Patent ID: 6181422
Filing Date: 2001-01-30
Classification: G01B

Abstract:
An apparatus for measuring the position of a point on the surface of a sample comprising,means for producing a spot of light to illuminate said point on the surface of the sample,a focus detector,a measurement head comprising a movable focusing element arranged to project onto the focus detector an image of a plane beyond the measurement head in the region of the sample surface,a measurement head drive constructed and arranged to translate the measurement head, so as to vary a distance between the movable focusing element and the plane imaged on the focus detector,a movable focusing element drive constructed and arranged to move the movable focusing element relative to the measurement head so as to vary a focal length of the measurement head,a controller responsive to the focus detector which operates the measurement head drive and movable focusing element drive such that an image of the illuminated point on the sample surface is kept in focus, or in a desired state of defocus, on the focus detector by simultaneously adjusting the focal length and the distance between the movable focusing element and the plane imaged on the focus detector.