Transcript Electron Microscopes Scanning electron microscopes (SEM) Transmission electron microscopes (TEM) System components Theory of operation Sample Preparation Vendors FNI 2C EM.

Electron
Microscopes
Scanning electron microscopes (SEM)
Transmission electron microscopes (TEM)
System components
Theory of operation
Sample Preparation
Vendors
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The Electron Microscope
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History of the electron
microscope
Types of electron
microscopes
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SEM
TEM
Cryo
Atmospheric
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Theory of operation
Electron beam specimen
interaction
Detectors
Specimen preparation
The electron gun
The five axis stage
Electron Microscope
Images
System components
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Introductory SEM Text
http://www.jeolusa.com/SERVICESUPPORT/Application
sResources/ElectronOptics/DocumentsDownloads/tabid/
320/DMXModule/692/Command/Core_ViewDetails/Defa
ult.aspx?EntryId=257
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Ernst Ruska
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Invented the electron microscope in 1931.
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The electron microscope obtains images
by scanning the surface of a sample with a
beam of electrons.
http://nobelprize.org/physics/laureates/1986/ruska-autobio.html
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Older SEM control panel
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Modern SEM in operation
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My Ant Rosie with a Micromachined Part
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Microscopic Crystal Garden in Concrete Sample
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Computer Chip Cross Section
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Leaf
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Red Blood Cells
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CVTC SEM
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Scanning Electron Microscope
Components
Column
Console
Display
Chamber
Control
Panel
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SEM Theory of Operation
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An SEM is usually operated under high vacuum of 10-4
Pa or 10-6 Torr.
There is a large voltage difference between the anode
and the electron gun cathode typically 10 kV to 40 kV.
When current begins to flow through the filament then
electrons begin to be emitted from the filament through
field emission.
The electrons are accelerated down the column by the
electric potential.
Electrons pass through a series of apertures and
magnetic lenses.
The final magnetic lens brings the electron beam into
focus on the surface of the sample.
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Scanning Electron Microscope
Components
Electron
Gun
Condenser
Lens
Scan
Coils
Final
Lens/
Focus
Electron
Beam
Final
Aperture
Sample
Secondary
Electron
Detector
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SEM Theory of Operation
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The scan coils raster the beam back and forth
over the surface of the sample.
Secondary electrons are drawn into the
secondary electron detector and amplified.
The signal is translated into bright and dark
areas on the monitor.
Magnification is determined by the amount of
area scanned.
Magnification can range from 10x to 500,000x
Resolution is approximately 1 nm.
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Electron Beam Specimen
Interactions
~3 nm
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Inside an SEM chamber
Final lens
Secondary electron
detector
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Backscattered
electron detector
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Inside CVTC’s SEM chamber
Final lens
Secondary electron
detector
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Backscattered
electron detector
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Sample Preparation
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Specimen Preparation
Specimen
Solid
Non-solid
Surface
preparation
Conductive
Dryable
Non-conductive
Biological
specimen
preparation
Non-dryable
Low moisture
High moisture
Metal coating
High vacuum
High accelerating voltage
High Vacuum
Low accelerating voltage
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Low vacuum
High vacuum
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Sample Preparation
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Samples will usually need to be prepared in some way for viewing in
an SEM.
Often a coating of gold, palladium or carbon is applied to the sample
if the sample is not conductive.
Sometimes a cross sectional sample is required.
To prepare a cross section sample it is first mounted in epoxy.
After the epoxy has hardened the sample is polished on a polishing
wheel with different grades of sand paper and grit.
Finally a slurry of sub micron particles can be used to prepare the
surface of the sample.
The sample may then be treated with different chemicals to highlight
different features of the sample.
These “stains” include HF and Wright etch for semiconductors.
4% nitric acid in methanol for stainless steel
After the sample is stained then they will be coated with a
conductive layer.
