electron energy-loss spectroscopy and energy dispersive x-ray … · 2015-12-16 · edx and wdx...
TRANSCRIPT
Electron Energy-loss Spectroscopy and Energy Dispersive X-Ray Analysis
Nano-Analysis
Techniques
• Inner-shell ionization – an emission of characteristic x-ray quanta or Auger electrons
• Electron energy-loss spectroscopy
– The electronic structure
– The elemental composition
– Efficient for low-Z elements
The volume irradiated by the electron
The volume from which the stimulated signal arises
Nanodimension
Low energy, 100-1000 eV
Resolution
Microanalysis in EM Technique Input
Signal Output Signal
Depth Sampled (nm)
Lateral Resolution (nm)
Elemental Range
Sample Detection Limit (at%)
Wavelength dispersive X-ray spectroscopy (WDX, WDS)
electron Photon (X-Ray)
~ 1000 nm ~ 1000 nm Z4 Flat polished surface
0.005% (50 ppm)
Energy dispersive X-ray spectroscopy in SEM (EDX, EDS)
Electron Photon (X-ray)
~ 1000 nm ~ 1000 nm Z5
Flat polished surface
0.1%
Energy dispersive X-ray spectroscopy in TEM (EDX, EDS)
Electron Photon (X-ray)
~ 10 -100 nm (Sample thickness)
~ 2 nm Z5 TEM foil 0.1%
Electron energy loss spectroscopy in TEM (EDX, EDS)
Electron electron ~ 10 -100 nm (Sample thickness)
~ 1 nm Z3 TEM foil 0.1%
Auger electron spectroscopy (AES)
Electron electron 0.5-2 nm 400 nm Z2
Surface 0.1%
Accuracy: 5%
Inelastic scattering of electrons
Through smaller angles than for elastic scattering The cross section varies linearly with atomic number
< 1 eV; ~ 10o, ~ 1um, heat 50eV – keV: EDX, EELS
5-30eV, ~ 100 nm, dominant
Interaction volume and information volume
Depends on materials and beam energy, Kanaya-Okayama range (empirical)
20 kV, Cu
Generation - escape
Limited range
889.0
67.1
00276.0
Z
AER
X-Ray Generation
• Bremsstarhlung: electrons being decelerated by the nucleus and electrons in the atoms in the materials - not useful, background
E
EEiZI
0
Beam current
• Characteristic X-rays: The energies/wavelengths of the emitted X-rays
Maximum energy is the beam energy
Low energy X-rays absorbed by the
specimen
Characteristic X-rays
Remove an electron from an inner shell: vacancy
An outer shell electron falling into the vacancy (~ 10-12 s)
The allowed transitions: n 1; l=1; j= 1 or 0
Angular momentum
K
Transition to
Transition is from..
• Sharp and element-dependent: Useful for analysis
2CZ
B
eB
CZhcE
2
• X-Ray wavelength:
• X-Ray Energy:
B, C = constants depending on the X-ray line
Moseley’s Relation:
EDX and WDX
WDX: •Use crystal monochromators to disperse the emitted X-ray spectrum in terms of Bragg diffraction angle and hence wavelength • Slow to acquire (30 min) • Increased spectral resolution: 10 eV • Sensitivity to all elements (especially light elements) • Confined to dedicated analytical SEMS (Electron Probe Microanalysers, EPMAs) • Detection limit: ~ 0.005%
EDX: • Detector: pn junction – e-h generation ~ the energy of the X-ray photon • Fast electronics: separate the pulses •Detectors collect X-rays in a near-parallel fashion (1 min) • Spectral resolution: 100-150 eV • Common attachment
EDX: poor energy resolution • Peak overlap • no chemical-state information can be extractive (< 10 eV)
Florescent yield
X-ray emission is very inefficient for low Z (and absorption), EEELs is more efficient technique for
analysing low Z elements
EDX
Detector: • reversed p-i-n (p-type, intrinsic, n-type) • e-h generation
Charge is not completely detected Collect e-h pair
Cooled by liquid nitrogen to • Reduce thermal e-h pair • Prevent the Li atoms from diffusing, • Reduce noise in the FET preamplifier
3.8 eV is required to form an e-h (not all energy creats e-h holes) A Cu Ka, 8040/3.8=2300 e-h holes ~ 10-16C – small signal – preamplifier (FET)
•Be(7-12um) > Na •Ultrathin (<100nm polymer) > C •Windowless (in vacuum)> Boron
Interpretation of X-ray Spectra Experiment • Beam energy > 2 X highest peak energy • Reproducible spectrum from the same area • Counts: enough to recognize peaks; not too high (<3000 counts/sec; dead time< 50%) to minimize sum and escape peaks
