pvd (physical vapour deposition) - tu wienultra high vacuum (uhv) 10-5!10-10 5•105!5•1010 0.027...
TRANSCRIPT
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Repetition: Physical Deposition Processes
PVD (Physical Vapour Deposition)
Evaporation
SputteringDiode-systemTriode-systemMagnetron-system ("balanced/unbalanced")Ion beam-system
IonplatingDC-glow-dischargeRF-glow-dischargeMagnetron- dischargeArc-dischargeIon-Cluster-beam
Reactive versions of the above processes
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Repetition: Rates and Cooling Rates PVD
These extremely high cooling rates show, that PVD processes (apart from being a direct transition from vapor !!!! solid state) always have to be considered as non equilibrium processes.
10
377
29
3.77·10
2.9·10 2.9·10
3.77·10
Sputtering
Evaporation
Effusion cell,Ion Gun
High ratemagnetron
High rateelectrongun
E =1eV
E =0.1eV
R[nm/s]
R [K/s]
R [K/s]
10 1010 101-2
T
T
3
particle
particle
4
3
7
6
-1
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Vacuum Physics
Mean free path: the way a gas particle (or a film particle) can travel without a collision with another particle.
Impingement rate: number of particles which hit a surface per second and unit area at constant pressure.
Coverage time: time needed for the formation of a full monolayer.
Central Termini:
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Mean Free Path ICollision of two particles, 1 and 2 with radius r = R/2:
2r
R
σ1
If both particles are considered as points then a collision happens, if particle 1 is located within a disk of the area σσσσ = ππππ·R2. σσσσ is called the collision cross section.
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Mean Free Path IIAparticle moves straight for a distance l through a gas. Within a cylinder of the volume V = l·σσσσ it will collide with each particle located in V.
l
σ
The cylinder contains N = n · V particles. For straight movement this is the collision number.
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Mean Free Path IIIOne collision happens if N = 1. This yields the mean free path λλλλ as:
1nVn1N =σ⋅λ⋅=⋅⇒≡
22 rn41
Rn1
n1
⋅⋅π⋅=
⋅⋅π=
σ⋅=λ
Macroscopic information: Particle density n,from the ideal gas equation.Microscoic information: Collision cross section σ,σ,σ,σ,contains energy dependent atom/molecule radii or the general interaction cross sections of the colliding particles.
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Mean Free Path IVState of movement of the background gas:
2rn41
⋅⋅π⋅=λ
Energeticcoating particle: relative movement may be neglected
Gas particle:relative movementmay not be neglected
2rn421
⋅⋅π⋅⋅=λ
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Mean Free Path - Example
Tkpn
VNTkNVp
rn41
BB2 ⋅
==⇒⋅⋅=⋅⋅⋅π⋅
=λ
p = 0.1 Pa kB=1,38·10-23J/KT = 300Kr = 1.5·10-10m
cm6.14]m[105.1]mJ[1.04
]K[300]K/J[1038.1rp4
Tk
2103
23
2B
=
=⋅⋅⋅⋅π⋅⋅⋅=
=⋅⋅π⋅
⋅=λ
−−
−
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Mean Free Path - Rough Estimate
λλλλp=5mmPap = 1 Pa !!!!λλλλ = 5 mm p = 10-4 Pa !!!!λλλλ = 50 m
10-5 10-4 10-3 10-2 10-1 100 101 1021E-4
1E-3
0,01
0,1
1
10
100
CO2; Ar; N
2
He
H2O
Mitt
lere
Fre
ie W
eglä
nge
[m]
Druck [Pa]
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Mean Free Path: Scale Consideration
λλλλp=5mmPaCERN � LHC:U = 2·4.3 ·ππππ= 27 km
mbar108.1Pa108.1107.2
5]mm[
5]Pa[p
]Pa[p5]mm[
97
7
−− ⋅=⋅=
=⋅
=λ
=
=λ
Within LHC a pressure of approx. 10-9 mbar has to be maintained, to exclude interparticle collisions.
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Gas Phase Transport
λ−⋅= xexp)0(N)x(N
A significant number of collisions happens before the mean free path is reached.Only approx. 37% of the particles reach λλλλ without a collision.The mean free path is only a statistical measure.
This means:
Clausius' law of distance:
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Gas Phase Transport - Statistics
λ=
λ−
λ−⋅
=
∫
∫∞
∞
0
0
xexp
xexpxd
d1 d2
dn
Consider large ensemble of single situations:
Calculate the expectation value of free throw distances:
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Areal Impingement Rate IInitial situation: Gas molecules hit surface
Wanted: number of gas molecules, which hit the unit surface within 1 second.
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Areal Impingement Rate IIApproach: cylinder with unit top areas, heigth u.
Only particles with a velocity component u in the direction of e1, which trespass the top cylinder surface reach the surface within unit time.
