Generating an image
Secondary electron (SE) images
For routine scanning electron microscope images, secondary electrons (SE) form the usual image of the surface. Secondary electrons are low energy electrons formed by inelastic scattering and have energy of less than 50eV. The low energy of these electrons allows them to be collected easily. This is achieved by placing a positively biased grill on the front of the SE detector, which is positioned off to one side of the specimen. The positive grill attracts the negative electrons and they go through it into the detector. This is the case for the Everhart-Thornley detector which is most commonly used but there is another kind of In-lens SE detector in some machines.
The major influence on SE signal-generation is the shape (topography) of the specimen surface. Secondary electrons provide particularly good edge detail. Edges (and often pointy parts) look brighter than the rest of the image because they produce more electrons. The image shows protuberances (bumps) on the wing of an insect. Notice the whiter edge to each bump.
To increase the yield of SE emitted from the specimen, heavy metals such as gold or platinum are routinely used to coat specimens. An extremely thin layer is applied (~10 nm). This coating is applied for two main reasons: (1) Non-conductive specimens are often coated to reduce surface charging that can block the path of SE and cause distortion of signal level and image form; and (2) Low atomic number (Z) specimens (e.g. biological samples) are coated to provide a surface layer that produces a higher SE yield than the specimen material.
Because secondary electrons have very low energies, only those produced at the surface of the sample are able to escape and be collected by the SE detector. Electrons emitted from a surface that faces away from the detector or which is blocked by the topography of the specimen, will appear darker than surfaces that face towards the detector. This topographical contrast due to the position of the SE detector is a major factor in the "life-like" appearances of SE images. In the image of the beetle, the electron detector is in the top left corner, hence that region looks brightest. It is, however, not the only factor that contributes to the contrast and brightness in an SEM.
Backscattered electron (BSE) images
Backscattered (BS) electrons are high-energy electrons (>50 eV) from the primary incident beam that are ejected back out from the sample. These BSE are used to produce a different kind of image. Such an image uses contrast to tell us about the average atomic number of the sample.
For example, a grain of sand that is made up of a titanium mineral looks whiter than a grain made of a silicon material (Ti versus Si). In the image, the left picture is taken using backscattered electrons. Here there is a difference in contrast between the grains labelled Si and Ti whereas in the right image, taken using secondary electrons, there is no difference in contrast between these grains. The sample is a mixture of mineral sand.
The higher the average atomic number, the more primary electrons are scattered (bounced) back out of the sample. This leads to a brighter image for such materials.
The backscattered electron has an energy up to the incident beam energy and is usually very near that energy. The greater energy of BSE, compared with SE, means that BSE produced from deeper within the interaction volume are able to escape from the sample and be collected by the BSE detector, so BSE images have lower spatial resolution than SE images. In other words, the BSE can travel further in the sample before coming out again and so the information they carry is less restricted to the surface detail. This results in reduced resolution.
Another BSE imaging technique used in a scanning electron microscope for example in studying defects in metals is electron channelling contrast imaging (ECCI). This can detect and characterise dislocation structures in bulk specimens. The change in diffraction of the backscattered electrons as they interact with a dislocation in the material results in a higher backscattering coefficient than for the matrix; so individual dislocations appear as bright lines in a darker matrix.
Coating: It is important to leave the sample uncoated (in its natural state) if compositional information is required because the practice of coating samples with metals obscures this. If the sample is non-conductive then it can be coated with carbon (a low atomic number material) which will enhance conductivity without obscuring the compositional detail from below.
BSE detectors: Typically, BS detectors are solid state and comprised of a piece of silicon wafer. The incident beam passes through a hole in the detector before striking the sample. The Silicon diode is divided into sectors (quadrants) that can be summed or subtracted depending on the nature of image required. The normal BSE signal is referred to as COMP (compositional) and provides information about the average atomic number of the sample. A TOPO mode (topographic) is also available which provides surface landscape information and includes no compositional contrast.
The differences between SE, BS COMP and BS TOPO modes can clearly be seen using a Cu grid on a carbon background. In the image below, the top third is a typical SE image, the middle third is BS COMP mode and the lower third is BS TOPO mode.
Topography and BSE: In order to get the best compositional information using BSE, it is preferable to use a flat sample. Otherwise the topography will interfere with the signal reaching the detector. For a smooth (e.g. polished) specimen the most dense material provides the highest (brightest) signal level and the least dense the lowest signal level. In this way, BSE images provide information on compositional heterogeneity through atomic number contrast. BSE greyscale differences indicate the average Z of the phases present and thus allow the recognition and classification of the phases, but they do not indicate either the elements present or the concentration levels.
As a general rule, differences in elemental composition or concentrations that can be observed clearly by BSE imaging can successfully be assessed with microanalysis using energy dispersive analysis. However, different phases in material can appear the same with BSE and these too, can successfully be assessed with microanalysis.
The contribution of BSE to images collected with the SE detector
The primary function of the SE detector is to attract low energy secondary electrons. These SEs are generated from approximately the top 15nm of the surface. Unless the SEM is specially set up to minimise the BSE contribution, the image produced by the detector will, however, always contain an amount of sub-surface information derived from high energy BSE. As a general rule, the higher the kV the more sub-surface information is picked up by the detector due to various backscattered effects (elastic scattering effects) (see Electron-Matter Interactions).
For example at 2kV you will see a lot more surface detail than at 20kV, but this surface detail may be due to contamination. One important skill in operating an SEM is to choose the correct kV for your specimen such that you gather information from the depth of the specimen that interests you, with the least contribution from surface contamination above or unimportant structures below. The following image is from the secondary electron detector and shows a metal surface at the same magnification but with different beam energies (different kVs): 5kV; 10kV; 15kV; 30kV (across 1-4). Note that subsurface and compositional information is apparent in frames 3 and 4 because the SE detector is also gathering backscattered electrons.
So far only the secondary electrons produced by interaction of the primary electron beam with the sample have been discussed. These are termed SE1 electrons. There are a number of different types of secondary electrons.
- Backscattered electrons can generate secondary electrons. These are termed "SE2"
- Interaction of the beam with the sample chamber, pole piece etc. can also produce secondary electrons. These are termed "SE3"