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8/2/2019 Section 3 Vacuum and Basic Science
1/71
University of Virginia, Dept. of Materials Science and Engineering1
Vacuum Science
8/2/2019 Section 3 Vacuum and Basic Science
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University of Virginia, Dept. of Materials Science and Engineering2
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
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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
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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
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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
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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:
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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
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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)
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Summary II
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Low Pressure Properties of AirSummary III
OHanlon
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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.
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Series
21
111
CCCT
C1
C2CT = C1 + C2
Parallel
C2C1
Pump Down and Conductance
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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
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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
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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
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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
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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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Pressure Measurement
Pressure Measurement
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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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Gas Sources in a Vacuum
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
Gas Sources in a Vacuum
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Gas Sources in a Vacuum
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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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
Gas Sources in a Vacuum
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University of Virginia, Dept. of Materials Science and Engineering 70
Gas Sources in a Vacuum
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