Vacuum systems Electron beam – mean free path: Gun – column - sample
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Transcript of Vacuum systems Electron beam – mean free path: Gun – column - sample
Vacuum systemsElectron 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 / N0 ρ Q A = atomic wt.
N0 = Avogadro’s number (6.02 x1023 atoms/mol)
ρ = densityQ = cross section (probability of an event)
Q = N/ntni N = events/vol.
nt = target sites
ni – 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 lensesSample 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 pumpsHigh-vacuum pump
At 1 atm (760 torr, 105 pa):~1019 molecules / cm3
mostly N2 and O2
molecule-molecule distance ~ 5nmmolecular mean free path ~ 0.1μm
At 10-2 torr:~1014 molecules / cm3
mostly H2O vapor, N2 and O2
molecule-molecule distance ~ 0.2μmmolecular mean free path ~ 1cm
At 10-7 torr:~109 molecules / cm3
mostly H2O vapormolecule-molecule distance ~ 10μmmolecular 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 ~ 100μmmolecular mean free path ~ 106m (about 50 miles)
Free molecules
Surface desorption
diffusion
permeation
10-1
101
10-5
10-9
10-13
105 109 1013
Pressure (torr)
Time (sec)
Mechanical pump operation
Mechanical pump (gas transfer pump)1) Rough pumps system from 1 atmosphere2) Backs high-vacuum pump
can use one pump for both purposes, or use two pumps3) 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 pumpTypical 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 pumpUse 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 pumps1) 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
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 (4th stage)
500 - 1000 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 pumpsPurely 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
PfeifferInlet 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 vacuumCold 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).
Measuring Pressure – Vacuum Gauges:
103 102 10-4 10-5 10-6 10-7 10-8 10-9 10-10101 100 10-1 10-2 10-3
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 lensesSample 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: