Question

) (a) “Briefly” explain the difference between secondary, backscattered and auger (French!) electrons. (b) What is the primary difference between a secondary electron detector and a backscatter electron detector?

Answer 1

a) Secondary electrons are produced when an incident electron excites an electron in the sample and loses most of its energy in the process. The excited electron moves towards the surface of the sample experiencing elastic and inelastic collisions until it reaches the surface, where it can escape if it still has sufficient energy.Backscattered electrons constitute high-energy electrons originating in the electron beam that are reflected or backscattered out of the specimen interaction volume. It is caused by an incident electron colliding with an atom in the specimen which is nearly normal to the incident's path. The incident electron is then scattered backwar 180 degrees. Auger electrons are caused by the de-energization of the specimen atom after a secondary electron is produced. Since a lower (usually K-shell) electron was emitted from the atom during the secondary electron process an inner (lower energy) shell now has a vacancy. A higher energy electron from the same atom can fall to a lower energy, filling the vacancy. This creates and energy surplus in the atom which can be corrected by emitting an outer (lower energy) electron; an Auger Electron.

b)

BSE images can also provide beneficial information on topography, crystallography and the magnetic field of the sample. 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. By enabling only particular quadrants of the detector, topographical information from the image can be recovered.

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.SE detector 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.