Surface inspection tool using reflected and scattered light

Abstract

A laser based inspection tool is described which is particularly useful for inspecting one or both planar surfaces of disks for use in disk drives. The tool uses low angle reflected light and large angle scattered (LAS) light from the surface which are separately detected and pixelated to create a reflected light image and a scattered light image. The tool uses a mechanical lifter which moves the disk through the laser scan lines to allow the surface on one or both sides of the disk to be scanned. The line scanning is performed using a rotating polygon mirror (scanner) which also captures the beam reflected from the disk surface. A telecentric lens assembly (TLA) acts to ensure that the laser beam is incident at a substantially constant, nearly perpendicular angle as the beam scans across the disk and to direct the reflected and scattered light back onto the scanner. The LAS light is captured by a pair of plano mirrors disposed above and below the scan line. The two cones of LAS light from the plano mirrors are directed back through the TLA along with the reflected light onto the polygon. The LAS light is then separated from the reflected light by a beam splitter and the intensities of the two separated beams are converted into sampled digital values which are processed to form two images of the surface. These two images are used in combination to detect defects in the disk surface and to avoid false rejections. The LAS image is useful in determining that deviations in the reflected light are likely to be a particle adhering to the surface and not a true defect. The LAS image and the reflected light image are available for various types of analysis and comparison making the tool adaptable for a wide variety of surface inspection techniques.