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The Surfscan installation is used to localize defects or irregularities on the surface of wafers. To simplify, particles with an expansion of >= 0.3 micrometers are recognized as a defect. The measuring principle of the installation is based on the comparisons of periodical structures. This means that only components of the same type are allowed on the wafer. A laser beam scans the surface of the wafer in a grid pattern, whereby the light is scattered on the structures and particles are registered by a photo- multiplier which is rotated by 90 degree against the incoming laser beam. As a rule these defects are not altogether spherical-symmetrical, so that the orientation and structure of the defect affect the intensity of the light scatter. As a defect is only illuminated from one side during measurement, various intensity values can be achieved by rotating the wafer. Therefore measurements with various wafer orientations were taken to assure that a possible defect is illuminated from all four sides. By using various intensity values information can be gained about the form, size and situation of a defect. Further the information can be used to obtain an optimal recipe design of a measurement procedure, and to better define the capture rate of the installation. This results in a more effective use of the installation.
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