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Chapter-3. Electron Microscopy.pptx
1. INDIAN INSTITUTE OF TECHNOLOGY ROORKEE
MATERIAL CHARACTERIZATION AND TESTING
Prof. Kaushik Pal
Professor
Department of Mechanical and Industrial Engineering
Indian Institute of Technology Roorkee
Lecture 03 : Electron Microscopy
2. 2
LIGHT MICROSCOPE ELECTRON MICROSCOPE
Use of vacuum No vacuum
Entire electron path from
gun to camera must be
under vacuum
The source of
illumination
The ambient light source is light
for the microscope
Electrons are used to “see” –
light is replaced by an electron
gun built into the column
The lens type Glass lenses Electromagnetic lenses
Magnification
method
Magnification is changed by
moving the lens
Focal length is charged by
changing the current through
the lens coil
Viewing the
sample
Eyepiece (ocular)
Fluorescent screen or
digital camera
Light Microscopy vs Electron Microscopy
• Microscopy is any technique for producing visible images of structures or details too
small to otherwise be seen by the human eye.
• As the things we are interested in get smaller and smaller (e.g. nanomaterials) we need
more better and more powerful microscopy
3. 3
Key Concepts:
Resolution is defined as the act, process, or capability of
distinguishing between two separate, but adjacent objects or sources
of light, or between two nearly equal wavelengths.
Resolving Power is the ability to make points or lines which are
closely adjacent in an object distinguishable in an image.
Resolution
Resolving Power
4. 4
How is Resolution Affected by Wavelength?
• Electron microscopes differ from light microscopes in that they produce an image of a specimen by
using a beam of electrons rather than a beam of light.
• Electrons have much a shorter wavelength than visible light, and this allows electron microscopes to
produce higher-resolution images than standard light microscopes.
6. 6
History of Electron Microscopy
1931- Ernst Ruska co-invents the electron microscope.
1938- 10 nm resolution reached.
1940- 2.4 nm resolution.
1945- 1.0 nm resolution achieved.
1981- Gerd Binning and Heinrich Rohrer invent the scanning tunneling
electron microscope (STM).
1986- The Atomic Force Microscope was developed in collaboration
between IBM and Stanford University.
8. 8
Core Technology: The Electron Gun
• Three main sources of electrons:
i. Tungsten
ii. LaB6 (lanthanum hexaboride)
iii. Field Emission Gun (FEG)
• Different costs and benefits of each
• Each selected primarily for their brightness
• Electron microscopes use a vacuum to make electrons behave like light.
9. 9
Scanning Electron Microscopy - Working
Principle
The scanning electron microscope (SEM) is the most widely
used type of electron microscope. It examines microscopic
structure by scanning the surface of materials, similar to
scanning confocal microscopes but with much higher
resolution and much greater depth of field.
An SEM image is formed by a focussed electron beam that
scans over the surface area of a specimen; it is not formed by
instantaneous illumination of a whole field as for a TEM
(transmission electron microscope).
The most important feature of an SEM is the 3-dimensional
appearance of its images because of its large depth of field.
For example, the depth of field can reach the order of tens of
micrometers at 103 × magnification and the order of
micrometers at 103 × magnification.
SEM also enables us to obtain chemical information from a
sample by equipping the X-ray energy-dispersive spectrometer
(EDS).
SEM image of pollen
grains
10. 10
A scanning electron microscope (SEM) produces
images of a specimen by scanning the surface with a
focussed beam of electrons.
The electrons interact with atoms in the sample,
producing various signals that contain information
about the surface topography and composition of the
sample.
The electron beam is scanned in a raster scan pattern
and the position of the beam is combined with the
intensity of the detected signal to produce an image.
In the most general mode, secondary electrons
emitted by atoms excited by the electron beam are
detected using a secondary electron detector.
The number of secondary electrons detected effect
the signal intensity of specimen topography.
SEM instrument
11. 11
Basic Principle:
When beam of electrons strikes the surface of specimen & interacts with the
atoms of sample, signals in the form of secondary electrons, back scattered
electrons & characteristic X-rays are generated that contain information about
the samples‟ surface topography, composition etc.
What we can see with SEM?
Topography: Texture/surface of a sample.
Morphology: Size, shape, order of particles.
Composition: Elemental composition of
sample.
Crystalline Structure: Arrangement
present within sample
12. 12
Three Modes
of
Operation:
Primary
•High resolution (1-5nm).
• Secondary electron imaging
Secondary
•Generates characteristic X-rays.
