1. RMS Summer School 2010, Leeds SEM ELECTRON BACK-SCATTERED DIFFRACTION
Geoffrey E. Lloyd
School of Earth & Environment, Leeds University
Acknowledgements:
Niels-Henrik Schmidt, Austin Day, Pat Trimby (formerly HKL Software)
Obducat CamScan (Dick Paden)
Dave Prior (University of Liverpool)

2. Plan Basic principals EBSD pattern recognition EBSD problems
Orientation contrast imaging
Automated EBSD analysis
Specimen requirements
EBSD applications
A few examples
Concluding remarks

3. Basic Principals

4. Electron:Sample Interaction
Single
nucleus
Multiple scattering
incident
electrons
‘lost’
emitted
‘lost’ electrons
emitted
electrons
‘focussed’
‘unfocussed’

5. Effects of Scattering
1. creates a ‘point’ source of electrons, with all possible trajectories, within the material
2. crystalline materials: electrons are diffracted by the crystal lattice planes when the Bragg condition is satisfied
note different
lattice spacing

6. Scattering from single lattice planes
Each lattice plane (hkl) gives rise to 2 diffraction ‘cones’
For SEM electron wave-lengths, opening cone angles are close to 180
(after A. Day)

7. Scattering from 3D lattice planes
A 20keV electron beam strikes a sample tilted at °
(after J. Hjeiling)
The crystal structure at the point of incidence diffracts the electron beam according to Bragg’s law, nl = 2dsinq
The image (EBSD pattern) is unique for the crystal orientation & composition at the point of incidence on the sample
Each lattice plane (hkl) gives rise to 2 diffraction ‘cones’
(after A. Day)

8. Typical SEM EBSD set-up
incident electron beam:
8-40kV, nA
EBSD detector - position
usually constrained by chamber geometry
emitted electrons
Specimen:
Surface normal
typically
inclined 60°-80°
to beam
EBSD detector
Camera
low-light sensitive (now digital; originally analogue)
Pattern
processing
EBSD detector distance
set to give ~90 °
angular range in EBSP

9. EBSD Pattern Recognition

10. EBSD pattern recognition
EBSD patterns are unique for a specific crystal orientation
The pattern is controlled by the crystal structure: space group symmetry, lattice parameters, precise composition
Within each pattern, specific ‘bands’ (i.e. pairs of ‘cones of diffraction’) represent the spacing of specific lattice planes (i.e. dhkl)
EBSD pattern recognition compares the pattern of bands with an ‘atlas’ of all possible patterns in order to index the crystal orientation depicted
This process WAS manual – it is NOW automated !
Example - next slide

11. diffraction from specific lattice plane
EBSD Patterns
pyrite
FeS2
diffraction from specific lattice plane
width = 1/d-spacing
Unique for crystal orientation & composition at the point of beam incidence
Can be >100° of total crystal projection - easy to index as symmetry decreases
Spatial resolution (1m)
Some pattern details:
major crystal ‘pole’
HOLZ ring
2nd order diffraction
1st order diffraction

12. Example: pattern indexing
Original pattern
Computer indexed pattern
manual/auto-indexed bands

13. EBSD pattern ‘range’
Determined by crystal symmetry – defines the crystallographic unit triangle that repeats the range of patterns over a sphere
Example: fcc (Cu)
Example: trigonal (quartz)
unit triangle
0001
11-20
-12-10
-2110
unit
triangle
001
101
111
011
symmetrically
equivalent
unit triangle
rotate

14. Indexing requirements
SEM geometry:
beam energy, specimen & detector positions & orientations
usually fixed per SEM
Crystallography:
sample lattice parameters & Laue/space group
input per phase (i.e. composition) as required
Diffraction characteristics:
relative diffraction intensities from different (hkl) lattice planes
calculated per phase as required

15. Creating a crystal database
1. Select a crystal
e.g. aluminium
2. Input lattice parameters
e.g. a = b = c = 0.405nm
a = b = c = 90
3. Input crystal symmetry
e.g. cubic
Laue group = m3m
Space group = 225
or Fm-3m
4. Input crystal unit cell
symmetry indicates 4 atomic positions
e.g. Atom x y z Occ Al Al Al Al

16. Create a diffraction database
To index EBSD patterns, we must know the relative intensities of the (Kikuchi) bands (reflectors) in the patterns
Most approaches use the kinematic electron diffraction model
This model calculates the structure factor (intensity) for each (hkl) reflecting plane:
intensity of
(hkl) plane
number
of atoms
lattice planes
structure factor
for (hkl) plane
atomic
scattering
factor
atomic position
of atom g

17. Diffraction database
Conventional software packages automatically calculate the diffraction (reflector) database of (relative) intensities
e.g. aluminium:

