plasma flow and plasma expansion around 3d objects in metal

Transcrição

PLASMA FLOW AND PLASMA EXPANSION
AROUND 3D OBJECTS IN METAL PLASMA
IMMERSION ION IMPLANTATION
Darina Manova & Stephan Mändl
Leibniz-Institut
für Oberflächenmodifizierung
1
Motivation
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Motivation
Visualisation of Water Flow from Dynamic Sand Dunes
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Motivation
Plasma Flow around Objects &
Interaction with Expanding Plasma Sheath
How to Measure?
Analytic methods
SIMS – very sensitive method,
absolute calibration is necessary
RBS –
absolute calibration, low
sensitivity
Spectroscopic Ellipsometry –
precise method for simple systems:
interference fringes from transparent
layer, extinction from adsorbing layer
Colour – fast and global measurement,
but very simplistic interpretation
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Table of Contents
Motivation
Generation of Supersonic (Metal) Plasma Flow
Experimental Set-up
Sampling of plasma flow at surfaces
Results
Influence of background pressure
Influence of high voltage
Conclusions
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MePIII: Vacuum Arc
Vacuum Arc
Self sustaining, high current,
low voltage gas discharge
Local heating (cathode
spots) + thermal emission of
electrons + ejection of metal
atoms
Plasma density near cathode
~ 1020 – 1024 cm-3
Properties of arc plasma
Small voltage drop between cathode and anode
Ion flux parallel to electron flux from the cathode
Fully ionised plasma, charge states 1+ – 3+
Initial kinetic energy of ions 10 – 100 eV
Supersonic flow
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HD vs. MHD
A. Anders, Surf. Coat. Technol., 136 (2001)
T. Arnold, Ph.D. thesis
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Diffusionsprozesse
Hydrodynamics
Re ≡
ρuL
η
transition from laminar to turbulent flow: Reynolds number
Magnetohydrodynamics
τ R µ 0 L2 / η µ 0 v A L
S≡
=
=
τA
L / vA
η
time scale for diffusion vs. convection: Lundquist number
for vacuum arc
S≈1
but resistive MHD dominant in plasma sheath regions
N 1/ 3 << λc << LH
mean free path similar to system dimensions in our case
⇒ transition between single particle and collective motion picture
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MePIII&D and MHD
Deposition vs. Implantation
small
& large
sheath
• Addition of high voltage pulses
change sampling volume of
plasma flux
• Pulse length and pulse voltage as
free parameters
• Modification of Child-Langmuir
law with non-stationary initial
plasma
Leibniz-Institut
P.V. Akimov et al., Physics of Plasmas 8 (2001) 3788
9
Experimental Set-up
30 mm
60 mm
“Tail”
Cathode
Front side
top
Back side
Gas inlet
Trigger
•
Vacuum arc with simple shield as filter
RF plasma source
•
Cathode materials: Al and Ti
39 cm
•
Ar flow: 0, 15, 35 sccm
•
Background pressure: 10-2, 0.9, 1.8 µbar
•
High voltage Pulses (f = 3 kHz): 0 to 10 kV
•
Pulse length: 2.5 to 50 µs
•
Substrates: Si and SiO2/Si
10 cm
Cathode
Pumping system
Sample
10 cm
Filter
(shield)
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Pulse generator
10
MHD Flow
800
800
600
600
4
2
-5
Al: 10 mbar
0 kV
5 kV
10 kV
5
10
15
20
25
Distance from Edge (mm)
400
200
0
0
Al_m
Al_e
Ti_m
Ti_e
30
Thickness (nm)
Thickness (nm)
18
-2
Area Density (10 cm )
6
1.8 µbar
Al_0kV
Al_10kV
Ti_0kV
Ti_10kV
400
200
0
0
5
-2
10 µbar
0.9 µbar
1.8 µbar
10
15
20
25
Distance from center (mm)
Front Side
• Minor pressure dependence as primary shield far away from substrate
• Thinner layer near edge caused by off-normal incidence and
correspondingly higher sputter rate
• Mapping of sheath width for different materials complicated by different
self-sputter yields and charge state distribution
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Back Side
top
inside
outside
• Schematic set-up of
coupons on the back
side of the sample
holder
bottom
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MHD Flow
Background pressure
10-2 µbar
Al
0.9 µbar
1.8 µbar
10-2 µbar
Ti
0.9 µbar
1.8 µbar
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Back Side
0
5
10
15
20
25
30
