How an SEM Works

The scanning electron microscope (SEM) is a microscope that works by scanning a focused beam of electrons on a sample of interest.  The main SEM components include:

  • Source of electrons
  • Column down which electrons travel with electromagnetic lenses
  • Electron detector
  • Sample chamber
  • Computer and display to view the images

Electrons are produced at the top of the column, accelerated down and passed through a combination of lenses and apertures to produce a focused beam of electrons which hits the surface of the sample. The sample is mounted on a stage in the chamber area and, unless the microscope is designed to operate at low vacuums, both the column and the chamber are evacuated by a combination of pumps. The level of the vacuum will depend on the design of the microscope.

Schematic of scanning electron microscope (SEM)

Schematic of a scanning electron microscope (SEM)

The position of the electron beam on the sample is controlled by scan coils situated above the objective lens. These coils allow the beam to be scanned 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 electron-sample interaction, a number of signals are produced. These signals are then detected by appropriate detectors.


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