The origins of the secondary electron signal in scanning electron microscopy

There are three possible modes by which the incident primary beam in a scanning electron microscope releases the secondary electrons that are detected. Secondary electrons may be released as the beam passes through the final aperture. These do not contribute any information about the specimen, instead they contribute only additional noise and background signal which reduces the contrast range available. From the specimen itself, there are two sources which contribute information about the specimen: namely the secondary electron signal released by the surface interaction of the incident primary beam, and the secondary electron signal released as the back-scattered electrons pass out of the specimen. This latter signal constitutes approximately 80% of the secondary electron signal strength from the specimen.