Electron Microscopy - Wikis introduction to em_01.pdf · TEM dark field image g=(200)dyn HRTEM zone...

Electron Microscopy

An Introduction

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Electron Microscopy

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1. Introduction, types of microscopes, some examples

2. Electron guns, Electron optics3. SEM, interaction volume,

contrasts4. Electron diffraction, X-ray

diffraction5. TEM, contrast, image formation

2

Microscopes…

http://www.ucmp.berkeley.edu/history/hooke.html

1665

2007

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Optical Microscopy

• Antique: first convex lenses• XII-XIII century: magnifying

power of convex lenses: magnifying glass, looking glasses

• 1590: Janssen, first comound microscope

• 1609 Galilei: occhiolino• 1665 Hooke: first image of

biological cell• 1801 Young: wave character of

light• 1872 (~) Abbe: resolution limit

linked to the wave length of the illuminating wave

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magnification

• Ratio between real size of an object and its apparent size on the image (paper, screen, eye)

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Which magnification…?

Who is lying ? The scale bar or the indicted magnification……?

Measure the size of a grain on the screen and calculate the magnification !

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resolution

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Why electrons…?

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Homework

How many electrons are at the same time in the column of a TEM…?

300keVL = 1.5 m

Beam current 1nA

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History

1897: J.J. Thomson: predicts the existence of electrons

1923: De Broglie: concept of wavelength associated to particles, confirmation by Young’s experiment

1927: Busch: focalisation low for magnetic fields, Davisson, Gremer, Thomson: electron diffraction

1931: Ruska, Knoll: first images by electron microscope

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History

FEI Titan2009

Siemens 1939

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Comparison of different microscopes

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Depth of field: photons and SEM

LM

~10-20°

SEM

~10-3 rad

10mm

1mm

100µm

10µm

1µm

0.1µm

10µm 1µm 100nm 10nm

10 102 103 104

1nm

105

SEMLM

=500nm

10mrad

1mrad

0.1mrad

Résolution

Pro

fond

eur

de c

ham

p h

Grandissement (grossissement) G

LM=0.5m

dept

h of

fiel

d h

resolution

magnification M

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Types of electron microscopes

TransmissionElectron Microscope

ScanningElectron Microscope

Slide ProjectorTV

What you see is what the

detector sees !!!

Scanning beam

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15

Types of microscopes

SEM• 0.3-30keV• Inelastic scattering: surface

information: topography• Chemical composition (EDX)• Pseudo-elastic (back-)

scattering: Compositional information (diffraction, EBSD)

• TEM• 60-300keV (up to 3 MeV)• “Projection” of the THIN

sample• Elastic scattering dominant:

Diffraction and diffraction contrast

• Interference between transmitted and diffracted beams: high-resolution (atomic resolution)

• Inelastic scattering: chemical composition (EDX)

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SEM, signals

X-rayselasticinelastic

Backscattered electrons

e-beam

Secondaryelectrons

photons Auger electrons

IR,UV,vis.

Absorbed current, EBIC

• Secondary electrons(~0-30eV), SE

• Backscattered electrons(~eVo), BSE

• Auger electrons

• Photons: visible, UV, IR,X-rays

• Phonons, Heating

• Absorbtion of incidentelectrons (EBIC-Current)

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Some typical SEM

At CIME:Zeiss NVision40

(FIB)

Zeiss UltraHR-SEM

Resolution

1.0 nm @ 15 kV, 1.7 nm @ 1 kV, 4.0 nm @ 0.1 kV

Magnification12 - 900,000x in SE mode Acceleration Voltage0.02 - 30 kV

Probe Current4 pA - 10 nA

Standard Detectors:EsB Detector with filtering gridHigh efficiency In-lens SE DetectorEverhart-Thornley Secondary Electron Detector

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SEM low kV imaging

No specimen preparation needed:Low kV imaging of non-conducting, low density samples

FEI MagellanOperator: Ingo GestmannSamples: Marco Cantoni

Al2O3 Nano-crystals

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Interaction e – matter in a TEM

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Signals from a thin sample

Specimen

Inc

ide

nt b

ea

m

Auger electrons

Backscattered electronsBSE

secondary electronsSE Characteristic

X-rays

visible light

“absorbed” electrons electron-hole pairs

elastically scatteredelectrons

dire

ct

be

am

inelasticallyscattered electrons

BremsstrahlungX-rays

1-100 nm

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Some “typical” TEMs

EPFL: Philips CM300

Japan: HITACHI H-1500

1’300’000V !!!