• Identification of elemental composition of sample by EDX technique
Tertiary
•Generates back-scattered electronic images.
• Clues to the elemental composition of sample
Electronic devices are used to detect & amplify the signals & display them as an
image on a cathode ray tube in which the raster scanning is synchronized with that
of microscope.
In SEM, beam passes through pairs of scanning coils or pairs of deflector plates in
the electron column to the final lens, which deflect the beam horizontally &
vertically.
The image displayed is therefore a distribution map of the intensity of the signal
being emitted from the scanned area of the specimen.
13. 13
Light vs Electrons
Human eye can detect detail of only 0.2 mm in size.
Optical microscope can distinguish about 200 nm size details.
Light microscope Electron microscope
λ = 0.4 – 0.7 μm
Refractive index = 1.5 (glass)
Angle of reflection = 70°
r (resolution) = 0.2 μm = 2000
Angstrom.
λ = 0.001 – 0.01 nm
Refractive index = 1 (vacuum)
Angle of reflection = 1°
r (resolution) = 0.00016 μm =
1.6 Angstrom (atomic
separation distance)
Strongly scattered by gas
14. 14
When high-energy electrons strike a specimen, the electrons are scattered by atoms of the
specimen. Electron scattering results in a change of direction of travel of electrons under
the sample surface.
The interaction between electrons and specimen atoms occurs within a certain volume
under the specimen surface.
Both secondary electrons and backscattered electrons generated by scattering are used as
signal sources for forming SEM images. However, secondary electrons and backscattered
electrons which are collected by a detector, escape from different locations in the specimen.
The interaction zone where electrons scatter under the specimen surface is described as
pear-shaped and its size increases with the energy of incident electrons in the probe.
Electron-Specimen Interactions
15. 15
Besides secondary electrons and backscattered electrons, characteristic X-rays
are also produced in the interaction zone and these are useful for chemical
analysis.
Secondary electrons are the products of inelastic scattering, and they have an
energy level of several electron volts.
In the interaction zone, secondary electrons can escape only from a volume
near the specimen surface with a depth of 5-50 nm, even though they are
generated in the whole pear-shaped zone.
Interaction zone
16. 16
In contrast, backscattered electrons are the
product of elastic scattering, and they have an
energy level close to that of incident electrons.
Their high energy enables them to escape
from a much deeper level in the interaction
zone, that is from depths of about 50-300 nm.
The lateral spatial resolution of an SEM
image is affected by the size of the volume
from where the signal electrons escape.
The image formed by secondary electrons
should have a better spatial resolution than
that formed by backscattered electrons.
The interaction zone of electrons and
specimen atoms below a specimen surface
17. 17
Optical arrangement
• A SEM consists of an electron gun and a series of electromagnetic lenses and apertures
similar to TEM systems.
• In an SEM, the electron beam emitted from an electron gun is condensed to a fine probe for
surface scanning.
• The electron gun for generating an electron beam is-
Thermionic gun
Field emission gun
In thermionic electron gun, electrons are emitted from a heated tungsten filament and then
accelerated towards an anode; a divergent beam of electrons emerges from the anode hole.
In field emission gun, very strong electrical field is used to extract electrons from a metal
filament. Temperature required is lower than those for thermionic emission.
Advanced SEM system use a field emission gun due to its high beam brightness. Beam
brightness plays an even more important role in imaging quality in an SEM than in a TEM.
Instrumentation and applications of SEM
18. 18
An SEM optical path goes through several electromagnetic lenses, including condenser
lenses and one objective lens.
The electromagnetic lenses in an SEM are for electron probe formation and not for image
formation directly as in a TEM.
The two condenser lenses reduce the crossover diameter of the electron beam; then the
objective lens focuses the electron beam as a probe with a diameter on the nanometer scale.
The objective lens should be considered as the third condenser lens in the SEM because it
functions more like a condenser than an objective lens.
Probe scanning is operated by a beam-deflection system incorporated within the objective
lens in an SEM.
The deflection system moves the probe over the specimen surface along a line and then
displaces the probe to a position on the next line for scanning so that a rectangular raster is
generated on the specimen surface.
The signal electrons emitted from the specimen are collected by a detector, amplified and
used to reconstruct an image.
19. 19
Signal detection
The electron signals from each pixel of the raster are collected by a detector in order to
generate a corresponding point-to-point image on a display screen.
To understand signal detection, we must know types of electron signal useful in an SEM.
