Patent ID: 7460210

Claim:
An auto focus system comprising: a stage to which a substrate is to be mounted; optical components including measuring light sources each of which emits a focus beam, and sensors each operative to output a signal indicative of a location at which a beam of light of the type emitted by the light sources is received thereon, the optical components being arranged to direct the focus beams emitted by the measuring light sources at different angles, respectively, towards a substrate mounted on the stage, wherein the angle at which the focus beam propagates towards the substrate mounted on the stage is defined as the angle subtended between the focus beam and a plane that is parallel to an upper surface of the stage, and such that the sensors receive the focus beams reflected from the substrate such that the sensors will output signals indicative of locations at which the reflected focus beams are received thereon; and control means operatively connected to said sensors so as to receive the signals output by the sensors, the control means comprising a calculation unit configured to incorporate together values representing the relative locations from which the focus beams emanate from the light sources, with values representing the angles at which the focus beams propagate towards the substrate, and with values representing the locations at which the reflected focus beams are received on the sensors, respectively, and thereby calculate a focus changing value indicative of the position of a surface of a substrate mounted to the stage relative to a reference plane, and the control means operatively connected to the stage so as to control the position of the stage based on the focus changing value.