Subμm registration of fiducial marks using machine vision

Sub/spl mu/m registration between circularly symmetric fiducial marks can be achieved simply by finding their centroids by fitting circles to distributed edge positions determined by the second-derivative zero-crossing method. However, because of diffraction fringes, substantial errors originating in the tilt of the microscope optical axis must be minimized. This method is used for laser-fibre alignment tolerance studies.>