Electron Beam Generation
- Electron generation
- Work function (escape velocity)
- Electron saturation
- Electron source
- Thermionic
- Tungsten hairpin
- Lanthenum hexaboride
- Field emission
- Tungsten crystal
- Generation of collimated beam
- Negatively-charged Wehnelt cylinder
- Positively-charged anode cap
- Accelerating voltage, is differential in charge
- Can be adjusted from 20 kV to 100 kV in Zeiss 10
- Electrons are correspondingly accelerated to 20 keV - 100 keV
- biased Wehnelt cap is more negative than source
- functions as electrostatic lens
- concentrates electron density
- extracts more electrons into column
- functions as spray aperture
- Focusing the beam
- Solenoid lens design = wire-wrapped tube
- windings. material constant
- lens current variable
- Pole piece = soft iron insert
- increases field density
- decreases focal length, aberration
- Apertures
- Spray apertures = fixed column location
- Less noise, improved contrast image
- Lens apertures
- are moveable, centerable
- range from 100 to 400 µm in diameter
- determine aperture angle
- affect depth of field
- affect spherical aberration strongly
- Field aperture = adjusts below the lens