Lecture Module #14
Introduction to Electron Microscopy
An overview
What are electron microscopes?
• Scientific instruments that use a focused beam of
electrons to examine objects on a very fine scale.
What is electron microscopy?
• Electron microscopy is the science and technology of
using an electron beam to form a magnified image.
• Advantages:
– The use of electrons rather than light provides a nearly 1000
fold increase in resolving power (i.e., ability to focus fine
details) over light.
• Disadvantages:
– High cost
– Time commitment
– Small areas of analysis
Magnification
• Refers to how large an object can be made (and still
resolved).
magnification = size of image
size of object
• Advantages:
– You get higher magnification with electron microscopes
techniques than you can with light.
– Smaller wavelength Æ higher resolving power.
Resolution is:
• The closest distance between two points that can
clearly be resolved as separate entities through the
microscope.
Resolution is defined by:
↓
Intensity
Consider light as
EM energy
transmitted as a
wave motion.
Abbe relationship
d
0.61λ 0.61λ
ro = 1 =
=
NA
2 n sin α
λ = wavelength of illuminant
α = semi-angle
We can consider
light as a series of
ripples impinging
upon an obstacle.
ro
Distance
d1
n = index of refraction
NA = numerical aperature
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
Depth of Field
Important Terms
• Depth of Field
• A measure of how much of the object that we are
looking at remains in focus at the same time.
– Height above and below the plane of focus that an image
remains sharp.
– DOF is a function of magnification, α, and probe size
• Advantages:
BEAM
α
– You get higher depth of field with (many) electron
microscopy techniques than you do with light.
Scan
Plane of focused image
DOF
– WHY?
Region of image
in focus
SEM
TEM
Other instruments we have or are getting
• DualBeam Focused Ion Beam (DB-FIB)
– Nano-machining platform
• X-ray photoelectron spectrometer / Auger (XPS/Auger)
EPMA
What do we get out of electron
microscopes?
• ???
Comparison of Selected Characteristics of Light
and Electron Microscopes
FEATURE
• Topography
– Surface features of an object. “How it looks.”
• Morphology
– Size and shape of particles making up object.
• Composition
– Relative amount of elements and compounds making up the
object.
• Structure
– Crystallography. How atoms are arranged in the object
Light Microscope
SEM
Uses
Surface morphology
and sections (1-40 µm)
Surface morphology
Source of
Illumination
Best resolution
Magnification range
Depth of field
Visible light
High-speed electrons
200 nm
10-1,000×
0.002-0.05 nm
(NA=1.5)
Glass
On eye by lens
3-6 nm
20-150,000×
0.003-1 mm
Lens type
Image rayformation spot
Information
generated
Limiting Factor
Phases
Reflectivity
Wavelength of light
Electromagnetic
On CRT by scanning
device
Topography
Composition
Crystal orientation
Brighness,
signal/noise ratio,
emission volume
TEM
Sections (40-150 nm)
or small particles on
thin membranes
High-speed electrons
0.2 nm
500-500,000×
0.004-0.006 mm
(NA=10-3)
Electromagnetic
On phosphorescent
screen by lens
Crystal structure
Crystal orientation
Defects
Composition
Lens quality
Adapted from: Scanning and Transmission Electron Microscopy, S. L. Flegler, J.W. Heckman Jr., K.L. Klomparens,
Oxford University Press, New York, 1993.
All of this information is related to
properties.
History of electron microscopes
• Developed due to limitations of light optical
microscopes (LOMs)
– LOM: ~1000x magnification; 0.2 µm (200 nm) resolution
properties
structure
• Transmission Electron Microscope (TEM) was
developed first.
processing
– M. Knoll and E. Ruska, 1931
– Patterned “exactly” like a LOM. Uses electrons rather than
light.
• Scanning Electron Microscope (SEM)
performance
– 1942
Mechanical, Physical, Thermal, Optical, Electrical, etc…
Goodhew, Humphreys & Beanland
Electron Microscopy and Microanalysis, 3rd Edition
Taylor & Francis, London, 2001
70X
How do electron microscopes work?
• Form a stream of electrons in the electron source
(thermionic emission) and accelerate them towards
the specimen using a positive electrical potential.
