Solid State Chemistry
Scanning Electron Microscopy
H.J. Deiseroth, SS 2003
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CamScan 44
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Interaction of a high energy electron beam with material
Primary Beam
Backscattered
Electrons
Cathode
Luminescence
Auger
Electrons
Characteristic X-Ray
Spectrum
Secondary
Electrons
X-Ray Retardation
Spectrum
Specimen
Absorbed Current
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Electron Gun
Magnetic Lens
Anode
Scanning Coils
Energy
Dispersive
X-Ray Detector
Objectiv Lens
Backscattered
Electron
Detector
Wave length
Dispersive
X-Ray Detector
Secondary
Electron
Detector
Stage with
Specimen
to the Vacuum
4
Pump
Comparison of W-, LaB6-, and Field emission-cathods
W
LaB6
FE
work function /eV
4,5
2,7
4,5
crossover /µm
(important for high
resolution images)
20-50
10-20
3-10
Tcathod /K
2700
<2000
300
1-3
25
105
gun Brightness
/A/cm2 sr
105-106
107
109
vacuum /mbar
10-5
10-7
10-9
service life /h
40-100
1000
>2000
emission current
density /A/cm2)
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Electron Gun (W-Cathode)
Cathode
Heating
Current
Tungsten
Wehnelt Cylinder
(500 V more negative
than the Cathode,„Bias“)
20000 V
Crossover (20-50 µm)
Electron beam source
Anode
grounding
The crossover is projected on the sample in a reduced size
by the electronic-optical system
(minimal diameter of the beam: ca. 5 nm)
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LaB6-Cathode (also: CeB6):
Graphite
LaB6 single crystal
- indirect heating (because of the low conductivity of LaB6)
- lower work function than the W-cathode ( higher brightness)
- demageable by ionic shooting ( high vacuum necessary)
- expensive!
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Field Emission-Cathode
- W – cathode with a fine apex
- two anodes:
1. one to bring up the work function
2. one for the acceleration
- high brightness
- high vacuum necessary
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Interaction volume of the electron beam (pear-like)
primary beam
Auger-electrons (up to 0,001m)*
Secondary Electrons (up to 0,01m)*
Backscattered Electrons (up to 0,1m)*
X-Ray (1-5m)*
*information depths
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Dependence of the information depth
on the acceleration voltage and material
(simulations)
Fe (10 kV)
Au (20 kV)
Fe (20 kV)
Al (20 kV)
1m
Fe (30 kV)
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Secondary Electrons:
- inelastic scattered PE (Primary Electrons)
- energy loss by interaction with valence
electrons or with the atomic nucleus
- Energy: < 50 eV
- maximal emission depth: 5-50 nm
- leads to high resolution images
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Backscattered Electrons:
- elastic and inelastic scattered PE
- Energy: 50 eV – Energy of the PE (e.g. 20 keV)
- maximal emission depth: 0.1 - 6µm (dependent on the
specimen)
- Intensity depends on the average atomic number of the
material ( material contrast images)
- high interaction volume ( low resolution images)
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Cu-wire imbedded in solder
SE-Image
(high resolution)
BE-Image
(high Z-contrast)
Cu
Cu
Cu
Cu
ZPb > ZSn > Z Cu
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Backscattered electron images are less
sensitive on charging:
BE-image
SE-image
reason: the average energy of the backscattered electrons is higher
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Auger Electrons:
- Energy characteristic for the Element
 Auger Electron Spectroscopy (AES)
3.
EAuger-Electron = E1-E2-E3
E3
E2
1.
2.
E1
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Comparison Auger / X-Ray
Rate
of
yield
Auger
X-Ray
Z
 small X-Ray yield for light elements (B, C, N, O, F)
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Cathode Luminescence
- not for metals
- visible and UV radiation
- special detectors necessary
PE
h
conduction band
valence band
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X-Ray Retardation Spectrum
Intensity
Energy /keV
- primary electrons are retarded by the electron clouds
of the atoms
- Emax of the X-Ray‘s: e × Uaccelerating voltage
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Characteristic X-Ray Spectrum
(without fine structure)
L
O
L
N
K
M
L
K
M
M
L
K
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Energy range of the main series as a function of
the atomic number:
energy
K
K
L
L
M
Mosley‘s law: 1/ = K
(Z-1)2
atomic number
(K: constant, Z = atomic number)
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Typical (characteristic) X-Ray spectrum (EDX)
intensity
Spectrum of Ag6GeS4Cl2
energy /keV
- sulfur (Z = 16) and chlorine (Z = 17) easily distinguishable
(in contrast to X-Ray diffraction)
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Electron Detectors
1. Secondary Electron (SE-) Detector
Szintillator-Photomultiplier-Detector
(Everhart-Thornley-Detector)
Amplifier
PhotoMultiplier
Light conductor
VideoSignal
Scintillator
Metal net (+ 400V)
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SE-Detektor:
SE-D
SE-D
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Principle of an EDX-Detector
P-
i-
n-conducting
Si (Li)
+
3,8 eV
h + Si

Si+ + e-
-
e.g. Mn K: 5894 eV
X-Ray
5894/3.8 = 1550
electron hole pairs
- +
high voltage
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WDX-Spectrometer:
Scanning of a -range with one monochromator crystal
Rowland
circle
Detector
Axis of
Crystal
movement
Monochromator
Crystal
Specimen
(X-Ray source)
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WDX Detector
4 Monochromator
crystals
Proportional
counter
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Comparison EDX - WDX
Irel
Ga L
Ga L
EDX
Ge L
Ge L
Irel
Ga L
WDX
Ga L
Ge L Ge L

(compound: GeGa4S4)
1.1
1.2
energy27/keV
Detector‘s for WDX:
two proportional counter switched in series
1: FPC, Flow proportional counter (for low energy X-Ray‘s)
- the counting gas (Ar / CH4) flows through the counter
(very thin polypropylen window, not leak-free for
the counting gas)
2. SPC, Sealed proportional counter (for high energy X-Ray‘s)
- counting gas: Xenon / CO2
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Comparison of EDX and WDX
EDX
WDX
spectral resolution
110-140 eV
10 eV
specimen current
10-10 A
10-7 A
analysis time
1-2 min
30-100 min
simultaneous
sequential
spectrum develops
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Sensitivity
of EDX and WDX
atomic
percent
EDX
Be - window
WDX
atomic
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number
Applications:
I) High resolution Images
II) Qualitative and quantitative analysis
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Principle of the image formation
synchronised scanning coils
Beam
Amplifier
Television image
Signal-Detector
(SE or BE)
Specimen
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Twinning of Crystals
PbS
Na2Zn2(SeO3)3 3H2O
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Morphology of crystals
CuGa3S5
K2In12Se19
SnIn4S4
CsIn3S5
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Quality control of small technical objects
Compact disc
Cantilever
of an AFM
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Large area mapping (X-Ray-images)
SE-Image
Cu-Kmapping
Ni-Kmapping
Zn-Kmapping
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256x256 pixel, moving of the specimen holder
typical sample holder equipment
typical preparation of
small crystals
conducting tabs
(adhesive plastic
with graphite)
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Special preparation for insulating material:
-metallisation with gold (sputtering process)
better for high resolution images
-carbon deposition (evaporation process)
better for quantitative analysis
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