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Scanning electron microscope (SEM)
Introduction
The Scanning Electron Microscope (SEM) is widely used in many industries and laboratories to investigate the microstructure and chemistry of many organic and inorganic materials.
The main components of the SEM are the source of electrons, the column down which electrons travel, which contains a number of electromagnetic lenses, and a sample chamber. The SEM is like an optical microscope, which can ‘see’ detail in samples at resolutions greater than is possible with the human eye. However, because an SEM uses electrons as the source of illumination rather than light as in an optical microscope, far superior resolutions are obtainable.
Electrons are produced at the top of the column, are accelerated down it, and pass through a combination of lenses and apertures to produce a fine beam or probe of electrons, which hit the surface of the sample. The sample is mounted on a stage inside the chamber area and, unless the microscope can operate at low vacuums, both the column and the chamber are pumped to achieve a vacuum. The level of the vacuum depends on the design of the microscope.
The position of the electron beam on the sample is controlled by scan coils, which are above the objective lens. The coils allow the beam to be rastered over the surface of the sample. This beam rastering or ‘scanning’, as the name of the microscope suggests, enables information about a defined area on the sample to be collected. As a result of the interaction of the electron beam with the sample, a number of signals are produced, which can then be detected by appropriate detectors.
These signals can provide information about the surface appearance or topography of the sample and also its composition and microstructure.
To summarize, the SEM is both a powerful imaging tool (backscattered electron and secondary electron images) because of its resolving power and high depth of field (greater than that from an optical microscope), and a powerful analytical tool when combined with appropriate detectors.
Low-vacuum and environmental microscopes facilitate the observation of non-conductive and water-containing samples. In these microscopes, the electron gun and electron optical system are maintained under high vacuum, while the sample chamber is maintained at a lower vacuum by another pumping system. A small aperture connecting the two parts allows this differential pressure to be maintained. Under these operating conditions, the electron beam ionizes the gas molecules surrounding the beam, and the charge on the sample surface is neutralized. The low vacuum also means that ‘wet’ samples can be observed.
Sumber: Oxford Instruments
