vacuum engineering intro

29/01/2016

Created by Dr Yasir F Joya 1

Lec-2

What is Vacuum?

Any space at sub-atmospheric pressure

Vacuum is actually the absence of gas

Vacuum is not absolute, but a continuous range of conditions covering magnitude in common usage (103 to 10-12 Torr)

Vacuum technology involves moving and removing gases

Created by Dr Yasir F Joya

Why do We Use Vacuum?

Dr. Fred Strnisa

[email protected]

Produce a cleaner environment Remove contaminants that can cause unwanted reactions

Increase mean free path (MFP) Allow sputtering, evaporation and ion implantation

Control number of surface collisions Sputtering of metal layers Control rate of film growth in CVD

Lower molecular density Reduce unwanted contaminants Allow plasma Increase evaporation rate without increasing temperature

(freeze drying) Reduce heat conduction (thermal insulation)

29/01/2016


Why do We Use Vacuum?

Dr. Fred Strnisa

[email protected]

Create a force Move solids or liquids through pipes

Reduce heat flow Reduced pressure reduces collisions between molecules and

hence heat transfer decreases

Increase vaporization Fewer molecules impacting surface or knocking vaporized

molecules back to surface

Protect materials from reactive molecules Pump out reactive molecules and backfill with inert gas

Units of Pressure


Pressure Force per Unit Area

lb/in2 PSI

N/m2 Pa

Atmospheric Pressure

760 Torr

1.01325x105 Pascal

1.10325 bar

14.7 PSI

Non-Si Units: (common units) Torr, mTorr Bar, mbar 1 Pascal = 10-5 atms

1.0 Bar = 105 Pascals = 750 Torr=0.987 atm

29/01/2016


Classification of Vacuum

760

10-3

1

10-8

750

25

7.5 x 10-4

7.5 x 10-7

7.5 x 10-10

7.5 x 10-13

105

3.3 x 103

10-1

10-4

10-7

10-10

Low

Medium

High

Very High

Ultra High

Extreme

Ultra High

Rough

Medium

High

Ultra High

25

Torr PascalTorr "Traditional"

AVS

Vacuum Ranges

Interstellar Space ~10-20 Torr

Pumping Variables:

Conductance


The molecular flow of gas through an orifice of area A, separating two large chambers maintained at low pressures, P1 and P2.

Gas flow is driven by the pressure difference,

“Q” is defined as the gas throughput with units of pressure x volume/s (e.g., torr-liters/s).

“C” is the conductance and has units of liters/s.

For air at 298K, conductance of an orifice is 11.7 A (lit/sec)

)(

)(

21

21

PPCQ

PPQ

P1

A P2

A

29/01/2016


Pumping Variables:

Conductance


For air at 298K (molecular flow), what will be the

conductance of an aperture of 5 cm dia?

What will be the conductance of pipe 3 cm long

under similar conditions?

What will be the conductance under viscous flow

conditions?

What is the difference between two flow regimes?

Pumping Speed


The pumping speed, S, is defined as the volume of

gas passing the plane of the inlet port per unit time

when the pressure at the pump inlet is P.

P

QS

29/01/2016


Conductance Vs Pumping Speed


Conductance implies a component of a given

geometry across which a pressure differential exists.

Pumping speed refers to a given plane that may be

considered a pump

Lec-3

Vacuum Systems

Vacuum systems variety of vacuum pumps, tubing, valves and gauges to establish and measure the reduced pressures.

Each pump may be used as a singly or in combination in a variety of pumping-system configurations

Created by Yasir Joya

Vacuum pumps Vacuum pumps

Gas transfer pumps

Gas transfer pumps

Entrapment pumps

Entrapment pumps

29/01/2016


a) Gas Transfer Pumps

Remove gas molecules from the pumped volume and

convey them to the ambient in one or more stages of

compression

Subdivided into positive displacement and kinetic

vacuum pumps.

Rotary mechanical and Roots pumps the positive

displacement variety.

Diffusion and turbomolecular pumps examples of

the kinetic vacuum pumps.


1) Positive Displacement Pumps:

i) Rotary Mechanical Pumps


Frequently used to produce the minimum vacuum level

required to operate oil diffusion and TMPs’ to

attain far lower pressures.

The rotary pumps divided into two types by design

Rotary Vane-Type

Rotary Piston-Type Inject/suck

confine

Compress

gas ballast

Discharge

+displ. pump

cycle

29/01/2016




The rotary-piston type: the gas is drawn into pumping

space by moving piston,

Gas isolated from the inlet after one revolution, then

compressed and exhausted during the next cycle.

