Electron Backscatter Diffraction

Electron backscatter diffraction (EBSD), also known as backscatter Kikuchi diffraction (BKD) is a microstructural-crystallographic technique used to examine the crystallographic orientation of many materials, which can be used to elucidate texture or preferred orientation of any crystalline or polycrystalline material. EBSD can be used to index and identify the seven crystal systems, and as such it is applied to crystal orientation mapping, defect studies, phase identification, grain boundary and morphology studies, regional heterogeneity investigations, material discrimination, microstrain mapping, and using complementary techniques, physicochemical identification. Traditionally these types of studies have been carried out using X-ray diffraction (XRD), neutron diffraction and/or electron diffraction in a TEM.

Experimentally EBSD is conducted using a Scanning Electron Microscope (SEM) equipped with an EBSD detector containing at least a phosphor screen, compact lens and low light CCD camera chip. Commercially available EBSD systems typically come with one of two different CCD cameras: for fast measurements the CCD chip has a native resolution of 640×480 pixels; for slower, and more sensitive measurements, the CCD chip resolution can go up to 1600×1200 pixels. However, with higher resolutions the readouts are more time-consuming. The biggest advantage of the high-resolution detectors is their higher sensitivity. For texture and orientation measurements, the images are binned in order to reduce their size and reduce computational times. Thus, the transfer and interpretation of up to almost 1000 images/s is possible if the diffraction signal is sufficient.

For an EBSD measurement a flat/polished crystalline specimen is placed in the SEM chamber at a highly tilted angle (~70° from horizontal) towards the diffraction camera, to increase the contrast in the resultant electron backscatter diffraction pattern. The phosphor screen is located within the specimen chamber of the SEM at an angle off approximately 90° to the pole piece and is coupled to a compact lens which focuses the image from the phosphor screen onto the CCD camera. In this configuration, some of the electrons which enter the sample backscatter and may escape. As these electrons leave the sample, they may exit at the Bragg condition related to the spacing of the periodic atomic lattice planes of the crystalline structure and diffract. These diffracted electrons can escape the material and some will collide and excite the phosphor causing it to fluoresce.

An electron backscatter diffraction pattern (EBSP) is formed when many different planes diffract different electrons to form Kikuchi bands which correspond to each of the lattice diffracting planes. If the system geometry is well described, it is possible to relate the bands present in the EBSP to the underlying crystal phase and orientation of the material within the electron interaction volume. Each band can be indexed individually by the Miller indices of the diffracting plane which formed it. In most materials, only three bands/planes which intercept are required to describe a unique solution to the crystal orientation (based upon their interplanar angles) and most commercial systems use look up tables with international crystal data bases to perform indexing.

While this 'geometric' description related to the kinematic solution (using the Bragg condition) is very powerful and useful for orientation and texture analysis, it only describes the geometry of the crystalline lattice and ignores many physical processes involved within the diffracting material. To adequately describe finer features within the EBSP, one must use a many beam dynamical model (e.g. the variation in band intensities in an experimental pattern does not fit the kinematic solution related to the structure factor).