Autofocusing Image System of CD-SEM

In order to realize auto focus during CD-SEM imaging process, the structure of image and scanning model is explained. The principle and the performance of image sharpness evaluation function, such as pixel gray variance function, gradient function, gray entropy function and SMD function, are analyzed in this paper. The experiment shows that all listed methods can evaluate the image sharpness correctly. But gray variance function excels in speed and evaluation effect, thus proving to be the ideal image sharpness function for CD-SEM auto focus.