TEM Imaging WWW
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Transcript of TEM Imaging WWW
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A.E. Gunns MENA3100 V10
Sample preparation for TEM Crushing
Cutting
saw, diamond pen, ultrasonic drill, FIB
Mechanical thinning
Grinding, dimpling
Electrochemical thinning
Ion milling
Coating
Replica methods
FIB
Plane view or cross section sample?
Is your material brittle or ductile?
Is it a conductor or insulator?
Is it a multi layered material?
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Grind down/
dimple
TEM sample preparation: Thin films
Top view
Cross section
or
Cut out a cylinder
and glue it in a Cu-tube
Grind down and
glue on Cu-rings
Cut a slice of the
cylinder and grind
it down / dimple
Ione beam thinning
Cut out cylinder
Ione beam thinning
Cut out slices
Glue the interface
of interest face toface together with
support material
Cut off excess
material
Focused Ion Beam
(FIB)
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Basic principles, first TEM
Wave length:
= h/(2meV)0.5 (NB non rel. expr.)
= h/(2m0eV(1+eV)/2m0c2)0.5(relativistic expression)
200kV: = 0.00251 nm (v/c= 0.6953, m/m0= 1.3914)
Electrons are deflected by bothelectrostatic and magnetic fields
Force from an electrostatic field (in the gun)
F= -e E
Force from amagnetic field (in the lenses)
F= -e (v x B)
Nobel prize lecture: http://ernst.ruska.de/daten_e/library/documents/999.nobellecture/lecture.html
a) The first electron microscope built by Knoll
and Ruska in 1933, b) The first commercial
electron Microscope built by Siemens in 1939.
http://ernst.ruska.de/daten_e/library/documents/999.nobellecture/lecture.htmlhttp://ernst.ruska.de/daten_e/library/documents/999.nobellecture/lecture.htmlhttp://ernst.ruska.de/daten_e/library/documents/999.nobellecture/lecture.html -
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A.E. Gunns MENA3100 V10
Basic TEM
Electron source:
Tungsten, W
LaB6
FEG
Electron gun
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Electron guns
Thermionic gunField emission gun (FEG)
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Technical data of different sourcesTungsten LaB6 Cold
FEG
Schottky Heated
FEG
Brightness
(A/m2/sr)
(0.3-2)109 (0.3-2)109 1011-1014 1011-1014 1011-1014
Temperature
(K)
2500-3000 1400-2000 300 1800 1800
Work function
(eV)
4.6 2.7 4.6 2.8 4.6
Source size
(m)
20-50 10-20
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Basic TEM
Electron gun
Vacuum requirements:
- Avoid scattering from residual gas inthe column.
- Thermal and chemical stability of the
gun during operation.
- Reduce beam-induced contamination
of the sample.
LaB6: 10-7torr
FEG: 10-10torr
Electron source:
Tungsten, W
LaB6
FEGCold trap
Sample position
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The lenses in a TEM
Sample
Filament
Anode
1. and 2. condenser lenses
Objective lens
Intermediate lenses
Projector lens
Compared to the lenses in an
optical microscope they are verypoor!
The point resolution in a TEM is
limited by the aberrations of the
lenses.
The diffraction limit on resolution
is given by the Raleigh criterion:
d=0.61/sin, =1, sin~
-Spherical
- Chromatic
-Astigmatism
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Spherical aberrations
Spherical aberration coefficient
ds= 0.5MCs3
M: magnification
Cs:Spherical aberration coefficient
: angular aperture/
angular deviation from optical axis
2000FX: Cs= 2.3 mm
2010F: Cs= 0.5 nm
r1
r2
Disk of least confusion
Cscorrected TEMs are now available
The diffraction and the spherical aberration limits on resolution
have an opposite dependence on the angular aperture of the objective.
