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Scanning Electron Mcroscopy (SEM)

Scanning electron microscopy is used to analyze surfaces. An electron beam scans the sample line by line. The generation of the electron beam is analogous to that of TEM. Since no large penetration depths are needed, the accelerating voltage is lower (from 1 to approx. 30 kV). The secondary and backscattered electrons which fly in every direction are recorded by a detector. A voltage is applied between the sample and the detector, whereby the electrons are drawn “into” the detector, in order to capture as many electrons as possible. Secondary electrons can be distinguished from backscattered electrons by the voltage level. The secondary electrons are recorded with a scintillator-photomultiplier, while a semiconductor detector is used for the backscattered ones. The signals emitted from the detectors are used for displaying an image. Unlike TEM, where electromagnetic lenses are used, the magnification is obtained by scaling down the area scanned. The resolution depends on the diameter of the primary beam (attainable diameters are approximately 5-10 nm).


The image’s contrast is a result of:

  • Relief contrast (surface topology)
  • Shadow contrast (shadow effects)
  • Surface slope contrast (the signal yield is a function of the surface’s slope angle to the primary beam)
  • Edge contrast (sharp edges or peaks have a higher intensity due to the increased discharge of secondary electrons. This is due to the larger surface with regard to the irradiated volume)
  • Material contrast (depends on the atomic number of the elements in the sample)

Foraminifera


Imaging methods

Secondary electrons (SE)

Secondary electrons are a frequently used source of information. The electrons of the beam (primary electrons) interact with the atoms of the object to be investigated and generate secondary electrons (SE). They possess energy of several eV. The secondary electrons stem from the sample’s top nanometers. Thus, they map the topography of the object.

Backscattered electrons

The detection of backscattered electrons (BSE) is another imaging method. This involves primary electrons, with the energy of several keV, reflected from the object. The mean atomic number of the material is responsible for the signal intensity. The heavier an element is, the more powerful the backscattering, which means that heavy elements are depicted brighter than lighter ones. The information contained within the BSE image (material contrast image) permit conclusions to be drawn about the chemical elements of the sample. The material contrast is, however, also influenced by the sample’s topography.

Sample current

The absorbed primary-beam electrons can also be used for the mapping of the surface.

Cathode luminescence

Some materials emit light when irradiated with electrons. Information about internal- and defective structures, as well as trace elements, can be determined with the aid of the light’s spectral analysis.

Auger electrons

Auger electrons can also be emitted through the interaction of the primary beam with the sample.

ESEM

A special type of scanning electron microscope is the ESEM (Environmental Scanning Electron Microscope). With this SEM type, only the electron beam is generated in a high vacuum. In contrast, the sample chamber is placed only under a slight vacuum. Thereby, the residual gas in the chamber acts as an amplifier, providing a charge compensation, so that no coating of the sample is necessary. Consequently, non-vacuum stable, degassing- or samples in a wet environment can also be analyzed.


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