section 3 vacuum and basic science

8/2/2019 Section 3 Vacuum and Basic Science

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University of Virginia, Dept. of Materials Science and Engineering1

Vacuum Science


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o Vacuum is obtained in a portion of space where matter and

radiation are absent

o Vacuum as obtained in laboratory chambers is a space withreduced pressure w/respect to the ambient

o Pressure is the force per unit area acting on a surface in adirection perpendicular to that surface.

o Mathematically:

P= F/Awhere:

Pis the pressureFis the normal forceA is the area.

What is Vacuum?


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Vacuum Science

1) Why are we studying vacuum science?

A: Reduce contamination by reducing the numerical density ofspecies

Kinetic energy of the species is maintained unaltered byreducing the probability of collisions

2) What is a vacuum system?

A combination of pumps, valves, and pipes, which create aregion of low pressure. It can be anything from a simplemechanical pump to complex ultra high vacuum systems.


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Pressure Units

o 1bar = 750.06 Torro 1 mTorr = 0.133 Pao In vacuum technology both mbar and Torr are used

Unit Symbol Pascals

Pa Pa 105 Pa

Millibar mbar 100 Pa

Torr Torr 133.32 Pa


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Three levels of vacuum normally recognized

Low vacuum 760 to 25 Torr = 100 to 3.3 kPa

Medium vacuum 25 to 110-4 Torr = 3.3 kPa to 13 mPa

High vacuum 110-4 to 110-8 Torr = 13 mPa to 1.3 Pa

Ultrahigh vacuum 110-9 Torr and less = 130 nPa and less

Each level is suitable for specific applications and obtained by special

pumping systems

Each pumping system rely on a different physical principle to produce

the vacuum and is working in a specific pressure range.

Working Conditions Another False Statement


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Collision Free ConditionsP < 10-4 Torr Maintenance of a Clean Surface P < 10-9 Torr

vacuum Pressure

(Torr)

Gas Density

(molecules m-3

)

Mean Free

Path (m)

Time / ML

(s)Atmospheric 760 2 x 1025 7 x 10-8 10-9

Low 1 3 x 1022 5 x 10-5 10-6

Medium 10-3 3 x 1019 5 x 10-2 10-3

High 10-6 3 x 1016 50 1

Ultra High 10-10 3 x 1012 5 x 105 104

Summary I


7/71University of Virginia, Dept. of Materials Science and Engineering7

Kinetic Picture of an Ideal Gas

Assumptions for this treatment of Gases:

A volume of gas contains molecules

Adjacent molecules are separated by distances that are largerelative to the individual diameters

Molecules are in a constant state of motion

Collisions are elastic



Gas Properties

Atmospheric Pressure at Room Temperature

Ultra High Vacuum at Room Temperature (10-9 Torr)

~2.5x1025 molecules/m3 (large number!)

~2.5x1013 molecules/m3, 2.5x107 molecules/cm3



Maxwell-Boltzmann Distribution

1) Average particle velocity

2) Peak Velocity (dn/dV= 0)

3) Root Mean Square Velocity (RMS)

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0 200 400 600 800 1000 1200

Velocity (m/s)

dn/dV

dN/dV

peak

averageRMS

2

1

8

m

KTavg

2

1

2

m

kTvp

2

1

3

m

kTvrms

particleofmassm

eTemperaturTConstantsBoltzman'K

velocity

:

where

Maxwell-Boltzmann Statistics

avg = 1.128 p and rms = 1.225vp

Used for particle flow

2

1

8

M

RTavg

2

1

2

M

RTvp

2

1

3

M

RTvrms



Pressure ( particle density), mean free pathRequirement for experiment in vacuum: Path length between surface anddetector might be 1 m, the pressure must be less than about 10-7 atm(7.67x10-5 Torr).

The Mean Free Path

Mean free path (), what does it mean?

densityparticlegasn

diametermolecular

:

2

1

221

d

where

nd

)(

105)(

3

TorrP

xcmmfp

(for air at room temperature)

-average distance a particle travels before it collides with another particle:



The measure of the frequency with which molecules impinge or collide

with a surface. The # of molecules that strike an element surface,perpendicular to the coordinate direction, per unit time and area is:

Collisions with Surfaces Particle Flux

Area A

n

Hertz-Knudsen eqn.

