NANO 225 Micro/NanoFabrication

Electron Microscopes

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Microscopes

Light Microscopes

• Magnification: 500 X to 1000 X

• Resolution: 0.20 µm

• Limits reached by early 1930s

• Color images

• Sample in air

Electron Microscopes

• Magnification: 1,000,000 X

• Resolution: <1 nm• down to 0.5 A (TEM)

• Use focused beam of electrons instead of light

• “Lenses” are coils, not glass

• Sample in vacuumScanning Electron Microscope (SEM)

Transmission Electron Microscope (TEM)

Electron Microscopes

What can electron microscopes tell us?• Morphology

– Size and shape

• Topography– Surface features (roughness, texture,

hardness)

• Crystallography– Organization of atoms in a lattice

Electron Microscopes: Crystallography

Crystallography:• Arrangement of atoms• Crystals have atoms arranged in ordered lattices• Amorphous: no ordering of atoms

Crystallography affects properties (electrical, strength, etc)

Scanning Electron Microscopy (SEM)

• Provides information about:– Topography of sample or structure– Chemical composition near the surface of

sample

• Magnification: ~30X to 500,000X

• Resolution– Nanometer scale– Dependent on:

• wavelength of electrons ()

• Numerical aperture of lens system (NA)

– Electron gathering ability of the objective– Electron providing ability of the condenser

R 2NA

SEM Instrument• Electron beam

– Spot size ~5 nm– Energy ~200 - 50,000 eV (electron volts)– Rastered over surface of sample

• Emitted electrons collected on a cathode ray tube (CRT) to produce SEM images

Sample Prep:• Attach to Al “stub” with conductive carbon tape

or paste• Sputter-coat non-conductive samples

SEM: How it works

1. Electron beam strikes surface and electrons penetrate surface

2. Interactions occur between electrons and sample

3. Electrons and photons emitted from sample4. Emitted electrons captured on CRT5. SEM image made from detected electrons

http://www.youtube.com/watch?v=bfSp8r-YRw0&feature=related

http://www.youtube.com/watch?v=fToTFjwUc5M&feature=related

SEM: Electron Beam InteractionsValence electrons

– Inelastic scattering: Energy transferred to atomic electron– If atomic electron has high enough energy can be emitted

from sample– “Secondary electron” if energy of emitted electron <50 eV

Atomic nuclei– “Backscattered electrons” – Elastic scattering: e- bounce off with same amount of energy– Atoms with high atomic numbers cause more backscattering

Core electrons– Core electron ejected from sample; atom becomes excited– To return to ground state, x-ray photon or Auger electron

emitted

Transmission Electron Microscopy (TEM)

• Provides information about:– Topography of sample or structure– Chemical composition

• Magnification: ~50X to 1,000,000X

• Resolution

– Dependent on:

• Electron mass (m) and charge (q)

• Potential difference used to accelerate electrons (V)

– Proportional to 3/4

– < 0.2 nm resolution (400 keV)

h

2mqV

TEM Instrument• Electron beam

– Energy: 100,000 - 1,000,000 eV (100 keV - 1 MeV)

– Projected onto thin sample using lens system by deflection coils

http://video.google.com/videoplay?docid=-5489601762301542658&q=transmission+electron+microscope&total=6&start=0&num=10&so=0&type=search&plindex=1

Sample Prep:• Need very thin sample!

• Slice of bulk material or cross-section of thin film

• Grind into power, dissolve, put on conductive grid to evaporate

http://www.vcbio.science.ru.nl/images/10-tem_grid_zoom.jpg

TEM: Electron Beam Interactions

Elastic Scattering– No energy loss– Diffraction patterns

Inelastic Scattering– Occurs at heterogeneties (defects, grain

boundaries)

1. Electron beam strikes surface and is transmitted through film

2. Scattering occurs during transmission3. Unscattered electrons pass through the

sample and are detected (along with elastic scattered electrons)

SEM and TEM Instruments

http://www.vcbio.science.ru.nl/en/image-gallery/electron/

SEM and TEM Comparison

• SEM makes clearer images than TEM

• SEM has easier sample preparation than

TEM

• TEM has greater magnification than SEM

• SEM has large depth of field

SEM and TEM Data Images

• Ag thin film deposited on Si substrate (thermal or e-beam evaporation)

• TCNQ (7,7,8,8-tetracyanoquinodimethane) powder and Ag thin film are enclosed in a vacuum glass tube, then heated in a furnace.

http://nami.eng.uci.edu/projects/Agtcnq.htm

Some definitions

• Stigmation: – correcting asymmetries in horizontal v. vertical

focus– seen as “streakiness”

• Collimation:– creation of parallel path particles– typically no control over

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Improving Images: Spot sizeSpot size:

electron spot radius (rms)

• Especially useful to improve focus at high mag

• Minimize spot size:– Decrease working distance– Increase current on focusing lens

Trade-offs:• Smaller area covered• Lower beam current (worse contrast)

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Improving Images: Depth of field Depth of field:

• How many planes are in focus at once

• Related to distance that beam stays narrow

• Especially useful to see detail on rough surfaces:

• Maximize DOF:• Decrease aperture size

• Decrease magnification

• Increase working distance

Trade-off:• Lower magnification

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Improving Images: Signal-to-Noise

Signal-to-noise ratio: contrast between interacting and non-interacting surfaces

• Especially useful to gain more fine detail• Maximize S/N ratio:

– High beam current– Slow scan rate

Trade-off:• Much larger spot size

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