Analysis • Prior knowledge of the sample: likely elements • Be aware of stray irradiation peaks: Fe Cr Ni Cu Zn Al Pt Mo • Confirm elements by looking for other peaks from that elements (KLM) • Work from high energy to low energy identifying peaks (at high energy, few peaks and better resolution of neighbouring peaks) • Peak shapes and energies (0-20 kV)
•K series – B (Z=4) to Ru (Z=44) • Z > 16(S) look for K;
• L series – Cl(Z-17) up • Z> 42(Mo) look for L
• M series - ~ Ag(Z=47) up • Peak overlaps:
• S K 2.31keV Mo L 2.29 keV Pb M 2.35 keV • N K 0.39 keV Ti L 0.45 keV
A Si K X-ray escape from the Si detector
Two X-rays arrive at the same time
EELS
• Compositional analysis: efficient for light elements
• Chemical analysis: shapes of energy loss edges depend on local bonding and oxidation state
EELS Spectrometer
Inelastic scattered electrons: Small
scattered angle < 1o
Typical energy loss: < 1 kV, a spectrometer can separate this.
Bends the electrons through 90o
with a magnetic prism.
Curved faces of the magnet: focus the electrons
Additional quadrupole and sextupole lenses: fine tune the focus
Parallel detection: a diode array with 1024 or more diodes to collect the
whole spectrum in parallel
Typical Energy Loss spectra YBa2Cu3O7 Log Scale
Zero Loss: Unscattered, elasticlly, phonon-scattered electrons
Plasmon Losses: low-loss (5-50 eV)
Core Losses
100 kV 99 kV Electron Energy
• Width of Zero peak: energy spread of the microscope: 0.3 eV (FEG) • Phonon: thermal diffuse scattering: < 1eV; ~ 50 mrad
• Plasmon: •oscillation of conduction band electrons •Characteristic plasmon energy • A few mrad • Thick sample: multiples of the plasmon energy
p
p
tII
exp0
Single electron excitation: background
~ 30 eV up - SEs
•Excitation of a core level electrons • Edge: excited to the lowest empty state or higher energies • Each shell has its own edge •It can be obtained from a much smaller volume than EDX
Quantifying EELS
• Thin specimen – to avoid multiple scattering which obscures the edges – the Plasmon peak < 1/10 the zero loss
• Edge: the intensity is spread over large range of loss rather than in a sharp peak… the edge area is difficult to measure… fit the background above the edge – Fit the background using I = AE-r above the edge and extrapolate this
background under the edge – Subtract the background from the edge – Calculate the edge area within a window: C – Calculate the ionisation cross-section and integrate over same window
(collection angle-the objective aperture, microscope voltage) : not accurate ~ 10%
– Repeat for other edges and the compositions
A
B
A
A
B
A
C
C
X
X
EDX and EELS
Techniques Signal to noise Low Z Detection limit Spatial resolution
Artefacts Compositional accuracy
EDX high Boron (5) 0.1 at% 2 nm Escape, stray, sum
Better < 1%
EELS low Better- Li and He (2)
0.05 at% (for some elements)
Better (1 nm) No stray etc. Poor 10%
ELNES and EXELFS
• ELNES: Energy loss near edge structure – The probability of the electron
ending up at a particular energy level in the conduction band depends on the conduction band density of states (DOS)
– Due to the selection rule, the near edge structure doesn’t show the full DOS
– DOS depends on the bonding state of the atom
• EXELFS: Extended energy loss fine structure – 50-200 eV beyond the edge – The structure depends on local
arrangement of atoms around the excited atom
– To get nearest neighbour distances and radial distribution functions
– Low noise spectra are needed
Al K, from Ni3Al, background subtracted
EFTEM • Energy-filtered Microscopy
– Collect an image from a given energy loss – Two ways:
• STEM: focused probe; collect an energy loss spectrum through a slit at the desired energy loss; scan to create an image
– collecting spectrum at each image point – Useful for the detection of low concentrations of
an element
• TEM: Use extra lenses after the energy selecting slit to allow the original image to be reformed (GIF, Gatan imaging filter)
– collecting an image at each energy loss
– Applications • Zero loss filtering
– Allow thick regions to be examined
• Thickness determination: t/ map – Thickness map: a zero loss image (I0) and an
unfiltered image (Iunf) or a loss image (Iloss), : total inelastic mean free path …. Non-uniform composition?!
