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 Q = cross section (probability of an event) Q = N/n t n i N = events/vol. n t = target sites n i – incident particles Smaller cross section & lower density = greater mean free path

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)

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

Ion pump Mechanical pumps High-vacuum pump

At 1 atm (760 torr, 10 5 pa): ~10 19 molecules / cm 3 mostly N 2 and O 2 molecule-molecule distance ~ 5nm molecular mean free path ~ 0.1μm At torr: ~10 14 molecules / cm 3 mostly H 2 O vapor, N 2 and O 2 molecule-molecule distance ~ 0.2μm molecular mean free path ~ 1cm

At torr: ~10 9 molecules / cm 3 mostly H 2 O vapor molecule-molecule distance ~ 10μm molecular mean free path ~ 10 5 cm (about 0.5 miles) At torr: ~10 5 molecules / cm 3 mostly H and He (can diffuse through walls of system) molecule-molecule distance ~ 100μm molecular mean free path ~ 10 6 m (about 50 miles)

Free molecules Surface desorption diffusion permeation Pressure (torr) Time (sec) Mechanical pump operation

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)

Indirect drive (belt drive) Direct drive (rotary-vane) 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 Kurt J. Lesker Co.

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

Edwards XDS10 Varian Triscroll 300

High vacuum pumps 1)Oil diffusion pumps (gas transfer) to torr 2) Turbomolecular pumps (gas transfer) to torr 3) Gas capture pumps ion pumps must operate in conjunction with other high vacuum pumps to torr

Oil diffusion pump 1)Oil heated – boils 2)Vapor streams up and is deflected out and down through baffles 3)The large oil vapor molecules transfer momentum to air molecules that randomly enter pump (3-stage stack at right) 4)Oil re-condenses on side of pump that is actively cooled by circulating chilled water 5)Air molecules build up at base of pump 6)Mechanical backing pump removes air from base (4 th stage) l/s pump rate Can’t operate above ~ 10-2 torr (can “crack” the oil) Kurt J. Lesker Co. Foreline Pump oil Heater Water cooling coils

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

Two basic types of turbomolecular pumps: Kurt J. Lesker Co. 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

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 1)Gas is ionized in tubes by electrons released from cathode 2)Ions strike cathode and sputter Ti 3)Results in chemical reactions and ion burial Generally used around electron gun Kurt J. Lesker Co.

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.

High vacuum Cold cathode (Penning) gauge Inverted magnetron gauge (to ~ 5 X torr) Hot Filament (Bayard-Alpert) gauge (to ~ 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).

Measuring Pressure – Vacuum Gauges: Pressure (torr) Bourdon (dial) Piezo Diaphragm manometer McLeod Pirani Capacitance manometer Thermocouple Hot cathode ionization (Bayard-Alpert) Cold cathode – (inverted magnetron, Penning) Residual gas analyzer (RGA)

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

Sample exchange sequence: