Transcript The Scanning Electron Microscope

The Scanning Electron Microscope
Alan Cheville
Electrical Engineering
Oklahoma State University
Outline of this presentation and reading assignments
• Overview of how a Scanning Electron Microscope (SEM) works
• Introduction to Basic Concepts
• How to use an SEM
• Beyond simple imaging - analytical techniques
What are the advantages of an SEM over an Optical Microscope?
Fundamental Limitations:
c
λ=
ν
h
λe =
me ve
Energy=1 2 m e v e 2
Practical limitations usually do not let you get to the theoretical resolution without a
lot of careful engineering.
Optical Microscopes: aberrations in the lenses
Electron Microscopes: charging effects, aberrations in the lenses, electron diffusion
Overview of how a Scanning Electron Microscope (SEM) works
The following web site has a nice Quicktime movie that describes how an SEM works:
http://www.mos.org/sln/sem/sem.mov
You should also read the following slide shows for a good simple overview:
http://www.mos.org/sln/sem/tour01.html
http://seallabs.com/hiw.htm
The SEM from this web site is shown below
To understand how an SEM works we
need to understand conceptually
each part of the SEM:
• How electrons are emitted (electron
gun).
• Components that act on electrons
the same way lenses bend light.
• How electrons interact with a target,
specifically secondary electrons.
• How electrons are detected.
• Why a vacuum system is needed.
Electron Emission Basics
The brief notes and images on this slide are taken from the following web sites:
http://www.chems.msu.edu/curr.stud/mse.sops/sem.comp.htm
http://www.uga.edu/caur/teaching.htm
A thermionic electron gun consists essentially of
a heated wire or compound from which electrons
are given enough thermal energy to overcome the
work function of the source, combined with an
electric potential to give the newly free electrons a
direction and velocity. Remember the energy
distribution of electrons is given by Fermi-Dirac
statistics. (see appendix at end of talk…)
Link to table of metal workfunctions:
http://www.pulsedpower.net/Info/WorkFunctions.htm
Field Emitters consists of a sharply pointed tungsten tip held at several
kilovolts negative potential relative to a nearby electrode. Because electrons are
quantum particles and have a probability distribution to their location, a certain
number of electrons that are nominally at the metal surface will find themselves
at some distance from the surface, such that they can reduce their energy by
moving further away from the surface! This transport-via-delocalization is called
'tunneling', and is the basis for the field emission effect. Final primary beam
probe size from a field emitter is 10-100X smaller than that of a thermionic
emitter but they are much more expensive.
Components that act on electrons the same way lenses bend light
The electron gun serves as the light source in a conventional
microscope. The cathode and anode serve to accelerate
electrons that are emitted from the filament so the have a
very constant speed. The force on electrons by electric and
magnetic fields is known as the Lorentz force:
Components that act on electrons the same way lenses bend light
Components that act on electrons the same way lenses bend light
Astigmatism
Spherical Aberration
Chromatic Aberration
How electrons interact with a target
X-rays
Incident Beam
When the electron beam strikes a sample, effects from emission of photons and
electrons occur. A summary of these effects can be found on these web sites:
http://www.unl.edu/CMRAcfem/interact.htm
http://www.chems.msu.edu/curr.stud/mse.sops/contrast.htm
Secondary
electrons
Primary
backscattered
electrons
Cathodoluminescence
Auger
electrons
Specimen
Specimen
Current
How electrons interact with a target
The actual INTERACTION VOLUME of an electron in a sample:
http://www.small-world.net/efs.htm
Secondary Electrons: Caused by an incident electron passing "near" an atom in the specimen, near enough to
impart some of its energy to a lower energy electron (usually in the K-shell). This causes a slight energy loss and
path change in the incident electron and the ionization of the electron in the specimen atom. This ionized electron
then leaves the atom with a very small kinetic energy (5eV) and is then termed a "secondary electron". Each
incident electron can produce several secondary electrons. Production of secondary electrons is very topography
related. Due to their low energy, 5eV, only secondaries that are very near the surface (< 10 nm) can exit the
sample. Any changes in topography in the sample that are larger than this sampling depth will change the yield.
