Question

Explain the differences between primary electrons, secondary electrons, Auger electrons, and backscattered electrons and how they are used in SEM imaging and analysis.

Answer 1

SEM (Scanning Electron Microscope) is a device used to study the structure at a molecular level.

In this technique, an electron beam is used which consists of high energy electrons. These electrons are called primary electrons. Backscattered electrons(BSE) are reflected back after elastic interactions between the beam and the sample. Secondary electrons(SE), however, originate from the atoms of the sample: they are a result of inelastic interactions between the electron beam and the sample. Auger electrons are originated from the core of the sample, which also releases energy.

BSE comes from deeper regions of the sample, while SE originates 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.

BSE IMAGING -

This type of electrons originate 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. The number of the 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.

SE IMAGING -

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.

Auger electron Spectroscopy -

When a beam of electrons is fired at the surface of the material, these beams simulate several interactions. One of those interactions is the Auger effect. The principle of Auger operates by allowing a high-energy electron from the beam to eject an electron from its orbit creating an empty hole in the orbit. As this occurs, another electron from a higher orbit moves to fill the empty space. As the electron changes from a higher to a lower orbit, it releases energy. This energy might eject the third electron from another orbit. By measuring the energy of the emitted electron, called the Auger electron, the atom can be identified. Different atoms have different atomic orbits and therefore different Auger energies. One of the techniques for measuring the energy of the Auger electrons is the same as the LEED setup. It consists of the three grids and instead of the screen present in LEED, an electron collector is located in the Auger. The electron gun in the Auger technique also operates at higher energy than in the LEED.