Scanning Electron Microscopy


At useful magnifications of 100,000 to 200,000 times, high topographic resolution becomes possible on bulk specimens with a secondary electron (SE) signal, generated by the probe at the site of incidence (SE-I signal), if SE, generated in the microscope chamber or the column by BSE or by electrons of the probe, are suppressed. SSE-dependent SE make up to 90% of the collected SE signal and add to the SE-I signal a high noise component that deteriorates topographic SE-I contrasts. SE-Ill, produced by BSE at the lower pole piece of the microscope, account for 60-70% of the SE signal. SE-Ill generation is eliminated by shielding the pole piece with an electron adsorption device. The SE-IV signal component, produced by the electrons of the probe at the final apertures is reduced to 2-3% of the SE signal by using a large final aperture. SE-II, generated by emerging BSE at the surface of the specimen at some distance from the probe, are collected together with SE-I. SE-(I+II) images obtained from bulk gold crystals under such improved conditions for signal collection show small particles of 4-5 nm in size and edge brightness effects 2-3 nm in width. The cores of ferritin molecules adsorbed on bulk carbon are imaged in appropriate size of ~ 5.5 nm. At high magnifications, contrasts of small topographic features are expected to be produced mainly by SE-I.

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