300X
Electron microscopy
provides significant
advantages over light
optical microscopy in
terms of resolution
and depth of field
• Use apertures and magnetic lenses to focus the
stream into a thin focused monochromatic beam
1400X
• Focus the beam onto the sample using another
magnetic lens.
2800X
• Interactions occur inside the irradiated area of the
sample. Collect results of interactions in a suitable
detector and transform them into an image (or
whatever you are interested in).
Electron Source
(i.e., electron gun)
The Electron Gun
• The electron gun provides an intense beam of high
energy electrons.
• There are two main types of gun. The thermionic
gun, which is the most commonly used, and the field
emission gun.
• Electrons are generated and emitted from a filament
by thermionic emission (W or LaB6) or from a sharp
tip by field emission (single crystal of W).
Thermionic electron gun
most widely used (~$70K-$150K)
Electrons are emitted
from a heated
filament and
accelerated towards
an anode
W Filament
Bias
resistor
Wehnelt cap
(negative potential)
Space charge
Electron
Beam
–
10-1000 kV
+
Crossover
Anode Plate
(positive potential)
• The electrons are accelerated through a potential
difference.
Wehnelt
A biased grid with a potential that is
a few hundred volts different than
the filament (cathode). This helps
to accelerate the electrons and
causes their paths to cross over.
W Filament
Wehnelt cap
Crossover
The effective source of
illumination for the
microscope. The size
is critical for high
resolution
applications.
(negative potential)
Space charge
Electron
Beam
Filament
Made from a high Tmp material
with a low work function (φ) in
order to emit as many electrons
as possible. The work function
is the energy needed by an
electron to overcome the barrier
that prevents it from leaking out
of the atom. φW = 4.5 eV, φLaB6 =
3.0 eV.
Ground
Operation of thermionic gun
•
Apply a positive electrical potential to the anode
•
Heat the cathode (filament) until a stream of electrons is
produced
Bias
resistor
–
10-1000 kV
+
Crossover
Anode Plate
– >2700 K for W
•
Apply a negative electric potential to the Wehnelt
– electrons are repelled by the Wehnelt towards the optic axis
•
Electrons accumulate within the region between the filament tip
and the Wehnelt. This is known as the space charge.
•
Electrons near the hole exit the gun and move down the column
to the target (in this case the sample) for imaging.
(positive potential)
Ground
Anode
Positively charged metal plate
at earth potential with a hole in
it. It accelerates the electron
beam to the high tension
potential.
Other Types of Electron Sources
(i.e., electron guns)
The function of the electron gun is:
Lanthanum hexaboride (LaB6)
Higher brightness than W. More $$$ (~$500K)
Field emission source
Highest brightness. Even more $$$ (~$750K)
ADVANTAGES
Provide higher current density → Higher brightness
To provide an intense beam of high
energy electrons
There are two main types of gun. The
thermionic gun, which is the most
commonly used, and the field emission
gun.
Brightness, B, is the beam
current density per unit solid
angle.
B = jcπβ2
jc = current density
β = convergence angle
http://www.matter.org.uk/tem/electron_gun/electron_sources.htm
Beam-Specimen Interaction
• This is what makes electron microscopy possible.
• Electrons strike the sample leading to a variety of reactions.
Interaction Volume
• Represents the region penetrated by electrons
– Signals must escape the sample to be detected
Incident high kV beam of
electrons
Secondary e(SE)
Backscattered
e- (BSE)
Auger e-
Characteristic
x-rays
Incident electron beam
Visible light
Bulk (SEM)
Absorbed e-
e- - hole pairs
Foil (TEM)
Bremsstrahlung
x-rays
(noise)
Interaction Volume
Elastically
scattered e-
Inelastically
scattered e-
Direct
(transmitted)
beam
• Use results of reactions to form image or generate other
information.
