Vacuum systems
Electron beam – mean free path:
Gun – column - sample
λ = A / N0 ρ Q
A = atomic wt.
N0 = Avogadro’s number (6.02 x1023 atoms/mol)
ρ = density
Q = 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
Signal detection – electron and Xray collection:
Scattering of emitted electrons and
X-rays reduces signal/noise…
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)
Complete vacuum system:
Ion pump
Gun
“manual” valve
Gun valve
High
vacuum
valve
Column
lenses
spectrometer
Sample
chamber
High-vacuum
pump
Sample exchange
airlock
Backing line
(and valve)
Air inlet
valves
Mechanical pump 2
Mechanical pump 1
Ion pump
High-vacuum
pump
Mechanical pumps
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
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)
Mechanical pump
operation
10-1
Free molecules
10-5
Surface desorption
Pressure
(torr)
diffusion
10-9
permeation
10-13
101
105
Time (sec)
109
1013
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)
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)
Kurt J. Lesker Co.
Direct drive (rotary-vane)
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)
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
Water cooling
coils
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)
Foreline
4) Oil re-condenses on side of pump
that is actively cooled by circulating
chilled water
Pump oil
5) Air molecules build up at base of
pump
6) Mechanical backing pump removes
air from base (4th stage)
Kurt J. Lesker Co.
Heater
500 - 1000 l/s pump rate
Can’t operate above ~ 10-2 torr
(can “crack” the oil)
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:
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.
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)
Thermocouple welded to filament, filament
temperature dependent on thermal loss to gas.
Thermocouple voltage responds to gas pressure
Pirani gauge (to ~10-5 torr)
Two filaments, one measurement, one reference
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 10-9 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.
Hot Filament (Bayard-Alpert) gauge
(to ~ 10-11 torr)
Essentially an electron gun. Thermionic
emission of electrons ionizes gas. Read ion
current (function of pressure).
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)
Complete vacuum system:
Ion pump
Gun
“manual” valve
Gun valve
High
vacuum
valve
Column
lenses
spectrometer
Sample
chamber
High-vacuum
pump
Sample exchange
airlock
Backing line
(and valve)
Air inlet
valves
Mechanical pump 2
Mechanical pump 1
Sample exchange sequence: