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Advances in EELS Instrumentation
C.T. Koch
Stuttgart Center for Electron Microscopy
Max Planck Institute for Metals Research
Stuttgart, Germany
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Outline
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How to sort electrons by energy
Spectroscopy & monochromation
Spatially resolved spectroscopy
Momentum resolved spectroscopy
Energy filtering
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
1
How to disperse
Electrons
(i.e sort them by energy)
3
C.T. Koch, MPI for Metals Research
ESTEEM Winter Workshop 2009
Lorentz Force Sorts Electrons by Color


  
FLorentz  e E  v  B


FLorentz
=> a Lorentz 
melectron

FLorentz L2
x 
=>
2melectron v 2

polychromatic
electron beam
FLorentz
L
fast
=> Electrostatic or magnetic fields or
the combination of both may be used
to construct a spectrometer.
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ESTEEM Winter Workshop 2009
slow
x
spectrum
C.T. Koch, MPI for Metals Research
2
Velocity of Beam Electrons
C
ARM1250
200
kV
0
5
ESTEEM Winter Workshop 2009
300
kV
C.T. Koch, MPI for Metals Research
Curved Magnetic Prism
Slower electrons experience a stronger
deflection by the magnetic field.
The momentum p of the electron may then be
determined from the radius of curvature R:
Focal plane of projector
lens system
e 
p  B R,
c

p2 
 E 

2
m


Slow electrons of energy E2 < E1
Magnetic
Prism
Energy
Dispersive
Plane
B
X
Fast electrons of energy E1
Remember, classically: xdetector ~ p-2 = 1/(2mE) => for small E x ~ -E
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Practical Design of Magnetic Prism
(a.k.a. “drift tube”)
Soft-magnetic
“mirror plates”
(reduce fringing fields)
A corrected spectrometer:
• The path length does not depend on the position within the entrance
aperture anymore.
• Fringing fields are minimized by mirror plates
• Possibility to make spectrum truly linear (?)
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Curved Electrostatic Prism
In first FEG-STEM (1968)
A.V. Crewe et al, J. Appl. Phys. 39, (1968) 5861 - 5867
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Principle of Wien Filter
Wien condition:



  
FLorentz  e E  v  B  0
for

E
v 
B
By combining electrostatic and magnetic
field electrons of the desired energy can
be kept on a straight path.
Figure from: W. Grogger et al. Top Catal (2008) 50:200–207
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Spectroscopy
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Motivation for Improving EELS Resolution
Potential applications for transmission EELS instrumentation
with improved energy resolution:
• Band gap and dielectric function mapping of novel
semiconductor structures
• Dielectric function mapping of nanostructures
(e.g. fast identification if carbon nanotube chirality)
• Phonon spectroscopy of nanostructures
(e.g. QD-Laser, QD-Logic / Quantum Computers, Energy dissipation in
single electron transistors, etc. …)
• Competition with optical spectroscopy, but: better spatial and
momentum resolution!
