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Lecture # - Electron Microscopy
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References
http://bama.ua.edu/~mweaver/courses/MTE481/Electron%20Microscopy.pdf
http://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdf
http://bama.ua.edu/~mweaver/courses/MTE481/Electron%20Microscopy.pdfhttp://bama.ua.edu/~mweaver/courses/MTE481/Electron%20Microscopy.pdfhttp://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdfhttp://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdfhttp://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdfhttp://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdfhttp://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdfhttp://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdfhttp://tpm.amc.anl.gov/Lectures/Zaluzec-1-Instrumentation.ppt.pdfhttp://bama.ua.edu/~mweaver/courses/MTE481/Electron%20Microscopy.pdfhttp://bama.ua.edu/~mweaver/courses/MTE481/Electron%20Microscopy.pdf -
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Timeline 1897 JJ Thompson - Discovery of the Electron 1926 H. Bush Magnetic/Electric Fields as a Lens 1931 Knoll and Ruska 1st EM built 1932 Davisson and Calbrick - Electrostatic Lens 1939 von Borries & Ruska - 1st Commercial EM
~ 10 nm resolution 1945 ~ 1.0 nm resolution (Multiple Organizations) 1965 ~ 0.2 nm resolution (Multiple Organizations) ~ 0.3 nm resolution probe - practical Field
Emission Gun 1986 Ruska et al - Nobel Prize 1999 < 0.1 nm resolution achieved (OM )
2009 0.05 nm (TEAM)
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Why Electrons?
Resolution in microscopes is limited by the wavelengthof imaging beam (Abbe Diffraction Limit)
=.
()
Where
= wavelength of the imaging radiation (400-800nm forlight and ~ 0.003nm upper limit for electrons)
= index of refraction of the lens= illumination semi-angle
NA = numerical aperture = sin ()
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Light vs Electrons
=0.6
sin()
Light:
=0.6 (400)
1.5sin(70)= 170nm
Electrons:
=0.6 (0.06)
1sin(1)= 0.4nm
Electrons offer much better resolution!
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Resolution
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Human eye
Optical
Microscope
X ray
Microscopy
Electron
Microscopy
Atoms in a lattice
Virus
Mosquito
MEMS Motor
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Electron interaction with matter
sample
Auger e-
Backscattered e-
Incident e- Beam
Secondary e-
Characteristic X rays
Visible light
Elastically scattered e-
Directly transmitted e- Beam
lnelastically scattered e-
Bremsstrahlung radiation
Electron hole pairs
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Backscattered electrons (BSE)
FormationCaused when incident electrons collide with an atom in a specimen
that is nearly normal to the path of the incident beam.
Incident electron is scattered backward (reflected).
UseImaging and diffraction analysis in the SEM.Production varies with atomic number (Z).
Higher Z elements appear brighter than lower Z elements.
Differentiate parts of specimen having different atomic number
Backscattered electrons are not as numerous as others. However,
they generally carry higher energies than other types of electrons
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Secondary Electrons Formation
Caused when an incident electron knocks an inner shellelectron (e.g., k-shell) out of its site.
This causes a slight energy loss and path change in theincident electron and ionization of the electron in the specimen.
The ionized electron leaves the atom with a small kineticenergy (~5 eV)
UseIMAGING!
Production is related to topography. Due to low energy, only
SE near the surface can exit the sample.Any change in topography that is larger than the samplingdepth will change the yield of SE.
More abundant than other types of electrons. They areelectrons that escape the specimen with energies below~50eV
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Auger Electrons
FormationDe-energization of the atom after a secondary electron is produced.
During SE production, an inner shell electron is emitted from the atomleaving a vacancy.
Higher energy electrons from the same atom can fall into the lower
energy hole. This creates an energy surplus in the atom which iscorrected by emission of an outer shell (low energy) electron
UseAE have characteristic energies that are unique to each element from
which they are emitted.Collect and sort AE according to energy to determine composition.
AE have very low energy and are emitted from near surface
regions.
Exploited in Auger Electron Spectroscopy
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X Rays
FormationSame as AE. Difference is that the electron that fills the inner shellemits energy to balance the total energy of the atom.
Use
X-rays will have characteristic energies that are unique to theelement(s) from which it originated.
Collect and sort signals according to energy to yield compositionalinformation.
Energy Dispersive X-ray Spectroscopy (EDS)
Foundation of XPS (X-ray photoelectron spectroscopy). XPS can
be used to determine the state of an atom and to identify chemical
compounds.
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Transmitted electrons
Can be used to determine:
thickness
crystallographic orientationatomic arrangements
phases present etc.
