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 = 𝐴𝑇2𝑒−
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 =
𝑖𝑒
𝜋(
𝑑0
2
)2𝜋(𝛼0)2
=
4𝑖𝑒
(𝜋𝑑0𝛼0)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
λ𝑐 =
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
𝑑𝑐 =
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 – 100 000
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
