Transcript LxxA, Overview of Microscopy methods, part a

Microscopy: Overview of Different
Methods
EML 5930 (27-750)
Advanced Characterization and Microstructural
Analysis
A. D. Rollett, P.N Kalu
Spring 2008
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(MACRO)TEXTURE
Review:
Recall that X-ray texture (macrotexture):
• Provides an overview of the crystallographic texture of
material - only texture information is obtained.
• Provides the volume fraction of the specimen, which has a
particular orientation.
• Does not tell how grains are distributed in the material.
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• Lack of a direct connection between the study of texture
and microstructure.
• Parallel but separate investigations are needed in order to
obtain microstructural data.
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MICROTEXTURE
• Microtexture technique provides concurrently the spatial
location and the orientation of individual grains in a
sample.
• This technique can be referred to as the modern approach
to texture investigation.
• Until recently, the orientation of individual crystals can
only be determined by Selected Area Diffraction Pattern
(SADP) technique on TEM - very tedious.
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• Phenomena that can be investigated using microtexture:
– Effect of property variation as a function of orientation
– Misorientation between neighboring grains or distribution of
grain boundary geometry, which can result in the grain
boundary/property relationship.
– Correlations between geometrical and orientational
parameters of the grains
– Orientation variations within individual grains
– Phase Identification/Relationships
– Direct ODF measurement
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• Macrotexture or Microtexture analyses techniques rely on
the diffraction of radiation by a crystal lattice.
• The exploring radiation can be used as an experimental
tool for microtexture measurement only if the probe size
is smaller than the microstructural unit.
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Diffraction:
• Crystal structure analysis is usually based on diffraction
phenomena caused by the interaction of matter with X-rays,
electrons, or neutrons.
• Therefore, when either X-rays or electrons interact with
crystalline material, they are:
(a) Subject to diffraction – have similar wave properties.
(b) Monochromatic radiation - produce a series of strongly
diffracted beams leaving the crystal in defined and
predicted directions.
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• The resultant diffraction pattern is given by Braggs Law,
and this is given by
n  2d sin 
where,
d:
:
n:
:
……..
1
Interplanar spacing
Grazing angle of incidence (Bragg angle)
Integer (0, 1, 2, 3 ….. )
Wavelength of the incident electrons
• Note
– With diffraction, we use Reciprocal lattice in which sets
of lattice planes are represented simply by a set of points
in reciprocal space.
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The Reciprocal Lattice
• Very useful in metric calculations
• Let a, b, c be the elementary translations of a space lattice
(direct lattice)
• A second lattice, reciprocal to the first one, is defined by
translations a*, b*, c*, which satisfy the following
conditions:
(2a)
a*. b = a*. c = b*. a = b*. c = c* . a = c* . b = 0
and
(2b)
a*. a = b*. b = c*. c = 1
• Equation 2a suggests that a* is normal to the plane (b,c),
b* is normal to the plane (a,c), and
c* is normal to the plane (a,b),
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• The magnitude and sense of a*, b*, c* are fixed by (2b)
• According to (2a), a* may be written as:
(3a)
a* = p (b ^ c)
where p is a constant.
The value of p is obtained by if the scalar product of both
sides of (3a) by a is taken:
a* . a = 1 = p (b ^ c . a) = pV
• Therefore, p = 1/V, and equation (3a) can be written as:
a* = { (b ^ c)}1/V ………
(3b)
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Table 1. The Characteristics of Light and Various Radiations Used for
Texture Measurement by Diffraction.
Light
Wavelength [nm]
400-700
Energy [eV]
1
Charge [C]
0
Rest mass [g]
0
Penetration depth [mm] 0
Neutrons
0.05 - 0.3
1.00E-02
0
1.67E-24
10 - 100
X-rays
0.05 - 0.3
1.00E+04
0
0
0.01 - 0.1
Electrons
0.001 - 0.01
1.00E+05
-1.60E-19
9.11E-28
1.00E-03
• Electrons are the only radiation in which their penetration
depth and interaction volume is small enough to allow
diffraction from individual grains (very small volume).
• Therefore, only electrons can be used for MICROTEXTURE
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• Microtexture technique can either be TEM-based or SEMbased.
• Until mid-1980, the TEM-based was the major microtexture
technique, although an SEM-based Selected Area
Channeling technique was available.
• Modern SEM-based technique known as can now be
classified into two:
– manual
– automated
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Principles of Electron Microscopy
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MICROSCOPY
Introduction:
• Several new microscopy-based microcharacterization
techniques have been developed over the last four decades,
which have significantly extended the ability to study the
microstructure of materials.
• In addition to Optical Metallography, there is a range of
Electron Optical techniques.
