Download Microanalysis in Science and Engineering

Microanalysis in Science and
Engineering - Electron Microscopy
A Workshop for Middle and High School
Teachers
sponsored by
Tennessee Technological University
Center for Manufacturing Research
Departments of Chemical, Mechanical, Earth
Sciences and Curriculum and Instruction
and The National Science Foundation
Faculty
Joseph J. Biernacki (Chemical Engineering)
June 16, 2003
What will we learn
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What is electron microscopy?
How are electrons generated?
How are electrons focused?
How do electrons interact with matter?
How are the electron/matter interactions used to
generate images?
What linkages can be made between the
“technology fundamentals” and the middle/high
school science curriculum?
What is electron microscopy?
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Electron microscopy is an
imaging technology that
uses the properties of
electrons rather than light.
A bit of history:
e- Source
Anode
1st lens
2nd lens
Final lens
von Ardenne (1938) – earliest recognizable work describing
scanning electron microscope (SEM)
Zworykin, Hillier and Snyder (1942) – basis for modern SEM
Cambridge Scientific Instruments (1965) – “introduction of
first commercial instrument”
http://mse.iastate.edu/microscopy/path.html
http://mse.iastate.edu/microscopy/elementary.html
Detectors
Backscatter eX-ray
Secondary e-
I’ve heard other terms used…
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Electron Probe Microanalyzer (EPMA)
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An electron probe microanalyzer utilizes X-rays emitted due to electron bombardment to
obtain qualitative and quantitative microanalysis.
Electron Microprobe (same as EPMA)
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Transmission Electron Microscope (TEM)
Uses transmitted electrons instead of emitted electrons.
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Scanning Transmission Electron Microscope (STEM)
Combines aspects of both SEM and TEM.
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Environmental Scanning Electron Microscope (ESEM)
Similar to a SEM, but does not require the high vacuum.
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Scanning Auger Microscope (SAM)
Similar to an SEM only it uses Auger electron emissions instead of secondary electron
emissions for imaging and compositional analysis.
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Microscopes.html
How are electrons generate?
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Thermionic emission
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Tungsten (W) filament
Lanthanum hexaboride (LaB6) filament
Field emission
The amount of electrons (flux or current density)
determines resolution.
The size of the electron beam (spot size) determines
resolution.
http://mse.iastate.edu/microscopy/source.html
Suggested Curriculum Links
Chemistry and Physics:
Work function
Thermionic emissions
Electrons will escape from heated metals when
the thermal energy of the electron is greater
than the work function.
E
Ew
Ew=E-EF
EF
Highest free energy state (Fermi level)
Lowest free energy state
Recall that the work function is the amount of energy required to remove an
electron from its highest free energy state to infinity.
Suggested Curriculum Links
Physics: current density
flux concept
Electron flux (current density)
current density = AcT2e-Ew/kT
Ac=a constant that depends on the material
To increase the current density at constant T, either Ac must increase of
Ew must decrease.
Material
W
LaB6
Ew (eV)
4.5
2.4
Suggested Curriculum Links
Physics: E-field near a sharp object
Electron tunneling effect
Field emissions
V1
V2
Benjamin Franklin discovered
that static discharges are
attracted to the sharp tip of a
conductor. He used this
phenomena to invent the
lightning rod which he gave as
his “gift to the world.”
An extremely high field is produced at the sharp tip of the cathode.
This reduces the potential barrier and permits electrons to tunnel out.
Suggested Curriculum Links
Chemistry and Physics: absolute and
relative pressure scales
The requirement of high vacuum
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Electrons have extremely low mass (~1/1000 that of
a proton) and easily give up their energy in collisions
with gas atoms and molecules. SEM technology is
not possible without a high vacuum in at least the
source and focusing column of the machine.
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Column vacuum ~10-7 torr
Sample chamber vacuum ~<10-5 torr
ESEM technology permits sample chamber vacuum ~<20
torr
Suggested Curriculum Links
Physics: force on a moving charged
particle in an B-field
Focusing a beam of electrons
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A magnetic field exerts a force perpendicular to the
plane formed by the vector velocity and the
magnetic field vector.
e- Source
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 
FB  ev  B
Anode
F
1st lens
B
v
2nd lens
x
y
Final lens
Detectors
Backscatter eX-ray
z
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Focus.html
http://mse.iastate.edu/microscopy/electro_lens.html
http://mse.iastate.edu/microscopy/path2.html
Secondary e-
Suggested Curriculum Links
Chemistry and Physics: kinetic theory,
collision dynamics, probability,
flux concept
How does the e- beam interact with matter?
