Download Scanning Electron Microscopy (SEM)

University of Vaasa – AUTO3160 Optics
Electron Microscopy (SEM & TEM)
By Thomas Höglund
15 May 2012
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Scanning Electron Microscopy
(SEM)
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The scanning electron microscope
• Versatile
• High resolution
• Many imaging modes
• Digital image processing
Example applications:
• Failure analysis and quality
control in metals
• Scientific research
• Comparison of blood and
tissue samples
• Comparison of samples as
evidence in forensics
Hitachi SU-70 Schottky field emission SEM
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What is the SEM used for?
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Microscopic imaging
Failure analysis
Semiconductor development
Quality control
– troubleshooting and finding production problems
– evaluating process parameters
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Morphology
Metrology
Crystallography
Criminalistics
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SEM in development and fabrication
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Infrastructural requirements:
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Sample requirements
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Power Supply
Vacuum System
Cooling system
Vibration-free floor
Room free of ambient magnetic and electric fields
Has to be solid
Has to fit into specimen chamber
Cannot be a living organism and biological samples require special treatment
Must be stable in vacuum
The surface has to be clean and clear
Possibility to image objects several centimeters in size
Shadows give the impression of 3D
Depending on the sample, it can be time consuming to prepare it for SEM
Low vacuum and environmental SEMs exist for more demanding samples
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The electron gun
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Scanning the beam
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Optics
d0 = filament diameter
d1 = diameter of intermediate image
d = probe diameter
v2 = working distance (WD)
I0 = initial current
I1 = probe current
r = resolution
λ = wavelength
μ = refractive index of the vacuum
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Aberrations
Coma
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Depth of focus/field
Ligth microscope
SEM
01.12.2009
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Resolution
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Electron beam – specimen interaction
• Samples must be
– appropriately sized
– mounted rigidly on a specimen holder
(specimen stub)
– electrically conductive
– electrically grounded
– coated (nonconductive specimens)
with an ultrathin coating of electricallyconducting material
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Ref: Modified Thiocarbohydrazide Procedure for Scanning Electron
Microscopy: Routine use for Normal, Pathological, or Experimental
Tissues by “Linda E. Malick; Richard B. Wilson; David Stetson”
Figure. The overview of Electron Beam’s path in SEM
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Electron Interaction Volume
The size of the interaction
volume depends on:
• Angle of incidence
• Accelerating voltage
• Atomic number (Z)
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Electron Beam – Specimen Interaction
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Secondary electrons (SE)
Backscattered electrons (BSE)
Auger electrons
Cathodoluminescence (CL)
Bremsstrahlung
Characteristic x-ray radiation
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Electron Interaction Volume
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Electron Beam – Specimen Interaction
Figure. a) Elastic scattering
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Figure. b) Inelastic scattering
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Electron Beam – Specimen Interaction
• The Auger effect, Auger electrons
– Energies between 280 eV (carbon) and 2.1 keV (sulfur)
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Energy Distribution of Emitted Electrons
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SE Detector
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The Everhart-Thornley detector
– Converts electrons into light, then back into electrical
signals
– Positive Faraday cage attracts low E SEs
– Also collects BSEs present within line of sight
The SEs emitted from the specimen are collected by a grid of +200V and
accelerated onto the scintillator with a high voltage (10kV).
Photomultiplier
Scintillator
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BSE Detectors
The generation region of backscattered
electrons is larger than that of
secondary electrons, i.e. several tens of
nm. Therefore, backscattered electrons
give poorer spacial resolution than
secondary electrons. But because they
have a larger energy than secondary
electrons, they are less influenced by
charge-up and specimen contamination.
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Imaging with Secondary Electrons
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Electron Beam – Specimen Interaction
• Secondary electron (SE) contrast and yield depends
on sample surface tomography and tilt, as well as
on the position of the SE detector.
• By detecting the SE from different angles, shadows
are created and this gives an impression of threedimensionality.
• Example: Cracks in the sample appear bright in the
SE image.
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Surface Tilt Contrast
• By combining images of the
specimen taken in different
angles, it is possible to construct
a 3D image.
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Influence of accelerating voltage on SE Image Quality
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Influence of accelerating voltage on SE Image Quality
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Summary
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The SEM is a very versatile technique that can be used to image at higher resolution and deeper
DOF than a light microscope.
The SEM has many imaging modes making it useful for many purposes.
Electron interaction volume increases with an increase in angle of incidence, accelerating
voltage, and a decrease in average atomic number.
Secondary electrons have lowest energy in a maximum of 50 eV, Auger electrons range between
50-2000 eV, while backscattered electrons can have an energy range of 50 eV to much higher
values when compared to those.
Beam damage occurs when a specimen area is irradiated with an electron probe for a long time
at high magnification which can be eliminated by use low accelerating voltage, decrease electron
beam intensity and shorten exposure time.
When a nonconductive specimen is directly illuminated with an electron beam, its electrons with
a negative charge collect locally called specimen charge-up, which can be decreased by properly
selecting the accelerating voltage, or by using low accelerating voltage.
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SEs are caused by inelastic interactions and BSEs due to elastic interactions.
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The image contrast depends on many things and can be adjusted to reveal different features of
the specimen.
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The contrast also depends on atomic number of the specimen and the angle of tilt.
