Improved defect signal-to-noise due to die-to-die process noise reduction

摘要

The gray level histogram of the test image and the gray level histogram of the reference image are adjusted by histogram scaling. The parameters of the histogram scaling are applied to the test image and the reference image. After the parameters are applied, the reference image and the test image are compared to generate a difference image, e.g., by subtracting the reference image from the test image. The noise in the differential image can be reduced, which improves the defect identification of the differential image. In addition, a noise structure in the differential image extending in the vertical or horizontal direction can be found. If the noise exceeds a certain threshold, the structure can not be inspected.