Microanalysis in Science and Engineering - Electron Microscopy
A Workshop for Middle and High School Teachers
sponsored byTennessee Technological UniversityCenter for Manufacturing ResearchDepartments of Chemical, Mechanical, Earth
Sciences and Curriculum and Instructionand The National Science Foundation
FacultyJoseph J. Biernacki (Chemical Engineering)
June 16, 2003
What will we learn
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?
Electron microscopy is an imaging technology that uses the properties of electrons rather than light.
A bit of history:
von Ardenne (1938) – earliest recognizable work describing scanning electron microscope (SEM)Zworykin, Hillier and Snyder (1942) – basis for modern SEMCambridge Scientific Instruments (1965) – “introduction of first commercial instrument”
e- SourceAnode
1st lens
2nd lens
Final lens DetectorsBackscatter e-
X-ray
Secondary e-
http://mse.iastate.edu/microscopy/path.html
http://mse.iastate.edu/microscopy/elementary.html
I’ve heard other terms used…
Electron Probe Microanalyzer (EPMA)An electron probe microanalyzer utilizes X-rays emitted due to electron bombardment to obtain qualitative and quantitative microanalysis.
Electron Microprobe (same as EPMA) Transmission Electron Microscope (TEM)
Uses transmitted electrons instead of emitted electrons. Scanning Transmission Electron Microscope (STEM)
Combines aspects of both SEM and TEM. Environmental Scanning Electron Microscope (ESEM)
Similar to a SEM, but does not require the high vacuum. 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?
Thermionic emission– 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
Thermionic emissions
Electrons will escape from heated metals when the thermal energy of the electron is greater than the work function.
EF
Lowest free energy state
Highest free energy state (Fermi level)
E Ew
Recall that the work function is the amount of energy required to remove an electron from its highest free energy state to infinity.
Ew=E-EF
Suggested Curriculum LinksChemistry and Physics:
Work function
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 Ew (eV)W 4.5LaB6 2.4
Suggested Curriculum LinksPhysics: current density
flux concept
Field emissions
Suggested Curriculum LinksPhysics: E-field near a sharp object
Electron tunneling effect
V1 V2
An extremely high field is produced at the sharp tip of the cathode. This reduces the potential barrier and permits electrons to tunnel out.
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.”
The requirement of high vacuum
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.
– Column vacuum ~10-7 torr– Sample chamber vacuum ~<10-5 torr– ESEM technology permits sample chamber vacuum ~<20
torr
Suggested Curriculum LinksChemistry and Physics: absolute and
relative pressure scales
Focusing a beam of electrons
A magnetic field exerts a force perpendicular to the plane formed by the vector velocity and the magnetic field vector.
Suggested Curriculum LinksPhysics: force on a moving charged
particle in an B-field
BveFB
Bv
F
yx
z
DetectorsBackscatter e-
X-ray
Secondary e-
e- SourceAnode
1st lens
2nd lens
Final lens
http://mse.iastate.edu/microscopy/electro_lens.html
http://mse.iastate.edu/microscopy/path2.html
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Focus.html
How does the e- beam interact with matter?
Incident electrons interact with matter in two ways
– elastic collisions– inelastic collisions
From these interactions, information regarding shape, composition, crystal structure, electronic structure, internal electric or magnetic fields, …
http://mse.iastate.edu/microscopy/beaminteractions.htmlhttp://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Interaction.html
Suggested Curriculum LinksChemistry and Physics: kinetic theory,
collision dynamics, probability,flux concept
eEo
Ei
QN
A
nn
NQ
oit ,
Q=collision cross-section (probability)N=num of collisions/unit volument=number of targets/unit volumeni=number of incident particles/unit area (flux)
http://biology.udayton.edu/SEM/Principle/2_Imaging.htm
Learning about secondary electrons.
Use the internet page below and any other web-based resource available to you and what you have learned thus far to answer the following questions about secondary electrons:
– 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
Download the software below and use it to answer the following questions:
– 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
Inelastic interactions result in a wide variety of emissions:– Secondary electrons– Characteristic X-rays– Bremsstarahlung (continuum) X-rays– Cathodluminescence radiation (IR, UV and visible
light)
How is a secondary image generated?
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.
eye
incidentlight secondary e-
detector
incident e- beam
emitted e-
~+12,000 V
Suggested Curriculum LinksPhysics: electrostatic phenomena
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Basics2.html
How is a secondary image generated?
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.
secondary e-
detector
incident e- beam
emitted e-
~+12,000 V
beam location
signal intensity
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_se2.html
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Basics3.html
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Basics1.html
Suggested Curriculum LinksAcross the curriculum: computers
and information processing
Elastic collisions
Elastic collisions produce backscattered electrons (BS).
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
eEo
Ei
In elastic scattering Ei~=Eo. The elastic collision is with the nuclei of an atom, partly obscured by the electron cloud.
Suggested Curriculum LinksChemistry and Physics: kinetic theory,
collision dynamics
2cot1062.1)( 2
2
220 o
o E
ZQ
Z=the atomic numberE=electron energy (keV)o=scattering angle
Detecting BS electrons
There are many types of detectors, only the solid state type is discussed here.
http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_bse4.html
sample
BS e-
solid state BS detector
incident e- beam
What are some unique properties of BS electrons?
Deeper penetration Intensity is function of
atomic weight of sample
(b)
Summary
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
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