Overlay measurement apparatus and overlay measurement method

An overlay measurement apparatus comprising:

• a light source configured to direct an illumination to an overlay measurement target in which a first filling unit formed in a first layer and a second filling unit formed in a second layer stacked on an upper portion or a lower portion of the first layer are positioned;
• a lens having an objective lens condensing the illumination on a measurement position of any one point in the overlay measurement target and a lens focus actuator controlling a distance between the objective lens and the overlay measurement target;
• a detector configured to acquire a sample image at the measurement position through a beam reflected on the measurement position;
• and
• a controller configured to align the sample image measured by the detector and a prestored setting model image, acquire CI information of the aligned sample image and the setting model image, measure a plurality of images by controlling the lens with a focus determined according to the CI information, and calculate an overlay with a difference value by comparing center points of the plurality of images.