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Incident electrons interact with
matter in two ways
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elastic collisions
inelastic collisions
From these interactions,
information regarding shape,
composition, crystal structure,
electronic structure, internal
electric or magnetic fields, …
Eo
N
Q
,
nt ni

A
N o Q
Q=collision cross-section (probability)
N=num of collisions/unit volume
nt=number of targets/unit volume
ni=number of incident particles/unit
area (flux)
Ei
fe
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Interaction.html
http://mse.iastate.edu/microscopy/beaminteractions.html
http://biology.udayton.edu/SEM/Principle/2_Imaging.htm
Learning about secondary electrons.
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Use the internet page below and any other webbased resource available to you and what you have
learned thus far to answer the following questions
about secondary electrons:
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Do secondary electrons originate only from the sample surface?
What is the kinetic energy of secondary electrons?
What type of interaction produces a secondary electron?
What type of information can be obtained from secondary electron
emissions?
Why is secondary electron emission independent of atomic
number?
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_SE1.html
Learning about electron interactions
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Download the software below and use it to answer the following
questions:
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What affect does atomic weight have on the interaction volume?
What is the nominal shape of the interaction volume?
What affect does beam voltage have on the interaction volume?
Design a computational experiment to answer each question. State
your design briefly, one or two sentences with a table, etc. Be
prepared to present your results in some understandable form.
Casino a software for performing Monte Carlo simulation of
electron-matter interactions.
Inelastic emissions
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Inelastic interactions result in a wide variety
of emissions:
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Secondary electrons
Characteristic X-rays
Bremsstarahlung (continuum) X-rays
Cathodluminescence radiation (IR, UV and visible
light)
Suggested Curriculum Links
Physics: electrostatic phenomena
How is a secondary image generated?
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Emitted electrons are not assembled by the electron
microscope in the way that light (visible photons) are
assembled by the human eye. Light reflecting from a given
spot enters the eye. Many points of such reflected light are
assembled in a pattern on the eye that exactly mimics the
reflecting source. This is not the case for electrons in the
electron microscope.
incident e beam
incident
light
emitted e-
secondary edetector
eye
~+12,000 V
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Basics2.html
Suggested Curriculum Links
Across the curriculum: computers
and information processing
How is a secondary image generated?
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Secondary electrons are generated by the interaction of the incident
electron beam and the sample. The secondary electrons emerge at all
angles. These electrons gathered by electrostatically attracting them
to the detector. Knowing both the intensity of secondary electrons
emitted and position of the beam, an image is constructed
electronically.
incident e- beam
beam location
emitted e-
secondary edetector
signal intensity
~+12,000 V
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Basics1.html
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_se2.html
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Basics3.html
Suggested Curriculum Links
Chemistry and Physics: kinetic theory,
collision dynamics
Elastic collisions
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Elastic collisions produce backscattered electrons
(BS).
E
i
fe
Eo
Q( fo )  1.62 1020
Z=the atomic number
E=electron energy (keV)
fo=scattering angle
Z2
2 fo
cot
E2
2
In elastic scattering Ei~=Eo. The
elastic collision is with the nuclei of
an atom, partly obscured by the
electron cloud.
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_bse1.html
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_bse2.html
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_bse3.html
Detecting BS electrons
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There are many types of detectors, only the
solid state type is discussed here.
incident e- beam
solid state BS detector
BS esample
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_bse4.html
What are some unique properties of
BS electrons?
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Deeper penetration
Intensity is function of
atomic weight of
sample
(b)
Summary
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SEM used the properties of e- to produce images.
e- are generated by a thermionic process wherein the work function
of the source must be exceeded. A strong electric field can also be
used to permit e- to tunnel out. W is the most common thermal
source.
Magnets are used to focus the e- beam.
The interaction of high energy e- with matter produces either
elastic or inelastic collisions. Elastic collisions are responsible
for backscattering of e-. Inelastic collisions produce secondary
electrons as well as characteristic X-rays and other forms of radiation
that give information about the surface morphology, composition,
electrical and magnetic properties and crystal structure.
Secondary images are not constructed by reflection as with light,
but require electrons to be attracted to a detector and assembled
using the signal intensity and beam location information.
SEM provides many opportunities to connect the science behind
the technology with curricular topics in chemistry and physics.
Some web links
How does and electron microscope work?
http://mse.iastate.edu/microscopy/choice.html
Electron microscopy basics.
http://biology.udayton.edu/SEM/
A more advanced web site about electron microscopy.
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_frontpage.html