a) vacuum systems, b) electron guns, c) electron optics, d ... fileresting for a longer time in the...

MSE-603 Electron guns, lenses, detectors Marco Cantoni 1

1.Components of an electron microscope

Marco Cantoni, 021/693.48.16

Centre Interdisciplinaire de Microscopie Electronique

CIME

a) vacuum systems,b) electron guns,c) electron optics,d) detectors


SummaryElectron propagation is only possible through vacuum. The vacuum level varies in the different areas of an electron microscope. The highest vacuum level (<10-7 Pa or 10-9mBar) is required in the gun where electrons are emitted through field emission. Also the specimen area requires a high vacuum level especially for chemical analysis when the electron beam is resting for a longer time in the same area. Hydrocarbon build up (contamination) on the observed area is often the result of a low system vacuum level. Turbomolecular and oil-diffusion pumps for high vaccum cannot work against atmospheric pressure and need a mechanical prevaccum pump in order to function.Electron beams can either be generated by thermal emission (thermionic sources, cheap) or field emission. Only field emission sources can provide the necessary low energy spread and coherence for modern high resolution electron microscopy and electron spectroscopy.Electrons are focused by simple round magnetic lenses which properties resemble the optical properties of a wine glass…. Unlike in light optics the wavelength (2pm for 300kV) is not the resolution limiting factor. However lens aberrations and instabilities of the electronics (lens currents etc.) limit the resolution of even the best and most expensive transmission electron microscopes to about 50pm.Recording an image means detecting electrons. Depending on theirenergy electrons can be detected by different detectors. A high detector efficiency and a high signal to noise ratio allows faster recording and reduces the exposure (beam damage) of the sample to the electron beam. A high linearity and high dynamic range permits to quantify images and to record high and low intensities in one image (important for diffraction experiments).


Components of an electron microscope

•Source: electron gun

•Lenses and apertures

•Sample holder (stage)

••Detector(sDetector(s))

common SEM and TEM

Specific for each technic

Vacuumsystem !


Pumping system

• Primary vacuum (>0.1 Pa)

– Mechanical pump

• Secondary vacuum (<10-4 Pa)

– Oil diffusion pump

– Turbomolecular pump

• High and ultra-high vaccumGun & specimen area (<10-6 Pa)

– Ion getter pump

– Cold trapVaccum level in space:

1 Pa at 100kmabove earth surface


Primary vacuum

• Rotary vane pump– Uses oil– noisy


Secondary vacuum• Oil diffusion pump

– Vibration free– Contamination possible oil vapor– High pumping capacity (>500 l/s)– Best with cold trap


Secondary vacuum

• Turbomolecularpump

– Rotation speed 20-50’000 rpm– Magnetic bearings– Pumping volumes50-500 l/s


High / Ultra-high vacuum

• Ion getter pump–no vibrations–No exit:improves vacuum !


Contamination

• Oil vapors from oil diffusion pump–Heat the sample up to 100°C–Cool the sample down to -200°C (danger ice!)–Clean the sample holders regularly

• Don’t touch samples and sample holders(even with gloves)

• Use a plasma cleaner before observation


SOURCES (gun)

http://www.feibeamtech.com

LaB6 Cathode


Emission of electrons

metalvacuum(with electrical field)

• Thermionic emission

• Shottky emissionfield-enhanced thermionicemission (108V/m)

• Extended Shottky emissionthermally assisted fieldemission

• Cold field emissiontunnel effect (quantum tunnelling)

tem

pera

ture

Electric field


Emission of electrons

metalvacuum(with electrical field)

• Thermionic emission

• Shottky emissionfield-enhanced thermionicemission (108V/m)

• Extended Shottky emissionthermally assisted fieldemission

• Cold field emissiontunnel effect (quantum tunnelling)

tem

pera

ture

Electric field


Electron gun

Important parameters

• Emitted current, energy

• Energy dispersion

• Brightnesscurrent per surface unit and solid angle

• Coupling to the column

• the gun incorporates often a first lens (Wehnelt, gun lens)


Thermionic gun

• Tungsten wireheated up to 2800K

• LaB6 crystalheated to 1900K

• Advantagesimple, cheapno high vacuum requiredmaintenance friendly

• Disadvantageslow brightnesshigh energy dispersionlarge source size (30um)



Field emission guns

Cathods

• Cold field emission (E≈109V/m)W monocristal with sharp tiptip radius ~100nm

• Thermally assisted emission:Shottky effectW/Zr tip at 1700-1800K

• AdvantagesSmall energy dispersion (<0.4eV)high coherence, high brightness-> higher resolution at lower energies

• Disadvantagesexpensivehigh vacuum necessarycold emission needs flushing (cleaning) after 8 hrs


Field emission gunsFirst anode (extractor)

• Some kV

• 5.109 V/m

Second anode

• Final acceleration

• Grounded

Characteristics

• Tip and anodes form an electrostatic condensor

• Cross-over (source) is virtualØ~5nm


Ion gun (FIB)

LMISSource: FEI Beam

Technology Division

Most common: LIMSLiquid Metal Ion Source

• W tip

• Liquid metal wets the tip through surface tension and electrostatic force

• Ionization and emission by field effect (~1010V/m)

• High brightness

• Small emitting surface (Taylor cone)

• Small Ion probes (~5nm) possible: FIB Focused Ion Beam



Optics, basics

tiré de Carter/Williams


Optics, basics

Carter/Williams

Image plane

Object plane

Focal plane


Optics, basics

tiré de Carter/WilliamsTEM: transmitted beam, diffracted beams


Optics, basics

FocusOver-focusUnder-focus



Optics, basics

Angle limiting aperturesCollection angle


Use:

