The Scanning Electron Microscope

1
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

2
What are the advantages of an SEM over an Optical
Microscope?
Fundamental Limitations
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
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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.or
g/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 h
ttp//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.

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Electron Emission Basics
The brief notes and images on this slide are
taken from the following web sites http//www.che
ms.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.p
ulsedpower.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.
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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
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Components that act on electrons the same way
lenses bend light
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Components that act on electrons the same way
lenses bend light
Astigmatism
Spherical Aberration
Chromatic Aberration
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How electrons interact with a target
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.ht
m http//www.chems.msu.edu/curr.stud/mse.sops/cont
rast.htm
Primary backscattered electrons
X-rays
Incident Beam
Cathodoluminescence
Secondary electrons
Auger electrons
Specimen
Specimen Current
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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 (lt 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.
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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.htm
l
The animated images are taken from this
website http//seallabs.com/hiw.htm
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How electrons are detected
Details on how a PMT works http//micro.magnet.fs
u.edu/primer/java/digitalimaging/photomultiplier/c
hannel/ http//quarknet.fnal.gov/projects/pmt/stud
ent/dynodes.shtml
Movie of a scintillator on a PMT http//www.tmater
na.com/programs/java/index.php?mov1 Scintillator
from Wikipedia http//en.wikipedia.org/wiki/Scint
illator
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Why a vacuum system is needed
Common Vacuum Units
1 atmosphere 760 mm Hg or torr 1.013 bar
1.013105 Pa 2.71019 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 101.3 kPa
Definition of the Mean Free Path is on this web
site http//hyperphysics.phy-astr.gsu.edu/hbase/k
inetic/menfre.htmlc1
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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
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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/Des
ktopModules/Bring2mind/DMX/Download.aspx?TabId320
DMXModule692CommandCore_DownloadEntryId1Por
talId2 A good general discussion on sputter
coating insulating samples for an SEM are found
here http//www.mse.mtu.edu/7Ejwdrelic/Lab6_MY42
00.pdf You are responsible for reading these
materials since there will be questions on the
on-line quiz covering using an SEM.
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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 K-shell)
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. X-rays 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.c
om/
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/s
cat5_2.htm
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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
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Creating Nanostructures using an SEM
Overview of E-beam lithography http//www.azonano
.com/details.asp?ArticleID1208 Commercial
Conversion Kit http//www.jcnabity.com/
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Appendix Extra Information
Fermi-Dirac Distribution
Maxwell Boltzmann Distribution
Lorentz Force