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Understanding the power of scanning electron microscopes (SEMs)

As the materials used in a range of products and items are becoming even smaller, a typical light microscope cannot always do the job. Scanning electron microscopes are highly powerful and help with the characterization of a range of materials. But, how a scanning electron microscope (SEM) works differs substantially from a light microscope.

As the materials used in a range of products and items are becoming even smaller, a typical light microscope cannot always do the job. Scanning electron microscopes are highly powerful and help with the characterization of a range of materials. But, how a scanning electron microscope (SEM) works differs substantially from a light microscope.

Light microscopes can be limiting as the wavelength they detect generally is between 400-700 nanometers (nm), so anything below this cannot be seen at a good resolution. As electrons have much shorter wavelengths, they can show them at a much higher resolution than a light microscope can. 

There are five main components of a scanning electron microscope:

  • Electron source
  • Column in which electrons travel down with electromagnetic lenses
  • Electron detector
  • Sample chamber
  • Computer and display to view images

The basics of how a scanning electron microscope works is that the electron beams that are generated scan the sample in a raster pattern. Electron generation occurs at the top of the column at the electron source. The electrons are pointed downwards through the column and onto a series of electromagnetic lenses. These lens tubes, which are also called solenoids, are wrapped in coils which can be adjusted to change the focus to suit the sample. As you adjust focus with the coils, it causes changes in voltage, thereby increasing and decreasing the speed at which the electrons make contact with the sample. 

A solid material sample can be placed onto the stage, which is typically a vacuum chamber. Whomever is operating the scanning electron microscope can control it with a computer, adjusting the coils for greater focus and magnification to help determine the surface area that is being scanned. The material inside the sample chamber will be exposed to kinetic energy from the electrons; when a connection is made between the electrons and the sample, energetic electrons are released from the surface of the sample, creating a scatter pattern. Detectors pick up on the scatter patterns to create a black and white 3D image. The scatter pattern will provide information about the size, shape, texture and composition of the sample in question. 

There are a range of industries that make use of scanning electron microscopes, specifically those that require characterizations of solid materials. Common fields that make use of this technology are those in the fields of biology, life sciences, medical and forensic science and gemology. Not only can these microscopes give characteristic information of a sample, but they can also provide information in regard to microstructures, recognize surface fractures, and even identify crystalline structures. 

Scanning electron microscopes are expensive and large but allow for easy operation and can provide analysis of a surface in as little as five minutes.  

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