Often employed to evacuate large systems and to

back roots blower pumps.

https://www.youtube.com/watch?v=fEO8LEGfdgc

i) Rotary Piston Pump

Created by web

https://www.youtube.com/watch?v=fEO8LEGfdgc

29/01/2016


i) Rotary Mechanical Pumps:

Types


Rotary vane type : Consists of vanes mounted to a

rotor that rotates inside of a cavity.

https://www.youtube.com/watch?v=AFHogF-9eGA

i) Rotary Mechanical Pumps:

Comparison


Piston pumps have

higher pumping speed

than vane type.




29/01/2016




Oil is employed as a sealant as well as lubricant between

components moving within narrow clearances

Single-stage vane pumps have an ultimate pressure of

10-2 torr, while two-stage pumps can reach 10-4 torr.

Both usually have a gas ballast to inject air into the

compressed gas a little before discharge.

The injected ballast prevents condensation, allowing

vapors to escape with the exhaust, reducing corrosion

Gas ballasts raise the ultimate pressure and thus reduce

the pump capacity.

ii) Root Pumps


This pump also called a blower mainly used as a booster or enhancer for positive displacement pumps.

Two figure-eight-shaped lobes rotate in opposite directions relative to each

The clearance between the rotors, and the rotors and housing is 75-300 microns rotation without contact and friction (eliminate oil lubrication)

Very high pumping speeds can attain ultimate pressures ~10-5 torr, however, a fore pump, e.g., rotary mechanical, is required

29/01/2016


ii) Root Pumps

Taken from vacuum engineering, © 2005 by Taylor & Francis Group, LLC

ii) Root Pumps


Pushes a high volume of gas at low pressure not

useful at high pressures

Roots pumps are popular in sputtering as well as low-

pressure CVD (LPCVD) systems where large gas

volumes continuously pass through reactors (~1 torr)

The pumping speed depends on the RPM

For heat generation water cooling is encased

29/01/2016


2) Kinetic Vacuum Pumps: i) Diffusion

Pumps

From www.furende-ltd.com

The gas molecules from the vessel are trapped by a

stream of vapors and carried towards the exit.

Ultimate pressure

10-9 torr

i) Oil Diffusion Pump: Design

From text book

http://www.furende-ltd.com/




29/01/2016


i) Diffusion Pumps


Has no moving parts, unlike positive displacement

Designed to operate in the molecular flow regime

The pumping medium, i.e., the vapor is produced at

the base of the pump by boiling synthetic oil.

As the vapor carrying the gas descends, it condenses

on the cold walls of the pump casing.

i) Diffusion Pumps


The pump inlet is essentially like the orifice, a pumping speed _________ is theoretically expected for air at room temperature.

The trapped gas molecules released near the lower end and discharge through the foreline.

Desirable properties of the pumping oils Low vapor pressure, High molecular weight, Low latent heat of evaporation and Noncorrosive and nontoxic.

29/01/2016


i) Diffusion Pumps


Achieving ultimate pressure (Po) depends on the back-

streaming of oil from the pump to the vacuum vessel.

It is controlled by using a cold trap or baffle at the

inlet to condense the oil vapor.

Can pump only from about 10−3 torr needs a

backing pump

Diffusion pumps with diameters of 25–1200 mm, air

pumping speeds from 10–20,000 (lit./sec) are

available.

ii) Turbomolecular pump (TMP): design

From text book

29/01/2016


ii) Turbomolecular pump (TMP)


Oil-free pumping has spurred the development and

use of TMPs’ (especially in electronics industry)

Gas molecules impact with a rapidly whirling turbine

rotor (20,000 to 30,000 RPM)

A vertical axial-flow compressor consisting of many

rotor/stator pairs or stages mounted in series.

Gas captured by the upper stages is transferred to

the lower stages where it is successively compressed

to the level of the fore-vacuum pressure.

ii) TMP


No traps or baffles are required and TMPs can be backed by rotary pumps and effectively achieve oilless pumping.

Expensive, but are increasingly employed in all sorts of thin-film deposition and characterization equipment

Pumping speeds of 103 liters/sec and ultimate pressures (Po) <10-10 torr.

Starts pumping from about 10−2 to10-3 torr down to 10−9 torr. A backing pump is required.

29/01/2016


Lec 6-7

b) Entrapment Pumps


Condense or chemically bind molecules to surfaces

situated within the pumping chamber.

They work in a different manner to the gas transfer

pumps, which remove gas permanently.

Reversible entrapment pumps release trapped or

condensed gas back into the system upon warmup.

Entrapment

Pumps Entrapment

Pumps

Adsorption Adsorption Cryogenic Cryogenic Sputter ion Sputter ion

i) Cryogenic pumps


Generate a clean ultrahigh vacuum in the pressure

range of 10-3 to 10-10 torr.

