Electron Microscopy

(1)

M i c r o a n a l

y s

i s in

S c i e

n c

e a

n d

Eng

inee

rin

g -

E l

e

c t r o n M i c r o s c

o p

y

A Workshop for Middle and High School

Teachers

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

h

school science curriculum?

(3)

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 SEM Cambridge Scientific Instruments (1965) – “introduction of first commercial instrument”

e-_Source Anode

1st_lens

2nd_lens

Final lens Detectors Backscatter

e-X-ray

Secondary e

-http://mse.iastate.edu/microscopy/path.html

(4)

 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.

(5)

How are electrons generate?



Thermionic emission

–

Tungsten (W) filament

–

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.

(6)

Thermionic emissions

Electrons will escape from heated metals when

the thermal energy of the electron is greater

than the work function.

E_F

Lowest free energy state

Highest free energy state (Fermi level) E E_w

Recall that the work function is the amount of energy required to remove an electron from its highest free energy state to infinity.

E_w=E-E_F

Chemistry and Physics: Work function

(7)

Electron flux (current density)

current density = A

_c

T

2

_e

-E_w/kT

A_c=a constant that depends on the material

To increase the current density at constant T, either A_c must increase of E_w must decrease.

Material E_w (eV)

W 4.5

LaB₆ 2.4

Suggested Curriculum Links Physics: current density

(8)

Field emissions

Physics: 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.”

(9)

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

–

Sample chamber vacuum ~<10

-5

torr

–

ESEM technology permits sample chamber vacuum ~<20

torr

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.

Physics: force on a moving charged particle in an B-field

B

v

e

F

_B

  







B v F y x z Detectors Backscatter e-X-ray Secondary e -e-_Source Anode 1st_lens 2nd_lens Final lens http://mse.iastate.edu/microscopy/electro_lens.html http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Focus.html

(11)

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.html

http://emalwww.engin.umich.edu/emal/courses/SEM_lectureCW/SEM_Interaction.html

Chemistry and Physics: kinetic theory, collision dynamics, probability, flux concept f_e E_o E_i

Q

N

A

n

N

Q

o i t





 

,

Q=collision cross-section (probability) N=num of collisions/unit volume

n_t=number of targets/unit volume n_i=number of incident particles/unit area (flux)

(12)



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?

(13)

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.

(14)



Inelastic interactions result in a wide variety

of emissions:

–

Secondary electrons

–

Characteristic X-rays

–

Bremsstarahlung (continuum) X-rays

–

Cathodluminescence radiation (IR, UV and visible

(15)

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 incident light _{secondary e} -detector incident e-_beam emitted e -~+12,000 V Suggested Curriculum Links

(16)

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_Basics1.html

Across the curriculum: computers and information processing

(17)

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 f_e E_o E_i

In elastic scattering E_i~=E_o. The elastic collision is with the nuclei of an atom, partly obscured by the electron cloud.

Chemistry and Physics: kinetic theory, collision dynamics

2 cot

10

62 .

1 )

(

₂ 2 2 20 _o o

E

Z

Q

f



f

  

Z=the atomic number E=electron energy (keV)

(18)



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

(19)

What are some unique properties of

BS electrons?



Deeper penetration



Intensity is function of

atomic weight of

sample

(b)

(20)

 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

(21)

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.