Electron backscatter diffraction in materials

Assignment No.1
Electron Backscatter Diffraction (EBSD)
Subject: Material Science
Assigned date: Dec. 20, 2015
Submission date: Jan. 10, 2016
Submitted to: Dr. Salamat Ali
Submitted by: Imtiaz Ali

Abstract
Electron Back-Scatter Diffraction (EBSD) is a powerful technique that captures electron
diffraction patterns from crystals, constituents of material. Captured patterns can then be used to
determine grain morphology, crystallographic orientation and chemistry of present phases, which
provide complete characterization of microstructure and strong correlation to both properties and
performance of materials. Key milestones related to technological developments of EBSD
technique have been outlined along with possible applications using modern EBSD system.
Principles of crystal diffraction with description of crystallographic orientation, orientation
determination and phase identification have been described. Image quality, resolution and speed,
and system calibration have also been discussed. Sample preparation methods were reviewed and
EBSD application in conjunction with other characterization techniques on a variety of materials
has been presented for several case studies. In summary, an outlook for EBSD technique was
provided.
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.
A brief history of EBSD
In 1954 Alam, Pashley and Nicholson6
recorded onto EM film patterns which we now call EBSD
patterns and observed that the pattern contrast was greatest when the sample was inclined so that
the beam was incident within ∼20 – 30° of the surface plane. They also noted that the part of the
pattern at low take-off angles gave the highest contrast which was at best 15 %. Today's EBSD
experiments adopt geometries that are in accord with these earliest observations with 20°
between the sample surface and incident beam routinely used.
In the 1970s Venables and co-workers implemented the technique on a VG STEM operated in
conditions more usual for an SEM and recording patterns on film either directly exposed to
electrons within the chamber or using an externally mounted film camera coupled to a scintillator
screen within the microscope chamber. Significantly they showed how the patterns could be used
to measure the orientations of micro-crystals selected with the scanned beam in the SEM and
gave a detailed error analysis estimating the orientation accuracy to be 0.5° to 1° very similar to
today's measurements.
Dingley who had been working on micro-Kossel diffraction picked up the EBSD technique and
pushed it forward by using a low light level ISIT camera to view the scintillator and overlaying

the video output from this with graphics from a BBC micro-computer allowing a significant
increase in the rate of pattern indexing and orientation measurements. This is one of the very
earliest attempts at integrating capture of video information at a microscope to enable
quantitative on-line analysis. Pattern indexing methods were improved so that minimal user input
of only 3 zone axes positions were required to index and calculate orientations for patterns from
cubic crystals, non-cubic crystals and full automation of the analysis set as a clear goal very early
on. The main materials science challenge EBSD was aimed at in these early days was in grain
boundary characterization, a theme that continues to this day, and in particular identifying any
significant special properties associated with low CSL boundaries, e.g. other topics that were
explored were possibilities for phase identification including combining EBSD with local
chemistry from EDX. A further notable study by Baba-Kishi and Dingley attempted to
distinguish between different space and point groups from symmetry elements and systematic
reflections in EBSD, again a topic that has been returned to more recently with much greater
image analysis and computing power available now.
The next big advance in the method came with automation of the band detection step which
finally allowed the whole process of pattern collection and indexing to be undertaken without
user intervention. By adding in computer control of the SEM beam position it was then a
relatively simple task to implement EBSD orientation mapping. The Yale group of Adams,
Wright, and Dingley (on sabbatical from Bristol) first implemented a Burns algorithm for band
detection but moved to the modified Hough transform which the Risø group (Krieger Lassen,
Juul Jensen and Conradsen) had shown to be successful. Both groups had successfully
implemented automated EBSD mapping by the early 1990s.
Goehner and Micheal showed that phase discrimination is possible by comparing measured
interplanar angles to those expected from the known crystallography for each of a wide range of
possible phases held in a database. The problem to be overcome here is in reducing the number
of possible phases to a manageable level either through knowledge of local chemistry or by the
primitive lattice volume.

FIGURE 2.1. Seishi Kikuchi (standing). Thanks to Professor Shun Karato, Yale University,
Geology Department. Originally published in Scientific American. Photo credit Nishina
Memorial Foundation, courtesy of Hiroshi Ezawa.
FIGURE 2.2. Kikuchi P-pattern from calcite cleavage.

FIGURE 2.3. Kikuchi P-pattern from mica.
FIGURE 2.4. Boersch 1937 Iron Kikuchi patterns.

How does EBSD work?
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.