Backscattered electrons (BSEs)
Secondary electrons (SEs)
When high energy electrons strike a specimen, they produces either elastic or inelastic
scattering. Elastic scattering produces the BSEs which are incident electrons scattered by
atoms in the specimen.
Inelastic scattering produces SEs which are electrons ejected from atoms in the specimen.
BSEs are deflected at large angles from the sample and with little energy loss; they retain
60-80% of the energy of incident electrons.
In contrast, SEs are deflected at small angles and show much lower energy compared with
incident electrons.
20. 20
Detector
A commonly used detector in an SEM is the Everhart-Thornley (E-T) detector. The SEs
travel with large deflection angles toward the detector while BSEs travel directly toward the
detector.
The Faraday cage in the front of detector is either positively or negatively charged (250 or -
50 V) depending on signal selection.
When given a positive charge, the detector attracts signal electrons mainly SEs.
When given a negative charge, it can screen out SEs with energy less than 50 eV.
The key element of the E-T detector is the scintillator, a disk of about 8-20 mm in diameter.
The scintillator coverts signal electrons into photons by accelerating the electrons with +12
kV and striking them onto a disk.
The photons then travel through a light guide and enter the photomultiplier tube for signal
gain. The photomultiplier output is further amplified for display on a screen.
21. 21
Probe Size and Current
The resolution of SEM imaging is determined by the cross-sectional diameter of the
scanning probe. Thus the size of the probe limits the size of features on the specimen
surface to be resolved.
To obtain high resolution, we should know how to minimize the probe size. The probe
diameter is expressed as 𝑑𝑝
Where ip is the probe current, β is the beam brightness which is controlled by the electron
source and 𝛼f is the convergence angle of the probe.
22. 22
The brightness of electron
illumination or beam brightness
depends on the type of electron
gun used.
A field emission gun is 1000 times
brighter than a tungsten thermionic
gun and 100 times brighter than a
LaB6 thermionic gun.
To obtain a minimal probe size, we
should increase the brightness as
well as the convergence angle. But
a large likely introduces other
optical problems like spherical
aberration.
Relationship between the probe diameter,
convergence angle and working distance
23. 23
The reduction of probe size alone is not
sufficient to obtain a high resolution image in
an SEM.
For getting a high resolution image, the probe
current must be larger than a minimum value
so that microscopic features of the specimen
are visible in the SEM image.
The relationship between the probe current and
image visibility can be understood by
analysing the basic requirements that an image
will not be obscured by background noise.
Background noise in an SEM system is
generated by fluctuation of the electron beam
current and signal amplification in the detector.
And cannot be completely eliminated.
Signals and background noise produced
when scanning a specimen
24. 24
Advantages:
Bulk-samples can be observed and larger sample area can be viewed.
Generates photo-like images.
Very high-resolution images are possible.
SEM can yield valuable information regarding the purity as well as
degree of aggregation.
Disadvantages:
Samples must have surface electrical conductivity
Non- conductive samples need to be coated with a conductive layer
Time consuming & expensive.
Sometimes it is not possible to clearly differentiate nanoparticle from
the substrate.
SEM can’t resolve the internal structure of these domains.
25. 25
Materials science
SEMs are used in materials science for research, quality control and failure analysis.
In modern materials science, investigations into nanotubes and nanofibers, high
temperature superconductors and alloy strength, all depend on the use of SEM for
research and investigation.
Nanowires for gas sensing
Researchers are exploring new ways so that nanowires can be used as gas sensors by
improving existing fabrication methods and developing new ones.
Semiconductor inspection
Reliable performance of semiconductors requires accurate topographical information.
The high resolution 3-dimensional images produced by SEMs offers a speedy, accurate
measurement of the composition of semiconductor.
Forensic investigations
Criminal and other forensic investigations utilise SEMs to uncover evidence and gain
further forensic insight.
Application of SEM
26. 26
Biological sciences
In biological sciences, SEMs can be used on anything from insects and animal tissue to
bacteria and viruses.
Soil and rock sampling
Geological sampling using a scanning electron microscope can determine weathering
processes and morphology of the samples.
Microchip assembly
Microchip production is increasingly relying on SEMs to help gain insight into the
effectiveness of new production and fabrication method.
With smaller and smaller scales materials as well as the potential of complex self
assembling polymers, the high resolution 3-dimensional capacity of SEMs is invaluable
to microchip design and production.
27. 27
Some Examples
• This image shows 50 µm of solar panel surface and highlights the pyramidal structures that help trap light and
reduce reflection.
• Photovoltaic researchers are seeking ways to optimize the texture of these surfaces, since the shape, size and
uniformity of the pyramids affect optical reflectance and energy capture.
