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