M

RTndnxx

20

AA N

nRT

N

nMP

3

2

remember:

scmmolesMRT

P

NA

2/2

scmmoleculesMT

P

x //10513.3

222

A useful variation is:

P in Torr



Monolayer Formation Times

Assuming a typical interatomic distance for a solid surface of 3.1:

2

10101.3

1#

mx

atomssurface

Requirement for experiment in vacuum: A clean surface quicklybecomes contaminated through molecular collisions, p must be less than about 10-12 atm (10-9 Torr).

10-10

to 10-11

Torr (UHV-ultra high vacuum) is the lowest pressureroutinely available in a vacuum chamber.

At 300K and 1 atm, if every N molecule that strikes the surfaceremains absorbed, a complete monolayer is formed in about t = 3 ns.If p = 10-3 Torr (1.3 x 10-6 atm), t = 3x10-3 secIf p = 10-6 Torr (1.3 x 10-9 atm), t = 3 sec

If p = 10-9 Torr (1.3 x 10-12 atm), t = 3000 sec or 50 min

= 1 x 1019 m-2

= 1 x 1015 cm-2

P

MT

x

atoms

S

tstick

c 22

10513.3

#1(sec)



Summary II



Low Pressure Properties of AirSummary III

OHanlon



Pumping Speed

where:C = Conductance, Units = m3/s: the ability of an object to transport gasbetween two pressures regimes.

Q=throughput, Units = l/s, Pa-m3/s: quantity of gas (the volume of gas at aknown pressure) that passes a plane in a known time.

)( 12 PPQC

Q = P(dV/dt) where: P = pressure and dV/dt = volumetric flow rateMass Flow - Units = kg/s: The quantity of a substance (kg) that passes a planein a known time.

Molecular Flow - Units = N/s: The quantity of a substance (number of

molecules N for example) that passes a plane in a known time.


16/71University of Virginia, Dept. of Materials Science and Engineering

16

Series

21

111

CCCT

C1

C2CT = C1 + C2

Parallel

C2C1

Pump Down and Conductance



17

Reduction ofpumpingspeeddepends ontube diameter

and length.

bPP

QS

C

S

SS

P

Pb

1

In General:

where SP is the intrinsic speed atthe pump inlet (SP=Q/PP) and S isthe effective pumping speed at the

base of the chamber. What doesthis tell us?

Pumping Speed - S



18

Pump Down Procedure

1.Start-up Turn on pumps Open foreline

valve 2.Close foreline valve 3.Open roughing

valve 4. Rough chamber

~100mTorr

5.Close roughingvalve 6.Open foreline valve 7.Open high-vac

valve Foreline Valve

high-vacuum

pump

chamber

mechanicalpump

high-vacuumValve

roughing valve

N2vent valve



19

Venting Procedure

1.Close high-vacvalve

2.Open vent valve Why N2 or Ar for

venting chamber??

Foreline Valve

high-vacuumpump

chamber

mechanicalpump

high-vacuumValve

roughing valve

N2

vent valve



20

Leaks:Real: a defect that allows room

gas into the vacuum systemVirtual, Screws, improper welds,sample jigging, etc

System Leaks:

< 10-6 TorrL/s: Very Leak Tight

~ 10-5 TorrL/s: Adequate> 10-4 TorrL/s: Needs work

Issues in Pump Down

Pump

PermeationRealleak

Virtualleak

Vaporization

Desorption

Diffusion

Back streaming



21

Real Systems

Pressure limits in vacuumsystems

1st term -- time dependence of

pressure that is due to the gasin the chamber volume (exp(-t))

2nd term -- pressure due tooutgassing (~ t-1)

3rd term -- pressure due todiffusion (~ t1/2 and later exp(-Dt))

4th term -- pressure due topermeation (constant)

eff

K

eff

D

eff

Oeff

S

Q

S

Q

S

Q

V

tSPP

exp0

101103 105 107 109 1011101310151017

10

10-1

10-310-5

10-7

10-9

10-11

10-13

103

Time (s)

Pressure(Torr)

Volume ~ exp(-t)

Outgassing ~ t-1

Diffusion ~ t-1/2

Permeation

V S t


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Mandatory

Working chamberSample holderPumping systemPressure controlTemperature controlFeed thru for thedeposition process

Optional

Residual gas analysisIn situ sample analysisAFM/STM, XPS, etc.

