Electron sources and guns for TEMs Different sources

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 La. B 6 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 monocromatic than thermionic sources • Emission varies with crystal orientation of the source • Best orientation for tip of La. B 6 crystal is <110> and <310> for tungsten crystalline tip

Thermionic emission •

Field emission •

Electron beam – brightness •

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 •

Electron beam – coherency •

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 La. B 6 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 La. B 6 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 • La. B 6 crystals should be operated just below saturation • This will increase lifetime of source without compromising signal • La. B 6 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 • La. B 6 crystals current densities are higher than that of tungsten, also brightness is significantly grater and operating temperature is lower • La. B 6 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 k. V is significanlty greater than that of La. B 6 source at 400 k. V • 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), La. B 6 source is better in these circumstances

Usable potential • The k. V axiom: • ”You should operate at the maximum available k. V (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 k. V While studying crystalline specimen by diffraction contrast, lower is better Below 100 k. V 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

Summary • Most TEMs use thermionic emission with La. B 6 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