Electron Microscopy

M i c r o a n a ly l y s i s i n S c i en e n c e an an d E ng i ne er e r in g - E le lec t r o n M i c r o s c o p y 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/hig middle/high h 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:

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)  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.

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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  –

Tungsten (W) filament

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Lanthanum hexaboride (LaB 6) 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 A c 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 h i g h v ac u u m 

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

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Sample chamber vacuum ~