Abstract
An automatic visual inspection system has been developed for Thin Film magnetic Head (TFH) wafers. Although there are only a few classes of defects to be detected, the difficulty of defect detection varies drastically depending on the location of the defect. When the optical characteristics of a defect and the underlying element pattern are similar, the defect becomes difficult to detect. To detect all defects reliably, we developed the following new techniques. (1) Optical enhancement: The wafer is illuminated by a slit-shaped light source from an oblique direction, and the scattered light is used for detecting flakes in the transparent protection layer. Reflected light from the surface is also used for detecting surface defects. Defects are easily extracted by thresholding the detected image. (2) Image processing: An element-to-element comparison method is employed to detect defects that cannot be enhanced optically. Many bright spots within the ceramic substrate cause discrepancies when compared. Local minimum processing is used for eliminating them and stabilizing defect detection.
The system has been evaluated in an actual production line and the defect detection rate achieved is approximately 13% higher than human performance.
