Glass substrate inspection tool having a telecentric lens assembly

Abstract

A glass disk substrate inspection tool uses a polarized laser beam that is directed to the first surface of the disk substrate at Brewster's angle and is then transmitted through the disk substrate to a light detector that generates a signal representative of the intensity of the light received. Because the light polarized parallel to the plane of incidence, i.e., the plane formed by the line of the incident beam and a line perpendicular to the surface of the disk substrate, is completely transmitted, there is no surface reflection at either the first or second surfaces of the disk substrate. The polarized beam is directed by a first rotating scanner to the input of a telecentric lens assembly that provides an output beam parallel to its optical axis as the beam is being scanned. The beam is then directed by a first fixed mirror to strike the first surface of the disk substrate at Brewster's angle as the beam is scanned along a line across the first disk surface. The beam is then transmitted through the substrate to the second surface of the disk substrate to a second fixed mirror that redirects the beam to a second telecentric lens assembly. The light passes through the second telecentric lens assembly in the direction reverse to the direction of passage through the first telecentric lens assembly so that the output beam from the second telecentric lens assembly is directed to its focal point. This focal point coincides with the point of contact of a second rotating scanner synchronized for rotation with the first scanner. The second scanner directs the transmitted beam to the light detector.