Imaging with a Commercial Electron Backscatter Diffraction (EBSD) Camera in a Scanning Electron Microscope: A Review
Nicolas Brodusch, Hendrix Demers, Raynald Gauvin
Journal of Imaging
Scanning electron microscopy is widespread in field of material science and research, especially because of its high surface sensitivity due to the increased interactions of electrons with the target material's atoms compared to X-ray-oriented methods. Among the available techniques in scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) is used to gather information regarding the crystallinity and the chemistry of crystalline and amorphous regions of a specimen. When
... -processing the diffraction patterns or the image captured by the EBSD detector screen which was obtained in this manner, specific imaging contrasts are generated and can be used to understand some of the mechanisms involved in several imaging modes. In this manuscript, we reviewed the benefits of this procedure regarding topographic, compositional, diffraction, and magnetic domain contrasts. This work shows preliminary and encouraging results regarding the non-conventional use of the EBSD detector. The method is becoming viable with the advent of new EBSD camera technologies, allowing acquisition speed close to imaging rates. This method, named dark-field electron backscatter diffraction imaging, is described in detail, and several application examples are given in reflection as well as in transmission modes. Technically speaking, in a SEM, a beam of focused electrons is scanned over a specimen surface in a raster fashion, and many different signals are collected to generate different images with various contrasts. These contrasts depend on the nature of the particle that is collected, as well as on the nature of the interaction that the particle has undergone during its path through the material. Secondary electrons are produced by the ejection of mostly valence electrons due to atomic ionization, and are characterized by their low energy and small inelastic mean free path, in addition to being highly absorbed before reaching the exit surface. Their emission depth is thus confined to surface layers-generally in the range of a few nanometers-depending on the atoms' band structures and absorption, and they provide topographic contrast that allows us to observe the relief of the surface in a 3D-like fashion. In contrast, the primary electrons that are backscattered towards the surface due to the atoms' Coulomb attraction forces retain sufficient energy to reach the exit surface with limited absorption, and carry information about the composition of the volume of material "seen" by these electrons. These are called Backscattered Electrons (BSE), and are responsible for compositional contrast (also known as material or Z contrast), which is in fact related to the mean atomic number of the material interacting with them. Their emission depth and lateral distribution depend on the material characteristics and the primary beam accelerating voltage. The energy distribution of these BSEs being material and SEM parameters dependent, energy filtration allows us to collect only high energy BSEs, i.e., those with low-loss of energy, which are associated with high spatial resolution and reduced interaction volume. These low-loss electrons suffer a small number of interactions, and originate from the close surrounding of the beam impact point on the surface. The depth resolution is of the order of the mean free path, but depends mostly on the energy-loss considered. Because they interact with the crystal lattice of the specimen through diffraction processes, the BSEs carry information about the crystallinity in their emission volume, but mostly from the exit surface. In the SEM, there are two ways of gathering the diffraction information carried by these electrons. In 1967, Coates evidenced the channeling of BSEs by imaging a Kikuchi-like patterns from Ge and GaAs crystals when the electron beam was scanned over the surface of the specimens at low magnification  . This type of pattern was later termed as an Electron Channeling Pattern (ECP); this led to the well-known electron channeling contrast imaging (ECCI). Later, in 1973, Venables and Hartland obtained similar patterns in spot mode when the specimen was highly tilted towards a phosphorescent screen, typically 60-70 • . At this time, the image was captured from a phosphorescent screen by means of an external camera. Currently, charge-coupled device (CCD) cameras are commonly used to capture these patterns with high speed rates -up to 3000 patterns per second-to produce phase and orientation maps based on the crystallography and crystallographic orientation of the different phases present in the analyzed material. This discovery resulted in one of the most important techniques in materials science, namely Electron Backscattered Diffraction (EBSD)  , that is now widely used in the materials community to characterize microstructures at the sub-micron scale with spatial resolution roughly ranging from 20 to 150 nm, depending upon the material's atomic number and density [7, 8] . In this technique, the bands detected on the EBSD patterns (EBSP) are a projection of the crystal planes on the EBSD camera screen. They are processed and compared to a look-up table to match the most probable phase and orientation at each pixel. However, it took many years before the raw signal collected by the CCD cameras was used to generate images, the information gathered so far being mainly related to the bands detected on the EBSD pattern. In parallel, Prior and co-workers reported compositional and crystallographic orientation imaging when attaching solid state diodes just below and above the CCD screen . They described a dramatic change in contrast when switching from the bottom to the top diodes to record the image. Top diodes provided compositional contrast, while those from the bottom resulted in orientation contrast. These findings were later confirmed and used by Payton et al. to help with phase identification when combined with EBSD indexing  .