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Transmission Electron Microscopy Transmisná elektrónová mikroskopia Elektrónová mikroskopia na priesvit

Alica Rosová Institute of Electrical Engineering SAS

Recommended literature:[1] P.B. Hirsch et al, Electron microscopy of thin crystals [2] D.B. Williams and C.B. Carter Transmission electron microscopy[3] J.W. Edington, Practical electron microscopy in material science

Why electrons?

• First motivation – higher resolution power

The smallest resolved distance :• Naked eyes ~ 0.1 – 0.2 mm• Optical microscopy uses the visible light with

wavelength ~ 380 - 750 nm ~ 400 nm

• Electrons: (E is energy of electrons)

𝐸for 100 kV = 4 pm = 0.004 nm

• 1960’s 1 MV, 3 MV – HVEM• However, electromagnetic lens are low quality lenses…. • Lens errors corrections!!!!! – lower voltage TEM with aberration correction

additional lens systems – up to subatomic resolution

Rayleight criterion for light microscopy

=0.61 𝑠𝑖𝑛

- wavelength - refractive index - semiangle of collection of the magnifying lens

Why electrons - resolution

[Pennycoock MRS Bull. 31 (2006) 36]

A grain boundary of SrTiO3 without and with CS correction

However, not only the better resolution, but also new analytical possibilities!!!

RuO2 thin films on Si substrate

Selected Area Electron DiffractionED, SAD, SAED

Bright Field (BF) TEM image

Diffraction TEM image

Why electrons – they scatter to relatively high angles electron diffraction

MgO in MgB2 Energy-dispersive spectrometry (EDS)MgO with [1-10] oriented parallel to the electron beam

Why electrons - Electrons are ionizing radiation Analytical TEM (AEM)

Limitations of TEM

• Destructive method

• Thin specimen preparation quality!!!!!

• Very local observation

• 2D projection of 3D (in exception of stereography) [2]

Grain boundary in Al reinforced by nano-Al2O3[2]

Dislocations in GaAs

Limitations of TEM

• Electron beam damage

• Defects, in-situ annealing, phase transformations, amortization

ocooling – cryo-TEM (C-TEM)

oLower doses and more sensitive detectors (CCD camera)

TEM techniques

DIFFRACTION IMAGINIG SPECTROSCOPY

Amplitude contrast

Phase contrast

Selected area

diffraction

Micro-nano-

diffraction

Convergent beam

diffraction

Energy dispersive

X-ray spectroscopy

Electron energy loss

spectroscopyEELS

X-ray mapping

Energy-filtered

TEMEFTEM

In-situ techniques -heating, deformation electrical current for

treatment or measurement…

Instrument

Electron gun – source of electron beam

Condensor – system of electromagnetic lenses + condensor aperture – shaping and/or scanning of incident electron beam

Objective lens + aperture – the most important for image creation and resolution

System of lenses + selected area aperture – for outcoming signal treating

Instrument – electron gun Thermoionic guns

Richardson law

T - temperatureΦ - work functionK - Boltzman constA - material const

W filament

LaB6 tip

Instrument – electron gun Field emission guns

• Anode 1 provides the extraction voltage to pull electrons out of the tip.

•Anode 2 accelerates the electrons to 100 kV or more.

Electric field near a sharp spherical electrode tip under voltage V

W FEG tip

Instrument – electron gun • Thermoemission gun

W wire tip – the simplest, low vacuum, low intensity, conventional TEM

LaB6 – a bit higher intensity, more attentive utilization,

• FEG – high vacuum, high intensity, monochromatic el. beam, nm probe size STEM, corrected TEMCold FEG – more attentive utilization, contamination problemSchottky FEG – the most frequent, heating helps to remove contamination and decreases the tunelling barrier

• Electromagnetic lenses – moving electron in magnetic field

• Lorenz force

F = - e (v x B)

