TEM Electron Microscopy

Transmission Electron Microscopy
I. Introduction
EMA 6518
EMA 6518
EMA 6518: Transmission Electron Microscopy
C. Wang

• Instructors:
Dr. Chunlei (peggy) Wang
EC 3463
Tel: 305-348-1217, Email: wangc@fiu.edu
http://web.eng.fiu.edu/~wangc/
Dr. Yanqing Liu
Advanced Materials Engineering Research Institute
Advanced Materials Engineering Research Institute
(AMERI)
Tel: 305-348-1371
• Time: Mon 2:00-4:00pm
• Location: MME conference room
• Office hours: by appointment
• Prerequisite: EMA 5507
C. Wang

• Reference:
Transmission Electron Microscopy
I: Basics
II: Diffraction
III: Imaging
IV: Spectrometry
by D.B.Williams and C.B.Carter
Plenum Press, New York and London, 1996
C. Wang
Plenum Press, New York and London, 1996
• Grading:
1 Written exam: 20%
Lab: 30%
Homework: 20%
Report & presentation: 30%

Extremely expensive equipment!
• A typical commercial TEM costs about $2 (up to $4-5) for
each electron volt of energy in the beam.
• Beam energy of a TEM: 100,000-40,000 eV
?
• Why use electrons?
• Why you need TEM to characterize materials?
• Advantages and Drawbacks?
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Brief History
• In 1801, Thomas Young passed a beam of light through
two parallel slits in an opaque screen, forming a pattern of
alternating light and dark bands on a white surface beyond.
This led Young to reason that light was composed of waves.
wave theory of light
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Thomas Young (1773-1829)

Brief History
• In 1897, J.J.Thomson discovered “corpuscles”, small
particles with a charge/mass ratio more than 1000 times
greater than that of protons, swarming in a see of positive
charge (“plum pudding model”).
Discovery of the ELECTRON
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Sir Joseph John Thomson
(1856-1940)
Nobel prize 1906
Thomson’s 2nd Cathode ray experiment

Brief History
• In 1924, Louis de Broglie first theorized that the electron
had wave-like characteristics. Application of the idea of
particle – wave dualism (only known for photons up to
then) for any kind of matter. (first person to receive a Nobel
Prize on a PhD thesis )
Electron=Particle & Wave
Louis Victor de Broglie (1892-1987)
Nobel prize 1929
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mv
h
p
h
=
=
λ
Electron=Particle & Wave

• In 1926, Hans Busch discovered that magnetic fields
could act as lenses by causing electron beams to
converge to a focus (electron lens).
Brief History
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Brief History
• In 1927, Davisson and Germer, Thomson and Reid,
independently carried out their classic electron diffraction
experiments (demonstration of wave nature of electrons)
Electron=Wave Electron gun
detector
θ
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Sir George Paget Thomson
(1892 – 1975)
Nobel Prize: 1937
(shared with C.J. Davison)
GP Thomson Experimental Apparatus and Results
Interference peak
Ni Crystal
θ
θ
θ
0 60
o
Davisson-Germer experiment

• In 1931, Knoll (inventor of SEM, 1935) and Ruska co-
invent electron microscope and demonstrated electron
images.
Brief History
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Max Knoll
(1897-1969)
Ernst Ruska
(1906-1988)
Nobel Prize 1986
Knoll and Ruska co-invent electron microscope

• 1938: M. von Ardenne: 1st STEM
• 1936: the Metropolitan Vickers EM1, first commerical
TEM, UK
– 1939z: regular production, Siemens and Halske,
Germany
– After World War II: Hitachi, JEOL, Philips, RCA, etc
• 1945: 1nm resolution
Brief History
• 1945: 1nm resolution
• 1949: Heidenreich first thinned metal foils to electron
transparency
• Cambridge group developed the theory of electron
diffraction contrast
• Thomas pioneered the practical applications of the TEM
for the solution of materials problems (1962)
• ……
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Microscope
– Bright-field microscope
– Dark-field microscope
– Phase-contrast microscope
– Fluorescence microscope
– Confocal microscope
•≥2 optical lenses
•Resolution: wavelength of light
laser
UV, violet, or blue light
– Confocal microscope
– Scanning Electron Microscope (SEM)
– Transmission Electron Microscope (TEM)
– Scanning Probe Microscope (SPM):
• Atomic force microscope (AFM)
• Scanning tunneling microscope (STM)
laser
Electron beam
Constant distance
Constant current
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Optical vs. Electron Microscopy
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First Place, Nikon'
s Small World 1995
Competition, Christian Gautier, Larva of
Pleuronectidae (20x), Rheinberg Illumination
and Polarized Light
First Place Winner, Nikon'
s Small World
2005 Competition, Charles B. Krebs,
Muscoid fly (house fly) (6.25x)
Reflected light
and Polarized Light
• Easy to use
• Samples in air or water
• Total magnification: ×100-1000
product of the magnifications of the
ocular lens and the objective lens
• Image processing by CCD
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SEM
!
!!
"#
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(a) Optical micrograph of the radiolarian Trochodiscus
longispinus (Skelton of a small marine orgnism). (b) SEM
micrograph of same radiolarian. (Taken from J.I. Goldstein
et al., eds., Scanning Electron Microscopy and X-Ray
Microanalysis, (Plenum Press,NY,1980).)
C. Wang

• Resolution
• Depth of Focus
Why electrons?
• Depth of Focus
Our eyes: 0.1-0.2 mm
Optical microscope: 400-700nm, resolution?
Electron microscope:100-1000 keV, resolution?
Resolution:
"The best possible resolution that can be resolved with a
light microscope is about 2,000 Angstroms" --Slayter, Elizabeth.
Microscope. Grolier Multimedia Encyclopedia Online. Grolier, 1998.

Diffraction
• Image formed by a small circular aperture (Airy disk) as an example
• Image by a point source forms a circle with diameter 1.22λ
λ
λ
λf/d
surrounded by diffraction rings (airy pattern)
• Diffraction is usually described in terms of two limiting cases:
Fresnel diffraction - near field.
Fraunhofer diffraction - far field.
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• Rayleigh suggested that a reasonable criterion for resolution
(R = distance between A and B) is that the central maximum
of one point source lies at the first minimum of the Airy
pattern of the other point (R = diameter of circle)
Rayleigh resolution
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• The numerical aperture (NA) of a lens represents the
ability of the lens to collect diffracted light and is given by
NA = n sin in this expression n is the index of refraction
of the medium surrounding the lens and a is the
acceptance angle of the lens ( n = 1 for air)
Rayleigh resolution
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Objects to be
resolved
diffraction
disks
D
a
a
d
f
• The ability to “resolve” tiny objects improves as the wavelength
decreases. Consider the microscope objective:
Why electrons?
disks
f
A good microscope objective has f/D
A good microscope objective has f/D ≅
≅ 2, so with
2, so with ~ 500
~ 500
nm the optical microscope has a resolution of d
nm the optical microscope has a resolution of dmin
min ≅
≅ 1 µm
1 µm.
.
D
f
.
f
d c
min λ
α 22
1
=
≈
Critical angle for
resolution:
The minimum d for which we
can still resolve two objects
is ac times the focal length:
D
c
λ
α 22
.
1
=
(not interference
maxima)
= focal length of lens if image
plane is at a large distance.
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• Wave Behaviors
– images and diffraction patterns
Resolution of Electron microscope: ?
λ
λ
λ
λ
λ
λ
λ
λ
Why electrons?
– images and diffraction patterns
– wavelength can be tuned by energies
• Charged Particle Behaviors
– strong electron-specimen interactions
– chemical analysis is possible
λ
λ
λ
λ
λ
=
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C. Wang

