electron microscopy lecture

7/22/2019 Electron Microscopy Lecture

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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)


16/78http://en.wikipedia.org/wiki/Electron_microscope


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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

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