• condensor lens system

• Diffraction contrast


Condensor lens system

Convergent illumination <-> parallel illuminationprobe mode (SEM ou STEM)

« projection » mode TEM


Condensor lens system, SEM

• Condensor aperture– Convergence angle– intensity (current)


Condensor lens system, SEM

• Condensor I (C1)– Defines probe size– Total Current


Projector lens system, TEM

mode DIFFRACTION mode IMAGE

TEM:• Intermediate and

projector lenses– Projection of the back focal plane to the screen“diffraction” mode– Projection of the image plane to the screen“image” mode(haute resolution)


Projector lens system, TEM

mode DIFFRACTION mode IMAGE

TEM:• Intermediate and

projector lenses– Projection of the back focal plane to the screen“diffraction” mode– Projection of the image plane to the screen“image” mode(haute resolution)


Lenses for electrons

• Light: glass lensesdeflection of light through changingrefraction index

• Charged particlesLorentz Force!Electrostatic lensesMagnetic lenses

• Particularity:Variable focusTunable correctors (astigmatisme)


Electrons in a magnetic field

• Homogeneus field, α small

• Component of v // B almost unchanged

• Component of v ┴ B: vr << |v|

• Spiral with radius r = m vr/eB

• All electrons crossing the axis in one point are focused into the same point, α, vr

• Focal length depends on Bincreasing B lowers f

www.x-raymicroanalysis.com

Optical axis

α


Magnetic lens• Field with rotational symmetry

• Lorenz Force : F = -e v ^ Be on optical axis: F = 0e not on optical axis : deviatedoptical axis: symmetry axis

Scherzer 1936:Magnetic lens with rotational symmetry:

Aberration coefficients:Cs: sphericalCc: chromatical– Always positive !!

4/14/366.0 sres CD λ=

Example: λ= 0.00197nm, Cs = 1 mmDres = 1.8 10-10 = 1.8ÅResolution limit:


Magnetic lens

• Electron optics: no sharp interface at lens « surface »

• No divergent lens !

• Electron beam diverges by itself– Electrostatic repulsion

• “multi-poles” lenses– Correction of aberrations

• “Pole piece”metal cone that confines the magnetic field

• Image rotation !

Pole piece

irone-beam

coil

www.x-raymicroanalysis.com


Aberrations:• Lens aberrations

– sperical and chromatical aberrations

– Astigmatism

– Can be corrected or minimised

• Physical limits

– Diffraction effect

Clic

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P.-

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chromatical aberration

Focal length varies with energycritical for non-monochromatic beams (advantage for FE guns)


Spherical aberration

Focal length depends on the distance from optical axis

Image of the object is dispersed along the optical axis

Circle of least confusion ds = ½ Cs α3


Aberrations: astigmatism

Astigmatism: focal length varies in different planes.


correctorsAstigmatism:

Light optics: correction withcylindrical lenses

Electron optics:

Correction with quadrupole lenses:2 quadrupole lenses under 45 degree allow to control strenght and direction of correction

Spherical Aberration:

Light optics: correction withcombination of convergent and divergent lenses

Electron optics:

Correction with hexapole or quadrupole and octopole lenses

Cs-corrector


Aberrations: diffraction


Résolution SEM

Limite SEM modèrne


Resolution: SEM vs TEM


Resolution SEM vs TEM


Resolution: SEM

100

50

10

5

10.5 1 2 5 10 20 30

FE LaB6

W

Tension d'accélération (kV)

Rés

olut

ion

(nm

)

Basse tension/haute résolution: - observation de la surface réelle - échantillons non-métallisés - faible endommagement dû au faisceau

Haute tension/haute résolution: - effets de bord - détails fins non-résolus - fort endommagement dû au faisceau

1985

2000

High voltage, high resolution

Edge effects, fine details not resolved

Beam damage

Low voltage, high resolution

Observation of the real surface

Uncoated samples

Very little beam damage


Resolution of a TEM

• Resolution depends on the aberration of the objetive lens:

– Chromatic:depends on ΔE/E; E @ 300 keV;Cc ~ 1 mm, not critical

– Diffraction: wave lenghtλ = 2 pm @ 300 keV

– Spherical Aberration: limiting !!!

• In 2000: a standard non-corrected TEM 300 keVprovides a resolution of ~2Å S. P

enny

cook

et a

l., M

RS

Bul

l. 31

, 36

(06)


Electron detectors

semiconductor

BSE semiconductor detector: a silicon diode with a p-n junction close to its surface collects the BSE (3.8eV/e--hole pair)

large collection angleslow (poor at TV frequency)

some diodes are split in 2 or 4 quadrants to bring spatial BSE distribution info

DetectsDetects higherhigher energyenergy (>5kV) (>5kV) electronselectrons: SEM : SEM backscatteredbackscattered electronselectrons


Electron detectors

CollectsCollects and and detectsdetects lowerlower energyenergy (<100eV) (<100eV) electronselectrons: SEM : SEM backscatteredbackscattered electronselectrons

PhotomultiplierEverhart-Thornley detector


Electron detectors

Caméra CCD(charge coupled device)1kx1k, 2kx2k, 4kx4k Pixel+ high dynamic range, sensitive- Slow (no TV rate)- expensive

From http://www.gatan.com


Electron detectors

Imaging plates

• High dynamic range 65’000 grey levels

• Absolutely linear senisitvity

• big surface

• Numerical Image


Electron detectors

Imaging plates

• High dynamic range 65’000 grey levels

• Absolutely linear senisitvity

• big surface

• Numerical Image


Electron detectors

Application: electron diffraction

a) vacuum systems, b) electron guns, c) electron optics, d ... fileresting for a longer time in the...

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