MT-0.6026 Electron microscopy
Scanning electron microscopy and electron probe microanalysis
Eero Haimi
Research Manager
Outline
1.
Introduction
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–
2.
Background of measurement principles and methods
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–
3.
Electron beam – specimen interaction
Equipment technology
Measurement techniques
–
–
–
4.
Basics of scanning electron microscopy (SEM) and electron probe
microanalysis (EPMA)
Introduction to sample preparation
Secondary (SE) and backscattering (BSE) electron imaging with SEM
X-ray spectrometry using EDS and WDS
Electron backscattering diffraction (EBSD)
Applications examples
1
2. Background of measurement principles
and methods
• Electron beam specimen interactions
• SEM+EDS+WDS+EBSD equipment technology
3
Electron beam specimen interactions
• Electron scattering
– Backscattered electrons
– Secondary electrons
• X-ray emission
– X-ray continuum
– Characteristic x-rays
• Interaction volumes and emission regions of
different signals
4
2
Electron beam specimen interactions
Electron beam: acceleration voltage 0,2 – 30 kV
Characteristic
x-rays
Auger
electrons
Electrons
Secondary
electrons (SE)
Photons
Continuous
x-rays
Back-scattered
electrons (BSE)
Cathode
luminescence
Heat
Electric
current
Sample material
Electron scattering
Electron beam
(Auger
Electrons)
Secondary
electrons (SE)
Backscattered
electrons (BSE)
Electron current
Sample
6
3
Electron scattering
• Scattering from electron
cloud
• Scattering from atomic
nucleus
• Elastic
• Inelastic
• Coherent
• Incoherent
7
Elastic electron scattering
•
Scattering with almost no energy loss is called elastic. In
this case, less than 1 eV energy is transferred from the
scattering electron to the sample.
•
Interaction takes place primarily between scattering
electron and atom nucleus. Scattering is for the most
part coherent.
•
However, as a result of scattering direction of electron
movement can change. Generally, the direction changes
only few degrees, but there is a small probability that the
degree of angular change can be anything between 0 180°. On average, change in the angle increases as a
function of atomic number Z.
•
If multiple elastic changes in direction of electron
movements takes place frequently times enough, some
of the primary beam electrons can escape the sample
without practical loss of energy. These electrons are
called backscattered electrons (BSE).
4
Elastic electron scattering
C
Fe
Monte Carlo simulation 25 electron trajectories (25 kV)
9
Backscattered electron yield as a function
of atomic number
h = backscattering coefficient
5
Backscattered electron yeild as a function of
sample tilt angle
Fe, 25 kV
0 °, h = 0,28
70 °, h = 0,54
11
Backscattered electron diffraction
• Coherently backscattered electrons are capable for
diffractive interaction.
• Intensity variation of backscattered electron diffraction
pattern is characteristic to crystal structure and
orientation of the sample.
12
6
Inelastic electron scattering
• In inelastic scattering electron • Secondary electron emission
loses kinetic energy.
• Auger-electron emission
• Several scattering
• Characteristic x-ray emission
mechanisms exists.
• Emission of x-ray continuum
• Cathode luminescence (with
certain materials)
• Fonon scattering
• Plasmon scattering
13
Secondary electron emission
•
When primary beam
electrons are interacting
with conduction band and
valence electrons,
secondary electrons (SE)
can be ejected from the
sample.
•
Kinetic energies of these
electrons are less than
50eV. Therefore, only
secondary electrons that
has been generated near
the sample surface are
capable of escaping the
material.
•
Secondary electrons are
also generated by
backscattering electrons.
l = free mean path
14
7
Effect of topography on secondary
electron emission
Energy distribution of emitted electrons
8
Charge ballance
Ibeam = Iscattered + Ispecimen
Ibeam = (h+d)Ibeam + Ispecimen
Electron beam specimen interactions
Auger
electrons
Secondary
electrons (SE)
Back-scattered
electrons (BSE)
Characteristic
x-rays
Continuous
x-rays
Cathode
luminescence
Heat
Electric
current
9
X-ray emission
• Continuous radiation (Bremsstrahlnung, white radiation)
• Characteristic radiation
19
Continuous x-rays
•
Continuous x-rays (bremsstrahlung )
are generated, when primary beam
electrons are decelerated by
interaction with Coulombic field of
atoms.
•
Energy distribution of this radiation is
continuous. The most energetic
radiation reaches so called DuaneHunt (short wave limit) limit. If the
sample is not charged, the limit has
same value as the whole kinetic
energy of primary electrons.
20
10
Characteristic x-rays
•
When acceleration voltage of
primary beam electrons is
increased adequately,
specific intensity peaks are
formed on top of continuous
x-ray spectrum at
wavelengths (energies) that
are ”characteristic” to each
element. After appearance,
these characteristic
wavelengths are independent
of acceleration voltage.
Mo
21
Emission of characteristic x-rays
Characteristic x-ray are emitted by the following
process:
•
•
a) The interaction of a high energy electrons
with an atom result in ejection of an electron
from inner atomic shell. (Also x-ray photons are
capable for the same process that result
fluorescence radiation).
