12.141 electron microprobe analysisweb.mit.edu/e-probe/www/slides1.pdf · 2020. 1. 13. · uses of...
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Massachusetts Institute of Technology
12.141Electron Microprobe Analysis 1
Electron Microprobe Facility
Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology
Nilanjan Chatterjee, Ph.D.Principal Research Scientist
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Uses of the EPMA 2
Complete quantitative chemical analysis at micron-scale spatial resolution:
• Be to U (10-50 ppm minimum detection limit)
High resolution imaging:
• compositional contrast (back-scattered electron)
• surface relief (secondary electron)
• spatial distribution of elements (x-ray)
• trace elements, defects (light)
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Function of composition
Thin section
An example: Compositional imaging with back-scattered electrons 3
Back-scattered electron Plane polarized transmitted light
Polished surface
Function of optical properties
High-Z
Low-Z elements
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Types of analysis 4
Qualitative analysis
Visual characterization (shape, size, surface relief, etc.)
Identification of elements in each phase
Semi-quantitative analysis
Quick and approximate concentration measurement of a spot
Elemental mapping (spatial distribution of elements)
Quantitative analysis
Complete chemical analysis of a micron-sized spot
Concentration mapping of all elements
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JEOL JXA-8200 Superprobe 5
Electron
gun
Condenser
lens
Optical
Microscope
WDS
LN2
EDSSE
Objective
lensBSE
Stage
High
vacuum
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Electron emitting source: Tungsten hairpin 6
Self-biased thermionic electron gun
• Cathode: Filament at negative potential
Tungsten has a high melting point and a low work-function energy barrier; heated by filament current, if , until electrons overcome the barrier
• Wehnelt Cylinder at a slightly higher negative potential than the filament because of the Bias Resistor
Bias voltage (Vbias) automatically adjusts with changes in ie to stabilize emission; the grid cap also focuses the electron beam
• Anode: Plate at ground potential
Potential difference (accelerating voltage, V0) causes electron emission (current, ie)
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Lens modes
Normal Large depth of
focus for rough
surfaces
7
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Depth of focus 8
Large depth of focus Small depth of focus
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Electron probe parameters 9
Sample
Accelerating voltage, V0(V0 of 15 kV generates electron beam with 15 keV energy)
Final beam current, or probe current, ipFinal beam diameter, or probe diameter, dp
Final beam convergence angle, or probe convergence angle, ap
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Field Emission
dp
(nm
)LaB6
W hairpin
Different electron sources
At 10 nA and 15 kV, a Tungsten hairpin filament produces a beam with dp ≈100 nm
10
At a fixed Probe current,
Probe diameter increases
• with source as
Field emission < LaB6 < W
• with decreasing
Accelerating voltage
As Probe current increases,
Probe diameter also increases
(signal improves, but image
resolution degrades)
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Width of interaction volume >> probe diameterSpatial resolution can be improved by using a lower accelerating voltage that reduces the interaction volume
High atomic number: small hemisphereLow atomic number: large tear-drop
Spatial resolution: electron interaction volume 11
Vertical electron beam, probe diameter = 0.1 mm
Horizontal surface of sample
Monte Carlo simulations of electron trajectories in the sample2.25 mm
0.4 mm
1
2
4
3
Depth
(mm
)
0.2
0.4
0.6
0.8
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Electron interaction depth (range) 12
R : Kanaya-Okayama
electron range
E : beam energy
A : atomic weight
r : density
Z : atomic number
Electron range increases with increasing E, and decreasing r and rZ
E.g., at 20 kV,
R = 4.29 mm in Carbon (Z = 6, A = 12.01, r = 2.26 g/cc)
R = 0.93 mm in Uranium (Z = 92, A = 238.03, r = 19.07 g/cc)
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Electron interaction depth (range) 13
Electron range increases with increasing E, and decreasing r and rZ
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Electronbeam
14
Other signals include phonon excitation (manifested by heating), plasmon excitation (generated by
moving electrons in metals), and auger electron (ejected from atom by internally absorbed x-ray)
• Characteristic
• Continuum
(Bremsstrahlung)
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The “onion shell” model:
Cu-10%Co alloy
Production volume is different for each signal
150 nm
450 nm
CuKa ~1 mm
CuLa ~1.5 mmFluorescence of
CoKa by CuKa ~60 mm
20 keV, W filament
diameter ~150 nm
(not to scale)
Spatial resolution for different signals (production volume) 15