• The DeBroglie wavelength of an electron:
Wavelength of an Electron
λ
λ
λ
λ = h/p
• the relation between the electron’s wavelength and its
kinetic energy E.
p and E are related through the classical formula:
2
-31
p
E m 9.11 10 kg
= = × -31
e
2
-15
2
p
E m 9.11 10 kg
2m
h
E h 4.14 10 eV s
2m
= = ×
= = × ⋅
λ
nm
eV
.
E 2
2
505
1
λ
⋅
=
E in electron volts
λ
λ
λ
λ in nanometers
λ
nm
eV
Ephoton
⋅
=
1240
Don’t confuse with for a photon !
For m = me:
(electrons)
C. Wang

• For a 100 keV electron:
λ
λ
λ
λ ∼0.004nm (4pm)
• BUT nowhere near building TEMs
that approach this wavelength limit
of resolution, because we can’t
make perfect electron lenses.
Why electrons?
C. Wang
make perfect electron lenses.
• HRTEM
• HVEM: 1-3 MV (1960s)
300-400 kV (1980s), very high
resolution close to that achieved at
1 MV

• You wish to observe a virus with a
diameter of 20 nm, which is much
too small to observe with an optical
microscope. Calculate the voltage
required to produce an electron
Imaging a Virus*
Electron
optics
D
electron gun
required to produce an electron
DeBroglie wavelength suitable for
studying this virus with a resolution
of dmin = 2 nm. The “f-number” for
an electron microscope is quite
large: f/D ≈ 100.
object
f
C. Wang

Solution
D
f
.
dmin λ
22
1
≈
nm
.
f
.
D
nm
f
.
D
dmin 0164
0
22
1
2
22
1
=
=
≈
λ
( )
eV
k
.
nm
.
nm
eV
.
m
h
E 6
5
0164
0
505
1
2 2
2
2
2
=
⋅
=
=
λ
f
.
f
. 22
1
22
1
To accelerate an electron to an energy of 5.6 keV requires
5.6 kilovolts .
C. Wang

• Depth of Field of a microscope is a measure of how
much of the object we are looking at remains “in focus”
at the same time.
• Depth of field is governed by the lenses in the
Depth of Focus & Depth of Field
• Depth of field is governed by the lenses in the
microscope.
C. Wang

• Depth of focus is a lens optics concept that measures
the tolerance of placement of the image plane (e.g. film
plane in a camera) in relation to the lens.
C. Wang
At f/32, background is distracting Shallow DOF at f/5 isolates flowers
from the background.
DOF = k2 λ
λ
λ
λ /(NA)2
• The very small angular aperture of the electron probe forming
system permits a large depth of field all in focus at once.

• We have to use very small limiting apertures in the lenses,
narrowing the beam down to a thin “pencil” of electrons (few
micronmeters across).
In SEM, to produce 3D-like images
In TEM, usually in focus at the same time, independent of the
specimen topography (as long as it’s electron transparent)
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“depth of field” refers to the specimen
“depth of focus” refers to the image

Interaction of high energy (~kV)
electrons with (solid) materials
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Electron Beam-Specimen Interactions
----------------Visualizing the interaction volume
•The interaction volume can be
observed in certain plastic
materials such as PMMA
•Undergo Molecular bonding
damage during electron
(Everhart et al., Proc. 6th Intl. Conf. on X-ray Optics and Microanalysis)
• Polymethylmethacrylate (PMMA)
• e-beam: 20 keV, ~ 0.5 m
damage during electron
bombardment that renders the
material sensitive to etching in a
suitable solvent
•This phenomenon is the basis for
EB lithography
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Electron Beam-Specimen Interactions
• EB lithography
C. Wang
Kartikeya Malladi, Chunlei Wang, and Marc Madou, “Microfabrication of Suspended C-MEMS
structures by EB Writer and Pyrolysis”, Carbon, 44(13), (2006) 2602-2607

kV and keV
• With kV is meant the high voltage. If an electron was
accelerated in this electrical field with e.g. 20 kV, the
electron has finally an energy of 20 keV.
• for SEM, acceleration voltage is the high voltage applied
• for SEM, acceleration voltage is the high voltage applied
to the filament. Acceleration voltage ranges from 100V
to 70kV (up to 100kV). Low acceleration voltage means
<1 kV.
C. Wang

