NANO 230
Micro/NanoFabrication
Scanning Electron Microscopes
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Microscopes
Light Microscopes
• Magnification:
500 X to 1000 X
• Resolution: 0.20 µm
Electron Microscopes
• Magnification: 1,000,000 X
• Resolution: <1 nm
• down to 0.5 A (TEM)
• Limits reached by
early 1930s
• Use focused beam of
electrons instead of light
• Color images
• “Lenses” are coils, not glass
• Sample in air
• Sample in vacuum
Scanning Electron Microscope (SEM)
Transmission Electron Microscope (TEM)
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:
0.612 
d
n sin 
• wavelength of electrons ()
• Numerical aperture of lens system (NA)
– Electron gathering ability of the objective
– Electron providing ability of the condenser
h

mv
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 sample
4. Emitted electrons captured on CRT
5. 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 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 <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
SEM instrumentation
Courtesy of the Science Education Resource Center,
Carlton College, Carlton, PA
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
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