0
50%
75%
10
15
20
25
30
80
140
9,5
120
8
60
40
6
8,5
40
8,0
Ti (µbar) 25%
-2
10
0.9
1.8
7,5
20
50%
75%
20
7,0
0
0
5
10
15
20
25
30
0
0
5
10
Position (mm)
15
20
25
30
Position (mm)
• Colour can be used as a substitute for layer thickness
• Reasonable agreement with layer thickness for Ti for thin films as long
as film is still partially transparent
• Optical properties of Al (i.e. n + k) apparently dependent of thickness
• Gradient of film thickness directly comparable to mean free path
(conversion factor still to be determined)
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Thickness (nm)
80
60
9,0
Color Index
100
Thickness (nm)
Al µbar 25%
-2
10
0.9
1.8
10
Color Index
5
10,0
12
Total Current
80
60
40
0.9 µbar
A)
10-2 µbar
Total Current (m
Ti
80
1.8 µbar
80
60
60
40
40
10
lse 15 2
Le 0 25
ng
3
th 0 4
(µ 0 5
s)
0
20
7.
1 .5 2
.0 2.
5
Volta 3.0 3.5 4.
0
g e (k
V) 4.5 5.0
2.
5
0
5
0
100
Al
10-2
µbar
0.9 µbar
100
0
5
20
Pu
20
100
1.8 µbar
80
80
80
60
60
40
40
20
20
0
0
60
40
20
0
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Back Side I: small pulse length
Ti, 10 kV, 0.01 µbar
225
45
200
40
175
35
150
30
125
25
100
20
75
15
50
10
25
5
0
0
0
5
10
15
20
25
30
Ti Thickness (nm)
SiO2 Thickness (nm)
Ti, 10 kV, 1.8 µbar
Ti, 3,5 kV, 0.01 µbar
• Deposition concentrated at high pressures on tail region
• Only sputter
removal of oxide
from back side
Position (mm)
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Back Side: SE
180
180
150
150
120
120
90
Exp
60
Sim
Psi
Delta
30
ψ, ∆(degree)
Winkel (Grad)
Exp
Sim1
Sim2
Sim3
Psi
Delta
0.1 nm Ti
5.1 nm Interface
141.3 nm SiO2
Si substrate
90
1.6 nm 0.0 nm
0.0 nm 0.0 nm
152.8 nm 153.5 nm
60
30
196.4 nm SiO2
Si substrate
0
400 500 600 700 800
0
Wellenlänge (nm)
400
500
600
700
800
Wavelength (nm)
• Surfaces subjected to sputtering by ion bombardment need modelling
with a graded interface (i.e. implanted layer) for reasonable results
(10 kV, 10-2 µbar series)
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Back Side II: large pulse length
1.5 kV, 50 µs
2.5 kV, 50 µs
3.5 kV, 50 µs
3.5 kV, 15 µs
• Peculiar behaviour at long pulse lengths
and intermediate voltages can be traces
to highly localized increase in ion flux
• Initial oxide of 200 nm is completely
removed at 3.5 kV and 50 µs
5 kV, 50 µs
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• Maximum depth could reach 500 nm
18
Discussion & Conclusions
• (Plasma) flow around obstacles is still a very modern problem
• Colour visualisations can be a very helpful tool
• Congruence of different analytic results from the same samples is not
always achieved as each method may measure something different
• Metal plasmas originating from a cathodic arc can be guided by
simple shields
• The fraction of lost flow depends on pressure and voltage (50 – 75%)
• MePIIID will lead to highly inhomogeneous deposition and
implantation distributions; especially around low symmetry objects
depending on pressure, voltage and pulse length
• Combinatorial materials science could be based on this approach to
vary energy flux and deposition rate independently from each other
• Prediction of actual distributions requires much more work
• Sheath expansion in 3D geometry is a very complex function, in
contrast to 1D geometries where a monotonous increase with time
and voltage is observed.
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Comparison to PIII Geometries
30 mm
2.9 ×1017cm-2
60 mm
30 mm
2.7 ×
1017
2.2 ×
cm-2
O
O
B
O
O
O
90 mm
1017
cm-2
100 mm
3D samples
Flat samples
influenced by
substrate holder
Steel, 1.4301, PIII, 10 kV,
front side, back side before and after
corrosion test
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Summary & Conclusions
Interplay of plasma flow, plasma sheath and geometry can
create very strange and highly localized effects
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Danksagung
Johanna Lutz
Susann Heinrich
Sabine Schirmer
Katharina Scholze
David Haldan
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plasma flow and plasma expansion around 3d objects in metal

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