OVERLAY ERROR DETECTION

摘要

An overlay target (24) with gratings thereon is illuminated and radiation scattered by the target is imaged onto detectors (32). A phase difference is then dectected between the outputs of the detectors to find the mis-alignment error. In another aspect, an overlay target with gratings or box-in-box (40) structures is illuminated and radiation scattered by the target is imaged onto detectors located away from the specular reflection direction of the illumination in a dark field detection scheme.