To obtain an image in a scanning electron microscope (SEM), a focused beam of high-energy electrons (3-30KV) is scanned across the surface of a sample. A detector measures the number of secondary electrons being emitted as the beam is rastered and forms an image on a TV screen. These secondary electrons have a much lower energy than the original beam (less than 50eV) so they come from only the top few nanometers of the surface of the sample. Their intensity is very sensitive to surface topography and texture: if the beam falls on a tilted surface or onto an edge more secondary electrons escape and likewise if the beam falls into a valley or a pit fewer secondary electrons escape.

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