18. EBSD Problems Pseudo-symmetry Spatial resolution Angular resolution
Specimen preparation - see later

19. Pseudo-symmetry
Occurs where 2 orientations cannot easily be distinguished due to an apparent n-fold rotation axis
Especially common in lower symmetry crystal structures:
e.g. the orthorhombic structure with a  b can appear to be tetragonal when viewed down the c-axis
Specific examples (minerals):
quartz trigonal but can appear pseudo-hexagonal
olivine orthorhombic but can appear pseudo-hexagonal
plagioclase triclinic but can appear pseudo-monoclinic &/or pseudo- hexagonal

20. Example Quartz: Often results in similar EBSD patterns distinguishes
60° rotation about the c-axis
negative
rhomb
positive
distinguishes
Often results in similar EBSD patterns
Recognition depends on the identification of ‘minor’ bands (black) - often not selected automatically

21. (e.g. http://www.gel.usherbrooke.ca/casino/index.html)
Spatial resolution
Example: R P
Depends on the penetration & deviation of electrons into a sample (plus beam diameter)
Typically ranges from ‘few’ mm for W-filament SEM to a few 100nm for FEG SEM
Penetration depends on:
sample atomic number
accelerating voltage
beam current
(plus, coating depth & surface damage - see later)
note ‘down slope’ effect of tilting – scan ‘uphill’
Several Monte Carlo based simulation packages are available via the Web
(e.g.

22. Angular Resolution
Angular resolution of an individual EBSD pattern is typically ~1
Also important when determining the misorientation between two (adjacent) crystal lattices (e.g. grains) – ‘misorientation analysis’ is becoming a popular application of EBSD as it provides information on sample properties & behaviour
But, calculations of misorientation axes from 2 individual measurements with misorientation of <15° contain increasingly large errors
Angular resolution depends typically on basic EBSD set-up configuration, EBSD pattern quality & hence indexing software (parameters, composition, pseudo-symmetry, etc.)
Examples (next slides)

23. Angular resolution 1: sample-detector considerations
Small detector distance
Large detector distance
Detector
poor for indexing but good angular resolution
good for indexing but poor angular resolution
important for constraining misorientation axes.

24. Angular resolution 2: effect of angle imaged
large angular spread:
low angular resolution
low angular spread:
good angular resolution
Changes in high resolution EBSD patterns can be used to define better rotation angles & more accurate misorientations

25. Orientation Contrast Imaging

26. Control of crystal orientation on emission signal
polycrystalline sample
note variation in image grey-scale level - depends on penetration & emission, which depend on crystal orientation

27. EBSD microstructural images
Electron beam is scanned over an area of a tilted sample, rather than positioning the beam on a point for EBSD patterns
Forescattered electrons (FSE) with intensities determined by penetration (i.e. crystal orientation) are emitted towards the EBSD detector
quartzite
FSE signal detected by silicon devices attached to EBSD detector
FSE Orientation Contrast image of variation in crystal orientation - contrast variations only qualitative (next slide)

28. Warnings
2 grains with the same orientation can have different OC signals
2 grains with different orientations can have the same OC signal
beam
beam
due to different
‘incident angles’
due to effectively
the same
‘incident angles’

29. OC images not quantitative !
(after D. J. Prior)
Grey-levels cannot be inverted to give orientation
Grains in the same orientation but different positions have different grey shades
Grains in different orientations could have the same grey shade
One image may show only 60% of boundaries – so, move the image slightly

30. Automated EBSD Analysis

31. Automated EBSD analysis
Computer controlled movement of the electron beam across a sample
EBSD pattern ‘captured’ at each point
Indexing of EBSD patterns is via pattern recognition software
Software writes the crystal orientation (3 Euler angles), & phase information per pattern to a data-base for later analysis
BUT – important to run a manual visual check of solutions before the automated analysis!

32. Automated EBSD analyses
provides a variety of information
crystal orientation variation
pattern quality - ‘strain’
orientation contrast P T
crystal orientation ‘pole’ figures
many other parameters:
e.g. ‘misorientation’
(becoming very important in microstructural analysis)

33. Specimen Requirements

34. Specimen Preparation
Polished blocks, thin-sections, natural fractured or grown surfaces
Surface damage (mm-mm) created by mechanical polishing must be removed:
chemical-mechanical (‘syton’) polish
etching
electro-polishing
ion beam milling
Insulating samples may require very thin carbon coat, but uncoated samples may perform OK - next

35. Charging Problems
Reduce charging by coating but only at expense of image detail &/or resolution
K-feldspar. 20keV ~15nA (after D.J. Prior)
Uncoated
3-5nm
C coat
Note: specimen damage can occur in absence of charging

36. Effect of coating on OC images
200mm
uncoated
~4nm C
~8nm C
(after D.J. Prior)

37. Interim Summary Orientation contrast images: EBSD patterns:
variations in crystallographic orientation & sample microstructure
EBSD patterns:
full crystallographic orientation of any point in OC image
Spatial resolution:
~100nm (FEG, metals) to ~1mm (W, rocks)
Angular resolution:
~1 - 2° (misorientation >5 °)
Materials:
most metals & ceramics; many minerals - depends on composition
Automated analysis:
100’s of EBSD patterns/second (record ~800/sec via stage scanning) but indexing accuracy may suffer (use of fast or sensitive EBSD detectors increasing depending on requirements)