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SEM is a very important technique for investigation of semiconductor materials
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References
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Maryland Nanocenter http://www.nanocenter.umd.edu/new_facilities/hitachi_su-70_sem.jpg
http://www.pvdproducts.com/services/brochures/sem.pdf
Susan Swapp, University of Wyoming:
http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html
Electron Microscopy and Analysis Third Edition, Goodhew, Humphreys, Beanland
http://www.youtube.com/watch?v=EXmaY2txEBo
Dr. Andriy Lotnyk, PTEM lecture slides 4.
http://bellaitalianaphotography.files.wordpress.com/2008/04/chicory-sem.jpg
http://www.engineering.arizona.edu/news/media/image/matsci_pollen.jpg
http://www.nhml.com/resources/1998/10/1/scanning-electron-microscopes-sem
Particle Beam Microanalysis E. Fuchs, H. Oppolzer, H. Rehme
http://www4.nau.edu/microanalysis/Microprobe-SEM/Signals.html
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High-Resolution Transmission
Electron Microscopy HRTEM
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Introduction to HRTEM
• HRTEM is used for imaging samples at atomic
resolution
• The accuracy of spatial measurements has reached a
few picometers
• Relies on quantum mechanical phase shifts caused
by the interaction between the electrons and the
interatomic potential for contrast
• A high acceleration voltage of a few hundred kV is
needed
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Dynamical Scattering
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The scattering of electron waves in the crystal potential
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An incident plane wave is modified in amplitude and phase by the potential of
the atoms in the object.
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Electron interactions with the scattering potential is very strong so the sample
thickness plays a very important role.
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The same crystal structure under the same microscope parameters lead to
different exit wave funtions and different images when the sample thickness is
altered.
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The Bloch-wave and multislice methods allow us to know object parameters to
compute exit plane wave functions for varying sample thicknesses.
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The multislice method
The multislice approximation mathematically converts a specimen (left) into many thin slices
(right). The electron wave is transmitted through the specimen and exits on the bottom. Each
slice is thin enough to be approximated as a simple phase shift of the electron wave which is
propagated between slices as a free space wave.
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Contrast Transfer Function CTF, H(u)
• The relationship between the exit wave and the
image wave is a highly nonlinear one and is a
function of the aberrations of the microscope
described by the contrast transfer function
• CTF:
• The CTF tells us how information in u space is transferred to
the image. H(u)=A(u)E(u)B(u) (A=aperture, E=envelope,
B=aberration functions)
• u=reciprocal-lattice vector, the spatial frequency for a
particular direction [1/length]
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Approximations for thin objects
• Phase grating approximation
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Image forming by a lens without aberrations
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Interpretation of the interference image
Distribution of maxima and minima of interference at
the exit of the crystal
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Problems in image interpretation
• Three major problems make it difficult to
interpret the images:
– The wavefunction is influenced by inteference,
which encrypts the object information
– The exit-plane wave function is distorted due to
optical aberrations
– The phase information is lost due to the quantum
mechanical properties of the travelling electron
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Lattice image
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• Zeros in the CTF represent
gaps in the image transfer.
• Oscillation causes contrast
inversion.
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Lattice imaging of crystals
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Micrographs and diffractograms
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Passbands
• One can find special defocuses that allow
higher spatial frequencies to contribute to the
image, while on the other hand introducing
zeros in the CTF at low u.
• Passbands occur periodically:
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Passbands (cont.)
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Computer simulation
• The image of the crystal structure is very sensitive to
the thickness of the specimen and the defocus.
• Computer simulation of lattice images is often
required for image interpretation.
• In order to compare experimental and calculated
images the thickness and the defocus have to be
known.
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Computer simulation (cont.)
• When calculating an image, three steps are reqired:
– A model of the crystal structure or the defect must be set up.
– Based on dynamic electron scattering, the wave function at the
exit surface of the specimen is then calculated.
– The optical transfer characteristic of the system is taken into
account.
• The most common method for calculating the wave
function is the multi-slice method.
– Weak-phase approximation is utilized by mathematically
splitting the crystal into numerous thin slices. The transmission
function for each slice can then be expressed by the weak-phase
approximation.
– The wave functions for each slice are then iterated to get the
function at the exit surface of the specimen.
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Example simulations
Tecnai F30 S-Twin
Titan T 80-300 kV
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Spherical aberration
• The objective lens compensates the distortion of the
electron wave due to the sperical aberration with a
negative defocus as a counter-distortion.
• Phase shift is introduced.
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Aberration correction
• Using multipole (hexapole) lenses it is possible to get a
combined effect of zero spherical aberration.
• Even though it is possible to as good as remove all spherical
aberration, a small amount of spherical aberration has to be
used in order to achieve optimum contrast.
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Applications
• “It is possible to observe how a nanocrystal grows or melts or
changes its structure atom by atom, or to investigate the structure
of nanocrystals embedded in microcrystals.” (U. Dahmen, 2007)
• Characterizing the atomic-scale structure, chemistry, and dynamics
of individual nanometer-scale structures
• Viewing grain boundaries
• Viewing lattice inhomogeneities
• Atomic resolution tomography
• Single atom spectroscopy
• Local electronic structure and bonding
• Dynamics and mechanisms of reactions
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Summary and outlook
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Relies on quantum mechanical phase shifts for contrast
Lattice structure and inhomogeneities can be imaged
Defocus and aberration greatly affect the image
Computer simulation is important for understanding
the images
• Aberration correction highly enhances the resolution
• The TEAM project has recently pushed the resolution
down to 0.5 Å
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Sources
• Fuchs, Oppolzer, Rehme (1990), Particle Beam
Microanalysis
• Williams, Carter (2009), Transmission Electron Microscopy
Second Edition
• Urban, Jia, Houben, Lentzen, Mi and Tillmann (2009),
Negative spherical aberration ultrahigh-resolution imaging
in corrected transmission electron microscopy (Article)
• U. Dahmen (2007), Aberration-corrected HRTEM and the
TEAM project (Article)
• http://www.fei.com/resources/image-gallery/
• http://www.fei.com/uploadedFiles/Documents/Content/20
07_10_semimarket_bro.pdf
• http://ncem.lbl.gov/TEAM-project
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Thank you
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