Imaging and Analysis
Techniques in Materials
Science
Pat Trimby
LINK Nordiska
A typical materials characterization course:
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X-Ray Fluorescence
Photoluminescence
X-ray diffraction
X-ray photoelectron spectroscopy
UV photoelectron spectroscopy
Fourier Transform IR spectroscopy
Solid State NMR
Optical Microscopy
Rutherford Backscattering
spectroscopy
Scanning electron microscopy
Transmission electron microscopy
Electron energy loss spectroscopy
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Auger electron spectroscopy
Electron diffraction
Scanning tunneling microscopy
Atomic force microscopy
Neutron activation analysis
Particle Induced X-ray emission
Secondary Ion Mass spectroscopy
Focused ion beam microscopy
This is the outline of a 20-lecture
course given at the University of
North texas, USA.
The topics we'll focus on:
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X-Ray Fluorescence
Photoluminescence
X-ray diffraction
X-ray photoelectron spectroscopy
UV photoelectron spectroscopy
Fourier Transform IR spectroscopy
Solid State NMR
Optical Microscopy
Rutherford Backscattering
spectroscopy
Scanning electron microscopy
Transmission electron microscopy
Electron energy loss spectroscopy
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Auger electron spectroscopy
Electron diffraction
Scanning tunneling microscopy
Atomic force microscopy
Neutron activation analysis
Particle Induced X-ray emission
Secondary Ion Mass spectroscopy
Focused ion beam microscopy
The scale of features is vital
The SEM is ideal for composite materials
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Phases can be distinguished by either structure or by chemistry
The SEM allows us to analyse both the crystallographic structure
and chemistry on a suitable scale (10nm-10mm), as shown by
this phase map from an Fe-meteorite
Talk outline
This Talk
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SEM overview
SEM imaging
SEM analysis – EDS and WDS
FIB-SEM
Talk 2
• EBSD Overview
• EBSD application examples
SEM introduction
• SEM – Scanning Electron Microscope
• First SEMs were developed in the 1930s, as
an offshoot from TEMs. The first true SEM was
developed in 1942.
• Early marketing experts, looking into the
commercial prospects of the SEM, estimated
that between 6 and 10 would saturate the
market!
• Today there are over 50,000 SEMs worldwide,
used in a wide range of applications
The SEM is sold as a high resolution imaging
tool
A modern SEM as seen in the product brochure
(courtesy of Hitachi)
but in reality the SEM is a perfect analytical
tool:
A modern SEM as seen in the lab – courtesy of
Gothenburg University
WDS
EDS
EBSD
CL
Filament:
- Field emission
- LaB6
- Tungsten
Scan coils
Lenses
Apertures
Chamber
(detectors)
SEM Components
Display and
Controls
SEM basics
Overview
Electron gun and
scanning principles
Specimen interaction
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The interaction between the
sample and the electron beam
determines the resolution of
the image or analysis
• Different electron types (SE,
BSE) and X-rays have different
interaction volumes
• The energy of the incident
electron beam and the density
of the sample material
determine the interaction
volume size
• Resolution varies from <1nm
to over 5µm
Imaging v Analysis
• The SEM is used both for imaging and analysis. Some
techniques (such as CL) can be a combination of both.
Imaging
• Secondary Electron (SE)
• Backscatter Electron (BSE)
• Forescatter Electron (FSE)
• Cathodoluminescence (CL)
• Scanning Transmission
Electron Microscopy (STEM)
Analysis
• Energy Dispersive
Spectroscopy (EDS)
• Wavelength Dispersive
Spectroscopy (WDS)
• Electron Backscatter
Diffraction (EBSD)
Secondary Electron Imaging
• SEs – secondary
electrons – come from a
small interaction volume
• SE images contain
topographic information,
with some z-contrast
• Ideal for imaging 3D
structures
• SE detectors can also be
located within the SEM
column ("in-lens") for
higher resolution imaging
SE image of a fly head
(courtesy of Tescan)
Backscattered Electron Imaging
• BSEs – Backscattered
Electrons – come from a
relatively large interaction
volume
• BSE Images contain
predominantly z-contrast
signal
• Ideal for imaging phases
• In single phase materials,
can be used for
orientation contrast (OC)
imaging
Z-contrast BSE image of a zircon grain in a
polyphase deformation zone in a rock sample
Forescatter Electron Imaging
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Utilises standard backscattered
electron diodes
Diodes are positioned below a
highly tilted sample – called
Forescatter Detectors (FSD)
Diodes can either be attached to
the EBSD detector (as shown)
or onto a sample holder
Sample-detector geometry is
critical
Z-contrast BSE image of a polyphase rock
sample
Forescatter orientation contrast image of the
same area
Forescatter orientation contrast image of the
boundary between aragonite and calcite (both
CaCO3) in a common mussel
High resolution forescatter OC image of a GaN thin
film, showing atomic steps and strain around
indivdual threading dislocations
Sample courtesy of Dr C. Trager Cowan, Strathclyde University
Cathodoluminescence Imaging
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CL – cathodoluminescence
CL occurs in many non-metallic
materials
CL can be stimulated by an
electron beam – therefore ideal
for SEMs
Signal can contain both structural
and chemical information
Commonly used in
semiconductor industry and in
geology
CL image of zoning in a magmatic
zircon grain
Image courtesy of Gatan UK
Plane polarised
light microscope
image of a
sandstone
Image courtesy of the
University of Texas
Crossed polarised
light image of the
same area
Colour CL image of
the same area
produced by
combining 3 CL
images collected
sequentially in the
SEM.
