Scanning Electron Microscope 10/27/2009

10/27/2009
Scanning Electron Microscope
– “SEM is a type of electron microscope that images the sample surface by scanning it with a high energy beam of electrons in a raster scan pattern.” (http://en.wikipedia.org/wiki/Scanning_electron_microscope)
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Geol 311
http://www.fhwa.dot.gov/pavement/pccp/pubs/04150/images/fig181.gif
http://img.directindustry.fr/images_di/photo‐g/microscope‐
electronique‐a‐balayage‐a‐emission‐de‐schottky‐sesem‐237074.jpg
http://mse.iastate.edu/microscopy/back
Basic Evolution y
How SEM Works
•Electron gun produces The first electron microscopes were made in the early 20th
century. These early electron microscopes could only study “thin specimens and the electron deflection that took place as the primary electrons passed through the thin specimen”. (1‐5 http://epmalab.uoregon.edu/lecture.htm)
y The largest advancement in electron microscopes is due to the detection and production of secondary electrons.
http://www.fhwa.dot.gov/pavement/pccp/pubs/04150/images/fig181.gif
an electron beam
•Condenser lenses and condenser aperture condense electron beam
•Set of coils scan the sample
•Objective lens guides the beam to desired location
•Electron detector collects the signal
•The data collected by the detectors is then sent to the SEM monitor http://www.wired.com/images/index/2009/04/electron_microscope.jpg
Calibration of SEM
y The sample chamber is always in a vacuum y This allows there to be minimum interference between scattered electrons and gas moplecules that would otherwise be inside the column.
y Use standards to determine if the calibration of the SEM is correct.
http://www.unl.edu/CMRAcfem/semoptic.htm
http://www4.nau.edu/microanalysis/Microprobe‐SEM/Images/Electron_Spectrum.jpg
Sample Preparation
y Most SEM’s need an electrically conductive sample.
y All metals are conductive and require no preparation.
y All non conductive samples must be covered with a thin layer of a conductive material, gold for imaging.
,g
g g
y
Done using a sputter coater
y Samples need to be cleaned prior to analysis.
y Use simple samples of known composition to check SEM imaging, magnification, and detection.
http://mse.iastate.edu/microscopy/prep2.html
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Secondary Electrons (SE)
y Secondary electrons are produced by ionizing an atom through inelastic interactions of high energy electrons (incident electrons) with valence electrons (typically K‐shell Electrons) of atoms in the specimen which cause the ejection of these electrons from the atoms. (http://www.emal.engin.umich.edu/courses/semlectures/se1.html)
Backscattered Electrons (BSE)
•
When the electron beam is emitted towards the specimen many of the electron emitted interact with the nucleus. When the angle is just right some of the electrons emitted will circle the nucleus, due to positive negative attraction, and be shot right back the way they came. They do not slow down, and because they are moving so fast they travel in straight lines. (http://mse iastate edu/microscopy/backscat2 html)
(http://mse.iastate.edu/microscopy/backscat2.html)
http://www.emal.engin.umich.edu/courses/semlectures/se1.html
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http://mse.iastate.edu/microscopy/back
http://mse.iastate.edu/microscopy/back
Scanning Electron Microprobe Electron Microprobe Calibration
y Instrument used to undergo Electron Microprobe y The electron microprobe is calibrated using standard microanalysis.
minerals of known composition.
y Essentially the same process as the SEM.
http://probelab.geo.umn.edu/assets/img/electron_microprobe.jpg
Signals Detected
y Like the SEM, the Scanning Electron Microprobe also uses an electron beam to excite (ionize) the electrons within the atoms of the sample. However, usually it does not detect the electrons released from the ionization, but instead the actual energy lost in the ionization process.
