Electron backscatter diffraction (EBSD)

About this technique

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Electron backscatter diffraction (EBSD) is a relatively new analytical technique that is increasingly common on scanning electron microscopes (SEMs). Inside the SEM chamber, flat and polished samples are tilted to a high angle (typically 70°) and the electron beam is rastered across a regular grid of points on the sample surface. At each point a diffraction pattern is projected onto an EBSD detector; the pattern is indexed by the EBSD software and the phase and the crystallographic orientation are stored. The whole process is fully automated and very fast (nowadays >100 points per second is commonplace), and can provide information on a range of scales varying from <100 nm to several centimetres.

Electron transparent samples can also be used for high resolution EBSD, more correctly known as Transmission Kikuchi Diffraction in the SEM (SEM-TDK), enabling rapid characterisation of nanostructure and nanotexture. This is now being routinely used to characterise a wide range of very fine-grained nanomaterials at high speed and with spatial resolution well below 10nm. Acquisition times of only 10–25ms, enable orientation mapping of over 100 μm² with measurement spacing in the 2­–20nm range.

EBSD and SEM-TDK can be combined  with energy dispersive X-ray spectroscopy (EDS) to allow identification of unknown phases in a sample. The chemical measurements using EDS are used to find a list of matching candidate phases from a phase database, and the corresponding diffraction pattern is then indexed using these candidate phases in order to find the phase that matches both the chemistry and crystallography. It is a simple and fast approach to phase identification, but the results are only as good as the phase database(s) that are used.

EBSD only works on crystalline materials, and so is generally limited to minerals, metals and ceramics. It is impossible to measure amorphous materials such as glass, plastics, wood and most biological materials, and the relatively high beam current needed for good diffraction patterns makes the analysis of beam sensitive samples (such as polymers) practically impossible. The measurement of the orientations is typically with <1° error, and the spatial resolution of the technique is in the range of 20–100 nm, or 2-10nm for SEM-TKD (dependent on the sample and the SEM). This relatively high resolution means that the preparation of the sample is very important, as standard mechanical polishing techniques general introduce damage into the crystal lattice close to the surface, degrading the quality of the EBSD data. It is usually necessary to have a final stage of polishing using one of the following four approaches:

• Mechanical-chemical polishing using colloidal silica or colloidal alumina.
• Electropolishing.
• Chemical etching.
• Ion polishing (either using broad ion beam or focused ion beam techniques).

For SEM-TDK a thin foil must be prepared as for transmission electron microscopy – imaging. Ideal sample thicknesses range from 50nm to 150nm, depending on the material.

Automated EBSD measurements can generate datasets with many millions of analysis points, so the processing of the data is an important part of the technique. The datasets can be reconstructed into maps of the surface of the sample showing orientations, phase distributions, grain and sub-grain boundaries, localised deformation, grains and so on. Quantitative measurements can be made of the local and global textures (the degree and nature of alignment of the crystal lattices), grain sizes and boundary populations. This wide range of measurements makes EBSD an ideal technique to study the microstructures in many different materials and across many fields of research, including metallurgy, microelectronics and geology.

A final, recent development of EBSD has been 3-D EBSD. This is a technique in which a series of automated EBSD datasets (orientation maps) are collected from serial sections through the volume of a sample; the resulting orientation and phase maps are reconstructed to form a 3-D dataset. A focused ion beam (FIB) SEM is usually used to section through the sample, limiting the technique to relatively small volumes (i.e. <20 x 20 x 20 mm) but providing high resolution in the z-direction (i.e. <50 nm between successive slices). 3-D EBSD can now be fully automated, although the collection of data can take a long time (>20 hours is typical) due to the relatively slow rate of material removal in the FIB-SEM.