Scanning Electron Microscopy

Question 1

What makes electrons more useful than light (optical microscope)?

Answer 1

Wavelength of moving electron decreases as speed increases.

Question 2

Optical microscopy vs Electron microscope

Answer 2

see notes

Question 3

What 4 things does electron microscopy provide information on?

Answer 3

1. Topography/surface features
2. Morphology (shape + size)
3. Elemental composition
4. Crystallographic info (arrangement of atoms)

Question 4

Describe the role of the filament in SEM

Answer 4

Filament (cathode) heated up using current -> electrons have enough energy to escape -> electrons emitted are guided by anode into focussed beam.

Question 5

Concisely describe general process of how SEM works

Answer 5

align electron beam -> hit sample -> electrons released + detected

Question 6

Name and compare 3 ways to produce the electron beam

Answer 6

1. Tungsten filament:

cheap + easier to maintain
gradually lose mass to evaporation + eventually break
lower brightness + broader beam spread (as operating at high temps), therefore reduced image res

2. LaB6 or CeB6 crystal:

lower temps required to emit electrons, therefore lower beam spread + higher brightness so higher image res.
Less volatile so last longer
BUT need vacuum so more expensive

3. Field Emission Gun:

Strong E field used to induce electron emission (instead of current) -> field applied to tip of tungsten wire -> Quantum Tunnelling allows high energy electrons to be released

Emission are smaller, so better brightness -> better spatial res and increase signal to noise ratio

Highest longevity
Ultra-high vacuum requires (as using E field) so v expensive

Question 7

Name the 3 parts of the magnetic lens system and describe what each part does

Answer 7

[In order:]
- Condenser lens: adjusts width of electron beam + controls intensity of beam

• Scan coils: deflect beam horizontally and vertically over specimen
• Objective lens: focusses beam onto specimen

Question 8

What functionality does the stage provide?

Answer 8

It holds he specimen and can be moved horizontally (to change field of view) and vertically (to change resolution)

Question 9

Why is a vacuum required?

Answer 9

reduces scattering of electron beam, therefore reduces probe size and increase resolution (as not losing electrons)

Question 10

Order of components in SEM machine

Answer 10

electron source, anode, condenser lens, scan coils, objective lens, (secondary electron detector), sample

Question 11

What is released from inner layer vs outer layer of the sample?

Answer 11

Electrons from higher layers
photons/X-rays from deeper layers

Question 12

Name the 3 things produced when electron beam interacts with sample

Answer 12

• Secondary electrons
• Backscattered electrons
• Xray fluorescence or Auger electrons

Note: have different detectors for each

Question 13

What are secondary electrons?

Answer 13

Electrons released from inelastic scattering b/w incident electrons from beam and electron in atom. The electron from the atom is released. They are low energy, therefore easily collected by detector.

Question 14

How are secondary electrons detected?

Answer 14

electrons attracted to high voltage -> hit scintillator -> photons produced -> go through light guide into photomultiplier tube -> converted to electrons -> amplified as an E field.

Question 15

What is sputtering and why is it used?

Answer 15

It is a heavy metal coating that is used on specimens to prevent losing electrons when they are released. It reduces surface charging of non-conducting samples, hence decreasing the number of secondary electrons produced.

Question 16

What information does secondary electrons provide?

Answer 16

Topographical information

As secondary electrons are produced from surface, therefore surface and edge sensitive in images.

Question 17

What information do backscattered electrons provide?

Answer 17

Atomic number and topographical info

Question 18

What are backscattered electrons?

Answer 18

High energy electrons that rebound back out of sample and are deflected by electrons in the atoms.

Question 19

Why are backscattered electrons useful for differentiating between parts with different average atomic numbers?

Answer 19

Higher avg atomic no. -> more electrons scattered -> therefore elements with higher atomic no. appear brighter -> provides better contrast in image.

Question 20

When does X-ray fluorescence occur?

Answer 20

Electron collides with electron in atom -> electron in atom exits out -> space filled by electron in higher energy level -> X-ray/photons released OR Auger electrons (outer shell electron) released

Question 21

What can happen if X-rays/photons are released from deep inside the atom?

Answer 21

They can be partially absorbed, inducing secondary x-ray emission.

Question 22

What information does Energy-Dispersive X-ray (EDX) spectroscopy provide?

Answer 22

• Characteristic peaks for different materials for elemental analysis
• Spatial resolution determined by size of X-ray generation volume

Question 23

What 2 factors does SEM resolution depend on?

Answer 23

• Spot size of electron beam: narrower beam, higher intensity
• Size of electron-specimen interaction volume

Question 24

Name and explain the 4 main types of SEM

Answer 24

• High vacuum SEM: dry, conductive sample required, which must be stable in a high vacuum
• Variable pressure/low pressure vacuum SEM: In low vacuum, the pressure in the chamber can be adjusted, eliminating surface charging -> allows imaging of non-conductive samples (e.g. biological samples)
• Cryo-Scanning Electron Microscopy (Cryo-SEM): Allows samples to viewed when frozen -> useful for imaging delicate samples

-Environmental Scanning Electron Microscopy (ESEM): Sample temp, humidity and surrounding pressure can be controlled to be able to view sample in its NATURAL state. Useful for studying biological processes and crystallisation dynamics, etc