What Is Electron Backscatter Diffraction (EBSD)?
Electron backscatter diffraction is a technique performed in a scanning electron microscope that measures the crystal structure and orientation of the material directly under the beam. At every point of a scan the microscope records a diffraction pattern, software identifies which crystal phase produced it and how that crystal is rotated, and the results are assembled into maps of grains, phases, boundaries and texture.
EBSD sits between imaging and diffraction. A secondary-electron image shows topography, and a backscatter image shows contrast that partly reflects grain orientation, but neither tells you the actual orientation in degrees. X-ray diffraction gives very precise crystallographic data, but averaged over a large volume. EBSD gives crystallography point by point, at a spatial resolution of tens of nanometres. If you are new to the instrument itself, our overview of how a scanning electron microscope works covers the beam, detectors and signals that EBSD builds on.
A short history
The bands that EBSD analyses are called Kikuchi patterns, after Seishi Kikuchi, who observed them with Shoji Nishikawa in 1928 using transmission of electrons through mica. Backscatter versions of these patterns were recorded in the following decades, but EBSD became a routine materials-science tool only once cameras and computers could capture and index patterns automatically, turning a single-point measurement into full orientation mapping.
How the Electron Backscatter Diffraction Pattern Forms
When the beam enters a crystalline sample, many electrons are scattered in all directions with little loss of energy. Some of those electrons then meet lattice planes at the Bragg angle and are diffracted. Because the scattered electrons travel in every direction, diffraction from each family of planes produces a pair of cones, and where these cones cut the flat detector screen they appear as a pair of nearly straight lines bounding a bright Kikuchi band.
- Each band corresponds to one family of lattice planes; its centre line is the trace of that plane projected onto the screen.
- Band width is proportional to the Bragg angle, so it relates to the spacing of those planes.
- Band intersections are zone axes, the crystal directions shared by several plane families.
- Angles between bands are the interplanar angles of the crystal, which is what makes the pattern identifiable.
The whole pattern is effectively a gnomonic projection of the crystal's geometry. Rotate the grain and the pattern shifts across the screen; change the phase and the band arrangement changes. That is why one pattern is enough to determine both phase and orientation.
Why the sample is tilted to 70 degrees
In a standard EBSD setup the sample surface is tilted to about 70° from horizontal, facing the detector. At this steep angle the interaction volume is elongated toward the surface, so far more diffracted electrons escape in the forward direction, and pattern intensity rises sharply. The cost is that the beam footprint is stretched along the tilt axis, so resolution is poorer in that direction than across it. Patterns come from a shallow layer, typically on the order of 10 to 40 nm deep at 20 kV, which is why surface condition matters so much.
The Electron Backscatter Diffraction Detector
A classic EBSD detector is a retractable unit that slides into the chamber close to the tilted sample. Its main components are a phosphor screen that converts electrons into light, a lens and a sensitive CCD or CMOS camera that captures the image, and often a set of forward-scatter diodes below the screen that record orientation-contrast images of the surface.
Camera choice is a trade-off between speed and pattern quality. Binning pixels allows hundreds to thousands of patterns per second for large maps, while full-resolution patterns are used for difficult phases or strain analysis. Newer direct electron detectors, which record electrons without a phosphor, offer cleaner patterns at lower beam energies and are increasingly used in research settings.
| Parameter | Typical starting point | Effect of changing it |
|---|---|---|
| Accelerating voltage | 15 to 20 kV | Higher kV gives brighter patterns but a larger interaction volume |
| Beam current | several nA, up to about 15 nA | More current speeds mapping but can reduce spatial resolution |
| Sample tilt | 70° | Lower tilt weakens patterns; higher tilt worsens resolution along the tilt axis |
| Working distance | set so the pattern centre sits near the screen centre | Affects pattern geometry and calibration |
| Step size | roughly a tenth of the average grain size or smaller | Smaller steps resolve fine features but multiply mapping time |
Electron Backscatter Diffraction Analysis: From Pattern to Orientation
Turning thousands of images into crystal orientations is a fully automated sequence that runs during the scan.
- Background correction. A static or dynamic background is subtracted so the bands stand out from the smooth intensity of the screen.
- Band detection. A Hough transform converts each straight band into a single bright point in Hough space, which software can find reliably even in noisy patterns.
- Indexing. Measured angles between detected bands are compared with a look-up table of interplanar angles for the candidate phases, and a voting scheme picks the best match.
- Solution quality. Each point receives a fit or confidence value, such as the mean angular deviation between measured and calculated bands, plus a pattern-quality value.
- Clean-up and reconstruction. Unreliable points are filtered, neighbouring points with similar orientation are grouped into grains, and boundaries are drawn where misorientation exceeds a chosen threshold.
