Scanning-Electron Microscope
TEM
SEM
Scanning Electron Microscope SEM
topographic contrast surface Transmission Electron Microscope TEM SEM-Introduction
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SEM-Introduction
density contrast slices
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Considerations in Microscopy Magnification M Resolution smallest separation of two points that are visible as distinct entities
high magnification without resultion is
JEOL 840
“nonsense” depth of focus or depth of field DOF damage preparation
Philips XL30 FEG
SEM-Introduction
LEO Supra 35
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SEM-Introduction
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History
for light: resolution in the visible ~ 200 nm with X-rays as small as 20 nm
1878 Abbe 1932 de Broglie 1926 Busch
light diffraction limit electrons are waves can focus e’s with magnetic field 1932 Ruska TEM 1938 von Ardenne first SEM 1938 Siemens first commercial TEM 1965 first commercial SEM
SEM-Introduction
operate at large NA (NA = numerical aperture) NA = n sin(α) up to 1.4
for electrons: resultion ~ 1 Å operate at small NA NA = α ~ 10-2 -10-3 100 keV, yields λ = 0.04 Å
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SEM-Introduction
Anatomy
Comparison light microscope vs SEM scanning optical microscope
optical microscope
SEM-Introduction
SEM
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SEM
SEM
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SEM-Introduction
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Radiolarien (biogenes Sediment, hier aus Opal)
SEM-Introduction
Radiolarien (biogenes Sediment, hier aus Opal)
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SEM-Introduction
magnified
SEM-Introduction
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further magnified
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SEM-Introduction
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SEM-Introduction
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SEM-Introduction
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SEM components Vacuum System low-, high- and ultrahigh vacuum together
Microscope Column
Electron Gun Condenser to shape the beam Electromagnetic Lenses to focus the beam Scan Coils to deflect the beam Apertures to limit the beam
Sample Chamber Motorized Stage (x,y,z, tilt, rotation…) Detectors SEM-Introduction
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SEM-Introduction
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Zooming capability another example
SEM-Introduction
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SEM-Introduction
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SEM-Introduction
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SEM-Introduction
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SEM-Introduction
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SEM-Introduction
Why Vacuum
Types of Pumps mechanical displacement
Electrons are scattered by gas molecules
rough-, rotary-, auxiliary pump
in ambient at 15 keV, mean-free path ∼ 10 cm it’s much worse for low-energy electrons need a mfp of order 1 m
oil diffusion pump diff pump
turbomolecular pump
Prevents beam induced chemical reactions Required for stable emission Required for some detectors and
turbo pump
sputter ion pump ion pum
Ti-sublimation pump cryopump
electrostatic lense
SEM-Vacuum
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SEM-Vacuum
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in an electron microscope
Rotary Pump initial pump e.g. to pump the chamber after sample change
good efficiency for high pressure 100 l/min and more
filled with special lubricant disadvantages:
SEM-Vacuum
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vibrations! oil vapor exhausts oil vapor maintenance
SEM-Vacuum
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Diffusion Pump invented by Gaede 1915 and Langmuir 1916 works by momentum transfer (not much to do with diffusion)
Considerations:
SEM-Vacuum
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SEM-Vacuum
must be used together with another roughening pump very high pumping speed needs warm up and special oil, which is evaporated needs cooling, too! needs to be mounted vertically
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Diffusion Pump Principle
Diffusion Pump Advantages
Disadvantages
oil vapor time to heat up and cool
simple cheap no moving parts pumps also light gases tolerant with particles
down
needs cooling water can overheat if roughening pump fails, oil may escape oil may be burnt or even distributed throughout the whole system can only work vertically
SEM-Vacuum
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SEM-Vacuum
Turbomolecular Pump
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Turbo Pump Construction
Becker, 1957 essentially a jet turbine, i.e. works also by momentum transfer multiple stages of rotating blades (rotor) spaced between fixed blades (stator)
Considerations: must be used together with another rough pump very high pumping speed (but not for light gases)
SEM-Vacuum
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SEM-Vacuum
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Turbo Pump Advantages
Disadvantages
clean no warm up high vacuum high pumping speeds can work in different orientation modern ones have magnetic bearings and are free of oil
Sputter Ion Pump can achieve ultrahigh-vacuum no moving parts ionization of gas by electrical fields and
relatively expensive not tolerant to particles can fail catastrophically high vacuum is pure hydrogen vibrations (for older ones) requires back-up pump
SEM-Vacuum
collisions ions sputter Ti, which reacts with residual gases and burries these under a film
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SEM-Vacuum
Ion Pump Principle
Ion Pump Advantages
