How does scanning electron microscopy work?

In scanning electron microscopy, a sample is scanned using a fine electron beam. Then the electrons arriving at the detector are depicted as grey scale values. SEM not only permits significantly higher resolutions than classic light microscopy, it also permits the production of three-dimensional images. This aspect makes possible the exact depiction of sample surfaces with high depth of field and resolution.

Scanning electron microscopy
at Quality Analysis

We utilise the capabilities of modern scanning electron microscopy to offer you the best possible, most detailed analysis. In this way we obtain insights into the interior of a sample and, at the same time, can remove material in the smallest quantities using the ion beam. In this manner we analyse the microstructures and coatings in your sample with high resolution up to the nanometre range.

Scanning electron microscopy

This is what quality analysis can do for you

  • Analysis of surface structures
  • Analysis of microstructures
  • Identification of foreign-matter inclusions
  • Verification of contamination
  • Depiction of flaws and fracture surfaces (Damage analysis)
  • Particle analysis

Image generation using a scanning electron microscope

Image generation in a scanning electron microscope is significantly more complex than in a conventional light microscope, however the scanning electron microscope permits significantly higher magnifications.

The electron beam in an SEM

Modern electron microscopes generate the electron beam necessary for the analysis using a field-emission cathode. This cathode generates electrons using an electric field applied to its fine tip. With a tungsten cathode, the field strength is sufficient to generate the beam, unlike a Schottky cathode that needs to be heated. With both cathodes there is a large voltage drop in the direction of the anode (up to 3,000 kV) which applies high acceleration to the electrons. For this reason, the uppermost part of a scanning electron microscope is also termed the electron gun.

Electron lenses – magnification in scanning electron microscopy

Lenses are also used in scanning electron microscopy to produce a magnified image of an object. Of course, these are not optical lenses but so-called electron lenses, which are actually magnetic fields. Every scanning electron microscope usually has three of these lenses. The first of the lenses is used to bundle the electron beam. When the electron beam is incident on the surface of the sample, it knocks further electrons out of the sample (so-called secondary electrons). Along with the secondary electron contrast, backscatter electron contrast and X-ray analysis (EDX or WDX) are used.
The image is magnified behind the sample using the objective coil and the projection coil. Then the electrons, as they arrive at the lower end of the coil, are registered by a detector either directly or by means of their interactions. The electron density in each area is depicted as a grey scale value. As such a snapshot from SEM is not an image or photograph in the classic sense, instead it is a distribution diagram.

What distinguishes scanning electron microscopy?

Three-dimensional visualisation using SEM and removal of material by ion beam

The process in scanning electron microscopes described above does not differ significantly from the process in older transmission electron microscopes (TEM). The difference is in the nature of the sensing. While in the TEM a broad, static electron beam is applied to the entire sample, a scanning electron microscope uses a significantly finer beam and guides this beam line by line over the analysis area. This method is termed scanning. It permits not only a high resolution image, but also the generation of three-dimensional depictions. It is also possible to use an ion beam to remove material in very small quantities and to analyse microstructures and coatings with an impressive resolution in the nanometre range.

Where is scanning electron microscopy used?

Scanning electron microscopy is used in various sectors. In industry it plays, for instance, a key role during damage analysis, here it is used for various types of components: mechanical, optical or electronic. It is also used for fracture analysis and in materialography to analyse microstructural characteristics and defects in materials at a very fine level. Essential in biosciences for the detailed analysis of cell structures and particles, scanning electron microscopy is also vital in the electrical engineering industry for the identification of contamination and for the analysis of signs of wear.

Scanning electron microscopy in use: 3 examples

Along with chemical and light microscopic methods, scanning electron microscopy is today one of the most important methods for the detailed analysis of samples of all types. Along with surfaces, it is possible to analyse cross-sections of the sample. Depending on the application, different methods are used for preparation in combination with different signal types and matching detectors.

Damage analysis using scanning electron microscopy

Damage analysis

Surface anomalies and deposits can be seen particularly clearly in the three-dimensional depiction provided by scanning electron microscopy. This advantage is also used for the depiction of fracture surfaces so that conclusions can be drawn as to the failure mode.  Signs of wear can also be characterised reliably during damage analysis.

SEM-EDX analysis to determine the chemical composition

Chemical composition

The chemical composition of a sample can be determined using energy-dispersive X-ray spectroscopy (EDX). It is used, for example, to verify the presence of contamination on samples or foreign-matter inclusions in samples. Even the finest traces of corrosion can be detected reliably.

Particle analysis using scanning electron microscopy

Particle analysis

During particle analysis, scanning electron microscopy makes an important contribution to the determination of the size, distribution and damage behaviour of particles. By using energy-dispersive X-ray spectroscopy, it is also possible to draw conclusions about the chemical composition of the particles.

In summary: scanning electron microscopy

Scanning electron microscopy is a method used for the analysis of organic and inorganic samples based on a bundled electron beam. This beam scans a pre-defined area of the sample. This high-resolution method is used today in many methods for material analysis.

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Julia Banzhaf


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