1. Electron sources and guns
for TEMs

2. Different sources Electron source affects image quality
Best available sources need to be used
Two types of sources
Thermionic
Modern sources are lantanum hexaboride LaB6 crystals, older are tungsten filaments
Field-emission (FE) sources
Fine tungsten needles
Carbon nanotubes are researched as alternative
Most advanced TEMs use FE sources, but most use still thermionic sources

3. Principles of electron sources
Thermionic sources produces electrons when heated
FE sources produce electrons when large electric potential is applied
FE source works as cathode
TEMs can use only one type of electron source
FE sources are more monocromatic than thermionic sources
Emission varies with crystal orientation of the source
Best orientation for tip of LaB6 crystal is <110> and <310> for tungsten crystalline tip

4. Thermionic emission
All materials can emit electrons if given sufficient energy
However, most materials either melt or vaporize when few eV of thermal energy is introduced
Viable source needs to have high melting point or low work function Φ
Thermionic emission can be summarized in Richardson’s law
𝐽=𝐴 𝑇 2 𝑒 − Φ 𝑘𝑇
K is Boltzmann’s constant (8,6 * 10-5 eV/K) and A is Richardson’s constant (A/m2K2)

5. Field emission Electric field E increases considerably at sharp points
When voltage V is applied to sperical point with radius r, then
𝐸= 𝑉 𝑟 .
One of the easiest materials to produce with sharp needle point is tungsten
Tungsten wire can be given with tip radius of < 0,1 µm
Two different FE methods: cold field emission (CFE) and thermal field emission (TFE)
FE to occur, the surface of the tip must be free of contaminants and oxide.
This can be achieved by operating on ultra high vacuum (UHV) (< 10-9 Pa)

6. Electron beam – brightness
Electron beam is affected by source
There is no best source for all applications
Brigthtness is relevant to intensity viewed on screen affecting, how easy it is to see images and operate the microscope
Characteristics of source are diameter d0, cathode emission current ie and divergence angle α0
Brightness can be defined as
𝛽= 𝑖 𝑒 𝜋( 𝑑 0 2 ) 2 𝜋( 𝛼 0 ) 2 = 4 𝑖 𝑒 (𝜋 𝑑 0 𝛼 0 ) 2

7. Electron beam – brightness
For thermionic sources Β increases linearly with accelerating voltage
Higher value of β allows more electrons to be put into beam of given size
More information can be generated from specimen
Sensetive spesimen can be damaged more easily
Electron density can be increased by using brighter source
This shortens exposure time minimizing image drift and instabilities

8. Electron beam – coherency
In coherent beam of electrons, electrons have the same frequency
This is called temporal coherency
Definition of coherence length λc is
λ 𝑐 = 𝑣ℎ ∆𝐸
V is electron velocity, h is Planck’s constant and ΔE is energy spread of the beam
Stable power supply is needed to have small ΔE
Temporal coherency is important for energy spread
Because of good high-voltage supplies, energy spread will not often limit aspects of TEM

9. Electron beam – coherency
Coherency is related to the size of the source
Smaller sources give better coherency
Called spatial coherency
The effective source size for coherent illumination can be calculated
ⅆ 𝑐 = 𝜆 2𝛼
λ is wavelength of electron, α is angle subtended by the source at the specimen
α can be controlled by using aperture
Coherency can be maximized by
Using smaller source
Using smaller illumination aperture
Decreasing accelerating voltage

10. Electron beam – stability
Stable high-voltage supply to source is needed
Also electron current from the source must be stable
Intensity on screen varies if system is not stable
Thermionic sources are very stable
Variations < ± 1 %
Better UHV conditions improve stability
Summary about electron source and beam
Important properties for sources: brightness, temporal coherency, energy spread, spatial coherency and stability

11. Electron guns
Electron guns are needed to be able to control the electron beam
Source is incorporated into a gun assembly
Assembly acts as focusing lens to electron beam
Different designs for thermionic and FE sources

12. Electron guns – thermionic
The LaB6 crystal source works as cathode
Cathode is in grid called Wehnelt cylinder
Anode is at earth potential
Cable is used to attach cathode to high-voltage
supply
Metal wire such as rhenium is bonded to LaB6
crystal
Wire is used to resistively heat source causing thermionic emission
To control electron beam, small negative bias is applied to Wehnelt cylinder
This converges electrons to a point called crossover

13. Electron guns – thermionic
The gun is designed to increase Wehnelt bias with increase of emission current
This is called self-biased gun
When increasing the current to heat source does not increase emission current, saturation condition is achieved
Thermionic sources should be operated just below saturation as operating above it, will reduce lifetime of source without any advantage
Brightess is also optimized when operating at saturation
Standard way of achieving saturation is to look at TEM screen for the image of source crossover

14. Electron guns – thermionic
LaB6 crystals should be operated just below saturation
This will increase lifetime of source without compromising signal
LaB6 crystals can break due to thermal shock if heated or cooled too rapidly
TEM computer often controls heating and cooling
Aligning the source may need to be done but sources are usually prealigned
Most modern TEMs have electronic corrections to ensure alignment
Only adjustments the user has to do to gun are alignment and saturation

15. Field emission gun (FEG)
FEGs are much simpler than thermionic guns
When switched on, the extraction voltage
must be increased slowly
Risk of fracturing tip by thermo-mechanical shock
Rest of the steps are computer controlled
FEG has two anodes
First anode has extraction voltage to pull electrons out of the tip
Second anode accelerates electrons to right potential
Two anodes of FEG work as electrostatic lens

16. Field emission gun (FEG)
CFE requires clean surface without contaminants or oxide
Contaminants build on the tip, even in UHV conditions
Contaminants are necessary to be removed bt flashing the tip
Potential can be reversed to blow off the surface layers of atoms
Tip can be quickly heated to ~5000 K to evaporate the contaminants
Most CFE guns do flashing automatically
Contaminants cause emission currents to decrease and extraction currents to increase

17. Guns compared
LaB6 crystals current densities are higher than that of tungsten, also brightness is significantly grater and operating temperature is lower
LaB6 sources are smaller resulting in better cohenrency and energy spread
In FEGs, the current densities are even greater
Brightness is correspondingly high
FEGs are best when brightness and coherency is required
Brightness of FEG at 100 kV is significanlty greater than that of LaB6 source at 400 kV
CFE has best spatial coherency without monochromation
TFE provides greater stability and less noise
CFE requires UHV and is cleaner

18. Guns compared
TFE tip is cleaned thermally, causing lower stress on tip compared to flashing and resulting in longer lifetime of the source
FEG source is too small for relatively low magnification (<50 – X), LaB6 source is better in these circumstances

19. Usable potential The kV axiom:
”You should operate at the maximum available kV (unless you shouldn’t).”
Usually highest possible potential should be used
However, high potential beam could cause damage to some specimen
Displacement damage threshold for most metals is less than 400 kV
While studying crystalline specimen by diffraction contrast, lower is better
Below 100 kV is not practical for most materials
High potential should be uset to
Get greatest brightness
Get shortest wavelength to get better resolution
Heating effects may be smaller due to less inelastic scatter
Observing thicker specimens
Peak to background ratio is improved

20. Summary Most TEMs use thermionic emission with LaB6 sources
Operating just below saturation optimizes image quality and source lifetime
Operate at the highes possible potential
Use FEG TEM for best resolution
High coherency is important for high resolution imaging
If source have to be changed, aligned or saturated, treat it carefully