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Dr. Philip D. Rack

Vacuum Technology

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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

Page 6

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

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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