F = e v B sin ϴ,

For ϴ≈90°

F= e v B = 𝑚𝑣2

𝑟 =𝑚𝑣

𝑒𝐵

Instrument – electromagnetic lenses

Apertures = holes in diaphragms -

• to limit the collection angle of lens

• To extract a part of electron beam tolimit el. beam intensity - condenser aperture

exclude some beams from image creation –different TEM techniques – objective aperture

select an area of interest – selection aperture

Instrument – apertures

Resolution limiting abberations

Spherical abberation

Chromatic abberationAstigmatism

- maximum collection angle of lensf – maximum defocus

Correction of spherical aberration

[Pennycook 2003]

Interaction with a thin sample

• Thin = transparent for electrons

• Scattering – e- are charged particles stronger scattering than VL or X-ray

Coherent At low angles

(1-10°)

“more” incoherent(>10°)

Almost always incoherent

(<1°≅ 20 mrad)

Reciprocal lattice

a*. b = a*. c = b*. c = b*. a = c*. a = c*. b = 0

a*. a = 1; b*. b = 1; c*. c = 1

ghkl = ha* + kb* + lc* reciprocal lattice vector of plane (hkl)

perpendicular to (hkl) plane

𝑔ℎ𝑘𝑙 =1

𝑑ℎ𝑘𝑙

Electron diffraction

k – wave vector

K . a = hK . b = k Laue diffraction conditionsK . c = l

KI incident

Electron diffraction

Ewald sphere

Radius = ൗ1

Laue zones

as λ is small Ewald sphere has big radius –reciprocal lattice cut

Excitation error

s – excitation error

Image contrast depends strongly on s!!!

SAD – diffraction spots

Fe- 2.9 at.% Mo alloy

Shape of “reciprocal spots”

TEM techniques

Amplitude contrast

Phase contrast

Selected area

diffraction

Micro-nano-

diffraction

Convergent beam

diffraction

Energy dispersive

X-ray spectroscopy

spectroscopyEELS

X-ray mapping

Energy-filtered

TEMEFTEM

DIFFRACTION

Selected area

diffraction

Micro-nano-

diffraction

Convergent beam

diffraction

The most frequently used, use parallel electron beam – cut of reciprocal lattice revealing also its fine structure – all examples used here

Small dimension electron beam used – the diffraction spots are larger, but possibility to analyze small grains, domains (in STEM)

Special technique, needs possibility to use highly convergent electron beam provided by all modern TEM microscopes but not by older ones (not in our institute) – rich in information concerning the lattice – lattice parameters, symmetry, strain….

SAD – Crystallinity

La0.67Sr0.33MnO3 (LSMO) thin films grown on Bi4Ti3O12(BTO)/CeO2/YSZ buffered Si

Monocrystalline polycrystalline amorphous

MgB2 Mixture of B-rich phases in MgB2 wire core

SAD – texture

Ag/Ni multilayers with different levels of texture Deformed Al wire – texture in grain orientation and in their shape, too

SAD – Crystallinity

Monocrystalline

La0.67Sr0.33MnO3 (LSMO) thin films grown on Bi4Ti3O12(BTO)/CeO2/YSZ buffered Si

two systems of twins

In YBaCuO

011110 110011

Fine structure of SAD

SAD – Superlattices

(110)YBa2Cu3O7- single crystal

Fine structure of SAD

SAD – Double diffraction

The blue spots were created by DD

020CeO2

200CeO2

2110Al2O3

Epitaxial CeO2 thin film grown on Al2O3

Phase analysis from SAD

𝑳 = 𝑅 𝑑

Bragg eq.

Camera constant

J.L. Lábar – free software ProcessDiffractionhttp://www.energia.mta.hu/~labar/ProcDif

TEM techniques

Amplitude contrast

Phase contrast

Selected area

diffraction

Micro-nano-

diffraction

Convergent beam

diffraction

Energy dispersive

X-ray spectroscopy

spectroscopyEELS

X-ray mapping

Energy-filtered

TEMEFTEM

TEM imaging – image contrasts

• Mass-thickness contrast – incoherent Rutheford scattering –strongly depends on atomic number Z – the principal contrast for amorphous materials, but present everywhere

• Diffraction contrast – coherent electron scattering – in crystalline materials – electrons are scattered by Bragg diffraction

Contrast = difference in intensity between two adjacent areas

Electron beam - changes its amplitude and phase

Phase contrast

Amplitude contrast

Every diffracted and transmitted wave pass diffeent way and has its own phase - if they ineract after sorting from specimen, 2D interferrence pattern is created on image plane Too many factors contrinute to the phase shift – thickness, orientation, scattering factor, focus, astigmatism – be carefull when interpreting them!!!