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Gallium Nitride
Production volume for cathodoluminescence 16
2 mm
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Electron-specimen interactions:Elastic scattering 17
Back-scattered
electron (BSE)
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Electron-specimen interactions:Inelastic scattering 18
Inner shell interactions:
• Characteristic X-rays
• Secondary electron (SE)
Outer shell interactions:
• Continuum X-rays
• Secondary electron (SE)
• Cathodoluminescence (CL)
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Back-scattered electron (BSE) 19
• Beam electrons scattered elastically at high angles
• Commonly scattered multiple times, so energy of BSE ≤ beam energy
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• Large differences between high- and low-atomic number elements
• Larger differences among low-Z elements than among high-Z elements
(Z)
Fraction of beam
electrons scattered
backward W Au
(74) (79)
Si (14)
high-Z
low-Z
Electron backscatter coefficient 20
K (19)
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Back-scattered electron detector 21
Objective LensBSE detector
A B
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Back-scattered electron detector 22
Solid-state diode
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Compositional and topographic imaging with BSE 23
A+B: Compositional mode A-B: Topographic mode
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Secondary electron (SE) 24
Specimen electrons mobilized by beam electrons through
inelastic scattering (causing outer and inner shell ionizations)
Emitted at low energies (mostly ≤ 10 eV for slow secondaries,
less commonly ≤ 50 eV for fast secondaries)
(recall BSE have high energies up to that of the electron beam)
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Secondary electron detector 25
Located on the side wall of
the sample chamber
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Secondary electron detector 26
(+ve bias)
When Faraday cage is positively biased,
secondary electrons are pulled into the detector
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Imaging with the Everhart-Thornley detector 27
Negative Faraday cage bias
only BSE
Surfaces in direct line of sight
are illuminated
Positive Faraday cage bias
BSE + SE
All surfaces are illuminated
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Cathodoluminescence (CL) 28
Caused by
inelastic scattering
of beam electrons
in semiconductors
Electron beam
interacts:
Valence electron
moves to the
conduction band
Electron recombines
with the valence band
to generate light with
energy Egap
Filled valence band is
separated from an empty
conduction band by Egap,
Trace element impurities expand the conduction band and enable additional electron transitions Emitted light has additional energy components E ≠ Egap
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Cathodoluminescence spectrometer 29
Optical spectrometer
(CCD array detector)
Optical microscope
light (turned off)
Optical microscope
camera (not used)
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Cathodoluminescence spectrum 30
Light wavelength (nm)
Inte
nsi
ty
Mn
(Ca,Mg)CO3 (dolomite) with Mn
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Hyperspectral CL imaging 31
Total intensities of 400-450 nm light (blue shades: 29-277 counts)
Total intensities of 500-550 nm light (green shades: 63-251 counts)
Total intensities of 600-750 nm light (red shades: 87-1018 counts)
A continuous spectrum covering all light wavelengths is collected at each point of the image area
Total intensities of200-950 nm light at each pixel (red shades represent 722-2090 counts)
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Hyperspectral CL imaging 32
Total intensities of
361-668 nm light at
each pixel in
multicolor scale:
Blue (≤ 0 counts):
no light
Red (9884 counts):
maximum intensity
Total intensities of
light of all
wavelengths at each
pixel in grey scale:
Black: no light
White: maximum
intensity
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The X-ray spectrum 33
Characteristic X-rays
Continuum X-rays
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Energy Dispersive Spectrometer (EDS) 34
• EDS detector: solid-state semiconductor, window and aperture
• Multichannel analyzer (MCA) processes the X-ray signal
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Energy Dispersive Spectrometer (EDS) 35
Lithium-drifted silicon, or Si(Li), “p-i-n” detector
X-rayelectron
hole
Neg
ati
vely
bia
sed
su
rfa
ce r
epel
s el
ectr
on
s
• A single crystal of silicon, coated with
lithium on one side
• Pure silicon is a semiconductor. But
impurity of boron, a p-type dopant,
makes it a conductor
• Lithium, an n-type dopant, counteracts
the effect of boron and produces an
intrinsic semiconductor
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Qualitative analysis withBSE and EDS 36
ilmenite
plagioclase
hornblende
Me
an A
tom
ic N
um
be
r