4 Vacuum Pump

Dr. G. Mirjalili, Physics Dept. Yazd University

Vacuum techniques

Pumps


Vacuum theory and pumping laws

Vacuum theory and pumping laws

How the vacuum is created?


• to reduce gas density in given volume to below atmospheric pressure with pump

• enclosed vessel has continuous sources which launch gas into volume and present pump with continuous gas load

• vacuum achievable at steady state is result of dynamic balance between gas load and ability of pump to remove gas form volume

Production of vacuumProduction of vacuum


Vacuum pumps and their characteristics

Vacuum pumps and their characteristics

• Gas transfer pumps:

(a) Positive displacement pumps that transfer repeated volumes of gas from inlet to outlet by compression ( e.g. rotary pump).

(b) Kinetic pumps that continuously transfer gas from inlet to outlet by imparting momentum to gas molecules (e.g. Diffusion pump, turbomolecular pump).


• Entrapment/capture pumps,

retain molecules by sorption or condensation on internal surfaces (e.g. sorption pump, sublimation pump, sputter ion pump, cryogenic pump).


Low vacuum pumps (1atm-10-3)

mbarRoughing Pumps

1


Ultrahigh Vacuum High Vacuum Rough Vacuum

Typical HighPressure

Typical Low Pressure

Vacuum (units)

1 atm.1.3x10-31.3x10-61.3x10-9

760 Torr1 Torr1 mTorr1x10-6 Torr

1 Torr =1 mm-Hg

101,333 Pa133 Pa0.133 Pa0.133x10-3 Pa

1 Pascal =1 N/m2


VACUUM PUMPING METHODS

Sliding VaneRotary Pump

MolecularDrag Pump

TurbomolecularPump

Fluid EntrainmentPump

VACUUM PUMPS(METHODS)

ReciprocatingDisplacement Pump

Gas TransferVacuum Pump

DragPump

EntrapmentVacuum Pump

Positive DisplacementVacuum Pump

KineticVacuum Pump

RotaryPump

DiaphragmPump

PistonPump

Liquid RingPump

RotaryPiston Pump

RotaryPlunger Pump

RootsPump

Multiple VaneRotary Pump

DryPump

AdsorptionPump

Cryopump

GetterPump

Getter IonPump

Sputter IonPump

EvaporationIon Pump

Bulk GetterPump

Cold TrapIon TransferPump

Gaseous Ring Pump

TurbinePump

Axial FlowPump

Radial FlowPump

EjectorPump

Liquid JetPump

Gas JetPump

Vapor JetPump

DiffusionPump

DiffusionEjector Pump

Self PurifyingDiffusion Pump

FractionatingDiffusion Pump

Condenser

SublimationPump


Name of Pump Mechanism of PumpingMechanical (roughing)* Compression of gasSorption Physical or chemical absorption Diffusion* Intermolecular collisions Turbo Molecular collisions with surfacesIon Ionization and implantation of gasCryo(genic) Solidification of gas by liquid He *used in lab


PUMP OPERATING RANGES

10-12 10-10 10-8 10-6 10-4 10-2 1 10+2

P (mbar)

Rough VacuumHigh VacuumUltra High Vacuum

Venturi Pump

Rotary Vane Mechanical Pump

Rotary Piston Mechanical Pump

Sorption PumpDry Mechanical Pump

Blower/Booster Pump

High Vac. PumpsUltra-High Vac. Pumps


VACUUM SYSTEM USE

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ChamberHigh Vac. PumpRoughing PumpForeline PumpHi-Vac. ValveRoughing ValveForeline ValveVent ValveRoughing GaugeHigh Vac. Gauge

7

33a


Mechanical pumps• Mechanical pumps (displacement pumps) remove gas atoms

from the vacuum system and expel them to atmosphere, either directly or indirectly

• In effect, they are compressors and one can define a compression ratio, K, given by

• K is a fixed value for any given pump for a particular gas species when measured under conditions of zero gas flow.

out

in

PK

P


Rotary Vane, Oil-Sealed Mechanical Pump


Pump Mechanism


Gas ballastting


The Molecular Sieve/Zeolite Trap


Rotary pump Trap


Single &Dual Stage


How 2-stage rotary pump Works


OIL BACKSTREAMING

2

PRESSURE LEVELS: LESS THAN 0.2 mbar


Other types of Mechanical pumps

Rotary Piston

Roots

Rotary Vane

Dry pump


Dry Vacuum Pumps


Root pump


How Root Pump works


One Stage Roots Blower Pump Assembly


Vacuum system use for Root pumps

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ChamberForelineRoughing ValveRoughing GaugeRoughing PumpForelineForeline ValveForeline GaugeHigh Vacuum ValveBooster/BlowerVent ValveHigh Vacuum Gauge