• Improved ELNES resolution in some materials and edges
(only a few)
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C.T. Koch, MPI for Metals Research
ESTEEM Winter Workshop 2009
Energy Resolution in Spectroscopy
PSFSpectrum = PSFHV-spread x PSFSource x PSFSpectrometer x PSFDetector
Detector: high spectrum
magnification
Spectrometer: high dispersion, high stability
Shottky-FEG:
E = 0.5 .. 0.8 eV
Cold-FEG:
E = 0.25 .. 0.5 eV
Carbon Nanotube: E = 0.05 .. 0.2 eV
Monochromator: E = 0.002 .. 0.2 eV
Very stable high voltage (ppm stability (10-6) => E = 0.2 eV @ 200kV)
Deconvolution of PSF only possible if:
• excellent signal / noise properties
• PSFSpectrum is known precisely (e.g. shape of ZLP)
=> Improvement of resolution by factor 3 has been shown to be possible
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Serial Acquisition Spectrometer (SEELS)
The spectrum is scanned across a slit aperture, only one energy recorded at a time
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Crewe’s Curved Electrostatic Prism
Electrostatic serial spectrometer
In first FEG-STEM (1968)
A.V. Crewe et al, J. Appl. Phys. 39, (1968) 5861 - 5867
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Parallel Spectrum Acquisition (PEELS)
Gatan “666” PEELS
Gatan “Enfina”
Spectrum illuminates a diode or CCD array => whole spectrum recorded at once
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Non-commercial TEELS Instruments
• Boersch and Geiger (1964): MC + Spectrometer
dE = 17 meV, dx = 10 µm, E0 = 25 kV
(1964)
(1972)
dE = 4meV, dx = 10 µm, E0 = 25 kV
• Batson (1986): cold FEG + Spectrometer [res.: 50 meV]
dE = 250 meV, dx = 1 nm, E0 = 100 kV
• Fink (1989):
dE = 80 meV, dx = 1mm, E0 = 170 kV
• Terauchi (1999): couple MC + Spectrometer
dE = 25 meV, dx = 110 nm, E0 = 200 kV, Emax <= 5 V
dE = 200 meV, dx = 110 nm, E0 = 200 kV, Emax = 300 V
H. Boersch et al. Phys. Letters 3, (1962) 64
R. F. Batson, Rev. Sci. Instrum. 57, (1986) 43–48.
J. Fink, Adv. Elec. & Elec. Phys. 75, (1989), 121–232.
M. Terauchi et al, Journal of Microscopy 194, (1999), 203–209
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Wien Filter Instruments
Highest (transmission) EELS Resolution has so far been
achieved by instruments using a Wien Filter as a
monochromator and spectrum analyzer:
• Boersch et al (1964): FWHM(ZLP) = 2 meV (25kV)
• Terauchi et al (1999): FWHM(ZLP) = 12 meV (200kV)
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Wien Filter – type Monochromator
Monochromatization of fast electrons in a Wien-type filter
monochromator (a); dispersed electron beam on the viewing screen of
the TEM before (b) and after (c) introduction of the energy-selecting
slit
Figure from: W. Grogger et al. Top Catal (2008) 50:200–207
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Fringe Field Monochromator
MC on
(61 meV)
Cold FEG
Test on a VG HB501 at IBM Watson
Research center
H.W. Mook and P. Kruit, Ultramicroscopy 81 (2000) 129 -139
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Improvement by Monochromation
dE = 700 meV (w/o MC)
dE = 100 meV (w/ MC)
dE = 40 meV (w/ MC)
EEL Spectra of Argon recorded at different energy resolutions
Incident beam energy: E0 = 25 keV (transmission EELS)
Peak interpretation
H. Boersch, J. Geiger and W. Stickel, Zeit. Physik. 180 (1964), 415–424
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Boersch’s Instrument
25 kV
Deceleration
Acceleration
Slowing down
electrons increases
dispersion of Wien
filter
E = 17 meV
H. Boersch, J. Geiger and W. Stickel, Zeit. Physik. 180 (1964), 415–424
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Phonon Spectroscopy in TEELS
0.1 mrad
Energy Resolution: 4 meV (2meV w/o specimen)
B. Schröder and J. Geiger, Phys. Rev. Lett. 28 (1972), 301 - 303
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ESTEEM Winter Workshop 2009
Low spatial
resolution
C.T. Koch, MPI for Metals Research
11
170 kV Spectrometer with E=80 meV
-170 kV
ground
electrostatic
analyzer
electrostatic
monochromator
Advantage:
• slow high voltage fluctuations
have no effect, because they
cancel during deceleration.
• variable energy resolution
• momentum resolution: 1 mrad
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Terauchi’s Approach
Deceleration of electrons inside
MC and Spectrometer
to potential U0
 Dispersion of MC = Dispersion of Spectrometer
 Energy resolution increases with decreasing U0
 Poor energy resolution @ high energy loss
M. Terauchi et al, Journal of Microscopy,194, (1999) 203
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Wien Filter designs
EZLP = 81 meV
@ E0 = 40V
EZLP = 15 meV
@ E0 = 20V
EZLP = 12 meV
@ E0 = 15V
K. Tsuno,
Rev. Sci. Instrum. 64 (1993) 659–666.
