Vacuum Technology Pumpdown and Vacuum Pumps - web.utk.edu
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Dr. Philip D. Rack
Vacuum Technology
Page 1
Pumpdown and Vacuum Pumps
Dr. Philip D. RackAssistant Professor
Department of Materials Science and EngineeringUniversity of Tennessee
603 Dougherty Engineering BuildingKnoxville, TN 37931-2200
Phone: (865) 974-5344Fax (865) 974-4115
Email: prack@utk.edu
Dr. Philip D. Rack
Vacuum Technology
Page 2
Conductances• Series conductances
• Parallel conductances
C1 C221
111CCCT
+=
C1
C2
CT = C1 + C2
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Dr. Philip D. Rack
Vacuum Technology
Page 3
Pumpdown Procedure• 1.Start-up
– Turn on pumps– Open foreline valve
• 2.Close foreline valve• 3.Open roughing valve• 4. Rough chamber
~100mtorr• 5.Close roughing valve• 6.Open foreline valve• 7.Open high-vac valve
ForelineValve
High-Vac Pump
Chamber
MechanicalPump
High-vacValve Roughing valve
N2
Vent valve
Dr. Philip D. Rack
Vacuum Technology
Page 4
Venting Procedure• 1.Close high-vac
valve• 2.Open vent valve• Why N2 or Ar for
venting chamber??
ForelineValve
High-Vac Pump
Chamber
MechanicalPump
High-vacValve Roughing valve
N2
Vent valve
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Dr. Philip D. Rack
Vacuum Technology
Page 5
Pressure Curves• Pressure versus time
ForelineValve
High-Vac Pump
Chamber
MechanicalPump
High-vacValve Roughing valve
N2
Vent valve
C1
C2
C3
VolumeVCSS
whereVtS
PP
Tpeff
eff
=
+=
−=
111:
exp0
Ignores Sources of Gas in a vacuum(Vaporization, Thermal Desorption, DiffusionPermeation, Backstreaming, Leaks)
Dr. Philip D. Rack
Vacuum Technology
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System Pumpdown• Roughing chamber
– Use Viscous flow equations
++=
32
11)(
11CCMPSS peff
High-Vac Pump
Chamber
MechanicalPump
High-vacValve Roughing valve
N2
Vent valve
C1
C2
C30.1
10
1000
100000
10000000
1 10 100 1000 10000time (s)
Cond
ucta
nce
S(eff)
Conductance
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Dr. Philip D. Rack
Vacuum Technology
Page 7
System Pumpdown
0.11
10100
100010000
100000
0 1000 2000 3000 4000
Time (s)
Pres
sure
(Pa)
100mTorr
Dr. Philip D. Rack
Vacuum Technology
Page 8
System Pumpdown• High Vac Pumping
– Use molecular flow equations
+=
1
1)(
11CHVPSS peff
High-Vac Pump
Chamber
MechanicalPump
High-vacValve Roughing valve
N2
Vent valve
C1
C2
C3
1E-11
1E-09
1E-07
1E-05
0.001
0.1
10
0 5 10 15 20 25
Time (s)
Pres
sure
(Tor
r)
Seff = 1m3/s
V=1m3
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Dr. Philip D. Rack
Vacuum Technology
Page 9
Real Systems• Pressure limits in vacuum
systems
– 1st term -- time dependence of pressure that is due to the gas in the chamber volume (exp(-t))
– 2nd term -- pressure due to outgassing (~ t-1)
– 3rd term -- pressure due to diffusion (~ t1/2 and later exp(-Dt))
– 4th term -- pressure due to permeation (constant)
eff
K
eff
D
eff
Oeff
SQ
SQ
SQ
VtS
PP +++
−= exp0
101103 105 107 109 101110131015 1017
1010-1
10-3
10-5
10-7
10-9
10-11
10-13
103
Time (s)
Pressure(Torr)
Volume ~ exp(-t)
Outgassing ~ t-1
Diffusion ~ t-1/2
Permeation
Dr. Philip D. Rack
Vacuum Technology
Page 10
Classificatoins• Pressure Ranges
– 760 torr - 1x10-3 torr (essentially viscous flow -roughing pumps
– 10 torr - 10-5 torr (transition flow range) - high throughput pumps
– 10-3 torr - 10-12 torr (molecular flow) - high vacuum pumps
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Dr. Philip D. Rack
Vacuum Technology
Page 11
Classifications• Pumping Action
– Entrainment pumps• Positive displacement
– Rotary Vane– Rotary Piston– Roots Blower
• Momentum transfer– Turbomolecular – Diffusion
– Capture pumps• Cryosorption• Ion sublimation• Titanium Sublimation Pumps
Dr. Philip D. Rack
Vacuum Technology
Page 12
Mechanical Pumps• Gas enters through
suction chamber (A)• Compressed by rotor
(3) and vane (5)• Expelled through
discharge valve (8)• 500-2000 rpm• Single stage pumps• Sp ~ 10-200 m3/hour• Ultimate pressures ~
1.4Pa (~10mtorr)
Single Stage Rotary Vane
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Dr. Philip D. Rack
Vacuum Technology
Page 13
Mechanical Pumps
• 500-2000 rpm• Single stage pumps• Sp ~ 10-200 m3/hour• Ultimate pressures ~
1.5x10-2Pa (~100µtorr)
Double Stage Rotary Vane
Dr. Philip D. Rack
Vacuum Technology