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9

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12

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11

5

2

678

10


Diaphragm pumps


Diaphragm pumps


Diaphragm Pump• Eccentric shaft produces

alternate expansion / compression process

• Inlet / outlet via reed valves

• Ultimate vacuum 100 - 0.1 torr - limited by external leakage past valves, internal back-streaming, dead volume

• Compression ratio typically 10 - 30

• Pumping speed: single unit 0.1-0.7 l/s, parallel units up to 5.3 l/s


Diaphragm Pump

• High resistance to chemical attack

• Oil free - used with roots blower or cryopump for completely oil-free system

• Lifetime ~ 5000 hours


Diaphragm pump


Sorption Pump Components


The sorption pump has no moving parts and therefore no oils or other lubricants. (5 liters of liquid nitrogen)

Sorption pumps


HIGH VACUUM PUMPS

2


VACUUM PUMPING METHODS

Sliding VaneRotary Pump

MolecularDrag Pump

TurbomolecularPump

Fluid EntrainmentPump

VACUUM PUMPS(METHODS)

ReciprocatingDisplacement Pump

Gas TransferVacuum Pump

DragPump

EntrapmentVacuum Pump

Positive DisplacementVacuum Pump

KineticVacuum Pump

RotaryPump

DiaphragmPump

PistonPump

Liquid RingPump

RotaryPiston Pump

RotaryPlunger Pump

RootsPump

Multiple VaneRotary Pump

DryPump

AdsorptionPump

Cryopump

GetterPump

Getter IonPump

Sputter IonPump

EvaporationIon Pump

Bulk GetterPump

Cold TrapIon TransferPump

Gaseous Ring Pump

TurbinePump

Axial FlowPump

Radial FlowPump

EjectorPump

Liquid JetPump

Gas JetPump

Vapor JetPump

DiffusionPump

DiffusionEjector Pump

Self PurifyingDiffusion Pump

FractionatingDiffusion Pump

Condenser

SublimationPump


PUMP OPERATING RANGES

10-12 10-10 10-8 10-6 10-4 10-2 1 10+2

P (Torr)

Rough VacuumHigh VacuumUltra High Vacuum

Roughing Pumps

Turbo Pump

Diffusion Pump

Cryo Pump

Ion Pump

Tit. Subl. Pump

Liquid Nitrogen Trap


VACUUM SYSTEM USE (high vacuum)

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ChamberHigh Vac. PumpRoughing PumpFore PumpHi-Vac. ValveRoughing ValveForeline ValveVent ValveRoughing GaugeHigh Vac. Gauge

7

33a

28

2


Diffusion pumps


Diffusion pumps

• diffusion pump is one form of a fluid entrapment pump– a fluid (usually oil) is heated and vaporized– the vapor is A sent through a nozzle with supersonic speed– the pump fluid vapor is condensed on a cooled surface

• Gas molecules are transported to the bottom of the pump by the pump fluid, where it is evacuated by a backing pump (usually a rotary vane pump) through the pump exhaust (the foreline)

• In order to work, the pump cannot be started until the foreline pressure is sufficiently low (~millitorr)


Water ejector pump (Liquid Jet pump)


Pump Construction


How the Pump Works


How the Pump Works

-A coil heater (1) raises the temperature of the oil pool (2) inside the pump body (3) with external cooling coils (4)

-The pump body is bolted to the vacuum system by a flange (5)

-The oil vapor rises through the housing that has 4 exit nozzles (A – D).

- The oil vapor exits the nozzles at high velocity (7) and collides with gas molecules (6), imparting a downward momentum to them.