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Sample mounting
Sample
Colloidal Graphite
or Silver
Carbon tape
Specimen
Stub
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Mounted sample preparation
Compression
mount sample
in plastic block
Sand sample on
successively finer sand
papers. Rotate 90°
between grits and
inspect under a
microscope to ensure
previous scratches are
polished out.
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Polish with successively
finer polishing slurry. (5
micron to .1 micron) until
no scratches are visible
under a microscope.
Sample may need to be
stained or etched with
acid after this.
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Sputter coating
Electrically insulating samples
must be sputter coated with a
layer of metal usually Palladium
or Gold.
This reactor uses argon excited
into a plasma. The argon ions
smash into the metal target
knocking atoms off like billiard
balls. The atoms deposit onto the
sample to form a continuous
layer.
This allows electrons from the
electron beam to flow to ground.
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Biological Sample Preparation
Obtain the sample. This may involving
toming or obtaining thins sections with a
razor blade
Fix the sample with gluteraldehyde
This is causes cross linking among proteins
similar to embalming
Displace the water with successively
more concentrated ethanol. This may
take several hours.
Displace the ethanol with liquid CO2 and
dry in the critical point dryer to avoid
drying with a meniscus.
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Critical Point Dryer – Caution this
operates under high pressure!
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Electron Gun
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Thermionic Emission
In thermionic emission a cathode is heated
to a high temperature, typically over 1000
K, by flowing electrical current through the
filament. This reduces the work function
for removing an electron.
 There are two main types of thermionic
cathodes.
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 Tungsten
(W)
 Lanthanum hexaboride (LaB6)
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Tungsten Cathode
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Tungsten Cathode – A filament of tungsten wire is
bent into a point. Current is passed through the
wire causing it to heat to 2800 K.
The work function for removing an electron from a
tungsten cathode is Ew = 4.5 eV.
Tungsten cathodes are inexpensive but they tend
to burn out eventually like a light bulb as tungsten
atoms evaporates from the surface of the filament.
Temperature: 2800 K.
Work function: Ew = 4.5 eV.
Current density: 5 x 104 A/cm2
Electron source size: 20 μm
Lifetime: 50-100 hours
Vacuum: 10-4 Pa
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LaB6 Cathode
Temperature: 1800 K.
 Work function: Ew = 2.4 eV.
 Current density: 2-3 x 105 A/cm2
 Electron source size: 10 μm
 Lifetime: 300-500 hours
 Vacuum: 10-2 Pa
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Cold Cathode Field Emission Gun
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A very sharp tip (<100 nm)
The electric field at the tip is > 107 V/cm.
The potential barrier becomes very narrow as well as reduced in
height.
Electron tunnel through the barrier and leave the cathode.
Brightness: 107 A/cm2
Temperature: Room temperature
Brightness: 2-3 x 105 A/cm2
Electron source size: 5-10 nm
Lifetime: One year or more
Vacuum: 10-8 Pa
High resolution
Can image nonconductive materials at low accelerating voltage
(1500 eV)
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Electron Acceleration Example
Consider an electron emitted from a 30 kV electron gun.
Calculate how fast it is going (compared to light speed).
Calculate its ultimate resolution.
Assume a tungsten cathode
KE = Eγ-w
h = 6.626 x 10-34 m2kg/s
KE = ½ mv2
c = 2.998x108 m/s
1 eV = 1.602 x 10-19 J
h
λ=
mv
me = 9.11x10-31 kg
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Sample Stage
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The sample is typically placed on a five
axis stage in the vacuum chamber. The
five dimensions in which the sample can
be moved are:
X
Y
Z
 Rotation
 Tilt
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A Five
Axis
Stage
Z
X
Y
Tilt
Rotation
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Venting the chamber
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Imaging
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Brightness and Contrast
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Brightness and Contrast
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Spot size 70
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Spot size 18
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Spot size 35
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Stigmatism
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Stigmatism
Stig X
Stig Y
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Secondary vs. Backscattered
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Backscattered Electron Images
Scan mode 2 (too fast)
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Backscattered Electron Images
Topographical Mode
Compositional Mode
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Burr
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