Integrating Research and Education > Geochemical Instrumentation and Analysis > Electron Backscatter Diffraction (EBSD)
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Electron Backscatter Diffraction (EBSD)

Susan Swapp, University of Wyoming

What is EBSD?

Accelerated electrons in the primary beam of a scanning electron microscope (SEM) can be diffracted by atomic layers in crystalline materials. These diffracted electrons can be detected when they impinge on a phosphor screen and generate visible lines, called Kikuchi bands, or "EBSP's" (electron backscatter patterns). These patterns are effectively projections of the geometry of the lattice planes in the crystal, and they give direct information about the crystalline structure and crystallographic orientation of the grain from which they originate. When used in conjunction with a data base that includes crystallographic structure information for phases of interest and with software for processing the EPSP's and indexing the lines, the data can be used to identify phases based on crystal structure and also to perform fabric analyses on polycrystalline aggregates.

How does EBSD work?

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The polished sample is placed in the SEM and inclined approximately 70o relative to normal incidence of the electron beam. The detector is actually a camera equipped with a phosphor screen integrated with a digital frame grabber. The camera resides on a horizontally mounted motorized carriage. It is inserted to within several mm of the surface of the inclined sample. The optimal arrangement results when the camera is as close to the sample as possible while avoiding the possibility of collision between the sample surface and the delicate phosphor screen. The pattern of Kikuchi lines on the phosphor screen is electronically digitized and processed to recognize the individual Kikuchi lines. These data are used to identify the phase, to index the pattern, and to determine the orientation of the crystal from which the pattern was generated. [insert ebsp_index.jpg here - improve this later with a picture of a crystal showing orientation below the indexed pattern] Individual mineral grains can be selected for identification and determination of crystal orientation, or data may be acquired on a grid over a selected area of the surface of the sample to determine the identity, orientations, and spatial relations between a large number of grains.
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These data can be used to make statistical studies of the microfabric of the sample, to reveal systematic textural relations between individual grains or phases, and even to determine relative abundances of phases in a polyphase sample.


Strengths

EBSD is unquestionably the fastest and most reliable way in which to acquire data for crystalline structure and orientation in a solid crystalline phase. Unlike optical techniques, it is possible to acquire data for phases of all symmetries (even isotropic phases) and for opaque phases. The data give true 3-dimensional orientations for individual crystals, which is superior to optical pole figures which give 2-dimensional orientations. The spatial resolution can be on the order of several microns, which is much superior to resolution attainable using selected area channeling (SAC) techniques. EBSD data acquired using either a scanned electron beam, or (better) an automated stage and a stationary electron beam can include analyses of thousands of individual grains in a run accomplished in hours; acquisition of data for 10's of thousands of individual spots in a single one-day run is routine in most laboratories. TEM can yield excellent diffraction data with exceptionally high spatial resolution for individual crystals, but sample preparation is considerably more involved than it is for EBSD studies, and most TEM mounts can only be examined over an area that is relatively small compared with areas accessible using EBSD.

Limitations


Results

Most systems include sophisticated software to generate graphical displays of the distribution of individual phases, of mis-orientation between individual grains of any particular phase and pole figures for displaying statistical distributions of orientations of individual grains of given phases.
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Sample Preparation, Operating Parameters, and Analysis Methods


Successful EBSD analysis requires careful sample preparation. A standard microprobe quality polish on a thin section is not enough!. This webpage provides the steps required for Sample Preparation to achieve successful EBSD results, along with advice on operating parameters and analysis methods, prepared by Rachel Beane, Bowdoin College. Or check out the Buehler, Inc. Tech-Note, vol 5 #2, on Specimen Preparation for Electron Backscatter Diffraction written by George Vander Voort of Buehler.[Note: login to a free Buehler online account is required].


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