Electron Channelling Contrast Imaging Technique (ECCI) and crystalline defects

The Electron Channelling Contrast Imaging (ECCI) technique, implemented in a scanning electron microscope (SEM), reveals crystalline defects such as dislocations or stacking faults close to the surface of a bulk sample. This technique is a direct consequence of the fortuitous observation of intensity modulation, in the form of bands delimited by dark lines, on a micrograph collected on a GaAs single crystal and recorded on a backscattered electron detector by Coates in 1967.

This is actually a channeling pattern, also known as the "Kikuchi pseudodiagram" or "ECP" for Electron Channelling Pattern. The extreme sensitivity of this diagram to the orientation of the crystal with respect to the incident electron beam is both a great advantage and a drawback. The advantage is that this ECP can provide precise information on the crystal's orientation in the microscope frame of reference, or even reveal very slight disorientations between two crystals.

In the vicinity of Bragg incidence for a given family of hkl planes, the intensity backscattered by a perfect crystal varies abruptly and passes through a minimum corresponding to the channelling of electrons when the angle of incidence increases slightly. Thus, a crystal imaged under this incidence condition will have a minimal yield of backscattered electrons. The presence of a defect such as a dislocation that produces lattice distortion in its vicinity will induce a sharp variation in intensity, contrasting the defect. ECCI is based on this phenomenon. To be able to carry out ECCI experiments, a scanning microscope must have a very precise means of controlling the orientation of the crystal with respect to the primary beam. ECP, whose spatial resolution is millimetric, can only be performed on a single crystal. To carry out this type of experiment on polycrystals, it is necessary to be able to carry out ECP on a reduced area by precession of the electron beam (so-called "SACP" diagrams for Selected Area Channelling Pattern). This is the major drawback of this technique.

Since Coates' observation (1967), solutions have been proposed to overcome this difficulty. Electronic columns are constantly evolving, and detectors are becoming more sensitive. The spatial resolution of SACPs has improved, opening the way to more precise ECCI characterizations in fine-grained polycrystals, as will be detailed in this article. Among the advantages of the technique is the ability to carry out successive defect characterizations in different areas of the bulk sample for better statistics compared with transmission electron microscopy. It is also possible to determine the nature of grain sub-joints of low disorientation or to reveal nanomacles. The latest developments concern in situ testing with fine characterization...