Amplitude contrast – Mass - thickness contrast

BF image of stained two-phase polymer exhibiting masscontrast due to the segregation of the heavy metal atoms to the unsaturated bonds in the darker phase. [2]

Selected Area Electron DiffractionED, SAD, SAED

RuO2 thin films on Si substrate

Bright Field TEM image Dark Field TEM image

Objective aperture

SAD aperture

Amplitude c. – Diffraction contrast

Diffraction TEM image

Diffraction contrast - Dark Field TEM{101} TiO2

d = 0.2487 nm

{101} RuO2

d = 0.2558 nm

RuO2/TiO2/RuO2/SiLocal epitaxial growth in polyrcystalline layers

Diffraction contrast – vizualization of extremely thin layers intercalated into MgB2

Dislocations in Si single crystal [1]

Diffraction contrast – characterization of defects

Invisibility criterion g . b = 0b - Burgers vector of dislocation

Two beam condition

Diffraction contrast – characterization of defects

Stacking fault in Cu + 7% Al alloy [1]

Phase contrast – Moiré fringes

CeO2 na Al2O3

CeO2/Al2O3 misfit 13,7 %

200CeO2

x – double diffraction

Phase contrast

Phase contrast – Moiré fringes

• Dislocation visualization

CeO2/Al2O3

Phase contrast – „HREM“

(100) a (010) oriented YBa2Cu3O7- thin film

(001) lattice fringes d=1.17 nm

Phase contrast - HREM

(012) Al2O3

Scanning TEM - STEM

STEM DF contrast

ADF ϴ > 45 mrads HAADF ϴ > 75 mrads

Nearly parallel e- beam

Focused scanned e- beam

corrected small probe with higher current –higher signal for all detectors (EDS, EELS, imaging) less noise for BF

The ability to tune thespherical aberration also provides bothoptimum phase-contrast imaging at small negative values of the spherical aberration and pure amplitude-contrast imaging at zero value, a mode which is not accessible in an uncorrected instrument

Z-contrast

(A) High-resolution phase-contrast image of epitaxial Ge on Si with an amorphous SiO2 surface. The bright array of dots common to the crystalline region represents atomic rows and the Ge and Si regions are indistinguishable.

(B) The high-resolution Z-contrast STEM image shows the atom rows but with strong contrast at the Si–Ge interface and low intensity in the low-Z oxide.

TEM techniques

Amplitude contrast

Phase contrast

Selected area

diffraction

Micro-nano-

diffraction

Convergent beam

diffraction

Energy dispersive

X-ray spectroscopy

spectroscopyEELS

X-ray mapping

Energy-filtered

TEMEFTEM

Energy dispersive spectrometry – EDS

As in SEM – mapping, line-scans, quantitative…, but from a thin foil – higher lateral resolution+ STEM + aberration corrected TEM – up to single atom resolution

InAlN/GAN/sapphire, IEE

TEM specimen preparation

• Good quality TEM specimen = major limitation in TEM analysis !!!

• Thin TEM specimen: thin = electron transparent

• Powders on carbon coated metalic grids• Replicas • Chemical jet etching• Electrolytical etching and jet etching• Grinding, polishing and final ion milling• Ultramicrotomy• Cleaving • FIB

Replica

+ carbon „shadowing“

Chemical jet thinning

Electrolytic thinning

Conventional TEM specimen preparation – thin films on substrates

Thin film

substrate glue

Dimpling

Ultramicrotomy

Cleaving

TEM lamella cutting off by focused ion beam – FIB

Dual beam equipment with FIB

Deposition of Pt protective layer using organometallic precursor

Thank you for your attention

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