Overlay measurement apparatus and overlay measurement method

An overlay measurement apparatus comprising:

• a light source configured to direct illumination to an overlay measurement target formed in a wafer;
• a lens assembly including an objective lens configured to condense the illumination on a measurement position and a focus actuator configured to control a distance between the objective lens and the overlay measurement target;
• a detection circuit configured to acquire a focus image at the measurement position through a beam reflected on the measurement position;
• a stage on which the wafer is seated;
• and
• a control circuit configured to control the lens assembly to acquire the overlay measurement target, processing a first sample image detected by the detection circuit for the overlay measurement target and a second sample image detected by the detection circuit for the overlay measurement target rotated at 180 degrees relative to the first sample image, normalizing respective pixels of the first sample image to generate a first normalization image and normalizing respective pixels of the second sample image to generate a second normalization image, rotating one of the first normalization image or the second normalization image at 180 degrees, and calculating a difference between the first normalization image and the rotated second normalization image to produce a correction image for correcting an image for measuring overlay.