Differential areal impingement rate:
u
e
1
1
{ { du)u(n1udzdensityparticle
volumecylinder
u ⋅Φ⋅⋅⋅=−−
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Areal Impingement Rate III
Differential areal impingement rate:
{ { du)u(n1udzdensityparticle
volumecylinder
u ⋅Φ⋅⋅⋅=−−
Total arealimpingement rate: ∫∫
∞∞
⋅Φ⋅⋅==00
u du)u(undzZ
Maxwell-distribution of one velocity-component:
Tk2um
B
B
2
eTkm2
m)u( ⋅⋅⋅−
⋅⋅⋅⋅π⋅
=Φ
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Areal impingement rate IVCalculation of the total areal impingement rate:
Tk2m
mp
Tkpn
VN
mTk
Tk2mndueu
Tk2mn
du)u(undzZ
BB
B
B
mTk
0
Tk2um
B
00u
B
B
2
⋅⋅π⋅⋅=
⋅===
=⋅⋅⋅π⋅
⋅=⋅⋅⋅⋅π⋅
⋅=
=⋅Φ⋅⋅==
⋅
∞⋅⋅
⋅−
∞∞
∫
∫∫
4434421
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Areal Impingement Rate V
Z = Z(p,T,m)=
Tk2m
mp
B ⋅⋅π⋅⋅=
Z = 2.7·1017 s-1cm-2
etwa 270 ML/s
m= 5.3·10-26 kg (O2)kB=1,38.10-23J/KT = 300K
p = 0.1 Pa
Z = 2.7·1014 s-1cm-2
etwa 0.2 ML/s
p = 10-4 Pa
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Areal Impingement Rate - Graphic
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Types of Vacuum
Name Pressure [Pa] Mean free Coverage timepath [mm] O2, 300K [ML/s]
Rough vacuum Atm!!!!1 5·10-5!!!!5 2.7·105 !!!! 2700Fine vacuum 1 !!!!0.1 5 !!!!50 2700 !!!! 270High vacuum (HV) 0.1 !!!!10-5 50!!!!5·105 270 !!!! 0.027Ultra high vacuum (UHV) 10-5 !!!!10-10 5·105!!!!5·1010 0.027 !!!! 2.7·10-7
Extreme UHV (XHV) <10-10 5·1010!!!! 2.7·10-7 !!!!
5·105 mm ≡ 500 m 5·1010 mm ≡ 50 000 km (!)
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Types of Pumps
Gas transporting:+ Rotary pump Rough vacuum/fine vacuum+ Diffusion pump High vacuum+ Turbomolecular pump High vacuum
Gas adsorbing:+ Cold traps Fine vacuum+ Cryo pumps High vacuum/UHV+ Sublimation pumps UHV+ Getter pumps UHV
reactive gases+ Ione getter pumps UHV
inert molecules (activation)
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Flow Types
Laminar/turbulent: Rough vacuum/fine vacuum
Flow through a pipe, diameter d:
Molecular: High vacuum, UHV
d<<λ
d>>λ
Particle collisionsprobable, global flow
Wall collisionsprobable,no flow
dλ
dλ
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Flow Types and Pumping Systems
Efficient in the laminar region:+ Gas transporting pumps:
Rotary pumpEjector pump
+ Rotary pumps, except Turbomolecular pumps
Efficient in the molecular region :+ Gas transporting pumps:
Diffusion pumpTurbomolecular pump
+ Gas adsorbing pumps
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Design Criteria for Vacuum Systems
Mean free path λλλλ:+ Choice of pump type+ Pump velocity+ Dimension of pipe diameters
Areal impingement rate Z:+ Coverage times (e. g. surface analytics)+ Impurity content in coatings (ratio
of impingement rate of the coating particles and of the background gas particles)
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Impurities
High sticking coefficient αααα ≈ 1 (ZDes ≈ 0):Reactive gases:
O2H20Complex carbohydrates (pump oil)
Low sticking coefficient αααα << 1 (ZDes ≈ Z):Inert gases:
Noble gasesN2CH4Carbohydrates without reactive groups
Sticking coefficient αααα: Z
Z1 Des−=α Z ... Ipingement rateZDes... Desorption rate
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Impurities: Example
Coating material: Al, m = 4.5·10-26 kgRate Al: 10 nm/s = 3 ·1019 At/(m2s-1)Impurity: O2, m = 5.3 ·10-26 kgSticking coefficient αααα: approx. 1 für Al und O2Temperture: 300KWanted: Background gas pressure, at which 1% Oxygen is incorporated unto the coating
Pa1011.1m
Tk2m10310p
Tk2m
mp
103110
ZZ
5
O
BO
192
B
O
O19
2
Al
O
2
2
2
2
2
−−
−
⋅=⋅⋅π⋅⋅⋅⋅⋅=
⋅⋅π⋅⋅⋅
⋅==
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Design Criteria: Summary
Mean free path λλλλ:Influences gas dynamics. Even at rather high pressures (10-2 Pa) the mean free path reaches the dimensions of average deposition chambers .
Areal impingement rate Z:Crucial for coating purity. The pressure of the background gas has to be at least in the medium high vacuum to guarantee sufficient film purity.