Laser-based inspection tool for disk defects and curvature

Abstract

A laser-based inspection tool (LIT) for disks that allows simultaneous inspection of disk surfaces for defects and curvature. The laser beam is directed by a rotating scanner, such as a rotating polygon mirror, 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 output beam from the telecentric lens strikes the disk surface substantially perpendicularly. The beam is then reflected from the disk surface and passes back through a collection lens to the sensing surface of an optical detector. The detector outputs analog signals that represent the X and Y positions on the sensing surface where the reflected light beam is incident, which thus correspond to the slope of the disk surface at the point where the laser beam was incident. A mechanical disk lifter moves the disk in a plane parallel to the disk surface so that different scan lines can be generated on the disk surface. A processor, such as a personal computer, receives the output signals from the detector and calculates the slope values, and from the slope values, the curvature of the disk surface. A large number of points on each scan line are sampled, and a large number of scan lines are generated, so that the disk surface curvature can be calculated at various locations and over various ranges of the disk surface. The large number of sample points and the rapid calculation of slope values enables the shape of disk surface defects to be determined, which allows the disk defects to be classified as pits or bumps.