First stage vapors are separated from others


Pumping Speed

10-10 10--3 10--1

Pu

mp

ing

Sp

eed

(A

ir)

1 2 3 4

Inlet Pressure (Torr)

Critical Point

1. Compression Ratio Limit2. Constant Speed3. Constant Q (Overload)4. Mechanical Pump Effect


Maximum Tolerable Foreline Pressure

(critical pressure)


LN2 reservoir with baffles


How the LN2 Trap Works

GasApproximate Vapor

Pressure (mbar)

Water (H2O)Argon (A)Carbon Dioxide (CO2)Carbon Monoxide (CO)Helium (He)Hydrogen (H2)Oxygen (O2)Neon (Ne)Nitrogen (N2)Solvents

10-22

500 10 -7

>760>760>760 350>760 760 <10 -10


Diffusion pump characters


Diffusion pump Fluids


Diffusion pumps -- additional information

• “The only justification for calling them diffusion pumps is due to the observation that the molecules of the pumped gas penetrate some distance into the vapor jet in a manner resembling diffusion of one gas into another.” (Hablanian, High Vacuum Technology)

• Original pumping fluid (before 1928) was mercury, since it did not break down and early oils did -- over 99% today use oil

• The boiler pressure inside a nozzle is 1 to 2 torr, while at the center of the vapor stream it is about 0.1 torr

• A cold trap is often used in the high vacuum side to reduce oil backstreaming


• Low cost per unit pumping speed, very high pumping speeds• Very well understood• Hard to destroy

• Continuous operating expense (LN2)

• Potential for serious vacuum accidents• “Open system”:Forbidden in certain applications

Diffusion pumps -- additional information


•Turbomolecular pumps


Turbomolecular pumps (high vacuum and UHV)

Turbomolecular pumps (high vacuum and UHV)

• Medium to high cost per unit pumping speed• Very clean, pumps rare gases• Requires periodic maintenance which can be

expensive• Difficult to reach very low UHV base pressures• “Open system”:Forbidden in certain

applications


Pump OperationMolecule V

Moving Wall with Speed V

Principle of the Turbomolecular Pump


Turbomolecular pumps• Operation can be extended to higher pressure

by adding a drag stage


Principal of Turbomolecular pump


Rotor - stator assembly


Turbomolecular pump principle• To maximise the compression ratio, blade tip velocities

need to be comparable to molecular thermal velocities.• For a single blade, at zero flow

• where α12 is the forward transmission probability

• and α21 is the reverse transmission probability

• It can be shown that

• where Vb is the blade velocity

12

21

out

in

PK

P

0

exp2bV M

KTkN


Compression ratio


high pressurestages

fore vacuum

high vacuum

low pressurestages

moving rotors impartdownward momentumto gas molecules

fixed stators decelerate the molecule for thenext rotor “hit”

without the stators,the next rotor couldnot impart additionalvelocity to the gasmolecule

med. pressurestages

moving rotors only:a “molecular dragpump”


Turbomolecular Pump

ROTOR BODY

HIGH PUMPING SPEED

HIGH COMPRESSION

EXHAUST

HIGH FREQ. MOTOR

INLET FLANGE

STATOR BLADES

BEARING

BEARING


A typical turbomolecular pumpA typical turbomolecular pump


Turbomolecular Pumps

• Similar in design to a jet engine. Alternating rotor and stator blade assemblies turn at 20,000-90,000 rpm to force out molecules. Requires a region of low or medium vacuum behind and in front of pump.

Pfeiffer Vacuum GmbH


Turbomolecular pump.

• Turbo pumps cannot pump from atmosphere and cannot eject to atmosphere, so they require: 1-roughing (fore vacuum) pumps to reduce the pressure in the vacuum system before they can be started and

2-backing pumps to handle the exhaust.• There are many types of roughing and backing

pumps. Most accelerators now use clean (dry) pumps to avoid oil contamination in the system.


Turbopump (con’t)

• contains no oil and is capable of reducing the pressure into the ultrahigh

vacuum range

• operates as a “molecular bat”

- rotor blades spinning at speeds as high as 6x104 rpm,

- gives a blade velocity at a radius of 10 cm of 3.8x106 cm/s.

- the mean velocity of a molecule of N2 at 300 K is 4.8x104 cm/s

• Because the rotor blades are slanted downward, the gas

molecules are driven towards the pump outlet

Blade sizes increase towards the high pressure exit port Stator (stationary) blade sets are placed between rotor blade sets

• Pumping efficiency is greatest when the spacing between blades is less than the mean free path of the molecules. (~5 cm at 10-3 Torr)


Turbo pumps speed


Vacuum system use for Turbo pumps

123456

ChamberTurbo PumpRoughing PumpVent ValveRoughing GaugeHigh Vac. Gauge

1

67

4

3

25

2

Rotary pump


Turbo pump &Rotary pump

Process chamber

Turbomolecular Pump

High rotation speed turbine imparts momentum to gas atoms

Inlet pressures: <10 mTorr

Foreline pressure: < 1 Torr

Requires a rough pump

Good choice for toxic and explosive gases –

-gases are not trapped in pump

All gases are pumped at approx. the same rate

Pumping Speeds:

