Scanning Electron Microscopy (SEM) Flashcards
What are the SEM components of the experiment?
- Electron gun (under vacuum)
- Electromagnetic Lens
- Scan Coils / Generator beam
- Electron/Xray detector
What is the function of the electron gun in SEM?
It is the speed of emitted electrons controlled by accelerating voltage applied to the cathode and anode plates.
What is the function of the Electromagnetic Lens?
Focuses the beam, by a current being applied to wire coiled around iron core, creating a magnetic field acting as a lens.
What is the function of the scan coils?
Made to scan lines across the samples.
What is the function of the electron detector and the x-ray detector.
Electron detector: image formation
X-Ray detector: elemental analysis
How is the image formation done in SEM?
- Electron detector placed in sample chamber at positive potential
- Attracts secondary electrons emitted from sample surface
- Incident beam scans across sample’s surface topography
- Some secondary electrons enhanced or reduced
- Regions of light and dark form contrast in image.
What are the three components of the triode electron gun?
- Filament/cathode held at negative potential
- Wehnelt/grid held at few hundred volt
- Anode held at earth potential
How do the electromagnetic lens function?
- Condenser affects number of electrons in the beam
- Lens focuses electrons on sample at working distance
- Lens strength varied by adjusting amount of current flowing through
What are the advantages of using SEM?
- High magnification
- Large depth of field
- Great resolution
- Compositional and crystallographic information
What is a depth of field?
The heigh over which a sample can be clearly focused. The reason for the production of images in 3D.
What are the properties required for the depth of field?
- Long working distance
- Small objective aperture
- Low magnification
What are the three main components of Electron Microscopes?
- Electron optical column
- Vacuum system
- Signal detection & display
What does the electron optical column in SEM consist of, and state the purpose of each?
- electron source - produce electrons
- magnetic lens - de-magnify beam
- magnetic coils - control and modify beam
- apertures - define beam, prevent electron spray
What does the vacuum system in SEM consist of, and state the purpose of each?
- chamber - holds vacuum,
- pumps - produce vacuum
- valves - control vacuum
- gauges - monitor vacuum
What do the signal detection and display in SEM consist of, and state the purpose of each?
- detector - collect signal
- electronics - produce image from signal
What are the interactions that occur when an electron beam strikes the sample?
- Elastic scattering: backscattered electrons
- Inelastic scattering:
- Secondary electrons
- X-rays
- Auger electrons
- Phonons
- Transmitted electrons
- Cathodoluminescence
What are secondary electrons?
Associated with non-elastic collisions, energy loss occurs.
What are backscattered electrons, and their purpose?
Associated with elastic collisions, no energy loss occurs.
- contribute to images which are atomic number dependent
What occurs at a low atomic no. for BSE?
Will not emit as many BSE, resulting in brighter areas corresponding to heavy elements.
What are the operating conditions for SEM?
- Accelerating Voltage - important is resulting interaction volume, 5 keV penetrate 1 μm.
- Spot size - improves SIN ratio, beam may damage sample
- Working distance - 5 - 40 mm
- Aperture size - define effective diameter of lens
- Beam current
- resolution - limited by beam diameter
What is the beam current?
The measure of the number of electrons making up the beam, and is seldom greater than 100 uA
What is the specimen interaction volume?
The volume inside the specimen in which interactions occur, while irradiated with electron beam.
What are the factors affecting the specimen interaction volume?
- Atomic number - high atomic no., absorb more electrons, hence small volume.
- Accelerating voltage - high voltage, penetrate further in sample, large volume.
- Angle of incidence of electron beam - larger angle, smaller volume
What is FESEM (or FEGSEM)?
Field Emission SEM:
1. produces clearer, less distorted images
2. Spatial resolution - 1 1/2 nm
3. 3 to 6 times better than SEM
How does FESEM work?
FE gun emits electrons from smaller area, coherency is higher, energy spread is smaller.
What are properties of FESEM?
- Fine, sharp, single crystal tungsten tip
- FEG emitter - more coherent beam
- Brightness higher than tungsten filament in SEM
- Electrons emitted from smaller area
- Source size - few nm
- Improved image quality
- Lifetime - 10,000 hours
- Better vacuum - 10-8 Pa (10-10 torr)
What are properties of SEM compared to FESEM?
- Source size of Electrons
SEM: 50 μm
FESEM: few nm - Lifetime
SEM: 100 -500 hours
FESEM: 10,000 hours - Vacuum
SEM: 10-3 Pa (10-5 Torr)
FESEM: 10-8 Pa (10-10 Torr)