Secondary electrons
Backscattered electrons
X-rays
Different signals come from
different depths in the target
(sample)
INCIDENT BEAM
Interaction Volume
~5–50 nm
Secondary electrons
~50 nm
TEM
specimen
thickness
~1 nm – Auger electrons
• X-rays:
– most escape
– sampling volume ≈ interaction volume
~300 nm
Backscattered
electrons
• Backscattered electrons:
~1.5 µm
X-rays
– originate further from the incident beam (some fraction of a µm)
• Secondary electrons:
~1.0 µm
X-ray
resolution
– escape from a region slightly larger than the incident beam
(several µm)
– yield the best spatial resolution
Z = 29 (Cu), Accelerating voltage = 20kV
Backscattered electrons (BSE)
• Formation
– Caused when incident electrons collide with an atom in a specimen
that is nearly normal to the path of the incident beam.
– Incident electron is scattered backward (“reflected”).
• Use
–
–
–
–
Imaging and diffraction analysis in the SEM.
Production varies with atomic number (Z).
Higher Z elements appear brighter than lower Z elements.
Differentiate parts of specimen having different atomic number
Backscattered electrons are not as numerous as others. However,
they generally carry higher energies than other types of electrons.
Secondary Electrons (SE)
• Formation
– Caused when an incident electron “knocks” and inner shell
electron (e.g., k-shell) out of its site.
– This causes a slight energy loss and path change in the
incident electron and ionization of the electron in the
specimen.
– The ionized electron leaves the atom with a small kinetic
energy (~5 eV)
• Use
– IMAGING!
– Production is related to topography. Due to low energy, only
SE near the surface can exit the sample.
– Any change in topography that is larger than the sampling
depth will change the yield of SE.
More abundant than other types of electrons. They are electrons
that escape the specimen with energies below ~50eV
X-rays
Auger Electrons (AE)
• Formation
– De-energization of the atom after a secondary electron is
produced.
– During SE production, an inner shell electron is emitted from
the atom leaving a vacancy.
– Higher energy electrons from the same atom can fall into the
lower energy hole. This creates an energy surplus in the
atom which is corrected by emission of an outer shell (low
energy) electron
• Use
– AE have characteristic energies that are unique to each
element from which they are emitted.
– Collect and sort AE according to energy to determine
composition.
– AE have very low energy and are emitted from near surface
regions.
Transmitted electrons
• Can be used to determine:
–
–
–
–
–
thickness
crystallographic orientation
atomic arrangements
phases present
etc.
• Foundation for Transmission Electron Microscopy (TEM)
• Formation
– Same as AE. Difference is that the electron that fills the
inner shell emits energy to balance the total energy of the
atom.
• Use
– X-rays will have characteristic energies that are unique to
the element(s) from which it originated.
– Collect and sort signals according to energy to yield
compositional information.
– Energy Dispersive X-ray Spectroscopy (EDS)
Foundation of XPS (X-ray photoelectron spectroscopy). XPS can
be used to determine the “state” of an atom and to identify chemical
compounds.
Specimen Interaction Volume
• Volume where electron beam - specimen interactions
occur.
• Depends upon:
– Z of material being examined
• higher Z materials absorb more electrons and have smaller
interaction volume
– Accelerating Voltage
• higher voltages penetrate further into the specimen and
generate larger interaction volumes
– Angle of incidence of electron beam
• larger angle leads to a smaller interaction volume
INCIDENT BEAM
~5–50 nm
Secondary electrons
~50 nm
TEM
specimen
thickness
INCIDENT BEAM
~100 Å
Secondary electrons
~1 nm – Auger electrons
~50 nm
TEM
specimen
thickness
~300 nm
Backscattered
electrons
10 Å - 30 Å – Auger electrons
<1µm - 2µm
Backscattered
electrons
~1.5 µm
X-rays
~5 µm
X-rays
~1.0 µm
X-ray resolution
???
X-ray resolution
Z = 29 (Cu), Accelerating voltage = 20kV
Z = 28 (Ni), Accelerating voltage = 20kV
Increasing incident energy (E0)
“Instruments of the trade”
~50 nm
• Primary Instruments
– Transmission Electron Microscope (TEM)
– Scanning Electron Microscope (SEM)
Z1
• Variants
Z2
Z1 < Z2 < Z3
Z3
Increasing atomic number (Z)
• Interaction volume is larger for materials that have
lower atomic numbers and for higher incident beam
energies!