Disadvantages
clean no moving parts can measure pressure
simultaneously
SEM-Vacuum
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SEM-Vacuum
need to bake out not very efficient for water low capacity gasses not permanently removed
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Types of Electron Guns
Ideal Emitter Characteristics
tungsten hairpin filament (thermionic emission) LaB6 cold field emission Schottky field emitter
SEM-Emitters
high current, i.e. low workfunction high melting point (stability), e.g. tungsten low outgasing chemically stable small source size (spatial coherence) high brightness “monochromatic” (temporal coherence) “infinite” life time “cheap”
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SEM-Emitters
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SEM-Emitters
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Thermionic Emission current is passed through the emitter in order to heat it there is typically a large angular spread in emission and a broad distribution in energy have to shape the beam and stabilize it through the “Wehnelt Cylinder” (grid gap) grid gap reduces space-charge and increases the brightness (beam shaping)
SEM-Emitters
Wehnelt Cylinder
Crossover and Aperture works at 10-5 mbar
cross-over of source is demagnified in the lenses following below W-hairpin
Wehnelt cylinder
the angle α is called aperture (though we call the limitting pin-hole the aperture)
SEM-Emitters
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SEM-Emitters
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LaB6 Emitter Advantages
Disadvantages
brighter than W hairpin smaller source size energy spread is lower
SEM-Emitters
needs < 10-6 mbar
reactive when hot higher vacuum required difficult to fabricate brittle must heat slowly must heat indirectly more expensive
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SEM-Emitters
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Field Emission
Field Emitter
proposed in 1954, achieved in 1966 needs UHV
single rystal wire of W, etched tip diameter 10-100 nm
large electrical field required, i.e. yielding 107 V/cm
SEM-Emitters
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SEM-Emitters
Field Emitter
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Electron Lenses
Advantages
Disadvantages
Electrostatic used in gun (only, few
cold source (no heating) very low spread very small “virtual” source
UHV (the higher the
exceptions) small fast response (e.g. beam blanker) it is conveniently uses in FIB’s
size very bright yields a large DOF
better) current is not very stable needs to be “flashed”
Electromagnetic solenoid high magnification possible needs cooling f∝H∝I rotates the image (need to be corrected)
there exists only converging lenses one cannot correct lens aberrations with compound lenses SEM-Emitters
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SEM-Lens
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Magnetic Lens (principle)
Einzellinse (electrostatic)
Davisson and Calbick 1931 Knoll 1932
SEM-Lens
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SEM-Lens
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Condenser and Objective Lens
SEM-Lens
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condenser
objective
first lens (often two
large demagnification,
condensers) used to control the source size and beam current
e.g. short focal length two sets of deflection coils are included stigmator is built in too objective aperture (at the end)
SEM-Lens
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Lens Aberrations
source size d0 (actual demagnified spot) spherical aberration (distance from axis) chromatic aberration (energy spread) diffraction (wavelength) astigmatism
astigmatism can be corrected (stigmator) aperture α is small in EM’s (unlike optical microscopes) at 10-30 keV (high voltage for an SEM), the limiting resolution is set by the source size and by spherical aberrations at 0.1-1 keV (low voltage for an SEM), the limiting resolution is set by diffraction and chromatic aberration
SEM-Lens
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SEM-Lens
Linsenfehler
d 0 = C0 / α d S = C Sα 3 / 2
α ∝ D/B d C = CC
smallest spot (resolution limit)
where D is the pin-hole diameter and B the brightness
∆E α E
d d = 0.6λ / α
at high energy, C0 and CS dominate and the optimal spot size is given by:
SEM-Lens
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(
d optimal ≈ C0 CS 3
)
1/ 4
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SEM-Lens
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Aperture (diamater) one or more used reduces the beam current lowers the angular spread and hence spherical aberration if too small, there is a lot of noise and the resolution is bad, because the virtual source size is increased must find a compromise!
SEM-Lens
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SEM-Lens
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same field of view with corrceted astigmatism
SEM-Lens
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SEM-Lens
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Depth of Field / Resolution Distance above and below plane of focus which appears to be in focus EM much larger than optical microscope DOF ∝ 1/α interrelated factors aperture working distance DOF
SEM-Parameters
resolution magnification
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SEM-Parameters
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Resolution vs WD
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small WD
large WD
small spot size large α small DOF high resolution
SEM-Parameters
large spot size small α large DOF low resolution
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aperture 120 µm, too large !
SEM-Parameters
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aperture 30 µm, good !
