Different signals result between interaction between electrons and matter, each of which carry useful information about the sample. In the case of a scanning electron microscope SEM , two types of electrons are typically detected: backscattered electrons BSEs and secondary electrons SEs.
BSEs are reflected back after elastic interactions between the beam and the sample. By contrast, SEs originate from the atoms of the sample; they are a result of inelastic interactions between the electron beam and the sample. BSEs come from deeper regions of the sample, whereas SE originate from surface regions.
Thus, the two carry different types of information. BSE images show high sensitivity to differences in atomic number; the higher the atomic number, the brighter the material appears in the image. SE imaging can provide more detailed surface information. The secondary electron yield depends on many factors, and is generally higher for high atomic number targets, and at higher angles of incidence. BSE can be used to generate an image in the microscope that shows the different elements present in a sample.
All elements have different sized nuclei and as the size of the atom nucleus increases, the number of BSE increases; therefore, the backscattered electron image contains two types of information: one on specimen composition and the other on specimen topography.
For example, minerals with 26 Fe will appear brighter than 12 Mg. The generation region of backscattered electrons is larger than that of the secondary electrons, namely, several tens of nm; therefore backscattered electrons give poorer special resolution than secondary, they are less influenced by charge-up.
In conjunction with the scanning electron microscopy, energy dispersive x-ray spectroscopy EDS or EDX is technique that utilizes X-ray emitted from the sample during the bombardment by the electron beam to characterize the elemental composition of material imaged in a SEM.
When the electron beam interacts with a sample in a scanning electron microscope SEM , multiple events happen. In general, different detectors are needed to distinguish secondary electrons, backscattered electrons, or characteristic x-rays. Depending upon the accelerating voltage and sample density, the signals come from different penetration depths. After Auger electrons, the secondary electrons come from the next most shallow penetration depth. SED images have high resolution that are independent of the material and is acquired from inelastically scattered electrons close to the surface.
No material composition information is available. A backscattered electron detector BSD detects elastically scattered electrons. These electrons are higher in energy from atoms below the sample surface. Using a BSD allows for lower vacuum levels, reducing sample preparation requirements and minimizing beam damage. In scanning electron microscopy SEM , samples are imaged using a focused electron beam that is rastered across a surface. Search Thermo Fisher Scientific. Search All.
Materials Science Scanning Electron Microscopy Electron microscopy reference focusing on the types of electrons that are detected using scanning electron microscopes. Contact us. Electron—matter interactions and the different types of signals that are generated.
Backscattered electron BSE imaging This type of electron originates from a broad region within the interaction volume. Copper atoms higher Z scatter more electrons back towards the detector than the lighter aluminum atoms and therefore appear brighter in the SEM image.
Typical position of the backscattered and secondary electron detectors. Secondary electrons SE In contrast, secondary electrons originate from the surface or the near-surface regions of the sample. Download application note. Style Sheet to change H2 style to p with em-h2-header class. Style Sheet for Support and Service footer.
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