–
–
–
–
–
–
Electron Probe Microanalyzer (EPMA)
Scanning Transmission Electron Microscope (STEM)
Environmental SEM (ESEM)
High Resolution TEM (HRTEM)
High Voltage TEM (HVTEM)
etc...
The TEM is analogous to the transmission LOM
TEM
TEM
LOM
Electron gun
• Patterned after transmission light optical microscopes
Vacuum
column
Light source
Condenser
system
Apertures
z
z
y
+ Tilt & rotation
• Yield Following Information:
Source
y
Stage
x
x
– Morphology
Objective
lens
• Size shape and arrangement of particles, precipitates, etc.
Final imaging
system
– Crystallographic information
• Atomic arrangement
Phosphor
screen; film;
digital recording
system
– Compositional Information
Eyepiece or TV
screen; film;
digital recording
system
Observation and
recording system
• If proper detector is present
1 in 1020 electrons
are collected
Components of the TEM
• Source – filament plus anode plates with
applied accelerating voltage.
Electron gun
Vacuum
column
• Condenser Lenses – electromagnetic lenses
adjusted by lens currents not position.
z
y
+ Tilt & rotation
x
• Specimen Stage – allows translations and tilts.
• Objective Lens – usually <50X.
• Imaging System – multiple electromagnetic
lenses below the objective: set magnification,
focal plane (image vs. diffraction pattern).
• Observation – fluorescent screen, plate film,
CCD camera.
Phosphor
screen; film;
digital recording
system
1 in 1020 electrons
are collected
Adapted from
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
<1 in 106 photons are
collected
Condensor Lenses
OPTICAL LENS
Light Source
MAGNETIC LENS
Electron Source
ELECTROMAGNETIC
lenses focus the
electron beam to as
small a spot as is
possible. They are
equivalent to convex
lenses in optical lens
systems.
Soft Fe
pole
pieces
Adapted from
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
Cu coils
N
N
S
S
Electrons in Magnetic Fields
Electron Lenses
• Use electrostatic or electromagnetic fields to focus
beams of charged electrons.
• ELECTROMAGNETIC – Most lenses are of this type.
Consist of Cu wire coils around soft Fe cores.
Sometimes an Fe pole-piece is used to “shape” the
field.
A. Electrons moving through a perpendicular magnetic field experience
perpendicular forces.
B. Electrons moving parallel to a magnetic field are unaffected.
C. Electrons moving nearly parallel to a magnetic field take a helical
path around the magnetic field direction (i.e., the microscope axis).
A
B
C
Nearly
aligned
electron
electron
• ELECTROSTATIC – Unusual. Only common
example is the Wehnelt aperture in the electron gun.
Force on
electron is
out of plane
unaffected
Magnetic
field
No Force
Electrons
follow a
helical path
Adapted from
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
Beam Tilting and Translation
Trajectories in Electromagnetic Lenses
• When you adjust the magnification (and the focal length),
you modify the lens strength by adjusting the current in the
electromagnetic lens coils.
• Since the magnetic field is changed,
MAGNETIC LENS
so are the helical trajectories.
Electron Source
• This ultimately leads to image
Soft Fe pole
rotation which must be corrected
pieces
for or calibrated.
N
N
S
Object
S
Cu coils
Rotated
image
Image is inverted
and rotated
• The electron beam can be positioned for fine
measurements (spot modes) or scanning (SEM, STEM)
BEAM TILTING
APPARENT
ORIGIN
Upper scan
coil
Lower scan
coil
BEAM TRANSLATION
APPARENT
ORIGIN
Condenser Alignment
• In TEM it is important to center the condenser aperture about
the optical axis.
• If the aperture is off center, the beam is displaced away from
the axis when the condenser lens is focused.
• You can try condenser alignment at:
Condenser Astigmatism
• Astigmatism in the condenser lenses distorts the beam to an
elliptical shape either side of focus and prevents the beam
being fully focused. It is corrected by applying two
orthogonal correction fields in the x and y directions.
• You can try stigmator adjustment at:
http://www.matter.org.uk/tem/condenser_astigmatism.htm
http://www.matter.org.uk/tem/condenser_alignment.htm
Objective Aperture
• Depending upon where you position the objective aperture
and whether or not you tilt the beam, a bright field image, a
dark field (i.e., scattered) image, or a diffraction pattern can
be collected.
