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)
~1.22
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
[2]
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)
[2]
TEM techniques
DIFFRACTION IMAGINIG SPECTROSCOPY
Amplitude contrast
Phase contrast
Selected area
diffraction
Micro-nano-
diffraction
Convergent beam
diffraction
Energy dispersive
X-ray spectroscopy
EDS
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
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
[2]
[2]
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
K
Electron diffraction
gi
Ewald sphere
[1]
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
DIFFRACTION IMAGINIG SPECTROSCOPY
Amplitude contrast
Phase contrast
Selected area
diffraction
Micro-nano-
diffraction
Convergent beam
diffraction
Energy dispersive
X-ray spectroscopy
EDS
Electron energy loss
spectroscopyEELS
X-ray mapping
Energy-filtered
TEMEFTEM
In-situ techniques -heating, deformation electrical current for
treatment or measurement…
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
and
In YBaCuO
011110 110011
90°
Fine structure of SAD
SAD – Superlattices
[2]
O5
O5
(110)
(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
CeO2
Al2O3
Phase analysis from SAD
𝑳 = 𝑅 𝑑
R
Bragg eq.
Camera constant
J.L. Lábar – free software ProcessDiffractionhttp://www.energia.mta.hu/~labar/ProcDif
TEM techniques
DIFFRACTION IMAGINIG SPECTROSCOPY
Amplitude contrast
Phase contrast
Selected area
diffraction
Micro-nano-
diffraction
Convergent beam
diffraction
Energy dispersive
X-ray spectroscopy
EDS
Electron energy loss
spectroscopyEELS
X-ray mapping
Energy-filtered
TEMEFTEM
In-situ techniques -heating, deformation electrical current for
treatment or measurement…
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
[2]
BF DF
Diffraction contrast - Dark Field TEM{101} TiO2
d = 0.2487 nm
{101} RuO2
d = 0.2558 nm
TiO2
RuO2
RuO2
RuO2/TiO2/RuO2/SiLocal epitaxial growth in polyrcystalline layers
Si
DF
BF
SAD
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]
BF DF
Phase contrast – Moiré fringes
Al2O3
CeO2 na Al2O3
CeO2/Al2O3 misfit 13,7 %
200CeO2
Al2O3
x – double diffraction
[2]
Phase contrast
Phase contrast – Moiré fringes
• Dislocation visualization
CeO2/Al2O3
[1]
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
DIFFRACTION IMAGINIG SPECTROSCOPY
Amplitude contrast
Phase contrast
Selected area
diffraction
Micro-nano-
diffraction
Convergent beam
diffraction
Energy dispersive
X-ray spectroscopy
EDS
Electron energy loss
spectroscopyEELS
X-ray mapping
Energy-filtered
TEMEFTEM
In-situ techniques -heating, deformation electrical current for
treatment or measurement…
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