Solar Panel Surface
28. 28
• Honey bees have been the subject of intense research, as their numbers are declining due to the spread of varroa
mites (Varroa destructor and V. jacobsoni).
• The mite attaches to hive larvae or the body of a bee and weakens the larvae or bee by sucking out fat bodies.
Symptoms include low body weight and deformed wings, both of which have been studied using SEM imaging.
A 2 mm Bee Head
29. 29
A hydrogen-absorbing alloy magnified 30,000 times
• Hydrogen-storage alloys are metallic materials that can reversibly absorb and release hydrogen from the gas phase
or electrochemically.
• These alloys are already being used in electrodes, particularly in electric vehicles, to improve performance and to
avoid using common alternative materials that contain toxic lead or cadmium, as these can leach into landfill.
30. 30
Cuboidal Ni3Al precipitates (Edge length ~400 nm)
• This image of cuboidal Ni3Al precipitates (edge length ~400 nm) in a Ni-based single-crystal superalloy.
• These precipitates are the source of the outstanding strength of superalloys at high temperatures, enable the
operation of turbine blades in the extreme environments within jet engines.
31. 31
• This microstructure shows the hydrogen induced cracking in the acicular ferrite of HSLA steel subjected to
aging treatment.
• The image presents the nucleation of crack through in the niobium carbide.
Hydrogen induced cracking
32. 32
• In a new study published in Nature Scientific Reports, researchers have fabricated a sheet of nickel with
nanoscale pores that make it as strong as titanium but four to five times lighter.
A sheet of nickel with nanoscale pores
35. 35
X-RAY Fluorescence Spectrometry
XRF analyses the chemical elements of specimens by detecting the characteristic X-
rays emitted from the specimens after radiation by high energy primary X-rays.
The characteristic X-rays can be analysed from either their wavelengths or energies.
Thus there are two types of XRF: WDS and EDS
An XRF instrument consists of three main parts: the X-ray source, detection system &
data collection and processing system.
Main components and
dispersive spectra of
(a) WDS and (b)EDS
Chemical analysis in SEM (EDS and WDS)
36. 36
EDS became a commercial product in the early 1970s and rapidly overtook WDS in
popularity.
An EDS system is structurally simple because it does not have moving parts such as the
rotation detector with WDS.
EDS systems are relatively faster because the detector collects the signals of characteristic
X-rays energies from a whole range of elements in a specimen at the same time rather than
collecting signals from X-ray wavelength individually.
For EDS, the typical resolution of energy dispersion is about 150-200 eV, worse than the
resolution of WDS and the lightest element that can be detected is O (Z=8) not C (Z=6).
But these disadvantages are not as important as the advantages of an EDS system which are
low cost and fast analysis.
Energy Dispersive Spectroscopy (EDS)
37. 37
Detector
The Si(Li) is the most commonly used detector in an EDS system. The detector
consists of small cylinder of p-type silicon and lithium in the form of Si(Li) diode.
X-rays photons collected by the detector generate a specific number of electron-hole
pairs.
The average energy of photons needed to generate as electron-hole pair is about 3.8 Ev
in the Si(Li) diode.
The higher the photon energy, the more pairs are generated. Characteristic X-ray
photons can be separated by their energy levels according to the number of electron-
hole pairs they generate.
The energy resolution of detector (R) in eV is
Where E is the energy of characteristic X-ray line, F is a constant called the Fano
factor (0.12 for Si(Li) and 𝜎noise is the electronic noise factor.
38. 38
Energy dispersive spectra
An EDS spectrum is presented as the intensity of characteristic X-ray lines across the
X-ray energy range.
A spectrum in a range from 0.1 to about 10-20 keV can show both light and heavy
elements because both K lines of light elements and M or L lines of heavy elements
can be shown in this range.
For example, the EDS spectrum of a glass specimen containing multiple elements
including Si, O, Ca, Fe, Al and Ba in an energy range upto 10 keV
EDS spectra are similar to WDS spectra but identification of individual elements from
EDS spectra is more simple than from WDS spectra because each characteristic line
generated by a specific element exhibits a unique X-ray energy.
However, the signal to noise ratio is lower than that of WDS and the resolution (in
terms of energy) is about 10 times lower than that of WDS.
40. 40
Wavelength Dispersive Spectroscopy (WDS)
XRF spectrometry was introduced as WDS in the early 1950s whereas the EDS came
along later years.
WDS provides better resolution and a wider range of elemental analysis than EDS, but its
instrumentation is more complicated.
