US Patent 9835956 Apparatus and method for overlay measurement

The present disclosure provides apparatus and methods for overlay measurement. An exemplary overlay measurement apparatus includes an illuminating unit configured to generate illuminating light to illuminate a first overlay marker formed on a wafer to generate reflected light; and a first measuring unit configured to receive the reflected light from the first overlay marker to cause the reflected light to laterally shift and shear to generate interference light, to receive the interference light to form a first image, and to determine existence of an overlay offset and an exact value of the overlay offset, according to the first image.