• You can look at this in more detail at:
http://www.matter.org.uk/tem/dark_field.htm
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005) AND http://www.matter.org.uk/tem/stem_images.htm
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
BF TEM Micrograph and SADP
NiAl-1.0Hf
Grain structure
Crystallographic information
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
[111] zone axis
BF TEM Micrographs
Defect Structures
NiAl-1.0Hf
β′
250 nm
1hr
NiAl-1.0Hf
100 nm
4hr
Precipitate distribution
Defect structures
Dislocations imaged in NiAl-0.5Zr single crystals deformed at elevated temperatures.
HAADF (Z-contrast imaging) analysis
Nano-scale X-ray Analysis
As-deposited
Annealed 1000°C/1h
1
2
Hf rich region
β′
Ni2AlHf
β′ -NiAl
EDX spectrum
β′ -precipitate
NiAl-1.0Hf
200 nm
NiAl-1.0Hf
20 nm
• Hf segregates to GB’s in as-deposited & annealed coatings.
• Hf precipitates as β´-Ni2AlHf phase after annealing.
How does a TEM work?
Electron Beam
• Pass a focussed beam of electrons through a thin foil
Thin Foil
< 500 nm thick
• As beam passes through sample, it is scattered
• Project the transmitted (scattered) beam onto a
phosphor screen to form an enlarged image
• Modes:
– Bright Field/Dark Field modes for visualization of structure
and defects
– Selected Area Diffraction/Convergent Beam Diffraction
for crystallographic information
Schematic
Representation
of optics in a
TEM
Technical Details
Electron Gun
Anode Plate
First Condenser Lens (C1)
•
Produce a stream of monochromatic electrons in the electron gun
•
Focus the stream into a small coherent beam using C1 and C2
– C1 determines the “spot size” (i.e., size of electron probe)
– C2 changes intensity or brightness
Second Condenser Lens (C2)
Condenser Aperture
Sample
•
Use condenser aperture to restrict the beam
•
Part of the beam is transmitted through the sample
•
Focus transmitted portion using the objective lens to form an image
•
Objective and selected area apertures are used to restrict the beam further
Objective Lens
Objective Aperture
Selected Area Aperture
First Intermediate Lens
Second Intermediate Lens
Projector Lens
Phosphor Screen
– allows examination of diffraction from specific atoms, crystals, features…
– SAD, CBD
Used to
magnify image.
•
Enlarge image with intermediate and projector lenses
TEM
SEM
SEM
Electron gun
Electron gun
• Patterned after reflecting light optical microscopes
Vacuum
column
• Yield Following Information:
Condenser lens
Condenser system
z
y
– Topography
Scan coils
Stage
Also tilts and rotates
• Surface features of an object. Detectable features limited to a
few nanometers depth.
x
Objective lens
– Morphology
Time
base
Probe lens
z
Signal
amplifier
y
Final imaging system
• Size shape and arrangement of particles, precipitates, etc
Stage
x
Also tilts and rotates
Detector and
processing
system
Observation
and recording
system
– Compositional Information
• Elements and compounds the sample is composed of
CRT
– Crystallographic information
• Possible using new techniques (OIM/BKD)
In the SEM you use secondary signals to acquire images.
Components of an SEM
• Source:
Schematic
Representation
of the optics in
an SEM
Electron gun
– same as TEM but lower V
Condenser lens
• Condenser:
Electron Gun
Anode Plate
First Condenser Lens (C1)
Condenser Aperture
Second Condenser Lens (C2)
– same as TEM
Scan coils
• Scan Coils:
Time
base
Objective Aperture
– raster the probe
Scan Coils
Probe lens
• Probe Lens:
– lens that forms a spot at the
specimen surface
• Detector & Processing System:
– collects signals such as X-rays and
electrons as a function of time and
position.
– Provides digital images for real-time
viewing, processing, and storage.
z
Signal
amplifier
y
Stage
x
Also tilts and rotates
Objective (probe) Lens
Detector and
processing
system
tor
Detec
CRT
Sample
SEM: Technical Details
•
Produce a stream of monochromatic electrons in the electron gun
•
Focus the stream using the first condenser lens
–
Coarse probe current knob
•
The beam is constricted by the condenser aperture (eliminates high-angle electrons)
•
Second condenser lens is used to form electrons into a thin, tight, coherent beam.