300’000V

resolution < 1.7Å

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22

Some “numbers”

• “Speed”of a 300keV electron: 76% oflight-speed

• Wavelength of a 300’000V beam:0.0197 Å~2pm (0.002nm)

• Image resolution: ~1.7Å• Typical scattering

angles: 10-3 rad

Pb

Mg/Nb

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Resolution TEM

TEM dark field image g=(200)dyn

HRTEM zone axis [001] HRTEM zone axis [001]

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High resolution….?!

electron at 300keV:

• v = 2.33 10-8 m/s (0.78 c)

• = 0.00197nm

• diff = 10-3 rad

Precipitate in a PbTiO3 ceramic

Pb

Ti

Atomic resolution…?

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Scherzer-defocus: “black-atom” contrast

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Cu

O2

Hg Hg

“White-atom” contrast

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experimental image compared to simulations

Space-charge Random-site

Ta

Mg

Pb

Pb

Cantoni, M; Bharadwaja, S; Gentil, S; Setter, N. 2004. Direct observation of the B-site cationic order in the ferroelectric relaxor Pb(Mg1/3Ta2/3)O-3.JOURNAL OF APPLIED PHYSICS 96 (7): 3870-3875.

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Light element analysis (O)

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Electron microscopy

• A very powerful and versatile tool• “nice images”• High resolution• Different information (signals) from the same

sample• Understanding the image contrast requires

understanding of the interaction of electrons with matter and electron optics (especially in TEM).

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Bibliography, SEM

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Bibliography, TEM

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Electron Microscopy

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33

Summary

•Electron 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 their energy 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).

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Components of an electron microscope

•Source: electron gun

•Lenses and apertures

•Sample holder (stage)

•Detector(s)

common SEM and TEM

Specific for each technique

Vacuumsystem !

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Pumping system

• Primary vacuum (>0.1 Pa)– Mechanical pump

• Secondary vacuum (<10-4 Pa)– Oil diffusion pump– Turbomolecular pump

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

– Ion getter pump– Cold trap

Vaccum level in space:1 Pa at 100km

above earth surface

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SOURCES (gun)

http://www.feibeamtech.com

LaB6 Cathode

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Emission of electrons

metalvacuum(with electrical field)

• Thermionic emission

• Shottky emissionfield-enhanced thermionic emission (108V/m)

• Extended Shottky emissionthermally assisted field emission

• Cold field emissiontunnel effect (quantum tunnelling)

tem

pera

ture

Electric field

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Emission of electrons

metalvacuum(with electrical field)

• Thermionic emission

• Shottky emissionfield-enhanced thermionic emission (108V/m)

• Extended Shottky emissionthermally assisted field emission

• Cold field emissiontunnel effect (quantum tunnelling)

tem

pera

ture

Electric field

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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)

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Thermionic gun

• Tungsten wireheated up to 2800K

• LaB6 crystalheated to 1900K

• Advantagesimple, cheapno high vacuum requiredmaintenance friendly

• Disadvantageslow brightnesshigh energy dispersionlarge source size (30um) MSE-621 2013 41

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 necessary

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Field emission guns

First 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

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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)

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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)

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Lenses for electrons

• Light: glass lensesdeflection of light throughchanging refraction index

• Charged particlesLorentz Force!Electrostatic lensesMagnetic lenses

• Particularity:Variable focusTunable correctors (astigmatisme)

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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

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

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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

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Aberrations:

• Lens aberrations– sperical and chromatical aberrations– Astigmatism– Can be corrected or minimised

• Physical limits– Diffraction effect

Clic

hés:

P.-

A.

Buf

fat

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chromatical aberration

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

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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 3

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Aberrations: astigmatism

Astigmatism: focal length varies in different planes.

