max-planck-institut für plasmaphysik euratom assoziation interaction of nitrogen plasmas with...
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Max-Planck-Institutfür PlasmaphysikEURATOM Assoziation
Interaction of nitrogen plasmas with tungsten
Klaus Schmid,A. Manhard, Ch. Linsmeier, A. Wiltner, T. Schwarz-Selinger, W. Jacob,
Stephan Mändl
K. Schmid 2009
• Introduction
• Summary
Outline
• Experiment & Results
N accumulation & sputtering of W
Nitride phase formation
K. Schmid 2009
Introduction
ASDEX Upgrade (now full W) experiments show performance increase for N2 seeding in the divertor compared to Ne or Ar [1]:
Why N and W ?
[1] A. Kallenbach, et. Al. Nuclear Fusion 49 (2009) 045007
Question: Influence of nitrogen ions on tungsten PFCs ?
Sputtering of tungsten by nitrogen
Nitrogen accumulation in tungsten
Thermodynamics and thermal stability of the tungsten-nitrogen system
Perform experiments on:
+ Improved energy confinement+ Smaller ELMs
N2 gas puffing is used for edge plasma cooling in high-Z PFC fusion experiments
K. Schmid 2009
Experiment & ResultsN-accumulation and Sputtering of W by N
grid
Sample holder
UDC, bias
• ECR plasma ion source with freely expanding plasma beam
• Homogenous irradiation of samples
• Ion energy by DC biasing up to 500V
• Water cooled sample holder
• Ion flux & energy distribution measured by retarding field analyzer: 3 to 4x1018 Ny
+ (m-2 s)
• Molecular ion distribution measured by plasma monitor Dominantly N2
+ ions
PLAQ
SamplesWCu
Si
PVD tungsten layer ~ 500 nmPVD copper interlayer ~100 nm
stress reliefSilicon substrate
well-defined surface for RBS analysis
Implantation setup
K. Schmid 2009
Experiment & ResultsN-accumulation and Sputtering of W by N
N2 plasma flux and composition
0 100 200 300 400 5000
1
2
3
4
Bias -500 VBias -400 V
Bias -300 VBias -200 V
d I
on
/dE
(1
01
7 c
m-2
s-1
eV
-1)
Ekin / ion (eV)
floating
Bias -100 V
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10
10
20
30
40
50
60
70
80
90
100
flu
x fr
acti
on
(%
)
pressure (Pa)
N2+
N+
N3+
Retarding field analyzer measurements
Flux :3 - 4 x 1014 N2+ / cm2 s in main
peak
Most ions have energy corresponding to bias
Plasma monitor measurements
Plasma dominated by N2+ ions
N3+ ions occur at highest
pressures ~1Pa
N+ ions maintain constant level with pressure
Implantations were performed at 0.25 Pa 90% N2+
K. Schmid 2009
0 5000 10000 150001
10
100
1000
p1+2
6x1016 N/cm2 in W
Cou
nts
Proton energy (keV)
40uC of 3.8MeV 3He
p0
p4 p3
Experiment & ResultsN-accumulation and Sputtering of W by N
Measuring N accumulation in W by nuclear reaction
NRA with 3.8 MeV 3He
Reaction used: 14N ( 3He, p ) 16O
Sensitivity: 1015 N/cm2 0.13 cts/C
Peak integral of p1 + p2 protons is
evaluated to obtain the N areal density
Measuring W sputtering via thickness change of W layer
Rutherford backscattering (2.3 MeV 4He)
Yields tungsten layer thickness
Sensitivity: 1012 W atoms / cm2
0 200 400 6000
2000
4000
6000
8000
10000
12000
Si-Substrate
Cu-Layer
Co
un
ts
Channel
W-Layer
K. Schmid 2009
Experiment & ResultsN-accumulation and Sputtering of W by N
0.0 0.5 1.0 1.5 2.00
2
4
6
8
10
Ret
aine
d N
(10
15 c
m-2)
Implantation fluence (1019 N/cm2)
20 V 50 V 100 V
TRIDYN prediction 20 V 50 V 100 V Unimplanted
Implantation in plasma ion source PlaQ
Low energy ions
Sample temperature room temperature
Quick saturation of retained N amount
TRIDYN calculations predict similar levels
Diffusion of N in W low (similar to C in W)
N accumulation controlled by the implantation range
For our low implantation energies one expects accumulation in the range of:
2cmN1030.5cm10cm106 15
n compositio nitrideW
7322
NN cr
K. Schmid 2009
Experiment & ResultsN-accumulation and Sputtering of W by N
0 100 200 300 400 500 600
1E-3
0.01
0.1
Sp
utt
ere
d W
pe
r N
or
Ne
Acceleration voltage drop (V)
Experimental N W Static TRIM N W Dynamic TRIM N W Experimental Ne W Static TRIM Ne W
Sputter yield of W by N from an N2 plasma
Sputter yield is much lower that expected from static TRIM
Accumulation of N in W surface reduces partial sputter yield
Good agreement between dynamic TRIM and experiment (dyn. Surface evolution)
For Ne no accumulation in W surface Ne Sputter yield matches static TRIM
N accumulation in W surface shields W from erosion by N
Could partly be the reason for the good AUG performance with N-puffing
K. Schmid 2009
Experiment & ResultsNitride phase formation
Bombardment of W by 3keV N ion beam
XPS analysis to identify nitride formation
Literature data on W4f shift due to nitride formation varies strongly
We find a shift of 0.45eV for nitride peaks. (lies within the literature data range)
The intensity of nitride phase decreases with temperature
K. Schmid 2009
Experiment & ResultsNitride phase formation
W-N Phase diagram calculated by ThermoCalcTM
Based on very little available data
N2 gas phase suppressed
At ambient pressures WN instable
above ~600K
XPS measurements confirm thermodynamic
modelingWN decomposes at high temperatures and
N is lost as degassing N2.
K. Schmid 2009
600 650 700 750 8005
6
7
8
9
6.0 0.5
4.6 0.4
3.2 0.3
Ret
aine
d N
(10
16 c
m-2
)
Temperature during implantation (K)
Decrease for high temperatures
1.7 0.3
Fluence
(1018 cm-2)
Plasma immersion ion implantation at IOM Leipzig
10 kV pulses
Sample heated by pulses
Implantation fluence comparable to plasma implantation
Retained amount of N decreases with temperature above 650K
Experiment & ResultsNitride phase formation
Decay above ~650K Nicely fits predictions by thermodynamic modeling
As expected: Total accumulated amount higher than in our plasma implantation due to higher energies
K. Schmid 2009
Summary
N accumulation quickly saturates once implantation range is filled with nitride phase
Total amount of N is determined by N energy via the implantation range
N accumulation in the surface leads to a reduction in the partial W sputter yield
Advantage over noble gas seeding species
XPS measurements of nitride fraction & measurements of total N amount indicate that the nitride decomposes at elevated temperatures
This is line with thermodynamic calculations by ThermoCalcTM
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