Electron microscopes are extremely versatile instruments, which can offer various types of information according to the user’s requirements. In this article, the different types of electrons that are created in a SEM, how they are detected and the type of information that they can provide will be described.
As the name indicates, electron microscopes make use of an electron beam for imaging. In Figure 1, the variety of products that are possible as a result of the interaction between electrons and matter can be seen. All these varied types of signals convey different beneficial information about the sample and it is the choice of the microscope’s operator which signal to capture.
Figure 1. Electron — matter interaction volume: the different types of signals which are generated.
For instance, in transmission electron microscopy (TEM), as the name indicates, signals such as the transmitted electrons are detected which will provide information on the inner structure of the sample. In the case of a scanning electron microscope (SEM), two types of signal are usually detected; the backscattered electrons (BSE) and the secondary electron (SE).
Backscattered-Electron (BSE) Imaging
The BSE type of electrons originates from a wide region within the interaction volume. They occur due to elastic collisions of electrons with atoms, which causes a change in the electrons’ trajectory. Imagine the electron-atom collision as the so-called “billiard-ball” model, where tiny particles (electrons) collide with larger particles (atoms).
Larger atoms are a lot stronger scatterers of electrons compared to light atoms, and thus create a higher signal (Figure 2). The number of the backscattered electrons reaching the detector is proportional to their Z number. This dependance of the number of BSE on the atomic number helps to distinguish between different phases, providing imaging that conveys information on the sample’s composition. Furthermore, BSE images can also provide beneficial information on topography, crystallography and the magnetic field of the sample.
Figure 2. a) SEM image of an Al/Cu sample, b), c) Simplified illustration of the interaction between electron beam with aluminum and copper. Copper atoms (higher Z) scatter more electrons back towards the detector than the lighter aluminum atoms and therefore appear brighter in the SEM image.
Solid state detectors are the most common BSE detectors which usually contain p-n junctions. The working principle is based on the production of electron-hole pairs by the backscattered electrons which escape the sample and are captured by the detector. The quantity of these pairs relies on the energy of the backscattered electrons. The p-n junction is linked to two electrodes, one of which attracts the electrons and the other the holes, thus producing an electrical current, which also relies on the quantity of the absorbed backscattered electrons.
The BSE detectors are positioned above the sample, concentrically to the electron beam in a “doughnut” arrangement, so as to maximize the collection of the backscattered electrons and they consist of symmetrically divided parts. When all parts are enabled, the contrast of the image shows the atomic number Z of the number. Alternatively, by enabling only particular quadrants of the detector, topographical information from the image can be recovered.
Figure 3. Typical position of the backscattered and secondary electron detectors.
In contrast, secondary electrons originate from the surface or the near-surface regions of the sample. They occur due to inelastic interactions between the primary electron beam and the sample and contain lower energy than the backscattered electrons. Secondary electrons are very beneficial for the inspection of the topography of the sample’s surface, as seen in Figure 4.
Figure 4. a) Full BSD, b) Topography BSD and c) SED image of a leaf.
The Everhart-Thornley detector is the most commonly used device for the detection of SE. It comprises of a scintillator within a Faraday cage, which is positively charged and attracts the SE. The scintillator is then used to speed up the electrons and change them into light before reaching a photomultiplier for amplification. The SE detector is positioned at the side of the electron chamber, at an angle, so as to boost the efficiency of detecting secondary electrons.
These two types of electrons are the frequently used signals by SEM users for imaging. Not all SEM users need the same type of information, so the capability of having many detectors makes SEM a highly versatile tool that can provide advantageous solutions for many different applications.
This information has been sourced, reviewed and adapted from materials provided by Phenom-World BV.
For more information on this source, please visit Phenom-World BV.