Note the original
quartz grains,
overgrowths and
fractures not visible
in the light images
CL spectroscopy
Advanced CL systems can
select specific wavelengths of
luminescence – used in
microelectronics industry
355nm
357nm
Wavelength specific CL images
from a GaN HVPE Growth sample
Example of a CL wavelength spectrum, showing
the frequency of different wavelengths of CL
STEM imaging in the SEM
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STEM detectors
(scanning transmission
electron microscopy) are
available for many
modern FEG SEMs
The principles are the
same as for normal
TEMs
Lower resolution and
lower accelerating
voltages than in a TEM
Ease of use much
greater, plus added
versatility of the SEM
detectors
Modified from Smith et al., Microscopy and Analysis, Jan 2006
Examples of
STEM imaging in
the SEM
(Images courtesy of Carl
Zeiss SMT)
Orientated Dark Field STEM
image of a nanocrystalline Al
sample (30kV)
High resolution
STEM image of
Mica (30kV)
Analysis – EDS
• EDS – Energy Dispersive Spectroscopy
• EDS is method of choice for chemical microanalysis
on the Scanning Electron Microscope.
• Why?
– Fast, convenient and flexible method for
determining the chemical constituents and
composition of phases on the microscale
– Also good for ‘mapping’ chemical variations
– Requires little sample preparation
– Is suitable for any SEM
– Works with most types of samples….
The EDS system
Images courtesy of Carl Zeiss SMT
How does EDS work?
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High energy electrons from the
SEM beam interact with the
sample
This may result in ejection of an
electron from an inner electron
shell => atom is in an "excited"
state
De-excitation occurs by filling the
vacancy with an electron from an
outer shell
This results in the release of a
characteristic energy, either by the
emission of an X-ray photon or an
Auger electron
Detection and measurement of the
characteristic X-ray shows the
element and the specific transition
Movie showing the X-Ray
generation of K-lines
Movie showing the X-Ray
generation of M-lines
The EDS detector
Background signal
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There is also the generation of
continuum X-rays –
"Bremsstrahlung"
• Caused due to interaction of
electrons with the atomic field
• Results in a background signal
on which the characteristic Xrays are superimposed
The resultant EDS spectrum
Metal Sample
Fe
Ni
Fe
Si
Ni
Ti
Ti
1
2
3
4
Full Scale 12713 cts Cursor: 5.825 keV (213 cts)
Fe
Ti
5
Ni
6
7
8
9
keV
Quantitative EDS Analysis
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EDS can be used to measure accurately the % of different
elements in a phase
This requires filtering, background processing and corrections
Usually it will also require comparison to known standards
ZAF corrections– Atomic
No, Absorption and
Fluorescence
Accuracy (using
standards) down to
0.1wt%
Spectra showing the effect of fluorescence
of Cr by Fe
EDS Mapping
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element mapping is a fast and common technique to show the
distribution of elements and phases in a sample
Good EDS maps can be acquired in just a few minutes
Al
Example BSE image and element
maps from a rock sample
Fe
"Spectral imaging"
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EDS mapping is often now referred to as "spectral imaging"
This is because a whole spectrum is collected at each point in the
map, and not just the counts for individual elements
Spectra can then be recompiled for single phases or regions in
the map (as below)
With care, these spectra can also be analysed quantitatively
offline
Spectrum compiled from
a Fe-Ti-O phase in the
previous dataset
EDS recent developments
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The most significant recent change has been the emergence of
LN2-free detectors
The most common type is known as a Silicon Drift Detector
(SDD) or Analytical Drift Detector (ADD – shown below)
