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What is Scanning Electron Microscopy (SEM)?


A scanning electron microscope (SEM) is a type of microscope which uses a focused beam of electrons to scan a surface of a sample to create a high resolution image. SEM produces images that can show information on a material’s surface composition and topography.

How Does a Scanning Electron Microscope Work?

SEM produces magnified detailed images of an object by scanning a focused beam of electrons. This works differently to transmission electron microscopes (TEMs) as the electron beam goes straight through the object.

Electrons are created and fired using an electron gun which accelerates down the microscope passing through a series of lenses and apertures creating a focused beam which then interacts with the surface of a sample.

The sample is positioned on a stage in the chamber of the microscope before a vacuum is created in the chamber via a series of pumps. The level of the vacuum depends on the microscope’s design, while some microscopes are designed to operate in low vacuum environments, meaning that the chamber does not need to be evacuated.

Scan coils control the position of the electron beam above the objective lens. These coils allow for the beam to scan across the surface of the sample, enabling information about a defined area to be collated. The interaction between the sample and the electron create a number of signals in the form of secondary electrons, backscattered electrons, and characteristic X-rays that are then detected by detectors. The detector creates images which are displayed on a computer screen.

Example Scanning Electron Microscope Image

Why Use Electrons Rather Than Light?

The distance that the human eye can distinguish two parts in visible light is 0.2mm apart, however this can be increased through the use of a lens. This distance is called the resolving power of the lens, which can magnify the distance many times over. However, a light microscope has a resolving power of about 1000x, as it is limited by the wavelength of the light as well as the quality and number of lenses used. However, electron microscopes provide a shorter wavelength, which creates a better resolution.

What is the Resolution of a Scanning Electron Microscope?

The spatial resolution is dependent on the size of the electron spot. Unlike conventional image forming cameras (film or CCD array), SEM uses an electron beam to rapidly scan over a specimen sample to yield an image. The resolution of SEM is normally around 10 nanometres (nm).

The maximum resolution for a scanning electron microscope image depends on factors such as the interaction volume of the beam and the electron spot size. SEMs can achieve a high resolution of below 1 nanometre (nm), although they cannot provide atomic resolutions. Most modern SEMS provide a resolution of 1-20 nm, while smaller desktop versions offer a resolution of 20 nm or more.

Scanning Electron Microscope Services

TWI carries out electron microscopy on behalf of our Industrial Members across a variety of industry sectors.


What are the Advantages and Disadvantages of a Scanning Electron Microscope?

The Advantages of a Scanning Electron Microscope:

  • Produces detailed three-dimensional and topographical images
  • Easy to operate with training
  • User-friendly software
  • Can perform quick scans (BSE, EDS and SEI analyses can be performed within a few minutes)
  • Samples require a small amount of preparation

The Disadvantages of SEM:

  • Expensive to buy and run
  • Large (takes up lots of space)
  • Needs to be in an environment where there is no electric, magnetic or vibration interference
  • Needs a trained operative to prepare the samples and operate the microscope
  • Small risk of radiation exposure
  • Restricted to samples small enough to fit into the vacuum chamber
  • Requires cool water and a steady voltage

How much is a Scanning Electron Microscope?

SEMs vary in cost. Table top models can cost anywhere between 50,000 to 70,000 USD and conventional SEMs (tungsten source) cost anywhere between 80,000 to 120,000 USD. These prices can be different depending on which country the SEM is bought from.

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