An auto focus system includes a stage on which a substrate is mounted, light sources that irradiate the substrate with a plurality of focus beams directed towards the substrate at different angles, sensors that detect the focus beams reflected from the substrate, and a controller that determines the...http://www.google.com/patents/US7460210?utm_source=gb-gplus-sharePatent US7460210 - Auto focus system, auto focus method, and exposure apparatus using the same
