Electron microscope techniques
Scanning Electron Microscope (SEM)
Transmission Electron Microscope (TEM)
Topics
• Electron sources
• Electro-magnetic lenses
• Interaction of the electron probe with the sample
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–
–
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Secondary electrons
Backscattered electrons
X-rays
Auger electrons
• Detection of electrons
• Various modes of operation
– Imaging
– Analytical
Brief historic overview
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TEM: 1931 by
Ernst Ruska (1906-1988)
Nobel prize 1986
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SEM: 1937-39 by
Manfred Baron von Ardenne
(1907-1997)
Electron sources
Electron guns:
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Various examples
of gun design
– Thermionic
– Schottky
– Field emission
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Cathode material
– Tungsten
– Lanthanum
hexaboride
(LaB6)
– Others…
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Cathode material
determines
emission current
density
Energy scheme of various gun types
Electron guns
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Wehnelt cup to concentrate emission on a small area of the cathode
Schottky emitter can be considered as field assisted thermionic gun
Field emission needs a better vacuum – otherwise the tip radius is
destroyed by Ion bombardment of residual gas
Electron guns
Electron guns
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Various methods to determine gun brightness
Electro-magnetic lens systems
sketch
Electro-magnetic lens systems
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•
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Axial magnetic field with rotational
symmetry
Radial component of the magnetic
field is related to the radial
component (Maxwell)
Electrons travel along screw
trajectories due to Lorentz force

 
F = e v ×B
•
1/f = 1/p + 1/q (lens equation) holds
true, therefore crossover is
demagnified by M= q/p
Lens aberrations
sketch
Resolution
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Rayleigh Criterion (a)
–
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Edge Resolution (b)
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Distance between points
corresponding to 25% and 75%
of total step height
Radial Intensity Distribution (c)
–
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Current density distribution
overlaps at half of separation
with an intensity drop ≤ 75%
Diameter that contains a given
percentage of the total probe
current
Maximum Spatial Frequency (d)
–
Contrast transfer drops so much
that periodicity Λmin cannot be
detected
Bild 2.27
Electron-sample interactions
•
Elastic scattering
– Classical scattering: Electron-Nucleus (Rutherford)
– Relativistic scattering: Schrödinger and Pauli-Dirac, Quantum mechanics of
scattering, exact Mott cross-section
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Inelastic scattering – Electron Excitation Processes and Energy Loss
– Plasmon interaction and inter- intraband transitions
– Electron-electron binary collisions
– Ionization of inner shells
Elastic scattering
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Classical scattering (a):
Electron-Nucleus (Rutherford)
Coulomb force:

e2 Z 
F=−
ur
4π ε 0 r 2
The ratio dσ/dΩ is called
„differential cross section for
scattering through an angle θ“
•
Relativistic scattering (b) – will
not be treated here, but results
in…
Elastic scattering
Inelastic scattering
•
Plasmon interaction and inter- intraband transitions
– Plasmon is a collective longitudinal charge density wave
– Plasmon energies are on the order of 5 – 30 eV
– Inter- and intraband transitions have a similar energy regime
→ Energy losses are in the range of 0 – 50 eV at scattering angles
below 10 mrad
•
•
Electron-electron binary collisions and inner shell ionization are usually
treated in a quantum mechanics way
The ratio ν of total inelastic to total elastic cross section decreases with
increasing Z
ν=
•
σinel 20
≅
σ el
Z
Modeling of multiple scattering is handled by complex Monte-Carlo
simulations
Elastic + Inelastic scattering
Elastic + Inelastic scattering
Al: Z=13
Au: Z=79
Elastic + Inelastic scattering
C: Z=6
Cu: Z=29
Au: Z=79
What kind of species are generated?
Probe-sample interaction results in the „generation“ of
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Secondary electrons
Backscattered electrons
X-rays
Auger electrons
Plasmons
Secondary electrons (SE)
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SE (exit energies < 50 eV) are generated if the energy gain of these species
is large enough to overcome the work function
This process needs to be treated quantum mechanically as the scattering of
an electron wave at a potential barrier
SE are only able to escape from a small surface range (probability of
escaping is based on their inelastic mean free path)
Backscattered electrons contribute to the SE yield δ
Backscattered electrons (BSE)
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BSE are present in the whole energy range from 50 eV (definition) to the
maximum acceleration energy of the primary electrons (PE)
Their spectrum shows a broad peak overlapped by SE and Auger peaks as
well as plasmon loss
BSE and SE are the most important signals for imaging. Knowledge about
the dependence of the backscattering coefficient and the SE yield on surface
tilt, material and electron energy is essential for any interpretation.
X-ray
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•
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Acceleration of a charged particle (electron) in the screened Coulomb
potential of the nucleus leads – with a low probability – to an emission of a
X-ray quantum (usually elastic scattering is observed)
Electron is decelerated by hν (energy of the X-ray quantum) → continuous
X-ray spectrum
This continuous spectrum is superposed on the characteristic X-ray
spectrum generated by filling of inner shell vacancies
X-ray
Auger electrons (AE)
•
De-excitation energy
produced when an inner shell
vacancy is filled does not
necessarily involve the
emission of a X-ray quantum
– its also possible that an AE
is emitted
•
The AEs are superposed on
the low energy tail of the BSE
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Only AEs from an ultra thin
surface layer contribute to
these energy peaks (multipleloss AEs are lost to the BSE
background)
How to detect SE and BSE
• SE detector: Scintillation & Photomultiplier combination (Everhart & Thornley)
How to detect SE and BSE
• In many cases BSEs
are detected by
semiconductor
detectors (p-n
junction diodes)
How to detect X-rays
• Wavelength dispersive X-ray
spectrometer (WDX)
Bragg:
2 d sin θB = n λ
High energy resolution
• Energy dispersive X-ray
spectrometer (EDX)
Whole spectrum can be
recorded simultaneously
Imaging modes
• SE mode / BSE mode
SE
BSE
Imaging modes – X-ray
• X-ray Line-Scan/mapping
– Limited resolution
– Usually bad signal to noise ratio
→ Limited situations were this
technique is applicable
Analytic modes - diffraction
• Diffraction mode
– By equipping the instrument with a camera or CCD detector system it is possible
to use a SEM or TEM in the diffraction mode
– According to Bragg‘s equation one will find constructive and destructive
interference from electrons of energy E scattered at adequate lattice planes
– This also works to analyze the texture or stresses in thin films
– In amorphous materials it is possible to gain insight into the nearest neighbor
distance
The general technique of diffraction will be treated in a separate lecture
Analytic modes - Auger
• This mode will be discussed in the next section as it is usually used in the
context of a sputtering process to gain insight into the 3dimensional element
distribution