1. NANO 230 Micro/Nano Characterization
Scanning Electron Microscopes

2. Microscopes Light Microscopes Electron 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 vacuum
Scanning Electron Microscope (SEM)
Transmission Electron Microscope (TEM)

3. 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
Most widely used techniques for characterization of nanomaterials and nanostructures.
Developed in 1942
First commercial instruments in 1965
Scans beam of electrons across sample
Easily adjusted magnification over a very large range
Resolution: down to a few nanometers
Numerical aperture of lens system is made of two parts: objective and condenser. For this reason, the numerical aperture of the lens system is dependent on two measures:
- electron gathering ability of the objective
- electron providing ability of the condenser
Numerical aperture is a number that should be engraved on the system

4. SEM Instrument Electron beam
Spot size ~5 nm
Energy ~ ,000 eV (electron volts)
Rastered over surface of sample
Emitted electrons collected on a cathode ray tube (CRT) to produce SEM images
Source of radiation in this technique is electrons. These electrons are focused into a very fine, focused beam that has a spot size ~ 5 nm and energy from a few hundred electron volts up to 50,000 eV (also known as 50 KeV)
Rastered? rectangular formation of parallel scanning lines that guide the electron beam
Deflection coils?
Sample Prep:
Attach to Al “stub” with conductive carbon tape or paste
Sputter-coat non-conductive samples

5. SEM: How it works
Electron beam strikes surface and electrons penetrate surface
Interactions occur between electrons and sample
Electrons and photons emitted from sample
Emitted electrons captured on CRT
SEM image made from detected electrons

6. 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
High Energy electrons from beam hit sample and have three different types of interactions, depending on what they hit.
If they hit a valence electron that belongs to an atom in the sample, inelastic scattering occurs. Called secondary electron because it is low energy and because the primary electron (one from the electron beam) is very high energy.
If the electron beam electrons hit an atomic nucleus, elastic scattering occurs.
Backscattering can NOT be used for chemical identification (even though it gives us relative information about atomic number of atoms in your sample). However, if you know your sample is made up of to different tpyes of atoms which differ greatly in atomic number, it is possible to see the contrast between these two types of atoms in the SEM image.
This “excited state” simply means that all of its electrons are not in their “ground state” (draw picture?)
These two emissions can then be used for chemical characterization (to be discussed later)

7. SEM instrumentation
Courtesy of the Science Education Resource Center, Carlton College, Carlton, PA

8. Some definitions Stigmation: Collimation:
correcting asymmetries in horizontal v. vertical focus
seen as “streakiness”
Collimation:
creation of parallel path particles
typically no control over

9. Improving Images: 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)

10. Improving Images: 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
Flower v. background

11. 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