The beam energy must be bigger than critical
excitation (ionization) energy E c:
Ue > Ec
•
Ionization leaves an atom in an excited state
that has higher energy than the ground state.
•
The critical excitation energy is larger than the
energy of corresponding x-ray photon.
(Fluorescence radiation do not excite same type
of atoms again.)
22
11
Characteristic x-ray emission
•
b) De-excitation (relaxation) takes place, when an
electron from an outer shell fills the empty state (Texcitation < 10-8s). The difference between the two shell
energies equals the energy of the characteristic x-ray:
K peaks
L peaks
hn = Ef - Ei
•
Consequently, if:
•
f=K
•
•
f=L
f=M
b
b
-->
K-series
i = L ->
Ka-lines
i = M ->
Kb-lines
i = N ->
Kg-lines
-->
L-series
i = M ->
La-lines
i = N ->
Lb-lines
-->
M-series
i = N ->
Ma-lines
g
a
a
M peaks
a
23
Quantum mechanics of electron transitions
•
Electron transitions that take
place as a result of relaxation of
excited state have not equal
probabilities. Some of the
transitions are even quantum
mechanically forbidden.
•
Calculation of quantum
mechanical transition probabilities
shows that transitions with:
•
Only with accurate spectrometers
fine structures of atomic energy
levels are detected in
charecteristic x-ray
measurements.
- Dl = ±1
- Dj = 0 tai ±1
are allowed.
24
12
Characteristic x-rays
Auger-electron emission
•
Atomic excitation state can be relaxed
instead of x-ray photon emission by
emission of Auger-electron that has also
characteristic energy.
•
Auger and x-ray yields per excitation state
equals one. The relative proportions depend
on atomic number.
•
Auger-emission is more probable in the
case of light elements. As a consequence,
characteristic x-ray emission of light
elements is not as intensive as it is in the
case of heavier elements.
•
Auger–electrons are measured in surface
analytics but not in SEM, because Augerelectrons generated deeper in the sample
loses their characteristic energy quicly in
inelastic scattering processes.
26
13
Physical background of x-ray spectroscopy
•
Inelastic electron scattering is capable of
producing characteristic x-rays, when
electron energies exceed the critical energy
of exitation.
•
Characteristic x-ray wavelengths are
specific to elements; they depend on
electron shell structure.
•
The wavelengths obey Moseley's law:
l -1/2 = C(Z-s)
or
E = D(Z-F)2
where C,s,D ja F are electron shell
dependent constants and E is energy of the
radiation.
•
Relation between wavelengths and energy:
E = hc/l
–
–
wavelength dispersive spectrometry (WDS)
energy dispersive spectrometry (EDS)
Interaction volumes
•
•
•
•
Electron range
Fluorescence
Absorption
Emission zones of
different signals
28
14
Primary electron range
Monte Carlo electron trajectory simulation
Acceleration voltage = 5kV
Acceleration voltage = 25kV
Carbon sample
29
Kanaya & Okayaman formula
R = (0,0276*M*E01,67)/(Z0,89r)
R = electron range (μm), M = atomic weight (g/mol),
Z= atomic number, ρ = sample density (g/cm3), Eo = incident beam energy (keV)
30
15
Electron ranges in different materials
C
Fe
5 kV
25 kV
31
Fluorescence
Cr-Ka
Fe-Ka
32
16
Absorption
• Part of electrons and photons are absorbed in the
sample.
• Electron absorption is stronger than photon absorption
• Absorption is dependent on electron or photon energy,
material thickness (length of scattering path), density
and mass absorption coefficients
33
Schematic illustration of interaction
volumes for various signals
34
17
Characteristic x-ray range
Empirical formula:
ρR = 0.064(Eo1.68 - Ec1.68)
R = x-ray range (depth of x-ray production) (mm)
Eo = beam energy (keV)
Ec = critical excitation energy (keV),
ρ = density (g/cm3)
(Anderson-Hasler)
35
Influence of acceleration voltage (beam energy)
and atomic number on interaction volume
Fe
18
Optimum resolution
empty
resolution
optimal
poor S/N
ration
Effect of magnification on optimum pixel size
Magnification
Scan area on sample
(for 10 x 10 cm display)
Pixel size
(1000 x 1000 pixel scan)
10
100
1 000
10 000
100 000
1 cm2
1 mm2
100 mm2
10 mm2
1 mm2
10 mm
1 mm
100 nm
10 nm
1 nm
38
19
Resolution and interaction volume
Electron beam specimen interactions
Auger
electrons
Secondary
electrons (SE)
Back-scattered
electrons (BSE)
Characteristic
x-rays
Continuous
x-rays
Cathode
luminescence
Heat
Electric
current
20
Overview of instrument capabilities
• High magnification
• Large depth of field
• Chemical information in
micrometer scale (BSE, EDS,
WDS)
• Crystallographic information
(EBSD)
• Special techniques (EBIC, CL,
voltage contrast)
• In-situ experiments
(temperature, strain, etc.)
More that just a microscope
More that just composition and
structure
21