• Core-loss mapping: Elemental map – Jump-ratio image: dividing the iamge beyond the
edge by an image below the edge – thickness variations are removed (at least approximately)
– 3 Window method (can be quantified): 2 pre-edge and one post-edge, the two pre-edge images are used to estimate the background under the post-edge image and the background can be subtracted … the thickness variations!
– Image spectroscopy: a series of images is collected both before and after the edge of interest – to get a better background (longer time larger dose)
Si 110 CBED at 100 kV
00
1lnlnI
I
I
It lossunf
Normal EFTEM: slice // xy plane,
low energy resolution
STEM Spectrum Imaging: sample drift
and distortion
Appendix
Inelastic Scattering
• Electron-Specimen Interactions with Energy Loss
• Differential Cross Section for Single-Electron Excitation
• Bethe Surface and Compton Scattering
• Approximation for the Total Inelastic Cross Section
Electron-Specimen Interactions with Energy Loss • Elastic preserves: Kinetic energy and momentum • Inelastic conserves total energy and momentum
– Energy conversion: atom-electron excitation – Energy loss: the primary beam
Excitation Mechnisms: • Oscillation in molecules and phonon excitations in solids:
• E ~ 20meV-1 eV • Monochromator • Low beam intensity – low spatial resolution • Infrared
• Intra- and interband excitation of the outer atomic electrons/ plasmons • Broad maxima: E ~ 3-25 eV • Concentration of C/V band electrons; chemical bonds, band structure • Visible and ultraviolet
• Ionization of core electrons • Edge E • Spectrum ~ eV beyond E
• low loss – less localized ( excite atom from ~ 10 nm – small angle) • For low atomic number: total inelastic cross section > total elastic cross section • Thick specimens: chromatic aberration due to energy loss • Energy - heat
Differential Cross Section for Single-electron Excitation
• Energy Transferred – E – Ei→Ef – Selection rules: l=1 – Scattering vector:
if kkq '
• Conservation of momentum, energy cos'2' 0
22
0
2 qkqkkn
cos'
cos'22
'cos'222
0
2
0
22
0
222
0
2
m
qkqk
mqqk
mkk
mE n
222
0
22
0
2 cos'' Ekqkq
2
0
2
2
0
2
2
0
2
2
0
2
0
2
2
2
cos'cos'
E
E
p
Em
k
Em
k
q
k
qE
Inelastic cross section: 2
4
2
if
i
ffirV
k
km
d
d
iijisi rkira exp)( ifjfsf rkira
exp)(
2
33*2
exp)(,)(exp jiiijisjijfsifif rdrdrkirarrVrarkirV
22
22
23*
23*2
'')(...'1)()('exp)()'( ifisjfsjjisjjfsjjisjjfs xqaruaqrdrarqirardrarqiraq
4
2
2
0
2
'
)'(
2 q
qme
d
d fi
2
22
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0
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222
2
0
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22
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0
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'2'
'
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)'(
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xme
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xqme
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d ififfi
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2
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42
'2 E
if
HE
if
HE
if
HE
ififfix
a
x
a
x
kak
xme
q
xme
d
d
Differential Cross Section for Single-electron Excitation
• Generalized Oscillator strength (GOS)
2
22
2
2
2
'
)'(2' ifif x
Em
q
qEmqf
222
0
4
2222
4
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'
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if
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if
HE
if
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if
HE
if
H
fi qf
EE
eqf
Emap
qf
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qf
a
x
ad
d
• Generalized Oscillator strength (GOS) per unit energy loss
Ed
Eqdf
EE
e
Edd
d if
E
fi
,'1
)4( 222
0
4
• Bethe Surface
Ed
Eqdfif
,'
• Bethe ridge – Compton scattering