The WDS systems can resolve relative change in wavelength in the range 0.002-0.02. This
range corresponds to energy range 0.01-0.1 keV which is about 1 order of magnitude better
than that of EDS.
Modern WDS system can detect element from upward of C(Z=6).
Parts of WDS-
There is a rotating X-ray detector system (an analysing crystal and X-ray photon
counter) to collect the diffraction beam, and collimators to align the characteristic X-
ray beam from the specimen and the beam diffracted from the analysing crystal.
41. 41
The rotating X-ray photon counter scans a range of 2θ to detect specific wavelengths of
characteristic X-rays from the specimen.
A WDS system may have one detector set (single channel) or a number of detector sets
(multichannel).
The multichannel system can partially overcome the drawbacks of sequential detection and
increase analysis speed.
WDS apparatus which includes the X-ray tube, specimen, primary collimator, analysing
crystal, flow counter, auxiliary collimator and scintillation counter
42. 42
Wavelength Dispersive spectra
A WDS spectrum is presented as a diagram in which the characteristic X-ray lines are
dispersed in a range of the X-ray wavelengths.
The relative intensities of the individual X-ray lines are represented by their heights in
the spectrum but there is no scale to indicate the real intensities of the X-rays.
Similar to the spectrum of X-ray diffraction, only the relative intensities among the
lines are important.
The relative intensities of spectrum lines representing individual elements provide a
rough idea of the relative concentration of those elements in the alloy.
WDS spectra of a nickel-based
superalloy
44. 44
Electron backscatter diffraction (EBSD) is a technique to determine crystalline
materials properties in electron microscopy (both SEM and TEM).
With special detector, an SEM system can record EBSD patterns of a crystalline
solid which are essentially the backscatter kikushi patterns.
With the EBSD patterns, we can determine crystalline orientations of individual
grains of a polycrystalline specimen and identify separate crystalline phases in a
multiphase specimen.
The EBSD technique is increasingly used for examining metallic and ceramic
materials, particularly for metals.
Electron backscatter diffraction (EBSD)
46. 46
EBSD pattern formation
EBSD requires incident electrons inelastically scatter in a specimen.
When the primary electron beam of SEM focussed on a location of a specimen, S (tens of
nanometers from the specimen surface), the electrons scatter from S to all directions in a
crystalline solid.
There must be an electron beam with the scattering angle (θ) from the scatter location to a
certain crystallographic plane that satisfies the constructive diffraction condition (Bragg’s
law).
The constructively diffracted electron beams can be recorded by a detector a short distance
away from the specimen surface.
In fact, the diffraction beams with the same θ angle form two symmetric cone surfaces with
respect to the scatter source location S, because of the three-dimensional nature of
specimen.
47. 47
Applications of EBSD
The most widely used applications of EBSD are determination of grain orientations and
identification of phase in crystalline materials.
By stepwise scanning through a specimen surface, the EBSD patterns of individual
microscopic areas will be collected and indexed automatically.
Differences of crystal orientation among grains can be revealed by the differences in
EBSD patterns.
This method is specially useful for analysing texture structures of metals resulting from
processing such as solidification, plastic deformation and heat treatment.
Commonly higher contrast between grains means a greater difference in grain angle. Grain
boundaries are effectively identified by small-step acquisition of EBSD patterns.
48. 48
Orientation of a local microstructural element compared to its surrounding.
Crystal orientation map reveal the positions of all grains and grain boundaries in the
sample microstructure.
In crystal orientation mapping a grain is defined by the collection of neighbouring pixels in
the map which have a misorientation less than a certain threshold angle.
The distribution of grain sizes can be measured from the data collected for the map.
Determination of grain orientation using EBSD
Crystal Orientation Map
Inverse pole figure
001 101
111
49. 49
Transmission electron microscopy (TEM) is a
microscopy technique in which a beam of
electrons is transmitted through a specimen to
form an image.
The specimen is often an ultrathin section less
than 100 nm thick or a suspension on a grid for
powder sample.
An image is formed from the interaction of the
electrons with the sample as the beam is
transmitted through the specimen.
The image is then magnified and focussed onto an
imaging device such as fluorescent screen.
TEM are capable of imaging at significantly
higher resolution than light microscopes owing to
the smaller de Broglie wavelength of electrons.
Transmission electron microscopy
A TEM image of a cluster of
poliovirus (30 nm is diameter)
50. 50
A TEM has the following components along its optical path:
Light source
Condenser lens
Specimen stage
Objective lens
Projector lens
The main differences are that in a TEM, the visible light ray is replaced by an electron tray
and glass lenses for visible light are replaced by electromagnetic lens for the electron beam.