SEM-Parameters
high DOF due to small aperture of 10 µm
low DOF due to large aperture of 120 µm
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high DOF due to small aperture of 10 µm
Signals and Interactions Electrons secondary (low energy) backscattered (high energy) transmitted Auger electrons (UHV) beam current Photons X-rays Cathodoluminescence
SEM-Parameters
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SEM Advanced Signals and Interactions
Contrast
Everhart-Thornley
topographic compositional (elements) potential adsorbates (low energy) recombination radiation efficiency conductivity crystallographic
SEM Advanced Signals and Interactions
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grid (usually at positive potential to attract the electrons)
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SEM-Basic Detectors
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ET-Detector
SEM-Basic Detectors
Origin of SE electrons
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SEM-Basic Detectors
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SEM Advanced Signals and Interactions
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SE contrast with standard Everhart-Thornley detector
polished steel spheres from the workshop
in-lens contrast
SEM-Basic Detectors
Emission Zones
SEM Advanced Signals and Interactions
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Backscattered vs Secondary esecondary electrons high resolution strongly topography sensitive little element sensitive sensitive to charging problems
SEM-Basic Detectors
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Backscattered vs Secondary e-
backscattered electrons lower resolution atomic number contrast, in particular strong signal for heavy atoms less sensitive to charging problems
SE image
BSE image
a solder
SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
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Excitation Volume
flat polished surface which one is SE and BE ?
Electron penetration volume and depth depends on: Electron beam energy
• •
more energy, deeper penetration more energy, less surface sensitive
Atomic number of specimen
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higher Z, lower pepentration and smaller volume
origin of topographic contrast in SE detection
SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
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polished steel surface at 5 keV
SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
polished steel surface at 1 keV
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X-ray Detection X-ray intensity ∝ Z wavlength dispersive detector energy dispersive detector
SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
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Wavelength Dispersive System (WDS)
SEM Advanced Signals and Interactions
SEM Advanced Signals and Interactions
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Potential Contrast on a Chip
SEM Advanced Signals and Interactions
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Crystallographic Contrast Electron Channeling Contrast Imaging (ECCI)
flat surface, small beam divergence BSE detector
SEM Advanced Signals and Interactions
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SEM Advanced Signals and Interactions
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Cathodoluminescence
SEM Advanced Signals and Interactions
Cathodoluminescence
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SEM Advanced Signals and Interactions
Signal Processing
Dynamic focusing & Tilt correction When sample is tilted a very large DOF would
signal averaging (by pixels, lines and
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be required to maintain the entire sample in focus Dynamic focusing corrects for this
frames) dynamic focusing tilt correction gamma control etc.
Tilting the sample results in distortion Correct the picture accordingly so that length are proper SEM-Lens
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SEM-Lens
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Tilt Correction Example
SEM-Lens
large WD and small aperture Î small beam current
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SEM-Lens
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large WD and small aperture Î small beam current need to integrate over long time
50 µm SEM-Lens
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SEM-Lens
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Preparation and Problems Preparation is an art, all kind of artefacts are possible charging contamination damage
SEM-Lens
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10 kV
SEM-Basic Detectors
SEM-Basic Detectors
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10 kV
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SEM-Basic Detectors
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2 kV
SEM-Basic Detectors
0.5 kV
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SEM-Basic Detectors
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Charging Effects
0.5 kV and twice WD
deflection of SE’s change in emission pattern periodic burst moving images jumps depends on beam energy
SEM-Basic Detectors
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SEM-Basic Detectors
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Metal Coating sputter coating Ar ions strike target quite homogeneous even if not in line of sight good adhesion
thermal evaporation target is heated shadow effect “cleaner”, but more grainy
SEM-Basic Detectors
Typical SEM sputter coater
often C, Au, AuPd artefacts in pictures: thermal damage etching damage roughness and grain of added film is imaged
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SEM-Basic Detectors
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Sample Preparation (organic) Fixation preserve fine structure by e.g. cross-linking
• formalin, chromic acid, glutaraldehyde, paraformaldehyde, Acrolein, OsO4
Dehydration
do it before and not in the column of the SEM:
• air drying (not prefered, because of surface tension) • ethanol, aceton, ether, chloroform… • freeze drying and critical-point drying Mounting Coating SEM-Basic Detectors
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SEM-Basic Detectors
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Critical Point Drying no heat of vaporization or rapid
SEM-Basic Detectors
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SEM-Basic Detectors
Phase Diagram
SEM-Basic Detectors
density change no surface tension fix samples first dehydrate in e.g. ethanol mix with liquid transitional fluid pressurize move around the critical point
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Transitional Fluids
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SEM-Basic Detectors
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CPD Maschine
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