[Wien condition also in fringing field]
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Spectrometer Aberrations
Diffraction pattern
Image
Spectrometer aberrations
• Reduce spectral resolution (by convoluting
diffraction pattern or image with the spectrum)
• Reduce the area that can be imaged (depends
on width of energy-selecting slit).
Energy Dispersive
Plane
(Spectrum x Image)
Magnetic
Prism
B
X
Energy-slit
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Effect of Spectrometer Aberrations
Without specimen: E = 12 meV
With specimen: E = 25 meV
M. Terauchi et al, Journal of Microscopy,194, (1999) 203
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
A STEM-capable Spectrometer
• Spectrometer capable of
E = 50 meV
• No Monochromator, but a cold-FEG
=> E = 250 meV
• E0 = 120 kV
• Can handle acceptance semi-angles
used in STEM spectroscopy
E = 70 meV at  = 12.5 mrad
P. E. Batson, Rev. Sci. Instrum. 57 (1986) 43–48.
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Improvements of Core-loss EELS by MC
Microscope: Tecnai F20 + MC (Graz)
– Al2O3
MC = on
MC = off
first to show spin-orbit coupling in
– Al2O3 by EELS
W. Grogger and G. Kothleitner,
Top. Catal. 50 (2008), 200–207
29
D.S. Su, G. Kothleitner, et al
Ultramicroscopy (2003)
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Improvements of Core-loss EELS by MC
Co L2,3 edge from CoO recorded with the Tecnai F20
(200 kV, Schottky emitter, background subtracted), without monochromator
(0.65 eV) and with monochromator (~0.20 eV)
W. Grogger et al. Top Catal (2008) 50:200–207
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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MC makes weak features visible
Calcite (CaCO3)
Ca-L2,3 edge
Aragonite (CaCO3)
Ca-L2,3 edge
Data recorded by V. Srot at Zeiss SESAM (StEM) with 0.2 eV energy resolution
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Energy Resolution Limits
MC useful
Natural width of K and L versus edge energy
Final-state energy broadening as a
function of energy above the ionization
threshold
R. Egerton, Micron 34 (2003) 127–139
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Phonon Spectroscopy in TEELS
0.1 mrad
Energy Resolution: 4 meV (2meV w/o specimen)
B. Schröder and J. Geiger, Phys. Rev. Lett. 28 (1972), 301 - 303
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ESTEEM Winter Workshop 2009
Low spatial
resolution
C.T. Koch, MPI for Metals Research
Current Monochromator Designs
(has residual dispersion)
(has residual dispersion)
(dispersion free)
electrostatic
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Design of the CEOS Monochromator
Gun Lens
Emitter Assembly
MC Slit
MC
Slit Selector
•
High Dispersion (12µm / eV @ 4.3 kV extractor potential)
•
No dispersion behind MC ! (No rainbow illumination)
•
Spot Size is preserved by the Monochromator
 full analytical capabilities, no effect on resolution in STEM
•
Ease-of-use: - Simple slit selection (electrical or piezo driven)
- Simple switch to unmonochromated beam (straight beam path)
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Performance of Electrostatic MC
0.5 µm slit ZLP compared to unfiltered ZLP
MC slit removed
MC slit inserted
48 meV
MC slit
Zero-loss peak (ZLP) recorded on Zeiss SESAM (StEM), E0 = 200 kV
=> Almost no reduction of intensity by MC
Slide courtesy E. Essers
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Spot size with and without monochromator
Slide courtesy E. Essers
MC off
dFWHM 
0,47nm
MC on
dFWHM 
0,5nm
Monochromator does not affect the probe size
Zero-loss peak (ZLP) recorded on Zeiss SESAM (StEM), E0 = 200 kV
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
48 meV ZLP @ 0.5 s exposure time
Slide courtesy E. Essers
48 meV
High Energy Resolution
Zero-loss peak (ZLP) recorded on Zeiss SESAM (StEM), E0 = 200 kV
Size of Filter entrance aperture: 50 µm
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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87 meV ZLP @ 100 s exposure time!