Vacuum Systems

Cl ifi i


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Classifications

Pumping Action Entrainment pumps

Positive displacement Rotary Vane (m) Rotary Piston (m) Roots Blower (m)

Momentum transfer or kinetic Turbomolecular (m) Diffusion (nm)

Capture pumps (entrapment) Cryosorption (nm) Ion -sputter sublimation (nm) Titanium sublimation pumps (nm)

Pressure Ranges1) 760 torr to 1x10-3 torr(essentially viscous flow -roughing pumps

2) 10 torr to 10-5 torr(transition flow range) -high throughput pumps

3) 10-5 torr to 10-12 torr

(molecular flow) HV, UHVpumps

mechanical (m) / non-mechanical (nm)

V R t P


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Compression ratio 106

Up to hundred liter/sec

Ultimate pump ressure10-2 Torrwith double stage 10-4

Oil is used as sealantpossible contamination

Main use: backing pump forturbo and diffusion pumps

Vacuum pumps: Rotary Pumps

Si l St R t V


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Gas enters throughsuction chamber (1)

Compressed byrotor (3) and vane (5)

Expelled throughdischarge valve (8) 500 to 2000 rpm Single stage pumps Speed ~ 10 to 200

m3/hour Ultimate pressures ~

1.4 Pa (~10 mTorr)

Single Stage Rotary Vane


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Two Stage Rotary Vane

500 to 2000 rpm Single stage

pumps Speed ~ 10 to

200 m3

/hour Ultimatepressures ~1.5x10 to 2 Pa(~100 mTorr)

Roots Pump (or lobe blower)


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2 lobed rotors mounted on parallel shafts and rotate inopposite directions

Not lubricated with oils: dry pump, (3000 to 3500 rpm) Pumping Speed 500 m3/hour

Ultimate pressure ~10 to 5 Torr (must be backed by a rotarypump because it can not pump at high pressures)

Roots Pump (or lobe blower)

Semiconductormanufacturersuse dry pumps

Pumping speed of single versus double stage


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Pumping speed of single versus double stagerotary vane (speed ~ 30 m3/hour)

Gas ballast introduces gas out exit port to keep gases fromcondensing (i.e. water, acetone)

Rotary vane and Rotary Piston Pump Issues


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Due to close tolerances (


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Contamination reduced bycold traps

Diffusion Pump

10-4 to 10-10 Torr

Pumping speeds, from30 L/s to 1000 L/s.

Working range 10-10down to 10-2 Torr.

Needs backingpump.

Diffusion Pump Operation


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Diffusion Pump - Operation

Diffusion pump pumping mechanism

Low vapor pressure oil is heatedto its boiling point Vapors flow up a chimney and

are ejected through a series ofnozzles (supersonic velocities)

The nozzles direct the vapor

stream downward The gas stream is directedtoward the water-cooled wallwhere it is condensed andreturned to the boiler

Gas particles that diffuse into

this region are on average givena downward momentum andeventually ejected through theoutlet

Need low vapor pressure oils Ultimate pressure ~ 10-11 Torr

Turbomolecular Pump


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Turbine rotating at 20,000 to 30,000 rpm High compression ratio for hydrocarbons(1010) and N

2(109), bad for H

2(103)

Oil back streaming negligible: its a cleanpump. Pumping speed 103l/s Ultimate pressure below 10-10 Torr

Turbomolecular Pump

Turbomolecular Pump Operation


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Pumping action by momentumtransfer

Can damage blades at highpressures (large viscousforces)

Must back turbo with amechanical pump

Pumping speed ~1000 l/s Ultimate pressure ~ 10-10 Torr

Turbomolecular Pump - Operation

blade

gas molecule

momentumxfer

turbomolecular pump blades atomic baseball bats

Cryosorption Pump


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Cryosorption Pump

Gas entrapment pump: gasmolecules condense on surfaces

cooled below 120KBare metalsMicro-porous surfacesChemically treated surfaces

Very clean vacuum between10-3 to 10-10 Torr

Ultimate pressure reached when theimpingement rate on the cooledsurfaces equals the impingement onthe chamber walls at 300K

TTPP Sult

300)(

where Ps(T) is the saturation pressure of the pumped gas (10-11 for N2 at 20K)