X‐ray Generation
y “In order to generate an x‐ray one must first excite an atom, in this case it is done by bombarding the atom with an electron beam.” (3‐5http://epmalab.uoregon.edu/lecture.htm)
y This causes energetic transitions (ionization)
y Most energetic transitions involve the k – shell electrons because they are the closest to the nucleus. (http://epmalab.uoregon.edu/lecture.htm)
y The ejection of an electron from the k‐shell causes an electron hole, at this current state the atom is unstable.
y The atom, to stabalize, will fill the hole with an electron from the nearest electron shell.
y
This lowers the energy of the atom by E(ka1) = Ek‐El
http://fp.okstate.edu/catlos/eprobe/what%20is%20microprobe_files/image002.jpg
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X‐ray Generation cont.
y To the left is a illustration of the process mentioned on the last slide.
y The energy lost is released in the form of an x‐ray photon. The wavelength of the photon will be determined by the energy transition. (6‐5 Wavelength Dispersive Spectroscopy (WDS)
y This is a method used to count the number x‐rays of a specific wavelength.
y X‐rays from different elements are separated from each other by their wavelength and diffracted at the detector using Bragg diffraction.
y nλ = 2dsinѲ where (n = 1, 2, 3... )
http://epmalab.uoregon.edu/pdfs/X_natur3%20
_Chap%203_.pdf)
http://epmalab.uoregon.edu/pdfs/X_natur3%20_Chap%203_.pdf
http://www.geoberg.de/text/geology/08030904.jpg
http://serc.carleton.edu/details/images/9512.html
Strengths and Limitations of WDS
Strengths
Weaknesses
y Has a very high resolution
y It is very expensive to use y Provides very accurate quantitative data
y
y
y
y
because it requires a high beam frequency
Sample has to be in vacuum
It is very slow, taking up to 30 minutes to analyze a few elements.
Each detector can only analyze one element at a time.
Must know what elements are in sample in order to analyze them.
http://images.google.com/imgres?imgurl=http://www.xos.com/wpcontent/uploads/wds.jpg&imgrefurl=http://www.xos.com/index.php%3Fpage_id%3D71%26m%3D2%26sm%3D3
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Strengths and Limitations of EDS
Energy Dispersive Spectroscopy (EDS)
y X‐rays from different elements are recognized and separated from each other by characteristic energy using a solid state detector and multichannel analyzer. http://epmalab.uoregon.edu/lecture.htm
y Does not require high x‐ray generation rate, so uses relatively low energy
y It creates these x‐rays in the same way as mentioned on the X‐ray Generation slide.
t c eates t ese ays t e sa e ay as e t o ed o t e
ay Ge e at o s de.
y Faster than WDS but less accurate.
http://www.thermo.com/eThermo/CMA/Images/Category/categor
yImg_11688.gif
http://www.ifw-dresden.de/institutes/ikm/organisation/dep-31/methods/energydispersive-x-ray-spectroscopy-eels/Graph_EDXNiO.JPG/image_preview
Sample Preparation for EDS and WDS
Strengths
Limitations
y The sample must be dry.
y IT is very fast, taking only y Will not give good quantitative y The sample must be highly polished minutes to analyze entire sample.
y It is inexpensive and versatile
y Because character x‐rays are proportional to atomic number, due to the low energy used by EDS, the sensor can’t detect elements with an atomic number lower than 5.
analysis for samples that are not flat or polished.
y Sample must be vacuumed.
y Does not have as high of a resolution capability as WDS
y The sample surface must be cleaned y If th
If the sample is not conductive than the sample must be l i t d ti th th l t b coated with carbon
http://dev.co2-research.ca/manager/gallery/pics/25_1132556871.jpg
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Scanning Electron Microscopy vs. Electron Microprobe Analysis References
Scanning Electron Microscopy
Electron Microprobe Analysis
y Non destructive to the sample
y Non destructive to the sample
y Relatively cheap, fast, and y Is very expensive and is slow y
easy to use
y Good for imagine surface topography and collecting quantitative data
and difficult to set up.
and difficult to set up
y Is very good for quantitative chemical analysis of samples
y
y
y
y
y
y
y
y
http://www.cea.com/techniques/analytical_techniques/eds.php
http://www.emal.engin.umich.edu/courses/semlectures/se1.html
http://en.wikipedia.org/wiki/Scanning_electron_microscope
http://epmalab.uoregon.edu/lecture.htm
http://mse.iastate.edu/microscopy/backscat2.html
http://mse.iastate.edu/microscopy/prep2.html
http://probelab.geo.umn.edu/specimens.html
http://www.unl.edu/CMRAcfem/semoptic.htm
http://www.unl.edu/CMRAcfem/glossary.htm
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