Vacuum Systems for Electron Microscopy
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Vacuum Systems for Electron Microscopy. Vacuum Systems for Electron Microscopy. They Suck!. Vacuum Systems for Electron Microscopy. Constraints on Specimens Specimens placed in the electron microscope must be able to withstand very high vacuum conditions.

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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 Can’t 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


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