Slide courtesy E. Essers
87 meV
 Excellent energy resolution also for low-intensity features!
Size of Filter entrance aperture: 50 µm
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Sum of 30 aligned ZLP: 43 meV @ 0.1 s
Slide courtesy E. Essers
43 meV
Spectra recorded and automatically
aligned by custom DM script
Size of Filter entrance aperture: 50 µm
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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52 meV ZLP @ 0.1 s with large aperture
Slide courtesy E. Essers
Size of Filter entrance aperture: 0.4 mm
100
90
80
70
x 10^4
60
52 meV
50
40
30
20
10
0
-0.08
-0.06
-0.04
-0.02
0.00
eV
0.02
0.04
0.06
0.08
High energy resolution even for large filter entrance apertures
=> lots of current in spectrum
41
C.T. Koch, MPI for Metals Research
ESTEEM Winter Workshop 2009
Steep drop of the ZLP (0.1 s, large aperture)
Slide courtesy E. Essers
1x10 6
10-1 @ E = 0.048 eV
2x10 5
1x10 5
10- 2 @ E = 0.091 eV
2x10 4
1x10 4
10- 3 @ E = 0.260 eV
2000
1000
10- 4 @ E = 0.570 eV
200
100
20
10
0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
eV
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
Sharp drop of ZLP tail
=> Good conditions to measure weak spectral features
and low-loss EELS
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Wien Filter Monochromator
Comparison of the zero-loss peaks of an unfiltered Schottky field emission microscope
(200 kV) and the monochromated electron beam of the Tecnai F20 (Graz).
• Wien-filter type monochromator in accelerating mode
• Exposure time = 1 s
W. Grogger et al. Top Catal 50 (2008) 200–207
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
HR-EELS of Surfaces
H. Ibach, PRL 24 (1970) 1416
ELS5000 (LK Technologies)
E >= 0.5 meV
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Energy Filtering
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
What are Imaging Energy Filters good for?
• EEL Spectrum
(you don‘t really need an energy filter for this,
a plain spectrometer will do)
• Energy Filtered Electron Diffraction (EFED)
• Energy Filtered TEM (EFTEM)
• Spectrum profiling (STRIPE-TEM)
• EEL-dispersion curves
(momentum-resolved EELS)
46
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
23
Zero-Loss Filtering in Electron Diffraction
Convergent beam electron diffraction (CBED) patterns become much clearer, if the
diffuse inelastic scattering background has been removed by zero-loss energy filtering.
Figure from: M. Tanaka et al, Journal of Microscopy, 194, (1999), 219–227
47
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Energy-Filtered Diffraction Patterns
unfiltered
Position of line
scan in following
slide
Unfiltered CBED Pattern of Si (110) [120kV, Zeiss EM912 @ ASU]
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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ZL-Filtering Removes Diffuse Inelastic Background
Holz-ring
unfiltered
zero-loss filtered
Linescans across diffraction pattern along the direction indicated by the
red shaded area in previous slide. The inelastic diffuse background
increases with specimen thickness.
Inset: Central part of zero-loss filtered CBED pattern.
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Linescans across interfaces
position
Sequentially acquired EELS Linescan across an interface in a Si3N4 ceramic (Si L edge)
intensity
Energy-loss
Data: K. van Benthem [VG501 @ StEM]
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ESTEEM Winter Workshop 2009
Energy-loss
C.T. Koch, MPI for Metals Research
25
Electron Spectroscopic Profiling (ELSP)
Single focusing spectrometers preserve
spatial information in the dirtection
normal to the dispersive direction.
E=0
AlGaAs
GaAs
• Less beam damage
• Better Signal/Noise
energy
interface
50nm
Data: Lin Gu on Zeiss Libra 200 (StEM),
equipped with corrected Omega filter
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Electron Spectroscopic Profiling (ELSP)
Concentration profiles across an interface from a single exposure!