• Electron microscope (developed in 1931) was initially used
for the study of biological systems, but thin foil techniques in
the mid-1950’s enabled microstructural investigations to be
undertaken on metals and alloys.
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Typical Information from Electron Microscope:
• Chemical composition of materials can be obtained using
electron microprobes to produce characteristic X-ray emissions
and electron energy losses.
• Imaging (surface) can be characterized using secondary
electrons, backscattered electrons, photo-electron, Auger
electrons and ion scattering.
• Crystallography or crystal structure information can be
obtained from backscattered electrons (diffraction of photons or
electrons).
The various studies of materials exploit at least one of the
above information, as well as the excellent spatial
resolution of electron microscopes.
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Figure 1. Summary of the various signals obtained by interaction of electrons
with matter in an electron microscope
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Comparison between Optical and Electron
Microscopy:
• In many ways, electron microscopes (Scanning and
Transmission) are analogous to light microscopes.
• Fundamentally and functionally, electron microscopes
(EM) and optical microscopes (OM) are identical.
• That is, both types of microscopes serve to magnify
minute objects normally invisible to the naked eyes.
• Basically, component terminology of an EM is similar to that
of an OM. Both microscopes consist the following (see
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Figures 2 and 3):
Figure 2. A simple optical, transmission microscope system
comprising a condenser and objective lens.
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Figure 3. Comparison of image formation.
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(a) Source of Illumination as light source
–Electron Gun produces an electron beam by thermionic or
field emission - EM
–Lamp produces light beam (including uv rays) - OM
(b) Condenser Lens system projects a near parallel radiation
on to the specimen
–Electro-magnets of variable focal length are the lenses in
EM.
–Curved transparent substance - OM
(c) Series of Imaging Lenses form the Image of the specimen
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• Although (a) to (c) above address the basic differences
between the two types of microscopes, a detailed
comparison is provided in Table 1.
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TABLE 1. EM and OM Comparison Chart
PARAMETERS
Illuminating Beam
Wavelength
Medium
Lens
Resolving Power
Aperture Angle
Magnification
Focusing
Contrast
Sample Type
Information
OPTICAL MICROSCOPE
ELECTRON MICROSCOPE
Light Beam
7,500Å (visible)
~2,000Å (ultraviolet)
Electron Beam
0.859Å (20 kV)
~0.0370Å (100 kV)
Atmosphere
Optical lens
(glass)
Visible: 3,000Å
Ultraviolet: 1,000Å
Vacuum
Electron Lens
(magnetic or electrostatic)
Point to point: 3Å
Lattice: 1.4Å
70o
10x ~ 2,000x
(lens exchange)
~35’
90x ~ 800,000x
(continuously variable)
Mechanically
Absorption,
Reflection
Electrically
Scattering absorption -SEM
Diffraction, phase - TEM
Bulk sample - SEM
Bulk sample
Thin foil ( 3 mm dia. and electron transparent,
i.e. 1000 atoms in thickness) -TEM
Grain size and shape
Grain size and shape
Distribution of phases (particles) Distribution of phases (particles)
Chemical composition, e.g Identify phases
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Crystal and defect structure
ELECTRON MICROSCOPES
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ELECTRON EMISSION
• The liberation of electrons from the surface of a solid into
vacuum.
• The process of electron emission is similar to that of ionization of
a free atom
– The energy of the electrons in an atom is lower than that of an
electron at rest in vacuum; consequently, in order to ionize an atom,
energy must be supplied to the electrons in some way or other.
• A solid can only emit electrons if some of the electrons have
energies equal to, or larger than, that of an electron at rest in
vacuum. This may be achieved by various means, such as by
– heating, irradiation with light (photoemission),
– bombardment with charged particles (secondary emission), or
– using of a strong electric field (field, or cold, emission).
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ELECTRON SOURCES
• Electron sources in electron beam instruments are required
to provide either
– a large total current beam of about 50 m diameter - low
magnification TEM, or
– a high intensity probe of electrons as small as 0.5 nm in
diameter - SEM
• There are three different types of electron source available
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a) Thermionic tungsten hairpin filament
This is usually heated to about 2800 K by direct
resistance heating. The surrounding grid, known as
the Wehnelt cylinder, and the anode, which is at earth
potential, act as an electrostatic lens.
For an operating condition of 100 kV, the brightness is
about
3 x 105 A cm-2 sr-1.
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(a)
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(b)
Figure 4. Schematic diagram of a conventional tungsten thermionic
source. (a) the filament F and Wehnelt cylinder (b) schematic ray path
showing focusing action.
