Patent ID: 7123345

Claim:
An automatic focusing apparatus, comprising: a stage which holds a substrate and which is movable in a horizontal direction and a vertical direction; an objective lens disposed facing a surface of the substrate; first illumination optics which illuminate the surface of the substrate with at least one incident light beam from an oblique direction; a first photodetector which detects light reflected from the surface of the substrate to output a first electric signal; a first position detection circuit which detects a vertical position of the surface of the substrate from the first electric signal to output a first position detection signal; a correction circuit which monitors the first position detection signal in real time and which subtracts a surplus signal change value exceeding a predetermined signal change value from the first position detection signal, when a difference between a change in the amount per unit time of the first position detection signal and a vertical position target value exceeds a first predetermined level, to output a corrected position signal; and a stage control circuit which controls the vertical position of the stage based on the corrected position signal.