These data can be used to make statistical studies of the micro-fabric of the sample, to reveal
systematic textural relations between individual grains or phases, and even to determine relative
abundances of phases in a poly-phase sample.
Application areas
EBSD has now been used for many aspects of materials characterization including
characterization of grain boundary types, establishing epitaxial relationships between layers on
substrates in metal, semiconductor and superconductor systems, characterizing texture and its
changes during deformation and annealing in metal alloys and geological samples, establishing
links between grain size and texture components during deformation and annealing, contributing
to determination of grain boundary energy through thermal grooving, and ex situ and in
situ experiments on grain boundary mobility.
Over the last two decades an ample of research has been conducted using EBSD technique on
various materials, initially carried at universities and research centers and lately in industry as
well. Today, besides continuous increase in research activity by employing EBSD worldwide,
the technique is also finding its way to complement other techniques and contributes to more
detailed microstructure characterization and better understanding of material properties. Several
such examples are presented below.
 EBSD and nano-indentation
 EBSD and energy dispersive spectroscopy
 EBSD and atomic force microscopy
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
 Specialized Sample Preparation Requirements: EBSP's are generated at very
shallow depths within the sample, so appropriate samples must be free of damage
to the crystal lattices at the surface of the sample. Mechanical grinding and
polishing in routine preparation of polished samples (such as microprobe samples)
results in significant damage to crystal lattices near the surface of most materials.
Therefore, it is necessary to perform additional chemical polishing on samples
after they have been polished using abrasives. This is a labor-intensive and time-
consuming process that requires experience with the materials of interest for
optimal results.
 Problems with Application of Conductive Coatings: Since EBSP's are generated
very near the surface of the sample, the application of a conductive coating to
electrically insulating materials is highly undesirable. SEMs capable of operation
in a low-vacuum mode can be used to examine electrically insulating materials
without application of a conductive coating and this capability is highly desirable
in EBSD work on many materials, including oxides and silicates of interest in most
geological applications.
 Data are best acquired with a stationary beam, or with the beam scanning over a
small area of the sample (high magnification). At lower magnifications, the angle
of incidence of the beam on the periphery of the area of interest creates artifacts in
the EBSP and the data become difficult to index accurately. The best solution is to
use a stationary beam (scanning mode off) and acquire data over the area of
interest by moving the sample. The procedure requires an automated specimen
stage, and data acquisition is considerably slower than acquisition using beam
scanning.
Conclusions
The use of the EBSD detector as an imaging device allows for a great amount of flexibility in
image formation. Through extreme binning of the EBSD CCD camera it is possible to collect
images at a rate comparable to slow scan imaging in the SEM. VFSD data can also be collected
concurrently with the usual orientation data during an EBSD scan. If patterns are recorded,
VFSD images can be formed with even more flexibility during post-processing.
By forming composite images of the simultaneously collected images it is possible to suppress or
isolate the contrast of interest. Determining the best recipe for highlighting a particular micro-
structural feature is not always obvious and changes with different samples, SEM/EBSD
geometry and SEM/EBSD operating conditions. Nonetheless, having the flexibility of utilizing

VFSD images provides a myriad of combinations for characterizing different aspects of
microstructure.
It should be emphasized that the EBSD patterns need not be of sufficient quality for indexing.
Images can be formed from surfaces too rough to produce index-able patterns. Thus, like SEI
and BEI, the VFSD images provide only visualizations of the microstructure—they do not
contain the quantitative data intrinsic to conventional OIM maps. However, VFSD images can be
combined with conventional EBSD and XEDS mapping and correlative imaging between all of
these techniques can be realized.
It is beyond the scope of this work to explore in detail all of the aspects of this imaging
technique. We have begun investigating the potential of using VFSD in phase differentiation,
automated directed scanning, and measuring crystallographic texture; however, these new areas
are just being explored and we expect other application areas, more detailed analysis of the
physical phenomena and more quantitative data processing approaches to emerge.
References
1. S. Kikuchi, ―Diffraction of cathode rays by mica‖,
Jpn. J. Phys., 5 (1928) 83–96.
2. S.I. Wright, Ph.D. Thesis, Yale University, USA
1992.
3. S. Nishikawa, S. Kikuchi, ―The diffraction of cathode
rays by calcite‖, Proc. Imperial Acad. Jpn., 4 (1928)
475–477.
4. R. von Meibom, E. Rupp, ―Wide angle electron diffraction‖,
Z. Physik, 82 (1933) 690–696.
5. H. Boersch, ―About bands in electron diffraction‖,
Physikalische Zeitschrift, 38 (1937) 1000–1004.
6. K. Artmann, ―On the theory of Kikuchi bands‖, Z.
Physik, 125 (1948) 225–249.
7. M.N. Alam, M. Blackman, D.W. Pashley, ―High-angle
Kikuchi patterns‖, Proc. Royal Society of London,
A221 (1954) 224–242.
8. J.A. Venables, C.J. Harland, ―Electron back-scattering
patterns – A new technique for obtaining crystallographic
information in the scanning electron microscope‖,
Philoso. Mag., 2 (1973) 1193–1200.

Electron backscatter diffraction in materials

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