These detectors are of
course much easier to use
They also cope better with
extremely high count rates,
so can collect data more
quickly
They are generally not so
good for analysing light
elements (e.g. Boron,
Carbon)
WDS – complementing EDS
• WDS – Wavelength Dispersive Spectroscopy
• Large detector that can be fitted onto most
SEMs
• Identifies characteristic X-rays based on their
wavelength, as compared to their energy (as
for EDS)
• More accurate and more sensitive
• But more expensive and more time consuming
WDS v EDS
EDS
WDS
Element range
Be upwards
Be upwards
Comp. range
~0.1-100wt%
<0.01-100wt%
Spec. Resolution
Poor ~65-150eV
~10eV
Speed
Fast- Parallel
Slow- Serial
Numbers
Common
Rare
Light Elements
Moderate
Good
How WDS works
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Detector comprises a
diffracting crystal and an X-ray
counter
• The crystal disperses the Xrays with different
wavelengths, as determined by
Bragg's law
• Different crystals are used,
each optimised for a specific
range of elements
• The counter and crystal lie
along a circle of constant
diameter – "Rowland Circle" –
this is a fully focusing
geometry
Why have WDS capability on SEM?
• Gives the best spectral resolution
Spectrum of W-Si.
Si Kα and W Mα are
approximately 35 eV
apart.
Yellow is EDS
spectrum, blue is
WDS spectrum.
• Far better sensitivity compared to EDS:
– Trace Elements
– Light Elements
• C and N in Steel
• B Glass
Spectrum 1
Si
Mg
Example from the
mineral garnet,
showing the
advantages WDS
has over EDS for
trace elements
Al
O
C
Ca
Ca
Cr
Cr
Mn
Fe
1
2
Full Scale 10992 cts Cursor: 7.459 keV (70 cts)
Garnet
WD
ED
Ca
Na
3
4
Cr
Mn
5
6
Fe
Mn
Fe
7
keV
Na2O
MgO
Al2O3
SiO2
CaO
TiO2
Cr2O3
MnO
FeO
Total
0.07
19.71
19.68
42.04
4.93
0.31
5.23
0.37
7.61
99.95
ND
19.67
19.52
42.04
5.04
0.36
5.29
0.53
7.92
100.37
...which is why WDS systems are used on
microprobes:
Note 5 WDS
systems,
and 1 EDS
system
Image of a JEOL JXA 8200 microprobe at the University of Calgary
EBSD
• EBSD – electron backscatter diffraction
• Relatively new SEM technique for
measuring crystal orientations and
identifying phases
• Will be dealt with in full detail in the next
lecture
FIB-SEM
• FIB – Focused ion beam
• FIBs have been used for many years, but
recently they have been integrated with SEMs
• A FIB allows the removal of material on a very
fine scale, the SEM allows the processed to be
viewed at high magnification
• Ideal for sample preparation and 3D analyses
• BUT – a destructive technique
FIB-SEM basics
• Both the electron beam and focused ion
beam are focused at a single point
• This allows the removal of material whilst
imaging at high resolution using the e-beam
• Very powerful combination for
manipulating samples on a fine (<<10µ
m)scale.
Image courtesy of Carl Zeiss
SMT
Schematic of chamber interior,
showing FIB and Electron guns
FIB example – serial sectioning through a
nanocrystalline Ni-superalloy
Movie courtesy of Hans Mulders, FEI
Company
FIB example – section through a 25µm Au
wire,showing ion-channelling contrast
Sample courtesy of Dr. J. Shischka, FIW, Halle
FIB-SEM applications
• Preparation of samples, especially for TEM
analysis
• 3D analysis of nano-crystalline materials –
serial sectioning and reconstruction (can be
combined with microanalysis techniques such
as EBSD)
• Nanolithography
• Failure analysis / defect review in
semiconductor industry
Summary so far
• There are many, many techniques for materials
analysis that can be used (far beyond the scope of this
talk)
• The SEM is one of the most powerful tools, providing
both imaging and analytical capability
• The SEM also covers the scales required for most
materials analysis (1nm to 10mm)
• Many imaging techniques can be used with the SEM
• Chemical analyses on the SEM are carried out using
the complementary EDS and WDS techniques
• The FIB-SEM offers a new dimension in material
manipulation and analysis
next.....EBSD