Based on the condensation of vapor molecules on

surfaces cooled below 120K (-153°C).

Temperature-dependent van der Waals forces are

responsible for physically binding gas molecules

29/01/2016


i) Cryogenic pumps


Surfaces that condense gas molecules are;

Untreated bare metal surfaces

A precooled surface (20K) containing a layer of

precondensed gas e.g., Ar for H2 and CO2 for He

adsorption

A microporous surface of very large area within

molecular sieve materials such as activated charcoal

or zeolite

i) Cryogenic pumps: Design

Taken from web.

29/01/2016


i) Cryogenic pumps: Design

Taken from text book

i) Cryogenic pumps


Require an initial forepressure of about 10-3 torr in

order to prevent a large thermal load on the

refrigerant and the accumulation of a thick ice

condensate on the cryopanels.

Cryopumps have the highest pumping speeds. For air

at 20 K is equal to 3 liters/s for each square

centimeter of cooled surface.

Expensive, more frequently used with other

conventional pumps (e.g., turbopumps).

29/01/2016


ii) Sputter-ion pumps


Relies on sorption processes initiated by ionized gas to

achieve pumping.

An electrical discharge between stainless-steel anode

and titanium cathode arrays maintained (few kVs’)

Electrons emitted from the cathode are trapped in the

applied magnetic field (few thousand gauss) high

electron density cloud

After impact ionization of residual gas molecules, the

gas ions travel to the cathode and sputter atoms of Ti.

ii) Sputter-ion pumps: Design

Taken from web and text book

29/01/2016




Ti atoms deposit within the pump to form films that

combine with reactive gases such as nitrogen, oxygen,

and hydrogen

These gases and corresponding Ti-compounds are

then buried by fresh layers of sputtered metal

A wide variation in pumping speeds for different

gases. Hydrogen pumped more effectively than

oxygen, water, or nitrogen

Gases are permanently removed (unlike cryo pump)



Quite expensive and have a finite lifetime that varies

inversely with the operating pressure.

Commonly employed in oilless ultrahigh (10-10 torr)

vacuum deposition and surface analytical equipment.

29/01/2016


iii) Ti-Sublimation pump


Ti metal is thermally evaporated (sublimed) onto

cryogenically cooled surfaces.

Vacuum Systems Design:


How to select the pump or pumps required to achieve

the given vacuum pressure in the given time ?

and to sustain it during the given process time?

The various available pumps have their characteristic

pressure range and pumping speed optimum

performance

29/01/2016



Created by Dr Yasir Joya and text book

No one pump can produce higher vacuum.

Only mechanical, i.e., positive displacement pumps,

can pump out at atmospheric pressure

In virtually all the pump combinations, the mechanical

pump will be the output or the last pump and will

have to be used as the backing pump.



Pumps applicable for low pressures have high pumping speeds.

Because at low pressures the volume of gas to be pumped out is large (PV = Constant).

At pressure of about 10−3 torr outgassing from the vessel, and components. The pumps should be capable of taking up this load and maintaining the vacuum.

The leaks and permeation will also pose an additional load

29/01/2016




The length of piping (conductance) will depend on the

system layout but should be the shortest possible.

Conductance will lower the actual speed available at

the input and, hence, will influence the pump selection.

Issues with Vacuum

Taken from web.

Enemies of Vacuum & Cleanliness

Backstreaming

Virtual Leaks

Permeation

Real Leaks

Particulates

Elastomer Seal on

Baseplate

Metal

Vacuum

Wall

Diffusion

Permeation

Vaporization

Desorption

Vacuum

EnvironmentAmbientCondensates

Grime

Rough

Medium

High

Ultra

High

Condensation

Particulate Generation

Large Leaks

Gross Contamination

Volume & Loosely

Bound Water

Elastomer Outgassing and

Permeation

Surface Desorption

Diffusion Through Metal

Permeation Through Metal

Vaporization

Admittance of

Room Air

Backstreaming

Enemies of Vacuum & Cleanliness

Backstreaming

Virtual Leaks

Permeation

Real Leaks

Particulates

Elastomer Seal on

Baseplate

Metal

Vacuum

Wall

Diffusion

Permeation

Vaporization

Desorption

Vacuum

EnvironmentAmbientCondensates

Grime

Rough

Medium

High

Ultra

High

Condensation

Particulate Generation

Large Leaks

Gross Contamination

Volume & Loosely

Bound Water

Elastomer Outgassing and

Permeation

Surface Desorption

Diffusion Through Metal

Permeation Through Metal

Vaporization

Admittance of

Room Air

Backstreaming

29/01/2016


Rate limiting pumping processes

From text book

Most of the time involved in pumping systems to high

vacuum is spent in removing gas from surfaces.

