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Vacuum systems Electron beam – mean free path: Gun – column - sample

Vacuum systems Electron beam – mean free path: Gun – column - sample Signal detection – electron and X-ray collection: Scattering of emitted electrons and X-rays reduces signal/noise…. λ = A / N 0 ρ Q A = atomic wt. N 0 = Avogadro’s number (6.02 x10 23 atoms/mol) ρ = density

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Vacuum systems Electron beam – mean free path: Gun – column - sample

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  1. Vacuum systems Electron beam – mean free path: Gun – column - sample Signal detection – electron and X-ray collection: Scattering of emitted electrons and X-rays reduces signal/noise… λ = A / N0ρ Q A = atomic wt. N0 = Avogadro’s number (6.02 x1023 atoms/mol) ρ = density Q = cross section (probability of an event) Q = N/ntniN = events/vol. nt = target sites ni – incident particles Smaller cross section & lower density = greater mean free path

  2. Essential system: 1) High vacuum pump (s) Establishes and maintains high vacuum in gun, column, and sample chamber Oil diffusion pump Turbo-molecular pump 2) Mechanical pump (s) Backing high vacuum pump “Rough” pumps entire system when required Rotary direct and indirect drive pumps Dry pumps (scroll, piston, claw, diaphragm)

  3. Complete vacuum system: Ion pump Gun Column lenses Sample chamber “manual” valve Gun valve High vacuum valve spectrometer High-vacuum pump Sample exchange airlock Backing line (and valve) Air inlet valves Mechanical pump 2 Mechanical pump 1

  4. Ion pump High-vacuum pump Mechanical pumps

  5. At 1 atm (760 torr, 105 pa): ~1019 molecules / cm3 mostly N2 and O2 molecule-molecule distance ~ 5nm molecular mean free path ~ 0.1μm At 10-2 torr: ~1014 molecules / cm3 mostly H2O vapor, N2 and O2 molecule-molecule distance ~ 0.2μm molecular mean free path ~ 1cm

  6. At 10-7 torr: ~109 molecules / cm3 mostly H2O vapor molecule-molecule distance ~ 10μm molecular mean free path ~ 105cm (about 0.5 miles) At 10-10 torr: ~105 molecules / cm3 mostly H and He (can diffuse through walls of system) molecule-molecule distance ~ 100μm molecular mean free path ~ 106m (about 50 miles)

  7. Mechanical pump operation 10-1 Free molecules 10-5 Surface desorption Pressure (torr) diffusion 10-9 permeation 10-13 101 105 109 1013 Time (sec)

  8. Mechanical pump (gas transfer pump) 1) Rough pumps system from 1 atmosphere 2) Backs high-vacuum pump can use one pump for both purposes, or use two pumps 3) Three general types Indirect drive (Welch) Direct drive (Alcatel, Edwards, etc.) Dry pumps (Edwards, Varian, Pfeiffer, Leybold, Anest Iwata)

  9. Mechanical pump Typical rotary pump creates low pressure by rotating cam or vane in oil Rotates away from inlet, compressing air on other side and forcing through the outlet port Indirect drive (belt drive) Direct drive (rotary-vane) Kurt J. Lesker Co.

  10. Dry scroll pump Use one fixed and one orbiting scroll to create crescent-shaped gas pockets Gas pockets are compressed and air is forced through central exit port No oil used for sealing or lubrication completely dry and contamination-free

  11. Edwards XDS10 Varian Triscroll 300

  12. High vacuum pumps • Oil diffusion pumps (gas transfer) • 10-3 to 10-10 torr • 2) Turbomolecular pumps (gas transfer) • 10-4 to 10-10 torr • 3) Gas capture pumps • ion pumps • must operate in conjunction with other high vacuum pumps • to 10-11 torr

  13. Oil diffusion pump • Oil heated – boils • Vapor streams up and is deflected out and down through baffles • The large oil vapor molecules transfer momentum to air molecules that randomly enter pump (3-stage stack at right) • Oil re-condenses on side of pump that is actively cooled by circulating chilled water • Air molecules build up at base of pump • Mechanical backing pump removes air from base (4th stage) Water cooling coils Foreline Pump oil Heater Kurt J. Lesker Co. 500 - 1000 l/s pump rate Can’t operate above ~ 10-2 torr (can “crack” the oil)

  14. Turbomolecular pumps Purely mechanical Very clean, fast Stack of rotors which deflect incoming gas molecules with rotating-angled blades Molecules hit underside of blade and are driven in direction of exhaust ~60,000 rpm Back with mechanical pump

  15. Two basic types of turbomolecular pumps: SNECMA (Société Nationale d'Etude et de Construction de Moteurs d'Aviation) Inlet at one end, exhaust at the other Pfeiffer Inlet between two rotor sets and exhaust at both ends Kurt J. Lesker Co.

  16. Ion pump (gas capture) • Principal: gases are taken up by reaction with fine particles of metal, or by ion implantation • Use parallel array of short stainless steel tubes (anode) • Plates of Ti (or Ta) near ends of tubes (cathode) • Generate strong magnetic field parallel to tubes • Gas is ionized in tubes by electrons released from cathode • Ions strike cathode and sputter Ti • Results in chemical reactions and ion burial • Generally used around electron gun Kurt J. Lesker Co.

  17. Measuring Pressure – Vacuum Gauges: Low vacuum Thermocouple gauge (to ~10-3 torr) Pirani gauge (to ~10-5 torr) Two filaments, one measurement, one reference Thermocouple welded to filament, filament temperature dependent on thermal loss to gas. Thermocouple voltage responds to gas pressure Filaments heated and the difference in temperature causes change in resistance of Wheatstone bridge (4 resistors, three known value). The current required to rebalance circuit is, therefore a measure of pressure.

  18. High vacuum Cold cathode (Penning) gauge Inverted magnetron gauge (to ~ 5 X 10-9 torr) Hot Filament (Bayard-Alpert) gauge (to ~ 10-11 torr) + ions released by HV discharge bombard metal cathode, releasing secondary electrons, which can, in turn, ionize gas atoms, adding to the discharge. Measure ion current and/or electron current. Essentially an electron gun. Thermionic emission of electrons ionizes gas. Read ion current (function of pressure).

  19. Measuring Pressure – Vacuum Gauges: Bourdon (dial) Piezo Diaphragm manometer McLeod Pirani Capacitance manometer Thermocouple Hot cathode ionization (Bayard-Alpert) Cold cathode – (inverted magnetron, Penning) Residual gas analyzer (RGA) 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 Pressure (torr)

  20. Complete vacuum system: Ion pump Gun Column lenses Sample chamber “manual” valve Gun valve High vacuum valve spectrometer High-vacuum pump Sample exchange airlock Backing line (and valve) Air inlet valves Mechanical pump 2 Mechanical pump 1

  21. Sample exchange sequence:

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