A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). Driven by the growing demand of manufacturing modern integrated circuits with ever shrinking sizes, lithographic technologies that can offer higher resolution and hotter reliability are under a fast-paced and extensive development. One of such advancing lithographic technologies is the extreme ultra-violet lithography (also known as EUV or EUVL), which allows for sub-10 nanometer (nm) scale resolution. In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. In order to keep pace wilh the advancement of lithographic technologies, various inspection and metrology tools have been made available. One of such inspection and mcirology tools is based on scanning electron microscopy. The basic principle of scanning electron microscopy is the scanning of an electron beam over a sample to build up an image.
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