Transcript ch5x - MyCourses

Electron sources and guns
for TEMs
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
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
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=
Φ
−
𝐴𝑇 2 𝑒 𝑘𝑇
• K is Boltzmann’s constant (8,6 * 10-5 eV/K) and A is Richardson’s constant
(A/m2K2)
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)
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
𝑖𝑒
4𝑖𝑒
𝛽=
=
𝑑0 2
(𝜋𝑑0 𝛼0 )2
2
𝜋( ) 𝜋(𝛼0 )
2
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
Electron beam – coherency
• In coherent beam of electrons, electrons have the same frequency
• This is called temporal coherency
• Definition of coherence length λc is
𝑣ℎ
λ𝑐 =
∆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
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
𝜆
𝑑𝑐 =
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
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
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
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
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
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
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
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
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
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
Usable potential
• The kV axiom:
• ”You should operate at the maximum available kV (unless you shouldn’t).”
• Usually highest possible potential should be used
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•
•
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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
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•
•
•
•
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
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