• SEM permits the observation and characterization of
heterogeneous organic and inorganic materials on a nm
to µm scale.
» Imaging capabilities
» elemental analysis
• In the SEM, the area to be examined or the microvolume
to be analyzed is irradiated with a fine focused electron
Scanning Electron Microscope
to be analyzed is irradiated with a fine focused electron
beam, which may be swept in a raster across the surface
of the specimen to form images or maybe static to obtain
an analysis at one position.
• The types of signals produced from the interaction of
electron beam with the sample include secondary
electrons, backscattered electrons, characteristic x-rays,
and other photons of various energies.
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Interaction of high energy (~kV)
electrons with (solid) materials
Cathodoluminescence
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What is Cathodoluminescence?
• Cathodoluminescence is an optical and electrical
phenomenon whereby a beam of electrons is generated by an
electron gun and then impacts on a luminescent material such
as a phosphor, causing the material to emit visible light. The
most common example is the screen of a television.
• Cathodoluminescence occurs because the impingement of a
high energy electron beam onto a semiconductor will result in
the promotion of electrons from the valence band into the
the promotion of electrons from the valence band into the
conduction band, leaving behind a hole. When an electron
and a hole recombine, it is possible for a photon to be
emitted. The energy (color) of the photon, and the probability
that a photon and not a phonon will be emitted, depends on
the material, its purity, and its defect state. In this case, the
"semiconductor" examined can, in fact, be almost any non-
metallic material. In terms of band structure, classical
semiconductors, insulators, ceramics, gemstones, minerals,
and glasses can be treated the same way.
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Interaction of Electrons with Matter
• Electrons are one type of “ionizing radiation”---capable of
removing one of the tightly bound inner-shell electrons from
the attractive field of the nucleus.
• “Ionizing radiation” produces many of the secondary signals
from the specimen are used in “analytical electron
microscopy” (AEM)
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microscopy” (AEM)

Abbreviations
• HEED: high energy electron diffraction
• LEEM: low energy electron microscope (many
variations with special names)
• EELS: electron energy loss spectroscopy
• EDXS: energy dispersive X-ray spectroscopy
• SEM: scanning electron microscope (electrons
• SEM: scanning electron microscope (electrons
do NOT normally transmit the sample)
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Interaction of Electrons with Matter
• Modern TEMs are very good signal-generating instruments.
• Electron beam: typically <10 nm and at best <1nm
• Combining TEM and SEM STEM
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Diffraction
• Electron diffraction is an indispensable part of TEM and is
the most useful aspect of TEM for materials scientists.
•Crystal structure, lattice repeat distance, specimen shape, point-
group and space-group determination, etc.
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Limitations of the TEM
• Sampling---0.3mm3 of materials
• Interpreting transmission images---2D images of 3D
specimens, viewed in transmission, no depth-sensitivity.
• Electron beam damage and safety---particularly in polymer
and ceramics
• Specimen preparation---”thin” below 100nm
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Different Kinds of TEMs
A wide variety of types:
• HRTEMs
• HVEMs
• IVEMs (intermediate high
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• IVEMs (intermediate high
voltage electron
microscopes,400kV)
• STEMs
• AEMs

Some Fundamental Properties of Electrons
• Typical electron beam current in a TEM is 0.1-1µA, which
corresponds to 1012 electrons passing through the
specimen plane.
• With 100-keV engery, these electrons travel at about 0.5c
(1.6 108 m/s), so they are separated by 0.16 cm and this
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(1.6 108 m/s), so they are separated by 0.16 cm and this
means that there is never more than one electron in the
specimen at any one time.
• Electron diffraction and interference occur, both of which
are wave phenomena, and imply interaction between the
different electron beams.

Some Fundamental Properties of Electrons
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TEM Electron Microscopy

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