Cryosorption Pump Operation


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Pumping action is byadsorbing gas moleculesonto cold surfaces Gas particles impinge on

cooled surface and do notdesorb

Typically two stages Liquid N2 (~80K)

Liquid helium (~20K) Need to rough chamber tomolecular flow or prematurepump saturation can occur must periodically regenerate

(ie heat up and desorb gas)

Pumping Speed ~ 1000 l/s Ultimate Pressure ~10-13 Torr

Cryosorption Pump - Operation

Sputter - Ion Pump


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Sputter - Ion Pump

Provides clean, bakeable and vibration freeoperation at pressure ranges of 10-6 to 10-11 Torr

The pump of choice for the surface analysischamber

Sputter - Ion Pump - Operation


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Pumping action Adsorption followed by

dissociation

Gettering from freshlysputtered cathodesurface

Surface burial undersputtered cathode

material Implantation of ionized

gas High energy neutral

implantation of

reflected ions Pumping Speed ~ 500 l/s Ultimate Pressure ~ 10-10

Torr

Sputter - Ion Pump - Operation

Titanium Sublimation Pump - TSP


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Pumping action -- adsorbed gases react withtitanium surface

Periodically evaporate a titanium filament whichdeposits a fresh film of Ti on nearby walls(typically cooled to inhibit desorption)

Ultimate pressure ~ 10-11 Torr

Titanium Sublimation Pump - TSP

Summary of Vacuum Pumps


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Summary of Vacuum Pumps

P M t V M it i


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Pressure Measurement Vacuum Monitoring

Pressure Measurement


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Vacuum System Dimensions


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These are some of the considerations we face indesigning a vacuum system.

Vacuum Chamber: Connections: Provide sufficient room for The smaller the diameterthe operation of the of the tube, the lower theanalytical techniques conductance

Large diameters increases the

Vacuum System Dimensions

O-Ring Seals


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O Ring Seals

Metal Gaskets


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Metal Gaskets

Leaks


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Where:

o O-rings sealso metal gasketso electrical feed-throughso shut-off valves with through

leaks,o internal welds/brazes onutility pipes

o chamber weldso porous flanges or deep-

drawn sheet

Leaks

Solution- Acetone- Helium

Pressure gauge or massspectrometer will react if thefluid enters the chamber.

Materials Compatibility I


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For depositions processed in vacuum, compatibility of materials with lowpressure is a delicate point.

The part of the equipment in direct contact with the vacuum must notevaporate at the pressure and temperature used for processing

If some gas is used for processing, materials should not react with it, or thereaction product must fulfill the previous point

Degassing: some materials release their gas content

Materials Compatibility I

Materials Compatibility II


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Material Property Requirements

Mechanical Properties

Thermal Properties

Gas Loading

Materials Compatibility II

Outgassing


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Outgassing

Approximate Rates(All rates are for 1 hr of pumping)

Vacuum Material Rate(Torr liter/sec/cm2)

Stainless Steel 6 x 10-9

Aluminum 7 x 10-9

Mild Steel 5 x 10-6

Brass 4 x 10-6

High-Density Ceramic 3 x 10-9

Pyrex 8 x 10-9

Other RateTorr liter/sec/linear cm

Viton (unbaked) 8 x 10-7

Viton (baked) 4 x 10-8


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THE END

Gas Pressure and Molecular Velocity


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If they move towards a wall of area A, and the number density is n(=N/V), the number of molecules that strike the wall in time t is:

Gas Pressure and Molecular Velocity

For molecules traveling withvelocity {x}, the distance they cantravel in time interval t is: {x} t

nA{x}t

(1/2)nA{x}tBut halfof the molecules move towards the surface, halfaway from the surface:

FYI - VRMS derivation

When a molecule collides with a surface, the particles


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Since force is the rate of change of momentum:

Pressure is the force per unit area:Generalizing:{2}= {x2} + {y2} + {z2} = 3 {x2}, P = (1/3)nm{2}

1 atm = 1013 mbar = 760 mmHg1 atm = 760 Torr = 101,325.00 Pa = 101,325 Nm-2

2/1

3 mkTvrms

momentum changes from m to - m x (total 2mx)(m=M/NA), hence the total momentum change is:= [(# of collisions)] (mom. change per collision)

= [(1/2)nA{x}t] (2m{x})= nmA{x2}tF =nmA{x2}

P = nm{x2}

where RMS is typicallyused in the calculation

Langmuir Units


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Langmuir Units

At 10-10

Torr a surface will stay clean for about 7.3 hr.

