Electrons in SEM
Electron microscopes are very versatile instruments that can provide different types of information depending on the user’s needs. In this blog, we will describe the different types of electrons that are produced in a SEM, how they are detected, and the type of information they can provide.
As the name implies, electron microscopes employ an electron beam for imaging. In Figure 1, you can see the various products that are possible as a result of the interaction between electrons and matter. All these different types of signals carry different useful information about the sample, and it is the choice of the microscope’s operator which signal to capture.
For example, in transmission electron microscopy (TEM), as the name suggests, signals such as the transmitted electrons are detected, which will give information on the sample’s inner structure. In the case of a scanning electron microscope (SEM), two types of signal are typically detected: the backscattered electrons (BSE) and the secondary electrons (SE).
Backscattered vs secondary electrons
In SEM, two types of electrons are primarily detected:
- Backscattered electrons (BSE)
- Secondary electrons (SE)
Backscattered electrons are reflected back after elastic interactions between the beam and the sample. Secondary electrons, however, originate from the atoms of the sample. They are a result of inelastic interactions between the electron beam and the sample.
BSE come from deeper regions of the sample, while SE originate from surface regions. Therefore, BSE and SE 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.
Electron–matter interactions and the different types of signals that are generated.
Backscattered electron imaging
This type of electron originates from a broad region within the interaction volume. They are a result of elastic collisions of electrons with atoms, which results in a change in the electrons’ trajectory. Think of the electron-atom collision as the so-called “billiard-ball” model, where small particles (electrons) collide with larger particles (atoms). Larger atoms are much stronger scatterers of electrons than light atoms, and therefore produce a higher signal (Figure 2). The number of backscattered electrons reaching the detector is proportional to their Z number. This dependence of the number of BSE on the atomic number helps us differentiate between different phases, providing imaging that carries information on the sample’s composition. Moreover, BSE images can also provide valuable information on crystallography, topography, and the magnetic field of the sample.
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.
The most common BSE detectors are solid state detectors, which typically contain p-n junctions. The working principle is based on the generation of electron-hole pairs by the backscattered electrons that escape the sample and are absorbed by the detector. The amount of these pairs depends on the energy of the backscattered electrons. The p-n junction is connected to two electrodes, one of which attracts the electrons and the other the holes, thereby generating an electrical current, which also depends on the amount of the absorbed backscattered electrons.
The BSE detectors are placed above the sample, concentric with the electron beam in a “doughnut” arrangement, in order to maximize the collection of the backscattered electrons. They consist of symmetrically divided parts. When all parts are enabled, the contrast of the image depicts the atomic number Z of the element. On the other hand, by enabling only specific quadrants of the detector, topographical information from the image can be retrieved.
Typical position of the backscattered and secondary electron detectors.
Secondary electrons
In contrast, secondary electrons originate from the surface or the near-surface regions of the sample. They are a result of inelastic interactions between the primary electron beam and the sample and have lower energy than the backscattered electrons. Secondary electrons are very useful for the inspection of the topography of the sample’s surface, as you can see
a) Full BSD, b) topography BSD, and c) SED image of a leaf.
The Everhart-Thornley detector is the most frequently used device for the detection of SE. It consists of a scintillator inside a Faraday cage, which is positively charged and attracts the SE. The scintillator is then used to accelerate the electrons and convert them into light before reaching a photomultiplier for amplification. The SE detector is placed at the side of the electron chamber, at an angle, in order to increase the efficiency of detecting secondary electrons.
These two types of electrons are the most used signals by SEM users for imaging. Not all SEM users require the same type of information, so the capability of having multiple detectors makes SEM a very versatile tool that can provide valuable solutions for many different applications.Itcan help you save valuable time, improve image resolution, and even automate your analyses.
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