Vacuum Systems for Electron Microscopy

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Vacuum Systems for Electron Microscopy
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Vacuum Systems for Electron Microscopy
They Suck!
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Vacuum Systems for Electron Microscopy
Constraints on Specimens Specimens placed in the
electron microscope must be able to withstand
very high vacuum conditions. This means that all
moisture and trace organics must be removed from
the specimen.
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Vacuum Systems for Electron Microscopy
Why do we need to operate under vacuum?
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Vacuum Systems for Electron Microscopy
1. Produce a coherent beam - The mean free path
of electrons at atmospheric pressure is only 1
cm. At 10-6 Torr they can travel several meters
(about 6.5 m) and eliminate electron scattering
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Vacuum Systems for Electron Microscopy
1. Produce a coherent beam - The mean free path
of electrons at atmospheric pressure is only 1
cm. At 10-6 Torr they can travel several meters
(about 6.5 m) and eliminate electron
scattering 2. Insulator - no interaction of beam
and gas molecules. Eliminate electrical
discharges, particularly between anode and
cathode and in area around field emitters
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Vacuum Systems for Electron Microscopy
1. Produce a coherent beam - The mean free path
of electrons at atmospheric pressure is only 1
cm. At 10-6 Torr they can travel several meters
(about 6.5 m) and eliminate electron
scattering 2. Insulator - no interaction of beam
and gas molecules. Eliminate electrical
discharges, particularly between anode and
cathode and in area around field emitters 3.
Increase Filament life - elimination of oxygen
prevents burning out of filament
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Vacuum Systems for Electron Microscopy
1. Produce a coherent beam - The mean free path
of electrons at atmospheric pressure is only 1
cm. At 10-6 Torr they can travel several meters
(about 6.5 m) and eliminate electron
scattering 2. Insulator - no interaction of beam
and gas molecules. Eliminate electrical
discharges, particularly between anode and
cathode and in area around field emitters 3.
Increase Filament life - elimination of oxygen
prevents burning out of filament 4. Reduce
interaction between gas molecules, e-beam, and
sample that leads to contamination
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Vacuum Systems for Electron Microscopy
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Vacuum Systems for Electron Microscopy
Different levels of vacuum are required for
different portions of the microscope Gun (10-9
Torr) Specimen (10-6 Torr) Chamber and
Camera (10-5 Torr)
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Abbreviations Pir Pirani Gauge V Valve ODP
Oil Diffusion Pump Pen Penning
Gauge Igp Ion Getter Pump PVP Pressure
Variable Pump (rotary)
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Vacuum Systems for Electron Microscopy
Vacuum Tube Gauge (Pirani Gauge) Uses a wire in
a sealed vacuum tube and a second wire in
specimen chamber. Apply a constant voltage of
6-12V to heat the wires. The hotter the wire,
the better the vacuum since fewer molecules are
hitting the wire to dissipate heat. The higher
the temperature of the wire, the greater the
resistance and the less the current flow. The
difference in current flow between the known
vacuum in the closed tube and the unknown vacuum
in the instrument gives an indication of the
vacuum in the chamber.
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Vacuum Systems for Electron Microscopy
Vacuum Tube Gauge (Pirani Gauge) Uses a wire in
a sealed vacuum tube and a second wire in
specimen chamber. Apply a constant voltage of
6-12V to heat the wires. The hotter the wire,
the better the vacuum since fewer molecules are
hitting the wire to dissipate heat. The higher
the temperature of the wire, the greater the
resistance and the less the current flow. The
difference in current flow between the known
vacuum in the closed tube and the unknown vacuum
in the instrument gives an indication of the
vacuum in the chamber.
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Vacuum Systems for Electron Microscopy
Ion discharge gauges (Penning Gauge) Get
current flow between anode and cathode (kept at
several thousand volt difference relative to
each other, which ionizes gas molecules in
instrument. As electrons hit gas molecules,
collisions form more ions. The more gas
molecules present, the more collisions to
generate more ions which leads to increased
current measured by the gauge
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Penning Gauge
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Rotary (mechanical) Pump
Used from atmospheric pressure to about 10-2 Torr
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Rotary (mechanical) Pump
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Go to Movie!
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Diffusion Pump
Boil Oil Condense Oil Vapor (cooling
coils) Condensing vapor sweeps gas
molecules down Reboiling releases gas molecules
which are then removed by mechanical pump
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Diffusion Pump
Diffusion Pump Considerations Must be used in
conjunction with another (usually rotary) pump
Cant be used at greater than 10-2 Torr. Hot oil
will deteriorate crack and form tar. Diffusion
oil is VERY expensive (1-2 per ml.) If cooling
system or backing pump fails oil will
backstream into the microscope by way of
diffusion Needs time to heat up and cool down
(30 min)
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Diffusion Pump
Disadvantages Oil Vapor Can crack Time to
heat up/cool down Needs coolant Can overheat If
lose RP, will have oil throughout system
Advantages Simple design Relatively cheap No
moving parts No vibration Pumps light gasses
well Tolerant of particles
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Turbomolecular Pump
Essentially a jet engine that pulls air instead
of pushing it. Turbine spins very fast
(20-50,000 rpm) and creates downdraft which
sweeps out gas Molecules. Multiple stages of
rotating blades (rotors) spaced between fixed
blades (stators). Usually requires rough
(backing) pump although in theory can go from
atmosphere
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Turbomolecular Pump
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Turbomolecular Pump
Disadvantages Must be vibration
damped Sensitive to movement Moving parts Very
expensive
Advantages Very high Vacuum 10-7 Torr. Very
clean (no oil) Relatively fast
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Entrainment Pumps
No moving parts Work by trapping gas
molecules to a
surface Ion Getter (sputter) Pumps Chemically
trap
molecules Cryogenic Pumps Freeze molecules
to a supercold
surface Vacuum Range 10-10 Torr
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Ion Getter Pump
Sputter ion pumps operate by ionizing gas within
a magnetically confined cold cathode discharge.
The events that combine to enable pumping of
gases under vacuum are Entrapment of electrons
in orbit by a magnetic field. Ionization of gas
by collision with electrons. Sputtering of
titanium by ion bombardment. Titanium gettering
of active gases.
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Ion Getter Pump
Permanent magnets (1) Surround an air tight
case (2). Titanium plates (3) are negatively
charged and act as cathodes and are separated by
anode cells (4). When a high voltage is
applied ionized gas molecules either
become entrapped directly in the Cathodes or are
trapped by sputtered Ti which acts as a getter
material.
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Ion Getter Pump
A getter Is a material that reacts with a
gas molecule to form a solid nonvaporizable
material
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Cryogenic Pump
Can be cooled with liquid nitrogen or liquid
helium 10-11 Torr. but must be recharged by
warming up
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The cold trap that immediately surrounds the
specimen in most TEMs acts as a mini cryopump,
trapping volatiles as they are produced from
interaction of the beam with the specimen. This
is an important way to keep the internal
components of the TEM clean. Once the beam is
off and the trap warms up the trapped gasses are
released and removed via the normal pumping
system
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Vacuum Pump Ranges