Scanning Electron Microscopy Flashcards
What makes electrons more useful than light (optical microscope)?
Wavelength of moving electron decreases as speed increases.
Optical microscopy vs Electron microscope
see notes
What 4 things does electron microscopy provide information on?
- Topography/surface features
- Morphology (shape + size)
- Elemental composition
- Crystallographic info (arrangement of atoms)
Describe the role of the filament in SEM
Filament (cathode) heated up using current -> electrons have enough energy to escape -> electrons emitted are guided by anode into focussed beam.
Concisely describe general process of how SEM works
align electron beam -> hit sample -> electrons released + detected
Name and compare 3 ways to produce the electron beam
- 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
- 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
- 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
Name the 3 parts of the magnetic lens system and describe what each part does
[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
What functionality does the stage provide?
It holds he specimen and can be moved horizontally (to change field of view) and vertically (to change resolution)
Why is a vacuum required?
reduces scattering of electron beam, therefore reduces probe size and increase resolution (as not losing electrons)
Order of components in SEM machine
electron source, anode, condenser lens, scan coils, objective lens, (secondary electron detector), sample
What is released from inner layer vs outer layer of the sample?
Electrons from higher layers
photons/X-rays from deeper layers
Name the 3 things produced when electron beam interacts with sample
- Secondary electrons
- Backscattered electrons
- Xray fluorescence or Auger electrons
Note: have different detectors for each
What are secondary electrons?
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.
How are secondary electrons detected?
electrons attracted to high voltage -> hit scintillator -> photons produced -> go through light guide into photomultiplier tube -> converted to electrons -> amplified as an E field.
What is sputtering and why is it used?
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.
What information does secondary electrons provide?
Topographical information
As secondary electrons are produced from surface, therefore surface and edge sensitive in images.
What information do backscattered electrons provide?
Atomic number and topographical info
What are backscattered electrons?
High energy electrons that rebound back out of sample and are deflected by electrons in the atoms.
Why are backscattered electrons useful for differentiating between parts with different average atomic numbers?
Higher avg atomic no. -> more electrons scattered -> therefore elements with higher atomic no. appear brighter -> provides better contrast in image.
When does X-ray fluorescence occur?
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
What can happen if X-rays/photons are released from deep inside the atom?
They can be partially absorbed, inducing secondary x-ray emission.
What information does Energy-Dispersive X-ray (EDX) spectroscopy provide?
- Characteristic peaks for different materials for elemental analysis
- Spatial resolution determined by size of X-ray generation volume
What 2 factors does SEM resolution depend on?
- Spot size of electron beam: narrower beam, higher intensity
- Size of electron-specimen interaction volume
Name and explain the 4 main types of SEM
- 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
