Scanning electron microscope and the type of radiation

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With a scanning electron microscope, the surfaces of a wide variety of materials can be examined and imaged. Various types of rays that arise in the microscope are used for this purpose.

The scanning electron microscope images surfaces.
The scanning electron microscope images surfaces.

How a scanning electron microscope works

  • A scanning electron microscope (SEM) is primarily used to examine the surfaces of materials.
  • Many devices are also equipped with an EDX detector for energy dispersive X-ray spectroscopy, with which the chemical composition of the sample can be examined.
  • With the scanning electron microscope a magnification of up to 100,000 times can be achieved. It is thus in the magnification range between the light microscope and the transmission electron microscope.
  • The cathode compartment is located in the upper part of the SEM. The cathode in it is heated up to such an extent that electrons emerge from it. These electrons are bundled and accelerated towards the sample with an acceleration voltage of 10-30 kV. This beam of electrons is called the primary beam (it is the first type of beam in the SEM).
  • The primary beam is guided (scanned) over the sample surface by deflection coils. If the electron beam hits the surface of the sample, the electrons interact with the Atoms in the sample and there are different types of radiation, which are measured by detectors can.
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The different types of rays

  • The most important source of information are the so-called secondary electrons. They arise from the interaction of the primary beam with the atoms near the surface and have a energy of a few electron volts. They depict the topography of the sample. Areas facing the detector appear light, areas facing away appear dark. One speaks of surface inclination contrast.
  • In addition, backscattered electrons can be used for imaging. These are electrons from the primary beam that are backscattered by the sample. The intensity of the backscattered electrons depends on the atomic number Z of the surface atoms. Heavy atoms scatter the electrons more strongly than light ones, so that areas with heavy atoms are shown lighter. This is called Z or material contrast. This means that conclusions can be drawn about the chemical composition of the surface.

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