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b) Lanthanum hexaboride crystal (LaB6)
The only difference between the conventional assembly
illustrated in Figure 4 and a modern LaB6 assembly is that extra
pumping holes are present in the Wehnelt cap to ensure a better
pumping speed near the tip.
Higher current (greater than the tungsten) is obtainable in small
probes.
The brightness of a LaB6 can be as high as 107 A cm-2 sr-1 at
100kV.
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c) Field emission source
• The emission of electrons from a metal or
semiconductor into vacuum under the influence of a
strong electric field.
• Field emission - electrons tunnel through a potential
barrier, rather than escaping over it as in thermonic
emission.
• The effect is purely quantum-mechanical, with no
classical analog.
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• In most cases, it is a <111> orientation crystal of
tungsten, and a Wehnelt cylinder, which is raised to an
extraction potential up to about 4 kV in order to cause
emission from the tip of the crystal.
• There is a requirement of high vacuum for this source
• The brightness of cold or thermal emission source can
be about 104 times of a conventional tungsten filament.
• The high brightness of this source make them preferable
for scanning instruments.
• Cold, thermal or Schottky
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Schotty Field Emission Source Electron
Gun for SEM
• Source:
– Schotty field emitter (ZrO/W
– High brightness
– Highly-stable electron beam
• High current density
• 100 nm spot size at 5 nA sample current
• The field emission source – current density can be
maximize and still produce a 100 nm spot size at 5 nA.
– Optimised for Auger analysis at very high spatial
resolution with SEM.
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ELECTRON LENSES AND OPTICS
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• Electron lenses in microscopes are generally electromagnetic.
• There are three types of magnetic lenses in uses (refer to
Figure 5):
(a) a multi-layer coil; i.e., an air-core solenoid
coil
(b) a coil enclosed by soft iron plates (in order to
reduce leakage flux) containing a gap (in order to
concentrate the induction field) and
(c) a coil enclosed by soft iron plates containing a
gap and internal soft iron pole pieces (in order to ensure a
high intensity magnetic field)
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Figure 5. Types of magnetic electron lenses.
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• Almost all modern electron microscopes use pole pieces
for high resolving power and high magnification.
• The function of such an electron lens is more or less the
same as that of horse-shoe magnets symmetrically
arranged about an axis.
• Accordingly, all the parallel electron beams incident to
the curved magnetic field converge at one point.
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ELECTRON OPTICS
• The action of a magnetic field on an electron is described
by a well-known right hand rule where thumb, first and
second finger are used to represent the terms in a vector
product
• The force F which an electron of charge -e experiences
when travelling with velocity v, due to a magnetic field B,
is given by:
F   e(v  B)
…………(4a)
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Figure 6. Schematic diagram of the action of a cylindrical
magnetic lens on the path of non-axial electrons.
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and the magnitude of the force is then given by
F  evB sin 
…………..(4b)
where  is the angle between B and v.
If the initial velocity of an electron is divided into two
components, vp parallel to B and vo perpendicular to B, then
the value of vp will be unchanged by B (since  will be zero)
and the force resulting from B and vo will result in a circular
motion of the electron about B (see Figures 7 and 8).
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Figure 7. Electrons passing through magnetic lens
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• Figs. 7 & 8 shows the trajectory of an electron passing
through such a magnetic field.
• Although the electron beam path in a magnetic lens is not
the same as the light ray path in an optical lens, the results
are similar.
• As shown in Figs. 7 & 8, the electron travels rectilinearly,
crosses the axis, moves through the magnetic field along a
spiral orbit, approaches the axis, crosses the axis again,
and travels rectilinearly.
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Figure 8. Schematic diagram showing the trajectory of an
electron through a magnetic lens.
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The radius r of this spiral motion is given by:
mv
r
sin a
eB
……….(4c)
Since generally, in electron microscopes, electron beams
near the axis are used for forming an image, a is extremely
small.
This effect is similar to the converging action of an optical
convex lens, and if the revolution of the electron about the
axis is omitted, the converging action of an electron lens can
be considered to be identical to that of an optical lens.
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• A magnetic lens containing pole pieces magnetized to
near-saturation for concentrating magnetic flux in a very
narrow space constitutes a thin lens.
• The focal length f and rotation angle  are given as
follows:
1   2
f


