backscattered-electron detector

A detector that is used for detecting backscattered electrons from the specimen surface by electron-beam illumination. In a TEM, a micro-channel plate (MCP) is used as the "backscattered-electron detector." (In a dedicated SEM instrument, a semiconductor detector that uses the p-n junction mechanism is adopted.) To improve the detection efficiency, an annular detector is placed just above the specimen. The MCP detects backscattered electrons and converts the electron signal into an electric signal. Since the energy of backscattered electrons is high, additional electron acceleration that is applied to secondary electrons is not required. A backscattered-electron image is obtained by displaying the intensity of the backscattered electrons on a computer monitor screen as a series of bright spots synchronized with the scan of the electron probe. A two-segment annular detector is often used for obtaining a composition (COMPO) image and a topographic (TOPO) image. That is, two signals acquired from each segment are amplified by with the preamplifier, and then processed with the main amplifier in such a way that COMPO image is obtained by addition of the two signals and TOPO image is obtained by subtraction of them. Furthermore, the use of a four-segment detector enables us to obtain a stereoscopic (SHADOW) image. To obtain SHADOW image, the composition signal from two segments and the topography signal from other two segments are synthesized.

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