Electromagnetic Lenses in TEM

Electron Sources

• There are two types of electron sources: Thermionic gun and Field Emission Gun (FEG)
• Thermionic gun:
  • Composed of a tungsten (W) filament, cathode shield (Wehnelt cylinder), and anode plate
  • Electrons are emitted when the filament is heated
• Field Emission Gun (FEG):
  • Electrons are pulled out of a very fine pointed tungsten tip by a high electric field
  • FEG has two anodes: first anode to pull electrons out of the tip and second anode to accelerate electrons to the right potential
  • FEG is more expensive and requires ultrahigh vacuum

Electromagnetic Lenses

• Made of electromagnetic coils (copper) that bend the path of electrons
• Used to focus and control the electron beam, ensuring precise placement of electrons
• Magnification of the lens can be varied by controlling the current through the coils
• There are three sets of electromagnetic lenses:
  • Condenser lens: doubles as a lens to control spot size and another to control intensity (brightness)
  • Objective lens: forms the intermediate magnified image
  • Projector (ocular) lens: produces the final further magnified image

Defects of Electromagnetic Lenses

• Chromatic aberration: magnification varies with electron wavelength, causing fringes around objects
• Spherical aberration: magnification differs between the center and edges of the field
• Astigmatism: circular shapes become elliptical in the image, correctable by using a Stigmator

Characteristics of the Electron Beam

• Has a very short wavelength (0.005nm)
• Can be focused by passing through a magnetic field
• Increasing accelerating voltage lowers the wavelength and increases electron speed
• Specimens must be very thin to allow electron penetration and image formation
• Thin sections of the specimen are mounted on copper grids of 3 mm diameter and attached to a holder for examination