38. EBSD Applications

39. What can EBSD be used for?
Measuring absolute (mis)orientation of known materials - most popular/obvious usage
Phase identification of known polymorphs - becoming popular
Calculating lattice parameters of unknown materials - difficult, only possible for relatively simple structures?
Measuring elastic strain
Estimating plastic strain on the scale of the electron beam activation volume
Estimate aggregate elastic stiffness matrix (Cij) from grain crystal texture – used to predict material properties (e.g. thermal, electric, acoustic, magnetic, etc.)

40. Recommended applications
Tremendous significance for many types of materials research, including:
- deformation & recrystallisation
- understanding processing histories
- effects of pre-heating & heat treatments
- identifying phases in multi-component systems
- microstructural characterisation & calibration (including boundary geometry, etc.)
- modelling microstructural processes
- constraining micro-chemical data
- estimating physical properties (e.g. elastic, thermal, sonic, electric, magnetic)
- etc.

41. Example applications

42. Crystal orientation data from SEM/EBSD
Individual orientation measurements related to microstructure:
crystal lattice preferred orientations/texture analysis (i.e. inverse/pole figures, orientation distribution functions
misorientation data (similar types of plots)
Non destructive
data be collected from representative samples
Automated
statistically large/viable data sets acquired
BUT! Samples must be oriented:
Materials - RD, ND, TD
Rocks – X, Y, Z or NSEW

43. CPO/texture analysis SEM/EBSD: BUT! only surficial analysis:
faster than optical & gives full crystallographic information
faster than X-ray goniometry & applicable to all crystals
faster & simpler than neutron diffraction & applicable to all crystals
does not suffer from problems associated with ODF calculations
BUT! only surficial analysis:
restricted to electron beam penetration depths
not 3D (unless incorporate serial &/or three orthogonal sections)

44. Example: quartz colour-coded automated pseudo-image
pole figures
colour-coded automated pseudo-image
inverse pole figures
colour-coded
misorientation
angle profile
subgrains
Dauphine
twins

45. Automated phase identification
Crystal
orientation
(Euler) image
100mm
Phase 1:
quartz
100mm
Phase 2:
feldspar
100mm
Phase 3:
mica

46. In Situ Experiments “Crystal-probe” HT (1200C) FEGSEM
(D.J. Prior, University of Liverpool)
Note: column tilted to 70° - allows horizontal sample movement & greater access to sample chamber (i.e. various other electron-specimen signal detectors)

47. projected 2010
(~3500)
April
2010

48. Selected bibliography
As of April 2010, searching for EBSD in Web of Knowledge results in ~3000 hits! So, here are some ‘early’ papers.
Lloyd, G.E Atomic number and crystallographic contrast images with the SEM: a review of backscattered electron techniques. Mineralogical Magazine 51, 3-19.
Schmidt, N.-H. & Olesen, N.O Computer-aided determination of crystal lattice orientation from electron channelling patterns in the SEM. Canadian Mineralogist 27,
Randle, V Microtexture Determination and its Application. The Institute of Materials, London 174pp.
Randle, V The Measurement of Grain Boundary Geometry. Institute of Physics Publishing, Bristol, 169pp.
Field, D.P Recent advances in the application of orientation imaging. Ultramicroscopy 67, 1-9.
Wilkinson, A.J. & Hirsch, P.B Electron diffraction based techniques in scanning electron microscopy of bulk materials. Micron 28,
Humphreys, F.J Quantitative metallography by electron backscattered diffraction. Journal of Microscopy 195,
Prior, D.J Problems in determining the orientation of crystal misorientation axes for small angular misorientations, using electron backscatter diffraction in the SEM. Journal of Microscopy 195,
Prior, D.J. et al The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks. American Mineralogist 84,
Trimby, P.W. and Prior, D.J Microstructural imaging techniques: a comparison between light and scanning electron microscopy. Tectonophysics 303,
Wilkinson, A.J Measurement of small misorientations using electron back scatter diffraction. Electron Microscopy and Analysis, Institute of Physics Conference Series, 161,

49. Crystallographic data sources
Pearson’s Handbook, Desk Edition, Crystallographic data for intermetallic Phases, ASM international, ISBN
American Mineralogist -
International Tables for Crystallography Volume A: Space Group Symmetry. Edited by T. Hahn, revised edition, ISBN ; see also:
Altwyk site:

50. Laboratory demonstration
Aims to show:
basic SEM/EBSD set-up
orientation contrast imaging
EBSD pattern capture & indexing (including crystal & diffraction database construction)
automation
questions & answers (?) plus general advice