The TEM has more lenses (the intermediate lens) and more apertures (including the
selected area aperture).
The TEM contains further features arising from using electrons as illumination. For
example- a vacuum environment is required in a TEM because collision between high
energy electrons and air molecules significantly absorb electron energy.
The modern TEM can achieve magnifications of one million times with resolutions of 0.1
nm.
52. 52
Principle:
Crystalline sample interacts with electron beam mostly by diffraction
rather than absorption.
Intensity of diffraction depends on the orientation of planes of atoms in a
crystal relative to electron beam.
High contrast image can be formed by blocking deflected electrons which
produces a variation in electron intensity that reveals information on the
crystal structure.
This generate both bright or light field & dark field images.
What can
be seen with
TEM ?
Crystalline Structure:
Arrangement of atoms in sample & defects in crystalline structure
Morphology:
Shape, size, order of particles in sample.
Composition:
Elemental composition of the sample.
53. 53
Advantages:
High magnification (ability to enlarge an image) & resolution
(ability to distinguish two very close object as separate images).
Provide information about internal ultrastructure of cells.
Images are high quality and detailed.
Disadvantages:
TEMs are large and very expensive.
Laborious sample preparation.
Operation and analysis requires special training.
Sample are limited to those that are electron
transparent.
TEMs require special housing and maintenance.
Images are black and white. Different TEM Images
54. 54
TEM SEM
Beam voltage 100-400 kV 1- 30 kV
Focus of analysis Internal or beyond surface Surface of sample
Modes Broad beams and scanning probe Scanning probe
Smallest probe 0.5 nm (5 Å) using STEM ~ 1 nm (10 Å)
Best resolution 0.14 nm (1.4 Å) lattice imaging ~ 1 nm (10 Å)
Contrast Forward scattered electrons Secondary emission and backscattered electrons
Insulators No charging Charging effects
Sample thickness 10-200 nm (100-2000 Å) 1-10 mm
Sample diameter < 3 mm across Full wafers
Minimum preparation time ~ 4 hours < 1 min
Image presentation 2-D 3-D
Display of image On TV monitor On fluorescent screen
Comparison of TEM vs SEM
55. 55
Electron sources
In a TEM system, an electron gun generates a high energy electron beam for
illumination.
In the electron gun, the electrons emitted from a cathode, a solid surface are
accelerated by high voltage (V) to form a high energy electron beam with energy E
=eV.
Because electron energy determines the wavelength of the electrons and wavelength
largely determines the resolution of the microscope, the acceleration voltage
determines the resolution to a large extent.
To achieve a high resolution, the TEM is usually operated under an acceleration
voltage of greater than 100 kV.
In practice, 200 kV is commonly used and meets most resolution requirements.
The general structure of an electron gun is composed of three main parts-
A cathode or electron source, a Wehnelt electrode and an anode
Components of TEM
56. 56
Electromagnetic lenses
The lens system of a TEM is more complicated than a light microscope besides the
electromagnetic lenses.
There are two or more condenser lenses to demagnify the electon beam emitted from
the electron gun.
The condenser lens system controls the beam diameter and convergence angles of the
beam incident on a specimen.
The TEM has three lenses to ensure a magnification capability of about 10-10 times:
objective, intermediate and projector lenses.
The intermediate lens is used to switch the TEM between an image mode and a
diffraction mode.
57. 57
For the image mode, the intermediate lens is focussed on the image plane of the objective
lens, and for the diffraction mode it is focussed on the back-focal plane of the objective lens
where the diffraction pattern forms.
The projector lens further magnifies the image or diffraction pattern and projects it onto the
fluorescent screen for observation.
Optical paths of (a) diffraction mode and (b) image mode
58. 58
The TEM has special requirements for specimens to be examined; it does not have the
same flexibility in this regard as light microscopy,
TEM specimens must be a thin foil because they should be able to transmit electrons means
they should be electronically transparent.
A thin specimen is mounted in a specimen holder in order to be inserted into the TEM
column for observation.
The holder requires that a specimen is a 3-mm disc. Smaller specimens can be mounted on
a 3-mm mesh disc.
The meshes is made from copper, prevent the specimens from falling into the TEM vacuum
column.
Also, a copper mesh can be coated with a thin film of amorphous carbon in order to hold
specimen pieces even smaller than the mesh size.