where,

8V 

B  x dx


8V 
…….. 5
B( x)dx
V => Accelerating voltage
 = specific charge of electron e/m
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RESOLUTION OF LENS
• Resolution defines the smallest separation of two points in
the object, which may be distinctly reproduced in the image.
Light Microscope: The resolving power for light microscope
is determined by diffraction aberration and can be defined
as 1,2
k

n sin a
……….. 6
• where  is the wavelength of the illumination,
n is the refractive index of the medium in front of the lens,
a is the semi-angle (aperture angle) subtended by the object at the lens
NA = n sinα = numerical aperture = measure of light-gathering ability
= 0.95 (max. with air). Higher (~1.515) for oil-immersion objtvs.
k is a constant usually taken to be 0.61.
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Optical Microscope –
 = 50 nm (for white light Illumination)
n sin a = 0.135 (when fitted with an oil immersion lens)
Therefore, it is possible to achieve a resolution of about
250 nm in Optical Microscopes.
• Filters can also be used to enhance the resolving power of
an objective. For light:
– The shorter wavelengths are at the violet-blue-green end of
the spectrum
– The higher wavelengths are at the orange-red of the
spectrum.
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Electron Microscope –
• De Broglie relationship relates the wavelength of
electrons, , to their momentum, mv (m is the mass and v
is velocity), by h – Planck’s constant, such that:
h

mv
……… 7
• Since electrons are accelerated by a potential difference
of V volts and have a kinetic energy K, where K is given
as –
1
K  mv 2
2
……...
8
Therefore,
mv 2  2eV
………
9
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By considering equations (7) and (9), we have –
h

1/ 2
2meV 
……. 10
The energy term eV is expressed in electron volts and
represents that energy required to pass an electron through
a potential difference of one volt ( 1 eV = 1.602 x 10-19 J).
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When the velocity of the electron approaches the speed of
light, v c, a relativistic correction is required for the
voltage, such that

