Scanning Electron Microscopy


Sometimes, the sample to be examined in the SEM will consist of a compositionally non-uniform substrate that is covered by an approximately uniform surface layer. With a low enough incident beam energy, only the surface layer can be seen in the SEM image. The underlying structure can be seen in the secondary electron (SE) image if the range of the incident electrons is greater than twice the thickness of the surface film. In the backscattered electron (BSE) image the threshold energy is higher because the BSE detector is insensitive to slow electrons. The information depth in the BSE image was investigated experimentally as a function of incident energy and BSE detector position using test specimens in which an Al layer of thickness either 210 or 1,100 nm was deposited onto an aluminised Si wafer covered by a pattern of gold lines. It was estimated that a lower limit to the surface mass-thickness that can be measured using a solid-state BSE detector is ~ I0μg/cm2 (=40 nm of Al) for the BSE method, as compared with ~ 0.25 μg/cm2 (=1 nm of Al) for the low-loss electron method. There would seem to be no reason why measurements by the BSE method could not be carried out automatically in a computer-controlled SEM equipped with image analysis and using the standard BSE detector systems, to measure the mass-thickness of a surface layer.

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