The specimen holder is a sophisticated rod-like device that not only holds the specimen but
also is able to tilt it for better viewing inside the TEM column.
Specimen stage
59. 59
A specimen holder for a TEM
A metal mesh disc supporting
small foil pieces of a specimen
3 mm TEM Grid
60. 60
Specimen preparation
Preparation of specimen is the most tedious step in TEM examination.
We have to prepare a specimen with at least part of its thickness at about 100 nm,
depending on the atomic weight of the specimen materials.
For higher atomic weight material, the specimen should be thinner.
A common procedure for TEM specimen preparation is described as follow:
Prethinning
Final thinning
1. Electrolytic thinning
2. Ion milling
3. Ultramicrotomy.
61. 61
Prethinning
Prethinning is the process of reducing the specimen
thickness to about 0.1 mm before final thinning to 100
nm thickness.
First, a specimen less than 1 mm thick is prepared. This is
usually done by mechanical means such as cutting with a
diamond saw.
Then, a 3-mm diameter disc is cut with a specially
designed punch before further reduction of thickness.
Grinding is the most commonly used technique to reduce
the thickness of metal and ceramic specimens.
During grinding, we should reduce the thickness by
grinding both sides of a disc, ensuring the planes are
parallel.
Generally, hand-grinding jig is used for prethinning. The
disc (S) is glued to the central post and a guard ring (G)
guides the grinding thickness.
Hand-grinding jig for TEM
specimen preparation
62. 62
Final thinning
Electrolytic thinning
Electrolytic thinning and ion milling are methods for reducing specimen thickness to
the scale of 100 nm.
These methods create a dimpled area on prethinned specimens because it is almost
impossible to reduce the thickness of specimens uniformly to the level of electron
transparency.
Electrolytic thinning is widely used for preparing specimens of conducting materials.
A specimen is placed in an electrochemical cell with the specimen as the anode.
A suitable electrolyte is used to electrochemically reduce the specimen thickness.
The electrolyte jet polishes both sides of the specimen until light transparency is
detected by a light detector.
63. 63
Ion milling
Ion milling uses a beam of energetic ions to bombard specimen surfaces in order to
reduce the thickness by knocking atoms out of a specimen.
The specimen does not need to be electrically conductive for ion milling. Thus, the
technique is suitable for metals, ceramics and other materials.
Before ion milling, the specimen is often ground with a dimple grinding device in order to
reduce the thickness in the central area of specimen.
Then, the ground specimen is cut as a 3-mm disc and placed in the ion-milling chamber
with a geometric arrangement.
The specimen is placed in the center at an angle of about 5-30° to the ion beam in order to
have a high yield of sputtering.
Light transparency is detected by a light detector aligned along the vertical direction.
66. 66
Selected-area diffraction (SAD)
A diffraction pattern is formed on the back-focal plane of the objective lens when an
electron beam passes through a crystalline specimen in a TEM.
In the diffraction mode, a pattern of SAD can be further enlarge on the screen or recorded
by a camera.
Electron diffraction is not only useful to generate images of diffraction contrast but also for
crystal structure analysis similar to XRD methods.
SAD in a TEM shows its special characteristics compared with XRD.
Selected-area diffraction characteristics
Constructive diffraction from a lattice plane (hkl) generates an intensity spot on the
screen when the TEM is in the diffraction mode.
The diffraction in the TEM is very small (θ<1°) because the reflecting lattice planes
are nearly parallel to the primary beam.
67. 67
tan 2𝜃 = 𝑟/𝐿
Since θ is very small
Also from Bragg’s law,
2𝑑 𝑠𝑖𝑛𝜃 = 𝜆
So,
Where, λ = wavelength, L = camera length, Lλ =
camera constant
L- is not a physical distance between the
specimen and camera lenses.
Distance of diffraction spot(r) from the direct
beam varies inversely as the d spacing of the
diffracting plane
68. 68
Why analyse electron diffraction patterns?
Various reasons
–To help identify an unknown material from diffraction geometry/ d spacing (similar to
X-ray Debye-Scherrer pattern)
•Knowing composition by EDS helps
–To accurately measure the camera constant of the microscope
–Unknown polycrystalline material
•Ring pattern gives info about
–crystal structure-e.g. bcc, fcc from diffraction rules; and
–Lattice spacing
•Composition from EDS
–A certain diffraction condition may be needed to obtain a particular diffraction contrast
image
–To determine exact orientation of the crystal needed to find out the habit plane of a
particular type of defect
•Defects are visible under particular diffraction conditions
69. 69
Comparison between X-ray diffraction and SAD in TEM
X-ray diffraction SAD in TEM
Scattering nature Scattering by shell electrons Scattering by atom nucleus
Wave paths Reflection (XRD) Transmission
Transmission (Laue)
Diffraction θ 0-180° 0-2°
Intensity Low 106-107 times higher
Precision High Relatively low
70. 70
Image mode (a) and Diffraction mode (b)
The image of the specimen in conventional microscopy is formed selectively allowing
only the transmitted beam (bright field imaging) or one of the diffracted beams (dark field
imaging) down to the microscope column by means of an aperture.