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Aberrations in a nutshell
Core of the M100 galaxy seen through
Hubble (source: NASA)
Before Cscorrection
After Cscorrection
Q.M. Ramasse
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Resolution limit
Year Resolution1940s ~10nm1950s ~0.5-2nm1960s 0.3nm (transmission)
~15-20nm (scanning)1970s 0.2nm (transmission)
7nm (standard scanning)
1980s 0.15nm (transmission)5nm (scanning at 1kV)
1990s 0.1nm (transmission)3nm (scanning at 1kV)
2000s
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A.E. Gunns MENA3100 V10
Chromatic aberration
v
v -vdc= Cc((U/U)
2+ (2I/I)2+ (E/E)2)0.5
Cc: Chromatic aberration coefficient
: angular divergence of the beam
U: acceleration voltage
I: Current in the windings of the objective lens
E: Energy of the electrons
2000FX: Cc= 2.2 mm
2010F: Cc= 1.0 mm
Chromatic aberration coefficient:
Thermally emitted electrons:E/E=KT/eV
Force from amagnetic field:F= -e (v x B)
Disk of least confusion
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A.E. Gunns MENA3100 V10
Lens aberrations
Lens astigmatismLoss of axial asymmetry
y-focus
x-focusy
xThis astigmatism can not be
prevented, but it can be
corrected!
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A.E. Gunns MENA3100 V10
Operating modes
Convergent beam Parallel beam
Can be scanned
(STEM mode)
Specimen
Imaging mode
orDiffraction mode
Spectroscopy and mapping
(EDS and EELS)
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Image or diffraction mode
1. and 2. condenser
lenses
Objective lens
Intermediate lenses
Projector lens
Spesimen
Filament
Anode
Diffraction plane
Image plane
Objective aperture
Selected area aperture
Image or diffraction patternSTEM detectors (BF and HAADF)
Bi-prism
Viewing screen
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Advanced nanotool
JEOL 2010F FEGTEMUltra high resolution version with analytical possibilities
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Imaging / microscopy
200 nm
Si
SiO2
TiO2
Pt
BiFeO3
Glue
TEM
- High resolution (HREM)- Bright field (BF)
- Dark field (DF)
- Shadow imaging
(SAD+DF+BF)
STEM
- Z-contrast (HAADF)
- Elemental mapping
(EDS and EELS)
GIF
- Energy filtering
Holography
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Simplified ray diagram
Objective lense
Diffraction plane
(back focal plane)
Image plane
Sample
Parallel incoming electron beamSi
c
ow
erCell2.0
1,1 nm
3,8
Objective aperture
Selected area
aperture
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Apertures
Selected area aperture
Condenser aperture
Objective aperture
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Use of apertures
Condenser aperture:Limits the number of electrons hitting the sample (reducing the intensity),
Reducing the diameter of the discs in the convergent electron diffraction pattern.
Selected area aperture:Allows only electrons going through an area on the sample that is limited by the SAD aperture
to contribute to the diffraction pattern (SAD pattern).
Objective aperture:Allows certain reflections to contribute to the image. Increases the contrast in the image.
Bright field imaging (central beam, 000), Dark field imaging (one reflection, g), High resolution
Images (several reflections from a zone axis).
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A.E. Gunns MENA3100 V10
Objective aperture: Contrast enhancement
All electrons contributes to the image. A small aperture allows only electrons in the
central spot in the back focal plane to contribute
to the image.Intensity: Thickness and density
dependence
Mass-thickness contrast
Si Ag and Pb
glue(light elements)
hole
50 nmOne grain seen along a
low index zone axis.
Diffraction contrast(Amplitude contrast)
Diffraction contrast: B i ht fi ld (BF)
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Diffraction contrast: Bright field (BF),dark field (DF) and weak-beam (WB)
BF image
Objective
aperture
DF image Weak-beam
Dissociation of pure screw dislocation
In Ni3Al, Meng and Preston, J.
Mater. Scicence, 35, p. 821-828, 2000.