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correctorsAstigmatism:

Light optics: correction with cylindrical 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 with combination of convergent and divergent lenses

Electron optics:

Correction with hexapole or quadrupole and octopole lenses

• Cs-corrector

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Aberrations: diffraction

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Résolution SEM

• Limite SEM modèrne

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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

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SEM modèrne

• Pour éviter la perte de brillance à basse tension, le canon travaille toujours à tension élevée.

• Pour amener l'énergie des électrons à la valeur souhaitée par l'opérateur, ceux-ci sont ralentis en sortie de colonne (LEO 1500 Gemini)

• La section bleue (beam booster) est au potentiel élevé V0. Noter le changement de polarité entre V0 et VB

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Resolution of a TEM

• Resolution depends on the aberration of the objetive lens:– Chromatic:

depends on E/E; E @ 300 keV;Cc ~ 1 mm, not critical

– Diffraction: wave lenght = 2 pm @ 300 keV

– Spherical Aberration: limiting !!!

• In 2000: a standard non-corrected TEM 300 keV provides a resolution of ~2Å S

. Pen

nyco

ok e

t al.,

MR

S B

ull.

31, 3

6 (0

6)

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Electron Microscopy

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SEM is easy! Just focus and shoot "Photo"!!!

Please comment this picture...

Any idea what itmight be???

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SEM is easy! Just focus and shoot "Photo"!!!

Does this one sound more obvious?

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Image formation

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SEM, signals

X-rayselasticinelastic

Backscattered electrons

e-beam

Secondaryelectrons

photons Auger electrons

IR,UV,vis.

Absorbed current, EBIC

• Secondary electrons(~0-30eV), SE

• Backscattered electrons (~eVo), BSE

• Auger electrons

• Photons: visible, UV, IR,X-rays

• Phonons, Heating

• Absorbtion of incident electrons (EBIC-Current)

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SEM imaging with electrons

• Energy spectrum of electrons leaving the sample

SE BSE

Auger electrons

SE: secondary electrons 0-50eVBSE: backscattered electrons E>50eV

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Electron detectors

Collects and detects lower energy (<100eV) electrons: SEM backscattered electrons

• Photomultiplier• Everhart-Thornley detector

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Electron detectors

• semiconductor

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

large collection angleslow (poor at TV frequency)

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

Detects higher energy (>5kV) electrons: SEM backscattered electrons

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SEM, secondary electrons

• Electrons with low energy (0-50eV) leaving the sample surface

• Intensity depends on inclination of the surface

• topography

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SEM, backscattered electrons

• Electrons with high energy (~Eo) backscattered from below the surface

• Intensity depends on density (atomic weight)

• ~compositionNb3Sn in Cu matrix

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The interaction volume:Monte-Carlo simulations

• Electron Flight Simulator ($$$ Small World / D. Joy)– old… DOS !!!!– http://www.small-world.net

• Single Scattering Monte Carlo Simulation (Freeware)– "Monte Carlo Simulation" Mc_w95.zip– by Kimio KANDA – http://www.nsknet.or.jp/~kana/soft/sfmenu.html

• CASINO (Freeware)– " monte CArlo SImulation of electroN trajectory in sOlids "– by P. Hovongton and D. Drouin– http://www.gel.usherbrooke.ca/casino/What.html

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Vacc = 20kV = cte

Depth of electron penetration vs Z and yield of electron backscattering BSE (Monte-Carlo simulation):

Cu 20keV

1

1

C 20keV

1

U 20keV

BSE=6%

BSE=33%

BSE=50%

Penetration and backscatteringvs elements (Z)

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Z = cteDepth of electron penetration in Cu vs energy E0and yield of electron backscattering BSE (Monte-Carlo simulation):

Cu 20keV

1

Cu 5keV1

Cu 1keV

Cu 1keV

1

BSE=32%

BSE=33%

BSE=35%

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Vacc = 1kV = cte


10nm

C 1keV

U 1keV10nm

Cu 1keV

10nm

BSE=14%

BSE=34%

BSE=44%

Penetration and backscattering

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Vacc = 5kV = cte


200nm

C 5keV

Cu 5keV

200nm

U 5keV

200nm

BSE=8%

BSE=33%

BSE=47%

Penetration and backscattering

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SEM: "true" secondary electrons SE1 and "converted BSE" secondaries SE2+SE3