–
Use fine probe current knob
•
Use objective aperture to limit beam (i.e., eliminate high-angle electrons)
•
Scan coils raster the beam across the sample, dwelling on the points for a
predetermined period of time (selected using scan speed)
•
Final objective lens focuses beam on desired region.
•
When beam strikes the sample, interactions occur. We detect what comes out of the
sample.
γ'
Substrate
80
γ + γ'
Ni
70
60
Ni-AD
Ni-Annealed
50
Substrate
IDZ
40
40
Coating
Al
Al-AD
Al-Annealed
30
Secondary electron mode
γ+γ′
γ
γ′
γ+γ′
Composition (at%)
20
10
0
40
Pt
Pt-AD
Pt-Annealed
30
20
10
0
1.0
10 µm
0.8
Could get
additional
information in
backscattered or
mixed mode
Hf
Hf-AD
Hf-Annealed
0.6
0.4
0.2
0.0
-15
-10
-5
0
5
10
15
20
Distance (µm)
Figure 4.8. Composition profiles of the major elements
measured by WDS in an EPMA.
25
X-ray Analysis in the SEM
EDS or WDS
Useful things to remember about
SEM operation
Al2O3 + ???
• Change magnification by changing the scan area
• Focus by changing the objective lens current
???
Ni
Al
O
Pt
Hf
O Hf
Pt
Al
Ni
• The probe current/spot size are controlled by the
condenser lens current
Elemental diffusion profile NiAlPtHf coatings on CMSX-4 substrates
for a temperature of 1150°C and times from (a) 20 hrs and (b) 100 hrs.
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
Microanalysis in Electron Microscopy
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
WDS
• Yields best discrimination of emitted X-ray signal
• Characteristic X-rays are always generated by
interactions between the incident electron beam and
the sample. They constitute a fingerprint of the local
chemistry.
• Use a series of bent crystals to cover the range of
wavelengths of interest
• Collect X-ray signal to determine local chemistry
• Scan wavelengths within each range by rotating the
crystal and moving the detector while keeping the
position of the crystal fixed.
• Common Techniques:
– Wavelength-Dispersive Spectrometry/Spectroscopy (WDS)
– Energy-Dispersive Spectrometry/Spectroscopy (EDS)
• X-rays are collected from the sample at a fixed angle.
The angle at the collecting crystal will vary with 2θ
and the diameter of the focusing circle will change
WDS
WDS continued
Bent crystal
Radius = 2R
• All peaks are scanned sequentially. You cannot
record more than one characteristic line at once
unless there are multiple spectrometers available.
Focusing Circle
• EPMA!
Specimen
Receiving slits
R
X-ray Detector
• Data is collected sequentially. Takes more time but
the results are much more precise than EDS.
EDS vs. WDS
• Pulse height is recorded by the detector. It is related
to the energy of the photon responsible for the pulse.
• Solid-state detectors are generally used.
• EDS is faster than WDS
• Problems with EDS:
Resolution of M lines in a WDS spectrum of a superalloy
(Oxford Instruments)
– Poor discrimination or energy resolution. WDS systems are
much better, particularly when characteristic lines from
different elements overlap.
– Need a windowless or thin window detector to pick up light
elements.
– WDS is more quantitative
Can use BSE signal in conjunction with SE signal to yield enhanced topographical
information
Images
• Topography and atomic number influence signal.
• Backscatter coefficient:
– η = -0.254 + 0.016Z - 1.86x10-4Z2 + 8.3x10-7Z3
SE image
Multiple elements of BSE
detector
“Overlapping shadows”
Reduced resolution
• Atomic number contrast:
– C = (η1 - η2)/ η1
Single segment of BSE
detector
The BSE sampling volume is large which limits resolution
TOPOGRAPHIC
CONTRAST
TOPOGRAPHIC
CONTRAST
Signals from 2
detectors
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)
B.D. Huey, MMAT322 Lecture Notes, University of Connecticut (2005)