Page 14
Mechanical Pumps• Pumping speed of single versus double stage rotary vane
(sp ~ 30m3/hour)
Gas ballast introduces gas out exit port to keep gases from condensing (ie water, acetone…)
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Dr. Philip D. Rack
Vacuum Technology
Page 15
Mechanical Pumps• Gas is drawn in during
1st revolution (A)• After 1st revolution,
that volume of gas is isolated from the inlet (B)
• During second revolution the gas is compressed and ejected
• ~40-600rpm• Sp~30-1500 m3/hour• Ultimate pressure ~
10mtorr
Rotary Piston Pump
Dr. Philip D. Rack
Vacuum Technology
Page 16
Mechanical Pumps• Rotary vane and Rotary Piston Pump Issues
– Due to close tolerances (<0.1mm) the pump surfaces are lubricated with oils
• Oil Properties– Vapor Pressure - sets ultimate pressure of the pump– Viscosity and wettability - lubrication
• Breakdown of oils and subsequent backstreaming can be a significant source of contamination
Semiconductor manufacturers are going to “dry pumps”
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Dr. Philip D. Rack
Vacuum Technology
Page 17
Mechanical Pumps• 2 lobed rotors mounted on
parallel shafts and rotate in opposite directions
• Not lubricated with oils: “dry pump”, (3000-3500rpm)
• Pumping Speed 500m3/hour
• Ultimate pressure ~10-5torr (must be backed by a rotary pump because it can not pump at high pressures)
Roots Pump (or lobe blower)
Dr. Philip D. Rack
Vacuum Technology
Page 18
High Vacuum Pumps• Pumping action by momentum
transfer– Blades spinning at 30,000-
60,000rpm (linear velocity~500m/s)
– Angled blades impart momentum to gas particles toward outlet
• Can damage blades at high pressures (large viscous forces)
• Must back Turbo with Mechanical pump
• Pumping Speed ~1000l/s• Ultimate pressure ~ 10-10torr
Turbomolecular Pump
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Dr. Philip D. Rack
Vacuum Technology
Page 19
High Vacuum Pumps
Blade
Gas Molecule
Momentumtransfer
Turbomolecular Pump Blades
Dr. Philip D. Rack
Vacuum Technology
Page 20
High Vacuum Pumps• Pumping action by momentum
transfer from a supersonic jet stream
• At viscous flow high particle density can scatter oil jet stream and cause severe backstreaming
• Need low VP oils• Pumping speeds ~ 1000l/s• Ultimate pressure ~ 10-11 Torr
DiffusionPump
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Dr. Philip D. Rack
Vacuum Technology
Page 21
High Vacuum Pumps• Diffusion pump pumping mechanism
– Low vapor pressure oil is heated to its boiling point– Vapors flow up “chimney” and is ejected through a series of
nozzles (supersonic velocities)– The nozzles direct the vapor stream downward – The gas stream is directed towared the water-cooled wall where is
is condensed and returned to the boiler– Gas particles that diffuse into this region are on average given a
downward momentum and eventually ejected through the outlet
Dr. Philip D. Rack
Vacuum Technology
Page 22
High Vacuum Pumps• Pumping action is by adsorbing
gas molecules onto cold surfaces– Gas particles impinge on cooled
surface and do not desorb• Typically two stages
– Liquid N2 (~80K)– Liquid helium (~20K)
• Need to rough chamber to molecular flow or pre-mature pump saturation can occur
– must periodically regenerate (ie heat up and desorb gas)
• Pumping Speed ~ 1000l/s • Ultimat Pressure ~10-13 Torr
Cryosorption Pump
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Dr. Philip D. Rack
Vacuum Technology
Page 23
High Vacuum Pumps• Pumping action
– Adsorption followed my dissociation
– Gettering from freshly sputtered cathode surface
– Surface burial under sputtered cathode material
– Implantation of ionized gas– High energy neutral
implantation of reflected ions• Pumping Speed ~ 500l/s• Ultimate Pressure ~ 10-10 Torr
Ion Pump
Dr. Philip D. Rack
Vacuum Technology
Page 24
High Vacuum Pumps• Pumpiong action -- adsorbed
gases react with titanium surface
– Periodically evaporate a titanium filament which deposits a fresh film of Ti on nearby walls (typically cooled to inhibit desorption)
• Ultimate pressure ~ 10-11 Torr
Titanium Sublimation Pump
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Dr. Philip D. Rack
Vacuum Technology
Page 25
Summary of Vacuum Pumps
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