Different types of SE from•SE1: incident probe•SE2: BSE leaving the sample•SE3: BSE hitting the surroundings

The SE signal always contain a high resolution part (SE1 from the probe) and an average (low resolution) part from SE2+SE3!

although this signal is gathered around the probe, its intensity is only attributed to the pixel corresponding to the actual probe position

x0,y0

intensity and delocalisation of SE stemming from the probe at x0,y0 (leadingto the X0,Y0 pixel intensity on the image)

(from L. Reimer, Scanning Electron Microscopy)

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Relative contribution of SE1 and SE2 (+SE3) vs primary energy

total

total

total

SE1

SE2

The total intensity (green+brown) is attributed to the (x,y) pixel, here at 0 nm on this 1-D model

(adapted from D.C. Joy Hitachi News 16 1989)

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Yield for SE and BSE emission per incident electron vs atomic number Zsample surface polished (no topography) and perpendicular to the incident beam direction

(intermediate energy E0 15 keV)

SE: low or no chemicalcontrast but for light elementsthe topographical contrast willdominate on rough surfaces

ISE=Ipe· +ISE3

Ipe(pe+pe·sur·)

with the total SE yield, pe the yieldfor SE1 and sur the SE3 yield for materials surrounding the sample (pole-pieces...)

SE1 SE2 SE3

Al Ni

0.11

0.28

(SE1)

BSE: chemical contrast for all the elements (sensitivity Z=0.5)A fast way to phase mapping

IBSE=Ipe· with Ipe the intensity of the primary beam, the BSE yield

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SE 25 kV BSE

Dust on WC (different Z materialslow Z materialflat material rough material low Z material

thin low Z material

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Relative yield of SE vs angle of incidence on the sample surface

Topographic contrast in SE mode

(0)( ) cos peII I

I0I()

1-10nm

penetration depth ("range") >>SE escape length

(adapted from D.C. Joy Hitachi News 16 1989)

This image cannot currently be displayed.

Autumn 2009 Experimental Methods in Physics Marco CantoniMSE-621 2013 80

Size and edge effects

Do not forget, in SEM:The signal is displayed at the probe position, not at the actual SE production position!!!

intensity profile on image

(adapted from L.Reimer, Scanning Electron Microscopy)

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(From L. Reimer, Image Formation in Low-Voltage

Scanning Electron Microscopy,

(1993))

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Tin balls

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Change in secondary electron contrast withaccelerating voltage

(from L.Reimer, Image formation in the low-voltage SEM)

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Contraste enhancement at low voltage: less delocalization by SE2. An example: a fracture in Ni-Cr alloy

SE, 5 kV SE, 30 kV

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SEM low kV imaging



Al2O3 Nano-crystals

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SEM low kV imaging



Al2O3 Nano-crystals

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SEM low kV imaging



Carbon nano-tubes(MWCT)

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SEM low kV imaging

No specimen preparation needed:Low kV imaging of non-conducting samples

Liquid filledorganic membranes

Zeiss Nvision 40Marco Cantoni

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SEM low kV imaging

No specimen preparation needed:Low kV imaging of non-conducting samples

Liquid filledorganic membranes

Zeiss Nvision 40Marco Cantoni

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SEM low kV imaging

Purely organic specimen: non-conductive, low density:Metal coating

HeLa Cells, Graham KnottMarco Cantoni, Nvision 40

15nm Ag/Pd coating

3nm Os coating

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SEM low kV imaging

“Easy” samples:

SC wireNb3Sn in Cu matrix

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SEM low kV imaging

“classical” conditions: 20kV Everhard-Thornley detector (SE)

Solid state BSEdetector

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SEM low kV imaging SE and BSE imaging

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Electron Microscopy

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95

Typical TEM

EPFL: Philips CM300

300’000V

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Electron Microscopy - Wikis introduction to em_01.pdf · TEM dark field image g=(200)dyn HRTEM zone...

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