Backscattered Electrons: Caused by an incident electron colliding with an atom in the specimen which is nearly
normal to the incident path. The incident electron is then scattered "backward" 180 degrees. The production of
backscattered electrons varies directly with the specimen's atomic number. This differing production rates causes
higher atomic number elements to appear brighter than lower atomic number elements. This interaction is utilized
to differentiate parts of the specimen that have different average atomic number.
Contrast Mechanisms from Electron Emission
You can see the effect of contrast and focus using this applet:
http://www.micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html
The animated images are taken from this website:
http://seallabs.com/hiw.htm
How electrons are detected
Details on how a PMT works:
http://micro.magnet.fsu.edu/primer/java/digitalimaging/photomultiplier/channel/
http://quarknet.fnal.gov/projects/pmt/student/dynodes.shtml
Movie of a scintillator on a PMT
http://www.tmaterna.com/programs/java/index.php?mov=1
Scintillator from Wikipedia:
http://en.wikipedia.org/wiki/Scintillator
Why a vacuum system is needed
Common Vacuum Units:
1 atmosphere = 760 mm Hg or torr = 1.013 bar = 1.013×105 Pa = 2.7×1019 cm-3
If an electron hits a gas molecule it will be scattered leading to loss of current and
resolution. Mean Free Path is the average distance between collisions.
1 atmosphere
= 760 mm Hg = 760 torr
= 1.013bar
Definition of the Mean Free Path is on this web site:
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html#c1
= 101.3 kPa
A Basic Vacuum System
You can find the figure and a much more complete
description of a basic vacuum system at this web site:
http://microlab.berkeley.edu/labmanual/chap6/vacuum.pdf
Types of pumps are found on this site:
http://www.chems.msu.edu/curr.stud/mse.sops/sem.comp.htm
How to use an SEM
Documentation on using the SEM for the lab can be found on the course web site.
General procedures for acquiring an SEM image are in one of your reading
assignments at:
http://www.jeolusa.com/DesktopModules/Bring2mind/DMX/Download.aspx?TabId=
320&DMXModule=692&Command=Core_Download&EntryId=1&PortalId=2
A good general discussion on sputter coating insulating samples for an SEM are
found here:
http://www.mse.mtu.edu/%7Ejwdrelic/Lab6_MY4200.pdf
You are responsible for reading these materials since there will be questions
on the on-line quiz covering using an SEM.
Beyond simple imaging - analytical techniques
X-rays: Caused by the de-energization of the specimen atom
after a secondary electron is produced. Since a lower (usually Kshell) electron was emitted from the atom during the secondary
electron process an inner (lower energy) shell now has a
vacancy. A higher energy electron can "fall" into the lower energy
shell, filling the vacancy. As the electron "falls" it emits energy,
usually X-rays to balance the total energy of the atom so it. Xrays or Light emitted from the atom will have a characteristic
energy which is unique to the element from which it originated.
These signals are collected and sorted according to energy to
yield micrometer diameter images of bulk specimens. You can
see typical X-ray spectra for the elements at this web site:
http://isotopes.lbl.gov/xray/
Electron binding energies are at: http://www.webelements.com/
Auger Electrons: The incident primary electrons cause ionization of
atoms within the region illuminated by the focused beam.
Subsequent relaxation of the ionized atoms leads to the emission of
Auger electrons characteristic of the elements present in this part of
the sample surface. As with SEM , the attainable resolution is again
ultimately limited by the incident beam characteristics. More
significantly, however, the resolution is also limited by the need to
acquire sufficient Auger signal to form a respectable image within a
reasonable time period, and for this reason the instrumental
resolution achievable is rarely better than about 15-20 nm.
The details of Auger spectroscopy can be found on this website:
http://www.chem.qmul.ac.uk/surfaces/scc/scat5_2.htm
Transmission Electron Microscopy
A TEM image is made up of
nonscattered electrons (which
strike the screen) and scattered
electrons which do not and
therefore appear as a dark area
on the screen
TEM
resolution is
not limited by
interaction
volume as an
SEM is
Creating Nanostructures using an SEM
Overview of E-beam lithography:
http://www.azonano.com/details.asp?ArticleID=1208
Commercial Conversion Kit:
http://www.jcnabity.com/
Appendix: Extra Information
Fermi-Dirac Distribution:
Maxwell Boltzmann Distribution:
Lorentz Force: