Scanning Electron Microscopy Flashcards
What is the resolution of a High-end SEM?
~ 1 nm
What can be studied with SEM?
Topography/ morphology
Chemistry
Crystallography
Orientation of grains
What is the equation for resolution?
r = 0.61 λ / n sin α
Where n sin α is the numerical aperture
What happens when wavelength goes down?
Resolution goes up
What is the voltage and subsequent Wavelength of Electrons
200 kV ~ 0.0025nm
r ~ 0.015 Å
How does the Aperture angle affect the depth of field?
Larger Aperture angle = Small depth of field
Which Depth of field is larger?
SEM
What is the equation for depth of field?
D = d/tan α
Where D = depth of field
and d = resolution at magnification
What passage do electrons take through an SEM
Electron gun, Condenser Lens, Beam deflector, Objective lens, Specimen, Detector
What can the electron gun sources be, and what are they suitable for (from least to most expensive)
Tungsten hairpin ($10), Lanthanum Hexaboride ($1000) - work on Thermionic emission
Field emission ($10000) - Strong electron field
What does the Condenser lens do?
Demagnifies the source - produces small beam of electrons
What does the beam deflector do?
Moves the beam of electrons to the correct position
What does the objective lens do?
Focuses the beam to a small spot, the size of which being controlled by the objective aperture
How does the Thermionic Emitter Electron source work?
- A negative cathode is heated to produce electrons (Up to 2700K for Tungsten)
- Electrons accelerated by positive anode potential
- Wehnelt Cap (-500V) repels electrons toward optical axis
- Electrons collect in space between filament tip and Wehnelt cap
- Electrons nearest to anode exit gun through small (<1mm) hole in wehnelt cap
What are the properties of an electron beam?
Point source, Similar energies (monochromatic) and parallel to optic axis
How does the FEG worl?
Source size = 50 nm
First anode is positively charged by several kV with respect to tip, extracting electrons by enabling them to tunnel out of the tip. Second anode accelerates the electrons to 100 kV or more
What are the benefits and drawbacks of using a thermionic source for electrons?
- Cheap, easy to handle, moderate vacuum required, good stability
-Not long lasting (100 hrs), low Brightness ratio, and bigger attainable resolution
What are some benefits and drawbacks of a field emission electron source?
- Large Brightness ratio, much smaller energy spread, longer lifetime
- Significant cost, Requires proper handling, very high vacuum required
What are backscattered electrons?
-Elastic interaction with the atom in the material
- Have a high energy as they are close to primary beam
- BSE emission is dependant on the avg atomic number of sample
What are secondary electrons?
- Generated from the collision between the incoming electron and loosely bonded outer electron
- Primary electron strikes electron in atomic shell
- this electron becomes a secondary electron (low energy)
- primary Electron continues colliding
- Only SE generated close to surface escape (topographic information is obtained)
What has higher energy, Secondary electrons or backscattered electrons?
Back scattered electrons (> 50 ev)
What happens to the interaction volume when the accelerating voltage decreases
It decreases, but increases the size of the beam as the wavelength of electrons is increased
How is a pixel brightness calculated?
The more a scanner is hit with electrons, the brighter that pixel from that scanner will be (Brighter pixel = more counts)
What is the equation for magnification?
magnification = Distance displayed on screen / Scan distance
What happens to the number of secondary electrons escaping from an edge as opposed to a flat area?
more
What creates a channeling contrast image and why?
BSE, as different atomic numbers will result in altered Backscattered electron energies
What is the resolution of Secondary and Backscattered electron imaging, and why?
Secondary = 1 - 50nm, lower energy and smaller specimen volume
BSE = 10 - 300nm, as much greater penetration depth
What penetrates more? BSE or SE
BSE (10 - 300 nm)
Why is resolution important?
Allows us to distinguish two close by objects instead of seeing just one
What info do you get in BSE?
The Z-Contrast