Foundation for Transmission ElectronMicroscopy (TEM)
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Components of a TEM
Source Electron gun Anode
Condenser lens assembly Electromagnetic lenses
Sample stage With provisions for tilt and translations
Objective lens Imaging assembly Detectors
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TEMElectron source Electron gun provides a steady stream of high
energy electrons
electron emission Thermionic
Field emission
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TEMElectron Source
Current density is given
by
= ATc2exp (-/KTc)
k is Boltzmanns constant,
TCis the cathode temperature and A and are a constants depending onmaterial. Note that jcT.
W has TCof 2500-3000 K (melting point 3650 K) LaB6 has a TCof 1400-2000 K
Heating usually produced by running a current through the material!
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Operation of thermionic gun Apply a positive electrical potential to the anode
Heat the cathode (filament) until a stream ofelectrons is produced>2700 K for W
Apply a negative electric potential to the Wehneltelectrons are repelled by the Wehnelt towardsthe optic axis
Electrons accumulate within the region betweenthe filament tip and the Wehnelt. This is known
as the space charge. Electrons near the hole exit the gun and move
down the column to the target (in this case thesample) for imaging.
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The width b of the potential barrier at the metal-vacuum boundarydecreases with increasing electric field E.
For |E|>107 V/cm the width b < 10 nm and electrons can penetrate
the potential barrier by the wave mechanical tunneling effect.
L
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Lenses Electric/Magnetic fields employed to
manipulate path of electron
Consist of Cu wire coils around soft Fe cores.sometimes an Fe pole-piece is used to shapethe field.
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Path of an electron in EM field
http://www.schoolphysics.co.uk/
http://www.schoolphysics.co.uk/http://www.schoolphysics.co.uk/ -
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Types of electron lenses
Condenser Lenses ~ Type A, Objective Lenses ~ Type A B or C, Stigmators Type D
L
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Lenses
Magnification is achieved byStacking Lenses
M= M1 * M2 * M3
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TEM sample preparation methods
Tissue sectioning
Chemical milling (etching)
Mechanical polishing
Sample staining
Ion milling
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http://www.docstoc.com/docs/67848646/TEM-sample-preparation-guide-Slide--Pips
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TEM Sample preparationIon milling
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Imaging Methods
Bright field imaging mode
Electron energy loss spectroscopy
Energy dispersive X ray spectroscopy
Selected area diffraction
Scanning transmission electron microscopy
Annular dark field imaging High angle annular dark field imaging
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TEM ImagingBright Field
Most common mode of imaging
Brightness contrast obtained by absorption ofelectrons by the sample
Higher z absorbs more and hence darker thanthe lower z regions leads to brightnesscontrast
Some examples of bright field imaging.
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Bright field imagingsome examples
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Electron energy loss spectroscopy
C
Inelastic scattering
Inelastically scattered electrons have a characteristic energy loss depending on the
material it passes through
Example: electrons passing through C have an energy loss of 285eV due to inelastic
scattering
The energies of the scattered electron are estimated using an electron energy loss
spectrometer and sample composition is determined
EELS Spectrum Example
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EELS Spectrum - Example
http://web.utk.edu/~gduscher/eels.html
EDX E Di i X t
http://web.utk.edu/~gduscher/eels.htmlhttp://web.utk.edu/~gduscher/eels.html -
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EDXEnergy Dispersive X ray spectroscopy
C C
Each element has a characteristic X ray wavelength that is emitted
following the above process
Detector measures the X rays to determine the composition of the
sample
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EDX - Spectrum
STEM Scanning Transmission
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STEM Scanning TransmissionElectron Microscope
focusing the electron beam into a narrow spot which is scanned over
the sample in a raster. Scattered electrons are collected by detectors
Atomic resolution possible using high angle detectors
contrast is directly related to the atomic number (z-contrast image)
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STEM imaging - Example
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Selected Area Diffraction
Electron beam undergoes Bragg scattering
wavelength of high-energy electrons is a fewthousandths of a nanometer whereas the
spacing between atoms in a solid is about ahundred times larger
the atoms act as a diffraction grating to the
electrons, which are diffracted.
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Imaging vs. Diffraction
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Imaging vs. Diffraction
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Example of SAD Imaging
http://www.2spi.com/catalog/standards/niox.shtml
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Scanning Electron Microscope
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References
http://www.eng.utah.edu/~lzang/images/Lecture_3_conventional-Microscope.pdf
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SEM - Basics
In scanning electron microscopy (SEM) an electron beam isfocused into a small probe and is rastered across thesurface of a specimen.