20 – 2000 liters per sec

foreline

adapted from Lesker.com


Ion Pumps


Ion Getter Pump

A getterIs a materialthat reactswith a gasmolecule toform a solid nonvaporizable material


Ion pump• The ion pump works by ionizing

gas molecules and accelerating them into walls coated with freshly-evaporated titanium– the gas ions strike a titanium

cathode and cause sputtering– the sputtered Ti is reactive and

will getter reactive gases (N2, O2)

– the gas ions can be buried by self-ion implantation

• A strong magnetic field is applied to cause the electrons to move in helical paths and increase the ionization efficiency


Ion pump (sputter- Ion pump, getter Ion pump)


Ion pumps

• Main components– Array of parallel

stainless tubes– Various charged

surfaces– Titanium or tantalum

coated surfaces

• Trap molecules with varying speeds via chemical reactions

Varian Scientific Instrumentation, Inc.


Ion pumps• Ion pumps have several serious disadvantages

– low pumping speeds (inert gases are pumped especially poorly)

– can only be started at low pressures (~ 10-4 torr)– can “arc-over” if pressure increases suddenly

• However, ion pumps are very clean and can produce very high vacuums (<10-12 torr)


Ion pump

• Expensive per unit pumping speed

• Low pumping speed

• Generates hydrocarbons

• Has a memory effect

• Very low maintenance

• Moderately difficult to destroy

• Excellent base pressures


• Does not pump rare gases well

• Does not pump hydrogen

• Closed system: very safe against vacuum accidents

A typical A typical ion-pumpion-pump


Ion Pumps• Current (per cell) – and hence pumping

speed – depends on voltage, magnetic field, pressure and history.

nI kP 1.05 < n < 1.2

Pump life depends on quantity of gas pumped

> 20 years at 10-9 mbar

Prone to generate particulates

Leakage current unpredictable, so pressure indication below 10-8 mbar unreliable


Ion Pumps


Ion PumpsDiode Differential

DiodeStarcell Triode

Voltage +7kV +7kV +2-5kV -5kV

Pumping Speed (Active gases)

Highest Good Good Lowest

Pumping Speed (Noble gases)

Lowest Good Higher Highest

Starting Pressure Lowest Lowest Good Highest

UHV Low Low Good Highest

Cost Lowest Higher Low Highest



Ion PumpsPumping in the basic diode Penning cell


Ion Pumps

• The Diode pump has poor pumping speed for noble gases

• Remedies– Differential Ion; Noble Diode

• “Heavy” cathode

– Triode– Special Anode shape e.g. Starcell


•Cryopumps


Cryo-condensation


Cryopumps


Pumping by Cryocondensation

Cold surface

molecules


Cryopumps

• Similar in principle to the ion pump but uses a cryogenically cooled surface of activated charcoal or zeolites to condense and trap gas molecules.

Kurt J. Lesker Vacuum Technology


Cryosorption in charcoal


Charcoal placement


CryopumpsCryopumps condense gases on cold

surfaces to produce vacuum

Typically there are three cold surfaces:

(1) Inlet array condenses water and hydrocarbons (60-100 Kelvin)

(2) Condensing array pumps argon, nitrogen and most other gases (10-20 K)

(3) Adsorption is needed to trap helium, hydrogen and neon in activated carbon at 10-12 K. These gases are pumped very slowly!

Warning: all pumped gases are trapped inside the pump, so explosive, toxicand corrosive gases are not recommended. No mech. pump is needed until regen.

adapted from www.helixtechnology.com

(Campbell)


CryopumpsCryopumps

• Expensive per unit pumping speed

• Very high pumping speeds are possible

• Pumping hydrogen (pumps everything)

• Requires periodic recharging

• Vibration can be a serious problem


Types of Cryogenic Pumps

• There are two major classes of such pumps– Liquid Pool

• Liquid helium temperature (~4K)

– Closed cycle• Refrigerator (~12K)• Supplemented by cryosorption


Cyro pump (Liquid Pool)


Cyro pump (Closed cycle )


Cryogenic Pump Speed


Getter Pumps

• When a gas molecule impinges on a clean metal film, the sticking probability can be quite high.

• For an active gas with the film at room temperature, values can be between 0.1 and 0.8. These fall with coverage.