eV 
V  V 1 
2
2
m
c

o 
…………………..
11
where mo is the mass of the electron at rest. It is important
to use this correction for cases when V  105 volts. Table 2
presents a chart of electron wavelength in relation to
applied voltage.
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Table 2. Variation of Electron Wavelength with applied voltage
Applied Voltage (keV) Wavelength, nm)
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0.008588
50
0.005355
80
0.00418
100
0.003702
200
0.002508
500
0.001421
1000
0.00872
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• If diffraction aberration is considered (see equation (6)),
the resolving power of 100 and 300 keV electron
microscopes will be about 0.0025 nm and 0.0017 nm
respectively. These values are not realistic!
• The ultimate resolution of an electron microscope
is dictated by the defects in the imaging system
rather than by the wavelength (diffraction
aberration) of the radiation employed.
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Lens defects:
• Chromatic aberration is related to energy losses within the
specimen. This is generally of little importance except for
very thick regions of thin foils or bulk specimens.
• Astigmatism may arise from
– intrinsic defects in the objective lens, or
– from contamination on the lens and the objective
aperture.
Microscope manufacturers have developed methods for
correcting this defect
– small set of coils are used to produce uniform magnetic
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field.
• Spherical aberration is the main factor, which limits the
performance of electromagnetic lenses used in microscopes.
– It is a function of the lens design and the acceptance
angle (a) for electrons entering the lens.
– This angle must be kept at a minimum, however this
decreases the resolution limit due to diffraction. The
best resolution is accomplished with a compromise
value of a, and is given by
 min  3 / 4cs1/ 4
and a  1/ 4cs 1/ 4 ………
12
– where cs is the coefficient of spherical aberration, and
min is typically less than 3 Å on modern microscopes.
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Current state-of-the-art TEM
capability
1.
2.
3.
4.
5.
Analytical Probe sizes less than 1A
TEM resolution 0.8A or better
Holographic reconstructions less than 1A
FEG: cold/thermal/Schottky
Energy resolution in EELS less than 20meV – more
typical values less than 100meV – routine values of 700
– 800meV. Monochromators help.
6. WDS resolution less than 1 eV (1000meV)
7. CCD Camera images 4K x 4K (8K under development
at UCSD-commercial version just delivered to Fujiyoshi)
8. Contemporary computer control – contemporary means
TODAY
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Current state-of-the-art TEM
capability
9. Accelerating voltages up to 3.0 MeV
10. A variety of energy filters: Omega, Mandoline, Gamma,
single-sector (GATAN)
11. Goniometer Automation (searching, recording, tilting
(tomography/kspace tilting and navigation))
12. Detector Automation - autoinsertion, autoconfiguration of
instrument modes
13. Piezo elements for nanopositioning
14. SPM – Nanofactory
15. Remote control – taking the operator away from the
microscope
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Resolution/Cs Correction
•
Before the Cs corrector became a practical invention,
the TEM/STEM went through the final transition to an
FEG instrument capable of uncorrected extended
information limit and smaller, higher current probes.
•
This happened in the early part of 1990 and set the
stage for the development of the TEM/STEM Cs
corrector.
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Directions for the future:
•
Areas of Future directions for Improvements include:
resolution (image/analytical – spatial/energy),
computer control, automation, vacuum, in-situ
capability, remote control
•
The question is “How will these improvements be
effected?”
Most instruments today are available with a variety of
resolution enhancers:
• Cs correctors for TEM and STEM image and
analytical resolution
• Monochromators for improved energy resolution.
•
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Directions for the future:
Other subjects to consider include:
A. Tomography: Image series acquisition, Image
reconstruction, and 3D rendering.
B. Lorentz: objective minilens with a resolution of
about 2nm that can be used as an objective
lens
C. Remote control
D. Data management
E. EDS and EELS/Imaging, and
F. Integration of all functions.
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Today Electron Microscopes represent stateof-the-art technology incorporating:
• Clean High vacuum and Ultra-high vacuum
• Extreme electronic stability - lenses and high
voltage
• Machining accuracy to 1um or better, and
• Unsurpassed mechanical stability
They remain the highest lateral spatial resolution
spectroscopy devices in the world
They can currently resolve sub-angstrom features and can
perform spectroscopy on single atomic columns or even
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single atoms.