(La,Ca)MnO3/SrTiO3 – 3.9A resolution
Spectrum profile recorded with a single CCD exposure using a Gatan Imaging Filter (GIF)
T. Walther, Ultramicr. 96, 401 (2003)
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
26
Momentum-Resolved EELS
Zero Loss Peak (ZLP
3.3eV band edge
interband
transitions
Diagram: P. Midgley,
Ultram. 76 (1999), 91
5eV
volume
plasmon
50μrad
53
Data: Lin Gu on Zeiss Libra 200 (StEM),
equipped with corrected Omega filter
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Physical Resolution Limits
• Energy Resolution: limited by inv. lifetime of
– The final state (1/Tf)
– The initial state (1/Ti) [the core hole]
=> Both increase with energy
• Spatial resolution (Heisenberg’s uncertainty
principle): p·x ≈ h
=>  x 
54
h
2 
 0 .8  / 
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
27
Spatial Resolution Limits
Calculated values of localization distance, as a function energy loss.
R. Egerton, Micron 34 (2003) 127–139
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Data: Lin Gu, MPI-MF
Microscope: Zeiss Libra 200FE+MC
Delocalization of Inelastic Scattering
Spatial-resolution of inelastic scattering (neglecting aberrations) using an AlGaN
edge (effective CCD pixel size: 2Å). d50 experimental reflects an average value
with energy-slit of about 2eV.
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
28
Energy Filtering
Instrumentation
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Energy Filtering Instrumentation
• Electrostatic mirror + magnetic prism
– Henry-Castaing filter
• Magnetic Prism(s)
– Omega filter
– MANDOLINE filter
– Gatan Imaging Filter (GIF)
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
29
Energy Filters
While dispersing electrons energy filters preserve their spatial
distribution in the filter entrance plane, allowing partial images
produced by electrons of a certain energy to be selected.
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Castaing-Henry-Ottensmeyer-Filter
sample
1st projector
lens system
Electrostatic mirror
Mirror must decelerate electrons
to v = 0
=> difficult to implement at higher
voltages.
2nd projector
lens system
Magnetic prism
Commercial implementation in the Zeiss EM 902A
Max. acc. voltage: E0 = 80 kV
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
30
Imaging Energy Filters
Intermediate Diffraction Pattern
Intermediate Image
Imaging Energy Filter
An optical which replicates the
image in O1 in O3, but electrons
of different energy cross the
image plane O3 at different
angles of incidence.
Achromatic image
D: diffraction plane
O: image plane
S: sextupole lens
Energy dispersed diffraction
pattern
Energy-selecting slit aperture
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Zeiss Omega Filter
In EFTEM mode, the energy filter is used to record images formed by electrons that
have experienced different energy losses.
The energy window is shifted by
changing the accelerating voltage of
the microscope. This ensures that
the TEM optics may remain
unchanged.
62
Slit aperture
C1 .. C7: correction elements
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
31
JEOL Omega Filter (200kV)
Omega Filter fitted to a JEM-2010F (200kV)
Slit width: 16 eV
Cs = 0.5 mm
E0 = 100 kV
M. Tanaka et al, Journal of Microscopy 194, (1999), 219–227.
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
MANDOLINE-Filter
Schematic arrangement of the deflection
elements and
the sextupoles within the MANDOLINE
filter, the distance between
the energy selection plane and the
diffraction image in front of the
filter defines the lengthening of the column.
MANDOLINE filter:
Magnetic Aberration-free Noticeably
Dispersive Omega-Like INhomogeneous
Energy filter
S. Uhlemann and H. Rose, Optik 96 (1994) 163 - 178
H. Rose, Sci. Technol. Adv. Mater. 9 (2008) 014107
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
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Diagram: G. Botton
Post Column Filter: GIF
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Actual Design of a Post-Column Filter
Gatan Imaging Filter (GIF): mounted below the column
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
33
In-Column vs. Post-Column
In-Column (Omega) Filter
• 4 Dispersive elements (4x90°)
• Symmetry cancels some
aberrations
• Integrated into microscope
column
• Electron Optics designed to
optimize filter performance
• Spectra and images projected on
viewing screen, image plate,
photographic film, TV-camera,
CCD, …
67
Post-Column Filter
• 1 dispersive element (90°)
• No axis of symmetry
• May be added to any existing
microscope
• Filter- and microscope optics
separately controlled
• Only detectors integrated into
energy filter possible (no film, no
viewing screen, no image plate,
…)
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
S. Uhlemann and H. Rose, Optik 96 (1994) 163 - 178
Energy filters (as of 1994)
ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
34
Energy Filter Aberrations
Diffraction pattern
Image
Imaging energy filter aberrations
• Reduce spectral resolution (by convoluting
diffraction pattern or image with the spectrum)
• Reduce the area that can be imaged (depends
on width of energy-selecting slit).
