Scanning Electron Microscopy

Question 1

wavelength of an electron

Answer 1

= ./sqrt of the voltage
therefore, apply more voltage, electron travels faster, the wavelength gets shorter, higher resolution

Question 2

creating an electron beam

Answer 2

1. heat pointed metal filament allows electrons to escape
   > tungsten filament, LaB6 crystal
2. field emission gun
   > electric field creates force to help electrons escape
3. making a beam
   > formed into beam, apply voltage to cylinder, higher voltage accelerates electrons❤️

Question 3

reason for resolution limit

which is ~ 0.1nm, can get it to 0.05nm at besttt

Answer 3

SPHERICAL ABERRATIONS

electrons travelling through the lens at different distances from the centre, are focussed at different distances away from the lens

therefore, have to add apeture > reduces wavelength so reduces resolution, but needed to remove abberations

can add more lenses > aberration correctors, but these add height and cost

Question 4

SEM components roles:
electromagnetic lenses form ____
apertures control ____

Answer 4

Beam of electrons

beam size

Question 5

Cathodoluminescence
X-rays

Answer 5

Emission of photons in the visible light range (MINERAL ZONING)
Photons with a range of energies up to the maximum energy of the primary beam (ELEMENT DATA)

Question 6

surface charging

Answer 6

e-s are -vely charged

sample must be conductive to dissipate the charge, else: surface charging

SE more sensitive to this than BSE

Question 7

Interaction Volume

Answer 7

• primary e- beam penetrates sample
• e- are scattered within sample
• volume of the sample into which they scatter = the interaction volume

depedent upon the ENERGY OF THE PRIMARY E BEAM and the DENSITY OF THE SAMPLE (E0 and Z)

Question 8

SE are created when _____
BSE are created when ____

Answer 8

SE: orbital electrons are ejected from specimen atoms
BSE: primary beam electrons are scattered by sample atom nuclei

Question 9

SEM operating parameters

Answer 9

Accelerating voltage = energy of electrons

Beam current = number of electrons

Working distance = distance between the sample and the last lens

Question 10

Accelerating voltage amount

Answer 10

kV > higher = larger interaction volume

therefore want to use lower kV for SE, higher for BSE

Question 11

Beam current amount

Answer 11

Ie, nA
Controlled by lenses and apertures(!!)

Lots of e- = good signal, some loss of res, can’t resolve small structures

Fewer e- = fewer e- in, fewer e- out = reduced signal BUT improved resolution

Question 12

Working distance amount

Answer 12

Affects: resolution, depth of field (the height over which a sample appears in focus)

Question 13

Stigmation aberration

Chromatic aberration

Spherical aberration

Answer 13

Asymmetrical beam caused by manufacturing errors and contaminants

Different e- energies focus at different points (limits practical res)

e- away from the central axis are deflected more (limits practical res) > apertures help

Question 14

Lenses of optical v em

Answer 14

Lenses for light microscopes are made of glass and work by refracting the
light to create a focusing effect. Lenses for electron microscopes are made from
coils of wire with currents passing through them to create a magnetic field that
deflects the electrons to create a focusing effect.

Question 15

Resolution depends on?

Answer 15

Interaction volume. Increasing the acceleration voltage results in shorter wavelengths (so would be increased res), but this means a greater interaction volume, so less e can come back out so decreased resolution for SEM