NANO 230 Micro/NanoFabrication

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NANO 230 Micro/NanoFabrication

• Scanning 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 lt1 nm
• down to 0.5 A (TEM)
• Use focused beam of electrons instead of light
• Lenses are coils, not glass
• Sample in vacuum

Scanning Electron Microscope (SEM) Transmission
Electron Microscope (TEM)
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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

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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

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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 sample
4. Emitted electrons captured on CRT
5. SEM image made from detected electrons

http//www.youtube.com/watch?vbfSp8r-YRw0feature
related
http//www.youtube.com/watch?vfToTFjwUc5Mfeature
related
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SEM Electron Beam Interactions

• Valence 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 lt50 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

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SEM instrumentation

• Courtesy of the Science Education Resource
  Center, Carlton College, Carlton, PA

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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 size

• Spot 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