Energy Dispersive
Plane
(Spectrum x Image)
Magnetic
Prism
Energy varies
across diffr. pattern
(non-isochromatic)
B
X
Energy-slit
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
Energy-filtered Electron Diffraction
Energy-filtered CBED pattern
(Slit width: 10eV)
Energy variation for CBED pattern
(Energy loss: 0eV [red] .. 10eV [blue])
(M.M.G. Barfels, M. Kundmann, C. Trevor, J.A. Hunt, Microsc Microanal 11(Suppl 2), 2005)
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ESTEEM Winter Workshop 2009
C.T. Koch, MPI for Metals Research
35
Transmissivity: an energy filter’s Q-factor
An energy filter’s transmissivity reflects the ability of the system
to image a certain field of view (2r2)
with a given spatial resolution (max scattering angle )
through an energy slit of width E.
TE 2r  
2
2
Assuming that the objective aperture acceptance angle  is matched to
the pixel size of the detector the desirable transmissivity depends only on
the number of detector pixels N along one side of a square detector:
TE N 
2
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Effect of Transmissivity on Diffraction Patterns
unfiltered
filtered
Zero-loss
Plasmon-loss
The transmissivity of an energy filter is the product of area and solid angle
that can be transmitted at a certain energy slit width.
Si (110) recorded in a Zeiss EM912 with an (uncorrected) Omega filter
E0 = 120 kV, slit width = 10 eV
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Non-Isochromaticity
Energy Filter
Filter slit width E
Energy slit limits
field of view
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Comparison of non – Isochromaticity
HR-GIF: 2210 meV (p-p)
(polar coordinates)
G. Kothleitner et al, Micron 34 (2003) 211–218
74
SESAM: 7 meV (peak to peak)
(cartesian coordiantes)
C.T. Koch et al,
Microsc. Microanal. 12 (2006) 506–514
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37
Post-Filter Magnification Helps
In column filters have the advantage of being able to optimize the pre-and post-filter
magnification w.r.t the overall performance of the microscope and filter.
Non-isochromaticity
of the JEOL
200kV in-column
energy filter
K. Tsuno, Journal of Electron Microscopy 47 (1998) 611-619
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Effect of non-Isochromaticity
100 nm
23 eV
22 eV
SiC
Si3N4
Image: A. Zern, W. Sigle
Libra 200 @ MPI-MF
Map of Plasmon energies in a Si3N4 / SiC ceramic determined by fitting the position of
the plasmon peak energy to an EFTEM series. The different plasmon energies stem
from different valence electron densities in these materials.
The shift in plasmon energy from left to right is due to an non-isochromaticity of 0.5eV.
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Evolution of In-Column Filter Designs
120kV
23eV
15eV
12nm2
1eV, 80kV
9nm2
1eV, 120kV
1984
77
0.5eV
0.007eV
190nm2
1eV, 200kV
11000nm2
1eV, 200kV
1992
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2003
2007
C.T. Koch, MPI for Metals Research
Transmissivity of MANDOLINE filter
Cs
= 1.2 mm
= 200 kV
E0
Slit width = 0.41 eV
83 mrad
0.41 eV slit
Si [111]
T0.41 eV
 T0.50 eV
 T1.00 eV
 T2.00 eV
= (0.25··Cs·max4)2 = 2.0·103 nm2
= T0.41 eV·(0.5/0.41)2.5 = 3.3·103 nm2
= T0.41 eV·(1.0/0.41)2.5 = 18.6·103 nm2
= T0.41 eV·(2.0/0.41)2.5 = 105·103 nm2
Calculation by Uhleman & Rose (1994): T0.50 eV = 0.89·103 nm2
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High Voltage Stabilization
HV cable
standard
HV generator
Generator
tank
feed-back
signal
Measuring
tank
HV
measurement
smart signal inverter
Both high-voltage stability and filter current stability are better than 0.1 ppm !
