1
Page 1
Vacuum Technology
Page 2
Gas Sources in a Vacuum
• Vaporization
• Thermal Desorption
•Diffusion
• Permeation
• Backstreaming
•Leaks
– Real
– Virtual Pump
Permeation Real
leak
Virtual
leak
Vaporization
Desorption
Diffusion
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Gas Sources in Vacuum - Thin Film Materials Processing - Lecture Slides, Slides of Material Engineering
These are the Lecture Slides of Thin Film Materials Processing which includes Vaporization, Vapor Pressure Curves, Thermal Desorption, Molecular Binding Energy, First Order Desorption, Desorption Rate, Real Surfaces, Diffusion of Gas Particles etc. Key important points are: Gas Sources in Vacuum, Vaporization, Vapor Pressure Curves, Thermal Desorption, Molecular Binding Energy, First Order Desorption, Desorption Rate, Real Surfaces, Diffusion of Gas Particles
Typology: Slides
2012/2013
1 / 11
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Page 1
Vacuum Technology
Page 2
Gas Sources in a Vacuum
• Vaporization
• Thermal Desorption
• Diffusion
• Permeation
• Backstreaming
• Leaks
– Real
– Virtual Pump
Permeation Real leak
Virtual leak
Vaporization
Desorption
Diffusion
Page 3
Vaporization
• Particle Flux (Γ) (from before)
• Similarly for Vaporization of a solid source
(or evaporation)
1 / 2
m
kT
n
π
T Temperatur e
M Molecularweight
A SurfaceArea
P VaporPressure
( /sec) 2. 6310241 / 2
Γ = × ×
where
MT
PA
molecules
Vaporization
Vacuum Technology
Page 4
Vapor Pressure Curves
Page 7
Thermal Desorption
• Heat stimulated release of gases or vapors
previously adsorbed on the surface of the
chamber walls
- Function of:
- Molecular binding energy
- Temperature of the surface
- Number of monolayers formed on the surface
Desorption
Vacuum Technology
Page 8
Thermal Desorption
• First Order Desorption
- Atoms or molecules that do not dissociate prior to desorption
1/vibrationalfrequency(~ 10 sec)
desorptionenergy
rate constant
asafunctionoftime)
desorptionrate(concentration
exp ()
- 12 0
1
0
0 1
τ
τ
D
D
E
K
dt
dCt
where
Ct NkT
E
KCt dt
dCt
moleculespendsona surface
averageresidencetimea
:
exp
(^11) exp
R
(^00)
1 0 0
=
= −
= −
τ
τ τ
τ
where
NkT
E
NkT
E K
R D
D
Integration
C concentrationat t 0
exp( ) exp( / )
0
1 0 1 0 1
= =
where
KC Kt CK t dt
dC t
τ R
Page 9
Thermal Desorption
• Temperature dependence of average
residence time (τR )
10
1000
100000
1E+
1E+
1E+
1E+
0 100 200 300 400 500 600
Temperature (K)
Average residence time (s)
H2O-metal ( MJ/kg-mol)
Vacuum Technology
Page 10
First Order Desorption
• Desorption Rate vs Time
1 0 50000 100000 150000 200000
Time (seconds)
Desorption rate (molecules/sec)
1st order BE=96MJ 1st order BE=192MJ
Page 13
First and Second Order Desorption Rate
• Desorption rate vs Time
1 0 50000 100000 150000 200000
Time (seconds)
Desorption rate (molecules/sec)
1st order BE=96MJ 1st order BE=192MJ 2nd order BE=96MJ 2nd order BE=192MJ
Vacuum Technology
Page 14
Real Surfaces
• Must consider multiple binding energies
- Particle - surface BE
- Particle - particle BE (for > 1 monolayer)
- Particle - particle BE changes as a function of the surface coverage
Surface (^) Surface Surface
Most systems: outgassing rate ∝ 1/t
Page 15
Diffusion
• Diffusion of gas particles inside chamber
wall to the interior of the chamber
– 2 step process
- Diffusion of gas to the interior of chamber surface
- Desorption of diffused species
– Diffusion << slower than desorption, therefore
diffusion is the rate limiting step
External Surface P >> Co
Internal Surface P < Co
Vacuum Technology
Page 16
Diffusion
- Outgassing rate (q) [Pressure-volume/sec)/surface area] ie
{(Torr-liters/s)/m^2 ]
d thicknessofthewall
D diffusioncoefficient
initialconcentrationofgasinsolidwall
1 2 ( 1 ) exp
0
1 / (^22)
0
=∞
=
C
where
Dt
nd
t
D
q C
n n
n o
Page 19
Permeation
• Three step process
– Gas adsorbs onto outer wall of vacuum
chamber
– Gas diffuses through chamber wall
– Gas desorbs from interior of chamber wall
• Permeability of a wall (K P)
– KP = DS’ where:
- D = diffusion coefficient
- S’ is the solid solubility of the gas in the chamber material
Vacuum Technology
Page 20
Permeation
Permeation
rate
time
Crossover point= d^2 /6D (time to reach Equilibrium for a chamber wall initially devoid of any gas)
Permeation Rate
d chamber wall thickness
P pressuredropacrosschamber wall
permeationofthewall
Kp
where
d
K P
q P^ Stainless Steel – H 2
Glass -- Helium
Page 21
Big Picture
Time (s)
Pressure
(Torr)
Volume ~ exp(-t)
Outgassing ~ t-
Diffusion ~ t-1/ Permeation