoing for mask alignment in photolithography positioning system

The technology of photolithography is getting more important as the critical dimension of micro-products getting smaller. In photolithography, "cycle time" and "positioning precision" of mask alignment are two of the most critical factors which affect the quality of photolithography. In this paper, three main factors that affect the cycle time and positioning precision of mask alignment have been investigated: (1) fiducial mark design, (2) image processing methods of the fiducial mark, (3) visual servoing control. For the fiducial mark design issue, three mark combinations other than the conventional pairs of cross marks have been designed to avoid the problems of image overlapping and cumbrous processing. Four image processing methods for extracting the positions of the fiducial marks have also been designed and investigated. The visual servoing for position alignment of the aligned panels in X, Y and thetas axis was then illustrated as well.