How to give a test or measurement of high accuracy is described in the bright field differential interference contrast (BF-DIC) system. The method can include the first light beam to generate a first and second beam. The first and second beams have a circular cross section, to form a first partial overlapping scanning spot is displaced in the radial direction on the substrate. The third and fourth beams are generated from the first light beam or the second light beam. The third beam and a fourth beam having an elliptical cross section, to form a second partial overlapping scanning spot is displaced in the tangential direction on the substrate. Substrate by using the first and second partial overlap scanning spot when being rotated, can be scanned at least a portion of the substrate. Using the measurements obtained from the scanning using the first and second partial overlapping scanning spot can determine the slope of the radial and tangential directions. To determine the wafer shape or any local morphological characteristics, it is possible to use the inclination of the radial and tangential directions.
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