Several interactions with the sample that result in theemission of electrons or photons occur as the electronspenetrate the surface.
These emitted particles can be collected with theappropriate detector to yield valuable information aboutthe material.
The most immediate result of observation in the scanningelectron microscope is that it displays the shape of thesample.
The resolution is determined by beam diameter.
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Operation of SEM
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Operation of SEM
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Electron interaction with matter
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Imaging methods
Secondary electron imaging
Backscattered electron imaging
EDXEnergy dispersive X ray imaging
SEM imaging - Secondary electrons
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SEM imaging - Secondary electrons
C Secondary electron
electrons generated as ionization products. They are called 'secondary' because
they are generated by other radiation (the primary radiation)
Secondary electrons are collected by the detector and used for imaging
Low energy ensures most of the collected secondary electrons are from thesample surface
brightness of the signal depends on the number of secondary electrons reaching
the detector
secondary electron imaging or SEI can produce very high-resolution images of a
sample surface, revealing details less than 1 nm in size.
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Backscattered electron Imaging
electrons that are reflected or backscatteredfrom the sample by elastic scattering
intensity of the BSE signal is strongly related to
the atomic number (Z) of the specimen BSE images can provide information about the
distribution of different elements in the
sample
B k tt d i i d
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Backscattered imaging vs. secondaryelectron imaging
Backscattered electron image
Secondary electron image
http://en.wikipedia.org/wiki/Scanning_electron_microscope
Examples of SEM images
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Examples of SEM images
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http://tysontrepidations.wordpress.com/2011/07/02/say-cheese-for-scanning-electron-microscop
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Helium Ion Microscopy
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References
http://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORIO
N.pdf http://www.imec.be/efug/EFUG2012_05_Gna
uck.pdf
http://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdf
http://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.imec.be/efug/EFUG2012_05_Gnauck.pdfhttp://www.imec.be/efug/EFUG2012_05_Gnauck.pdfhttp://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdfhttp://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdfhttp://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdfhttp://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdfhttp://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdfhttp://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdfhttp://www.fibsem.net/web_documents/2010Presentations/2010DCFIBUGM-HeIM.pdfhttp://www.imec.be/efug/EFUG2012_05_Gnauck.pdfhttp://www.imec.be/efug/EFUG2012_05_Gnauck.pdfhttp://www.imec.be/efug/EFUG2012_05_Gnauck.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdfhttp://www.zeiss.com/C1256E4600307C70/EmbedTitelIntern/ArticlePhotonics-Helium-IonMicroscopy/$File/Photonics_Spectra_ORION.pdf -
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State of Affairs
Optical microscope Very easy to set up, low cost, color images Low resolution due to large wavelengths
Scanning Electron microscope Use electrons instead of photons, much better resolution
(~2nm) Stagnated at 2nm for the past few years
Transmission electron microscope Resolution lower than 2nm easily achieved
Expensive, detailed sample preparation required Enter - Helium ion microscope Sub nanometer resolution achieved Better contrast
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HIM - Basics
Uses a beam of He ions
Larger mass than e and hence smaller debroglie wavelength and hence much better
resolution than possible with electrons
=0.6
sin()
HIM - Schematic
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HIM Schematic
http://www.medgadget.com/2007/09/the_orion_helium_ion_microscope.html
HIM Helium source
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HIM Helium source
He atom
He ion
Intense E Field
Needle at
cryo temp
+V
HIM imaging - Secondary electrons
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HIM imaging Secondary electrons
C Secondary electron
electrons generated as ionization products. They are called 'secondary' because
they are generated by other radiation (the primary radiation)
Secondary electrons are collected by the detector and used for imaging
Detected number of secondary electrons varies with material composition andshape
Provides excellent topographical and compositional imaging
HIM imaging- Backscattered electrons
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g g
Small fraction of incident He ions arebackscattered
Probability of backscattering depends on theatomic number of the target element
Hence we get an contrast image withbrightness dependent on the atomic number
C
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HIM Vs. SEM
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HIM vs. SEM
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Helium beam generates
contrast from atomically thin
layers as well as chemical
contrast (light grey isNH2
and dark grey is -NO2)
Self Assembled Monolayer of 4 -
nitro-1,1 -biphenyl-4-thiol (NBPT)
exposed with E-beam Lithography
which modifies the terminal group
from NO2 (dark grey) to NH2 (light
grey)
HIM Nanomilling and patterning
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HIM Nanomilling and patterning
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HIMMilling and Patterning
Ga FIB ~ 20nmHIM ~ 1nm
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Some nanopatterns
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ExamplesHIM nanopatterning
ExamplesHIM Nanopatterning
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p p g