Vacuum Systems for Electron Microscopy

1. Vacuum Systems for Electron Microscopy

2. Vacuum Systems for Electron Microscopy They Suck!

3. 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.

4. Vacuum Systems for Electron Microscopy Why do we need to operate under vacuum?

5. 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

6. 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

7. 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

8. 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

9. Vacuum Systems for Electron Microscopy

10. 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)

11. Abbreviations Pir = Pirani Gauge V = Valve ODP = Oil Diffusion Pump Pen = Penning Gauge Igp = Ion Getter Pump PVP = Pressure Variable Pump (rotary)

12. 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.

13. 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.

14. 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

15. Penning Gauge

16. Rotary (mechanical) Pump Used from atmospheric pressure to about 10-2 Torr

17. Rotary (mechanical) Pump

18. Go to Movie!

19. 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

20. 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)

21. 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

22. 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

23. Turbomolecular Pump

24. 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

25. 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

26. 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.

27. 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.

28. Ion Getter Pump A getter Is a material that reacts with a gas molecule to form a solid nonvaporizable material

29. Cryogenic Pump Can be cooled with liquid nitrogen or liquid helium 10-11 Torr. but must be recharged by warming up

30. 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

31. Vacuum Pump Ranges