Surface exposureto gas is measured in Langmuir (L) units ofpressure - time, e.g. Torr-s.

1 Langmuir (L) = 10-6 Torr-s, which means that gas exposurecould occur at 10-6 Torr for 1 s, at 10-7 Torr for 10 s, etc..

Since a monolayer typical forms after 7.3 hr (26,280 s) at 10-10

Torr ( or 2.63 L), 1 L corresponds to about 0.38 monolayer.

Boyles Law (1622)


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Boyle s Law (1622)

P1/V (T and N constant)

P

V

Amontons Law (1703)


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Amontons Law (1703)

PT (N and V constant)

T

P

Charles Law (1787)


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Charles Law (1787)

VT (P and N constant)

T

V

Daltons Law (1801)


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Dalton s Law (1801)

Daltons Law of Partial Pressures

Pt = n1kT + n2kT + n3kT + ... nikT

where Pt is the total pressure and ni is the number

of molecules of gas i

Pt = P1 + P2 + P3 Pi

where Pt is the total pressure and Pi is the partial

pressure of gas i

Avogadros Law (1811)


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Avogadro s Law (1811)

PN (T and V constant)

N

P

Diffusion


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Diffusion

Example: diffusion in stainless steel H2 is a common diffuser gas in stainless steel

Typically we perform a Bake while undervacuum for stainless steel chambers

D=Doexp(-Ed/kT) increase T, increase D, remove H2from stainless steels and decrease q (diffusion)

C0 t0H

H

H2

Vaporization


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p

Particle flux ():

retemperatuT

weightmolecularMpressurevaporP

:

whereVaporization

Similarly for vaporization of a solid source (or

evaporation):

M

RT

n 2

scmmoleculesMTPx

222 /10513.3

Vapor Pressure Curves


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p

Vapor Pressure Curves


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p

Gas Sources in a Vacuum


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Permeation

Realleak

Virtualleak

Vaporization

Desorption

Diffusion

Vaporization

Thermal Desorption Diffusion

Permeation

Backstreaming

Leaks

Pump

Thermal Desorption


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Thermal Desorption

Heat stimulated release ofgasesor vapors

previously adsorbed on the surface of thechamber walls Function of:

Molecular binding energy

Temperature of the surface Number of monolayers formed on the surface

Desorption



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Permeation

Realleak

Virtualleak

Vaporization

Desorption

Diffusion

Vaporization

Thermal Desorption

Diffusion Permeation

Backstreaming

Leaks

Pump

Diffusion


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Diffusion

Diffusion of gas particles

- 2 step process Diffusion of gas to the interior of chamber surface

Desorption of diffused species

Diffusion > Co

InternalSurface

P < Co

Diffusion

walltheofthicknessd

tcoefficiendiffusionD

wallsolidingasofionconcentratinitial0

C

Diffusion


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University of Virginia, Dept. of Materials Science and Engineering 68

Diffusion

Outgassing rate (q) [Pressure-volume/sec)/surfacearea] i.e. [(Torr-liters/s)/m2]

walltheofthicknessd

tcoefficiendiffusionD

wallsolidingasofionconcentratinitial

:

exp)1(21

0

22/1

0

C

where

Dt

nd

t

D

Cq

nn

on

Diffusion


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Diffusion

21

2/1

0

~

tq

t

DCq

)exp(~

2exp

22

2

0

aDtq

d

Dt

d

DCq

Short times Long times

(infinite series solution)

Log (q)

Log (time)

t1/2

exp(-t)

Crossover point

t=d2

/6D



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Permeation

Realleak

Virtualleak

Vaporization

Desorption

Diffusion

Vaporization

Thermal Desorption

Diffusion

Permeation Backstreaming

Leaks

Pump

Permeation


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Permeation

Three step process

1) Gas adsorbs onto outer wall of vacuum chamber

2) Gas diffuses through chamber wall

3) Gas desorbs from interior of chamber wall

Permeability of a wall (KP)

KP= DS where:

D = diffusion coefficient

S is the solid solubility of the gas in the chamber

material Non-Dissociative vs. Dissociative

Permeation

section 3 vacuum and basic science

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