After leaving the electron source, the beam follows the central (optic) axis of the lens system and is sequentially defined by apertures and focused by the magnetic and/or electrostatic fields of the lens system. Within the final (objective) lens, a system of scan coils acts to displace the beam off the optic axis so that it can be addressed to a location on the specimen, as illustrated schematically for single deflection scanning in Fig. 6.1. At any particular time, there is only one ray path (solid line) through the scanning system and the beam reaches only one location on the specimen, for example, position 3 in Fig. 6.1. The SEM image is a geometric construction created under computer control by addressing the focused beam to a sequence of discrete x-y locations on the specimen and measuring the effect of the interaction of the beam with the specimen at each location. For a single gray-scale SEM image, this interaction could be the output from a single electron detector, such as the Everhart–Thornley detector. It is also possible to measure the output from more than one detector simultaneously while the beam is addressed to a single x-y location. When this is done, multiple gray-scale SEM images are built up at the same time during the scan. It is essential to realize that even when these multiple signals are being collected simultaneously and multiple images are produced, only a single scan is needed; the parallel nature of the acquisition arises from parallel detection, not parallel scanning. Note that no “true image” actually exists within the SEM in the same sense as the image created in a light optical microscope, where actual ray paths extend from each point on the specimen through the lens system to a corresponding point on the image recording medium, whether that is the eye of a human viewer or the positionally sensitive detector of a digital camera. In the SEM, at each location sampled by the incident electron beam, each signal is measured with an appropriate detector and the analog measurement is converted to an equivalent digital value (using an analog-to-digital converter, ADC). The beam x-y location and the intensity(ies) Ij of the signal(s) of interest generate a digital stream of data packets (x, y, Ij), where j represent the various signals available: backscattered electrons (BSE), secondary electrons (SE), absorbed current, x-rays, cathodoluminescence, etc.
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