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Applications
• Spectrum profiles across interfaces by EFTEM
• Mapping Surface Plasmon Resonances (SPRs) by EFTEM
• Momentum-resolved EELS
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Spectrum profiles across
interfaces by EFTEM
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Extraction of line profiles from EFTEM maps
Image size:
MC slit:
Filter slit:
Energy step:
Energy:
SrTiO3 (100)
1k x 1k
0.4 eV
0.4 eV
0.4 eV
0 .. 39.6 eV
(100 frames)
Acquisition: 40 eV -> 0eV
17 GB
SrTiO3 (100)
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EELS profiles from EFTEM
Avg. profile
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Analysis of a single spectrum profile
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Profiles of sample edge
Average spectrum
profile
Edge profile at 1.2eV loss
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Spatial extend of STO surface plasmon
4.4 eV
5.6 eV
10 eV
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20 eV
C.T. Koch, MPI for Metals Research
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SrTiO3 sample edge
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Mapping Surface Plasmon
Resonances (SPRs)
by EFTEM
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SPR mapping by spatially-resolved EELS
Access to full spectral and spatial information with
nm resolution. BUT,
• long acquisition time (~ 15 mins)
Experiment
D
C
Simulation
(Nelayah et al. Nature Physics 3, 348 - 353 (2007))
89
C.T. Koch, MPI for Metals Research
ESTEEM Winter Workshop 2009
SPR imaging by low-loss EFTEM
A
A
ω = 1.00 eV
B
ω = 1.45 eV
C
ω = 2.00 eV
A
C
B
1.00 eV
1.45 eV
2.00 eV
B
C
Energy loss (eV)
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45
EFTEM imaging of a 250 nm long nanoprism
vacuum
SiNx
ω = 0.9 eV
91
ω = 1.5 eV
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ω = 2.0 eV
C.T. Koch, MPI for Metals Research
Low-loss EFTEM v/s STEM-EELS
ω = 0.9 eV
ω = 1.5 eV
ω = 2.0 eV
EFTEM
452 nm
E
STEM-EELS
Nelayah et al. Nature Physics 3, 348 - 353 (2007)
For SPR´s mapping, STEM-EELS and low loss EFTEM offer comparable
possibilities. BUT, with EFTEM
- quick access to SPR´s map
- no intensive post- acquisition data analysis
- Higher spatial sampling
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Momentum-resolved EELS
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First -q maps (1973)
Microscope: Hitachi HU11A, E0 = 75 keV,  = 8µrad, E = 0.5 eV
Spectrometer: Wien filter
R. Vincent and J. Silcox, Phys. Rev. Lett. 31 (1973) 1487
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Removing Cerenkov artifacts
(a)
150
(b)
Cerenkov bump
130
Counts (102)110
90
70
50
30
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Energy loss (eV)
(a) ω–q map of h-GaN at a thick region with strong Cerenkov losses; (b) lineprofiles extracted at different q values with a linewidth of about 5 µrad. The
energy loss of Cerenkov radiation has a narrow angular distribution.
L. Gu et al, Phys. Rev. B 75 (2007) 195214, data recorded on Zeiss Libra (StEM)
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Conclusions
• Electron energy loss spectroscopy with 4 meV
resolution has already been possible in the
1960s (=> EELS phonon spectroscopy).
• Early spectrometers could only handle small
acceptance semi-angles (of order 1 mrad)
• Modern energy filters will hopefully reach
energy resolution of pure spectrometers,
combined with high spatial and momentum
resolution.
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Future Energy Filters?
K. Tsuno, Nucl. Instr Meth. Phys. Res. A 519 (2004) 286–296
97
H. Rose, Sci. Technol. Adv. Mater. 9 (2008) 014107
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