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•
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Improved accuracy
large acceptance angle => stable indexing
sufficient number of consistent bands => reliability
bands from several zone axes => reliability
less pattern coarsening or pixel binning => precision
dynamic correction of scan field rotation with focus
(scan field rotation does not affect grain orientation measurement but distorts crystal orientation maps only).
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•
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High speed of measurement reduces demands on long-term stability of the SEM
(FastEBSD at present more than 1000 patterns/sec acquisition and >2000 orientations/sec indexing of pattern sequences).
Digital cameras enable pixel binning (combination of several pixels on the chip => increases sensitivity,
reduces volume of data transfer, increases speed).
If intragranular structure is of no concern, it is sufficient to acquire the orientation of each grain only once.
Iterative mesh refinement has proven very effective by concentrating measurement along grain boundaries. Mesh refinement is inadequate if the microstructure
contains twinned grains or if a broad grain size distribution is present.
R.A. Schwarzer, Microscopy and Microanalysis 5, Suppl. 2 (1999) 242-243 .
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|
•
|
Low crystal symmetry is still a challenge.
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|
•
|
Fast phase discrimination has to be expanded to fast phase identification.
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|
•
|
Improved quantification of pattern quality parameter
=> correlation with stress, dislocation density.
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|
•
|
Radon transform and peak shape analysis
=> Evaluation of band intensities.
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|
•
|
3-D reconstruction of the microstructure at subgrain resolution by consecutive sectioning.
An excellent depth resolution is achieved with a dual-beam SEM which is a conventional SEM attached with an additional FIB (field ion beam source) column for in-situ surface
sputtering.
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|
The Scanning Electron Microscope (SEM)
|
|
•
|
Clean vacuum ==> less
contamination
|
|
•
|
Variable Pressure SEM ==> no
charging, no contamination
|
|
•
|
Schottky Field Emission ==> high beam brightness
|
|
· High current in small probes ==> improved spatial resolution
(but be aware of damaging the specimen and increased contamination rate)
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· Small beam aperture
==> large depth of focus
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|
•
|
Adequate camera port (wide, perpendicular to x-y stage movement, low
position)
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|
•
|
External computer control, computer interface
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|
The Acquisition System
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|
•
|
Actual digital CCD camera
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|
· fast: ~ 1000 frames/sec and more with a GigE Vision
or USB3
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|
· high sensitivity by pixel binning on the chip.
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|
· high dynamics (12-14 bit), no flat image for background correction
required
|
|
•
|
Actual digital CMOS camera |
| |
· fast: ~ 7500 frames/sec and more with proprietary frame grabber
|
|
•
|
Coming up is a lensless direct exposure of the sensor chip (high quantum efficiency, low beam voltage).
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| |
· extremely high sensitivity
|
| |
· high sample tilt is not nessary: low tilt angles from a few to 0° avoid image distortions and shadowing. In this configuration the solid state detector is mounted on the bottom of the probe forming lens.
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•
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Parallel processing of the Radon Transform on the GPU (dedicated graphic board replacing the computer's CPU) enables extremely high speed (analyzing 10.000 to 45.000 pps https://doi.org/10.1093/micmic/ozad067.224).
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|
Type of pattern
|
Equipment
|
Resolution
|
Main applications
|
|
spatial
|
angular
|
|
Spot pattern
|
TEM
("SAD" = selected area diffraction)
|
0.5 - 1.5 μm
|
5° (1°)
|
thin foil specimens
dark-field imaging
dislocations (weak beam)
Burgers vector analysis
precipitates, nanomaterials
orientation relationships
(estimate of) crystal orientation
|
|
Transmission Kikuchi pattern
(TKP)
|
TEM
(SAD)
|
0.5 - 1.5 μm
|
0.2°
|
thick foil specimens
medium size grains
crystal orientations
orientation differences
dislocation density
|
|
TEM, STEM
("MBD" = microbeam diffraction)
SEM TKD, t-EBSD
|
< 10 nm
|
0.2°
|
thick foil specimens
fine grain structures
crystal orientations, ACOM
orientation differences
indexing of grain boundaries
deformed materials
grain growth
|
|
Backscatter Kikuchi pattern
(BKP, "EBSP")
|
SEM
with an EBSD appliance
(low light level camera, computer control)
several commercial EBSD systems are available
|
< 30 nm
(FE gun)
0.05 - 1 μm
(W filament)
|
< 0.5°
|
bulk specimens
coarse grains, mesostructure
crystal orientations, ACOM
dynamic experiments
(e.g. hot stage, tensile stage)
fracture surfaces
(residual stress)
phase identification (Phase ID)
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|
Channeling pattern
(ECP)
|
SEM
(as an option for some SEM commercially available)
|
10 - 50 μm
(SAD)
|
0.5°
|
bulk samples
semiconductors (gentle method)
crystal orientation
orientation differences
residual stress
fracture
|
|
TEM pole-figure measurement
|
TEM
with a side-entry goniometer,
high-resolution camera or computer control
|
1 μm (SAD)
0.1 mm (RHEED)
|
|
thin film specimens (SAD)
bulk surfaces, layers (RHEED)
very fine grain structures
high degree of deformation
shear bands
texture fields
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|
Convergent Beam Electron Diffraction (CBED)
(Zone axis pattern;
Kossel-Moellenstedt pattern)
|
TEM
with cooling stage,
energy filter
|
5 nm
|
0.01°
|
inadequate for texture analysis.
determination of lattice constants
residual stress
phase identification
space groups
structure potentials
|
|
conventional X-ray diffraction
X-ray scanning apparatus
|
Euler cradle, x-y stage
" and ED detector
|
0.1 mm
50 μm
50 μm
0.1 mm
|
|
local pole figures
texture mapping
element mapping (micro XRFA)
lattice strain mapping
|
R.A. Schwarzer: Advances in the analysis of texture and microstructure. Archives of
Metallurgy and Materials 50 (2005) 7-20