Methods for Edge Detection on CCD or CMOS Linear Sensor Arrays enabling Subpixel Resolution

Optical high-precision measuring devices are used where fast and contactless measurements of object surfaces and edges with sub-micrometer resolution is needed. Wafer inspection in the semiconductor industry is an important application. The proposed measurement setup relies on exposure of an object’s edge onto a light sensitive detector with a point light source. The X/Y position of the edge, or geometrical object parameters like thickness or diameter could be determined from the position of sequentially projected edges. When using narrowband point light sources for exposing an object, a diffraction pattern is generated, which can be seen by a photosensitive detector. Thresholding is commonly used for determining the projected edge position on the sensor. Since its lack of performance, additional information contained in the diffraction pattern can be used by other methods to achieve a much higher resolution than the pixel size of the detector is. Other Algorithms outperform thresholding quite well. A prediction error for the theoretical projected edge position on the sensor of less than 200nm could be achieved, still when using a modelled image sensor pixel size of 3.8µm. To compare the different algorithms, and additionally, take the influence of variables such as measurement noise, sensor pixel size, and the pixel value ADC resolution into account, a simulation framework was set up.