The origin of the image contrast is the variation of intensities of transmitted and diffracted
beams due to the differences in diffraction conditions depending on the microstructural
features on the electron path.
71. 71
Common modes of operation of TEM
Bright field (BF) microscopy
Selected area diffraction
Dark field (DF) microscopy
Bright field imaging – objective aperture stops all diffracted beams
- undeflected electrons contributes to image
Dark field imaging – displace aperture/ tilt beam to allow chosen diffracted beam to travel
down
73. 73
Schematic of electron diffraction patterns
The types of diffraction pattern arises from different specimen microstructures (a) A single
perfect crystal (b) A polycrystalline structure (7-8 grains)- the spots have now tending to
form rings (c) polycrystalline (50 grains) – spots now merged into rings.
76. 76
Indexing electron diffraction pattern
To obtain a diffraction pattern from individual crystallites, two modes can be used
SAD (selected area diffraction)- approx. 500 nm diameter
CBED (convergent beam electron diffraction)
Region as small as 10 Angstrom
2-D plot of diffraction spots
Indexing- labelling individual diffraction spots with their appropriate h, k, l values.
Identify the transmitted beam i.e. (000) diffraction
Brightest spot in the center
Index two independent non-co-linear diffraction spots nearest to the (000) spot
Linear combinations of these two vectors give indices of the remaining spots.
Specify the ‘zone axis’ (normal to the plane of the spot pattern, pointing towards the gun
by convention.
Indexing of DP- By hand, by computer program, by ‘manual’ comparison.
77. 77
Reciprocal Lattice
To understand the intensity of electron diffraction.
A set of parallel planes (hkl) in real lattice is represented by a point at a distance 1/d from
origin perpendicular to that plane.
Real lattice
Planes normal to the paper
Corresponding reciprocal lattice
A plane is represented by a
point at 1/d from origin on the
plane normal.
78. 78
Use of Reciprocal lattice in diffraction
Diffraction occurs from planes parallel to the electron beam.
Diffraction pattern consists of points, spaced at a distance (r) in a direction normal to the
planes.
Diffraction pattern is approximately a scaled section through the reciprocal lattice
normal to the beam.
Can construct DP for any crystal for any orientation of crystal.
79. 79
The Ewald sphere construction
Relationship between reciprocal lattice and
Diffraction pattern.
Diffracting crystal is represented by its
reciprocal lattice.
Electron beam by a vector of length 1/λ.
Bragg’s law is satisfied for this
construction.
Ewald sphere
80. 80
Implications
Diffraction occurs when the Ewald sphere
touches a reciprocal lattice point.
Radius of the Ewald sphere is large 270 nm-1 ,
for 100 kV electron as compared to reciprocal
lattice vectors (about 5 nm-1 )
Hence for small θ, Ewald sphere surface can
be approximated to be a plane.
Hence, a Diffraction pattern is considered to
be section through the reciprocal lattice.
The Ewald sphere surrounds the incident
beam and is affixed to it. Tilting the direction
of the incident beam is performed by tilting
the Ewald sphere by the same amount.
SAD pattern for different
orientation
81. 81
TEM provide topographical, morphological, compositional and crystalline information.
The images allow the researchers to view samples on a molecular level, making it possible
to analyse structure and texture.
This information is useful in the study of crystals and metals, in addition to industrial
applications.
TEM can be used in semiconductor analysis and the production and manufacture of
computer and silicon chips.
TEM offer the most powerful magnification of over one million times or more.
Technology based companies use TEM to identify flaws, fractures and damages to micro-
sized objects. This data can help to fix problems and help to make a more durable efficient
product.
Applications of TEM
85. 85
Cr23C6 - FCC
a = 10.659 Å
Ni2AlTi - Primitive cubic
a = 2.92 Å
• Symmetry of diffraction pattern reflects symmetry of crystal around beam direction
• All diffraction patterns are centrosymmetric, even if crystal structure is not centrosymmetric