• For noble gases and hydrocarbons sticking coefficients are very low (essentially zero)

• Evaporated films, most commonly of titanium or barium, are efficient getters and act as vacuum pumps for active gases.


Getter pumps

• In recent times, thin films of getter material have been formed on the inside of vacuum vessels by magnetron sputtering

• These have the advantage of – pumping gas from the vacuum chamber by gettering – and of stopping gases from diffusing out of the walls

of the vessels


Getter Pumps

• For vacuum use, the most common getter pump is the titanium sublimation pump


Titanium sublimation pumps (HV and UHV)

Titanium sublimation pumps (HV and UHV)

• Very inexpensive and simple

• Requires periodic maintenance, which is cheap

• Often misused, which limits their performance

• Selective in what it pumps (good for oxygen, N2, air, not for rare gases)


A typical titanium sublimation pumpA typical titanium sublimation pump



Others Getter Pumps

• An important class of getter pumps are the Non Evaporable Getters (NEGs)

• These are alloys of elements like Ti, Zr, V, Fe, Al which after heating in vacuo present an active surface where active gases may be gettered

• Traditionally, the getters take the form of a sintered powder either pressed into the surface of a metal ribbon or formed into a pellet


Getter Pumps


Getter Pumps


•Vacuum cycle


Pumpdown Curve• Conditions:

– Chamber closed and sealed– Vacuum pump on and all isolation valves open– No gas flowing into the chamber

• What would an ideal pumpdown curve look like?• What effect would the following have on the ideal

curve?– Real (Gross) Leak– Virtual Leak– Permeation Leak– Outgassing– Backstreaming


Pumpdown procedure


Venting procedure






System pumpdown


Standard Vacuum Cycle• Step 0: Start at atmospheric pressure at t=o

– load wafer and close chamber– alternative, start at loadlock pressure (~100mT)

• a loadlock is a separate vacuum chamber that prevents the chamber from being exposed to atmosphere

• Step 1: Pump down to base pressure– remove atmospheric contaminants from the chamber– verify system integrity– continue to next step: when pressure falls below a trigger

point– abort: if base pressure is not reached within a certain

amount of time, indicating a leak or a pump problem


Standard Vacuum Cycle• Step 2: Introduce gasses and stabilize pressure

– pressure increases from base pressure to process pressure

– most reactive gas is introduced last– throttle valve controls conductance to achieve desired

process pressure (effects residence time of gasses)– continue to next step: when pressure reads within a

specified range– abort: if process pressure is not reached within a

certain amount of time, indicating a pressure control problem


Standard Vacuum Cycle• Step 3: Process

– equilibrium is maintained through controlled gas flow and controlled (throttled) pressure

– RF power (if applicable) is introduced– continue to next step: when pre-set time is reached,

or endpoint is detected (for etch process)– abort: if pressure drifts outside of desired range


Standard Vacuum Cycle• Step 4: Pump Out

– gas flows and RF Power (if applicable) are turned off– throttle valve opens wide– purpose is to remove the majority of the reactive gasses from the

chamber– continue to next step: when base pressure is reached for a

minimum length of time

• Step 5: Purge– inert gas (usually nitrogen - why?) is introduced into the chamber– pressure inside the chamber increases to a trigger point– presence of nitrogen restores viscous flow allowing residual

reactive gasses to be efficiently pumped (rinsed) out– continue to next step: when a minimum pressure is reached

indicating adequate nitrogen has entered the chamber


Standard Vacuum Cycle• Step 6: Second Pump Out

– turn off nitrogen– pump out nitrogen and residual reactive gasses to base pressure– continue to next step: when base pressure is reached for a

minimum length of time– Note: steps 5 and 6 may be repeated

• Step 7: Vent– close all valves between chamber and pump– flow nitrogen directly into chamber– pressure increases from base pressure to atmospheric (or

loadlock) pressure– continue to next step: when atmospheric pressure is reached

• Step 8: Open Chamber and Unload Wafer


Abort Conditions• Abort = Failure to meet all conditions required to

continue processing.– Pressure not in range, Gas flow not in range, Electrical or

mechanical malfunction, Timeout, Interlock tripped.– Accompanied by an audible and visible alarm.

• Abort Priority:– 1. System immediately goes to safest possible state.– 2. Possible recovery of product material.

• Safest Possible State:– Shut off all gas flows.– Shut off RF power (if applicable).– Pump(s) on, all isolation and throttle valve(s) wide open.

Documents

4 Vacuum Pump