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Bending contours
BF image
DF image
DF image
Obj. aperture
Obj. lens
sample
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Thickness fringes/contours
Sample (side view)
e
000 g
t
Ig=1- Io
In the two-beam situation the intensity
of the diffracted and direct beamis periodic with thickness (Ig=1- Io)
Ig=(t/g)2(sin2(tseff)/(tseff)
2))
t = distance traveled by the diffracted beam.
g= extinction distance
Sample (top view)Hole
Positions with max
Intensity in Ig
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Thickness fringes,bright and dark field images
Sample Sample
DF imageBF image
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Phase contrast: HREM and Moire fringes
2 nm
http://www.mathematik.com/Moire/
A Moir patternis an interferencepattern created, for example, when
two grids are overlaid at an angle, or
when they have slightly different mesh
sizes (rotational and parallel Moire
patterns).HREM image
Long-Wei Yin et al., Materials Letters, 52, p.187-191
200-400 kV TEMs are mostcommonly used for HREM
Interference pattern
http://www.mathematik.com/Moire/http://www.mathematik.com/Moire/ -
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Moire fringe spacing
Parallel Moire spacingdmoire= 1 / IgI = 1 / Ig1-g2I = d1d2/Id1-d2I
Rotational Moire spacing
dmoire= 1 / IgI = 1 / Ig1-g2I ~1/g= d/
Parallel and rotational Moire spacing
dmoire= d1d2/((d1-d2)2+ d1d2
2)0.5
g1
g2
g
g1g2 g
Simulating HREM images
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A.E. Gunns MENA3100 V10
Simulating HREM imagesContrast transfer function (CTF)
CTF (Contrast Transfer Function) is the function which
modulates the amplitudes and phases of the electron
diffraction pattern formed in the back focal plane of theobjective lens. It can be represented as:
k = u
The curve depend on:
Cs (the quality of objective lens)l(wave-length defined by accelerating voltage)
Df(the defocus value)
u(spatial frequency)
In order to take into account the effect of the
objective lens when calculating HREM images, the
wave function (u) in reciprocal space has to be
multiplied by a transfer function T(u).
In general we have:
(r)= (u) T(u) exp (2iu.r)
T(u)= A(u) exp(i), A(u): aperture function 1 or 0
(u)= fu2+1/2Cs3u4 : coherent transfer function
Si l ti HREM i
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A.E. Gunns MENA3100 V10
Simulating HREM images
Contrast transfer function (CTF)
Effect of the envelope functions can be represented as:
where Ecis the temporal coherency envelope(caused by
chromatic aberrations, focal and energy spread,instabilities in thehigh tension and objective lens current), and Eais spatial
coherencyenvelope(caused by the finite incident beam
convergence).
http://www.maxsidorov.com/ctfexplorer/webhelp/background.htm
http://www.maxsidorov.com/ctfexplorer/webhelp/spatial_coherency.htmhttp://www.maxsidorov.com/ctfexplorer/webhelp/spatial_coherency.htmhttp://www.maxsidorov.com/ctfexplorer/webhelp/background.htmhttp://www.maxsidorov.com/ctfexplorer/webhelp/background.htmhttp://www.maxsidorov.com/ctfexplorer/webhelp/spatial_coherency.htmhttp://www.maxsidorov.com/ctfexplorer/webhelp/spatial_coherency.htm -
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Scherzer defocus
http://www.maxsidorov.com/ctfexplorer/webhelp/effect_of_defocus.htm
f = - (Cs)1/2
f = -1.2(Cs)1/2
Scherzer condition Extended Scherzer condition
http://www.maxsidorov.com/ctfexplorer/webhelp/effect_of_defocus.htmhttp://www.maxsidorov.com/ctfexplorer/webhelp/effect_of_defocus.htm -
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HREM simulations
One possible model for which the simulated HREM images match rectangular region I
HREM simulation along [0 0 1] based on the above structures. The numbers before and after the slash
symbol / represent the defocus and thickness (nm), respectively
The assessment of GPB2/S structures in AlCuMg alloys
Wang and Starink, Mater. Sci. and Eng. A, 386, p 156-163, 2004.
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HAADF image of an icosahedral FePt particle (false colors): thanks to the small
probe size, it is possible to probe precisely the chemical structure of samples atthe atomic level, revealing here a small crystalline layer of iron oxide
surrounding the outermost shell of the particle.
Combined HAADF and EELS
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Energy filtering
A. Thgersen et al., Collaboration with Prof. T. Finnstad, UiO, S. Diplas, SINTEF and
UniS, UK and NIMS, Japan