Conventional Hough-based indexing determines absolute orientation to roughly 2°, while relative misorientation between neighbouring points is resolved much better, around 0.1 to 0.5°. Cross-correlation methods known as high-angular-resolution EBSD compare patterns at sub-pixel level and reach misorientation sensitivity near 0.006° and elastic strain sensitivity around 10⁻⁴. Pattern-matching approaches, including dictionary indexing against simulated patterns, help with noisy data and with phases whose patterns look alike.
Indexing is only as good as the phase list you give it. If a phase is missing from the candidate list, the software will often force those points onto the wrong phase rather than leave them unindexed.
Electron Backscatter Diffraction in Materials Science: What the Maps Show
The real value of EBSD lies in the maps and statistics produced from millions of indexed points. The same dataset can be displayed in many ways, each answering a different question.
- Inverse pole figure maps colour each grain by which crystal direction is parallel to a chosen sample axis, making texture visible at a glance.
- Grain maps and size distributions give grain size by area or equivalent diameter, often more reliably than line-intercept methods on etched samples.
- Phase maps separate phases with different crystal structures, such as ferrite and austenite in duplex steel.
- Boundary maps classify boundaries as low angle (commonly 2 to 15°) or high angle (above 15°), and identify special boundaries such as Σ3 annealing twins in copper, nickel or austenitic steel.
- Local misorientation maps, such as kernel average misorientation, highlight plastic deformation and stored strain energy.
- Pole and orientation distribution figures quantify crystallographic texture for rolled sheet, wire, coatings or additively manufactured parts.
Typical applications include tracking recrystallisation after annealing, checking texture in aluminium can stock or electrical steel, studying weld heat-affected zones, analysing failure origins in fatigue and corrosion studies, and mapping microstructure in geological samples and ceramics.
Combining EBSD with EDS
EBSD and energy dispersive X-ray spectroscopy are often run at the same time from the same tilted surface. EDS adds chemistry, which is especially useful when two phases have similar crystal structures but different compositions. The underlying method is explained in our guide to energy dispersive X-ray spectroscopy in the SEM.
| Question | EBSD | EDS |
|---|---|---|
| What it measures | Crystal structure and orientation | Elemental composition |
| Signal | Diffracted backscattered electrons | Characteristic X-rays |
| Sample geometry | Tilted to about 70° | Usually flat, works at tilt too |
| Surface demands | Very high, deformation-free | Moderate, flat and clean |
| Typical output | Grain, phase, boundary and texture maps | Spectra and element maps |
Sample Preparation for EBSD in Metals and Ceramics
Because patterns come from only the top tens of nanometres, any damaged layer left by cutting or polishing destroys pattern quality. Preparation that is perfectly acceptable for optical metallography is often not enough for EBSD.
- Section gently with a slow precision saw and generous cooling to limit deformation depth.
- Grind progressively through silicon carbide papers, for example from 240 to 4000 grit, removing the damage from each previous step.
- Polish with diamond suspensions from about 9 µm down to 1 µm.
- Finish with colloidal silica, typically around 0.05 µm, which combines mild chemical attack with mechanical polishing and removes the last deformed layer.
- Use alternatives where needed: electropolishing for many metals, or broad ion beam milling for soft, multiphase or sensitive materials.
- Manage charging on ceramics and minerals with a very thin conductive coating or low-vacuum operation, keeping in mind that thick coatings blur patterns.
General cleaning, mounting and coating steps are similar to those in routine SEM sample preparation, but the final polish is where EBSD work succeeds or fails.
Limitations and Related Techniques
EBSD struggles with heavily deformed metals, where patterns become diffuse, with very fine nanocrystalline structures below the spatial resolution, and with phases that are crystallographically almost identical. Maps of large areas can take hours, and sample drift or charging during long scans distorts the data. Results are also two-dimensional sections of a three-dimensional microstructure, unless serial sectioning is used.
For finer grains, transmission Kikuchi diffraction uses a thin foil in the SEM and collects patterns transmitted through it, improving spatial resolution to a few nanometres. For true atomic-scale crystallography, transmission electron microscopy remains the reference method.
Treat EBSD quality as a preparation problem first and a software problem second. A well-polished sample at modest settings will outperform a poorly prepared one at any detector speed.
Conclusion
Electron backscatter diffraction turns an SEM into a crystallographic mapping tool. Kikuchi bands from a sample tilted to 70° are detected, indexed against known phases and converted into maps of grains, phases, boundaries and texture with nanometre-scale spatial resolution. Combined with EDS and backed by careful surface preparation, EBSD analysis has become one of the most informative everyday techniques in materials science.