Outgassing


All materials, when exposed to low pressure, release gases, called “outgassing”.

During manufacturing, processing and handling, materials absorb gases water vapor, hydrogen, and oxygen.

Bonded with the surface layers said to be “absorbed.”

May be dissolved within the material and evenly distributed

Clean surfaces would also have absorbed gases

29/01/2016


Outgassing: Sources


Construction of vacuum vessels, accessories metals,

polymers and rubbers (gaskets), glasses (view ports),

ceramics, etc.,

These materials have absorbed gases mainly on their

surfaces.

The pressure at which a material outgasses, and the

rate of evolution varies for each material type

Outgassing: Measurement


Outgassing rates throughput per unit area, i.e.,

torr × liters/sec × cm2 and are published.

29/01/2016


Outgassing: Remedy


The outgassing problem can be reduced by heating the vessel during the roughening stage. This is called “bake out.”

A temperature of 200–400°C is used. Due to the combination of heating and vacuum, the exposed surfaces outgas quickly

At elevated temperature the material outgasses at a higher pressure and at a faster rate.

It is limited to surface outgassing (desorption) only. Surfaces will reabsorb gases if exposed to the environment.

Outgassing and Leaks:


What is vacuum leakage?

For outgassing, its rate first increases and then

diminishes with time.

Gas leakage causes a linear rise in pressure with time

Most materials outgas in 10−2–10−4 torr pressure

range.

29/01/2016


Lec-08

Monitoring of Vacuum


Vacuum Gauges, ordinary Bourdon tube gauges

can be used from 760 to 0.5 torr,

Every gauge has its applicable pressure range.

Vacuum Gauges


Vacuum Gauges

Rough

Medium

High

Ultra

High

Thermal

Conductivity

of Residual Gas

Ionization of Residual Gas Drag Induced by

Residual Gas on

Moving Object

Force Applied

to Surface

Hot &

Cold Cathode

Ion Gauges

Residual

Gas

Analyzer

Gas

Composition

Analysis

System

Total

Pressure

Measurement

Spinning

Rotor

Gauge

Capacitance

Manometer

Ranges of Vacuum Gauges

Thermo-

couple &

Pirani

Gauges

Convection

Pirani

Atm

100

10-3

10-8

29/01/2016


Monitoring of Vacuum:


Direct Pressure Gauges

Capacitance manomater consists of a flexible

circular metal diaphragm that is welded in place

symmetrically between two fixed electrode plates

Vacuum Gauges Vacuum Gauges

Direct Gauges Direct

Gauges Indirect Gauges Indirect Gauges

1) Direct Vacuum Gauges:

Capacitance Manometer


There are two chambers

One chamber permanently sealed and evacuated to

a pressure of ~10-7 torr, while the other opens to the

vacuum chamber

The capacitance change is a direct measure of the

vacuum chamber pressure.

Operate in the range from atmospheric pressure to

about 10-5 torr

29/01/2016


2) Indirect Vacuum Gauges

Relatively high-pressure regime (~10-4 to 760 torr),

Thermocouple gauges and Pirani gauges mostly used

For Knudsen numbers, 10 > Kn > 0.01, the rate of

heat transfer through gases is linearly proportional to

pressure.

Sense the rate of heat transfer between a heated

wire and a nearby wall

i) Pirani Gauge

The filament resistance varies linearly with

temperature proportional to the gas pressure.

As the pressure decreases the number of impacts

decreases resistance increases

29/01/2016


ii) Thermocouple Gauge

http://www.rdmag.com/images/0410/FEVac_a.jpg

In thermocouple gauge, a thermocouple placed in

close thermal but not electrical contact with the

heated filament.

iii) Ionization Vacuum Gauges

From the vacuum engg. ref book

In the high and ultrahigh vacuum regimes, gas

molecules are ionized and the ion current is used

measure of the system pressure.

Hot-cathode gauges Baird-Alpert

ionization tube universally employed

Monitor pressure in the ~10-5 to 10-13

torr

29/01/2016



Adopted from vacuum engg book

Vacuum port

Anode Electron source

Cathode

Glass envelope



Thermionic emission of electrons from a heated

filament

Attracted to a positively biased grid design to

increase the collisional impact with gas molecules.

Positive gas ions generated and drawn to a fine wire

collector positioned coaxially within the grid.

The resulting ion current is proportional to the number

of gas ions proportional to the gas pressure

29/01/2016



Ionization gauges also available as Cold cathode or Penning Gauges

Tube is like a diode. A permanent magnet is placed outside the tube.

The combined electric and magnetic fields increase ionization and the resulting current.

Penning gauges are cheaper but bulky

Good for general vacuum duty

vacuum engineering intro

Engineering