laboratorio nacional de fusión 1/13 cleaning efficiency of carbon films by oxygen plasmas in the...
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1/13
Laboratorio Nacional de Fusión
CLEANING EFFICIENCY OF CARBON FILMS BY OXYGEN PLASMAS IN THE PRESENCE OF METALLIC GETTERS
Francisco L. Tabarés, J.A. Ferreira, D. Tafalla1, I. Tanarro, V. Herrero2, C. Gómez-Aleixandre and J.M. Albella3
1 Laboratorio Nacional de Fusión (CIEMAT)
2 IEM. CSIC3 ICMM. CSIC
Laboratorio Nacional de Fusión
2/13
MOTIVATION
Tritium retention in PFM in ITER is a concern for:– Safety issues: maximum T allowed in vessel 350 g– Tritium global inventory in plant– Operation under carbon-dominated PFC scenarios
Urgent need of in-situ de-tritiation techniques– Full oxidation of co-deposits by plasma techniques suited for
cleaning areas exposed to the plasma: CO, CO2 and T2O generated/pumped.
– O and O+ species generated by GD,ECR and ICR plasmas in He/O2 mixtures
But: Unknown effect of mixed materials (Be, W..)
Laboratorio Nacional de Fusión
3/13
Previous works: Examples
Silicon: Thermo-oxidation: Higher T required for C/Si. SiO2 remains in sample.
Ref. Balden and Mayer. JNM(2000) O+ irradiation: Enhanced O retention. Lower erosion yield. Ref. A. Refke et al. JNM(1997) Tungsten: Expected to catalyze oxidation of C. Deposited on top of
a:C-H film and thermo-oxidized: Inhibition of C removal (surface effect?)
Ref. Davis et al. JNM(2002) Lithium: Etching of C/Li films by O2
+ ions: At low E(<500): lower etching rate vs C
At high E: same rate. Surface segregation of Li during oxidation. Ref. J.U. Thiele and P. Oelhafen, U. Basel Boron: Ample experience in boronized Tokamaks ( AUG, Textor, lab.
experiments…) Inhibition of etching in O2/He plasmas. Ref. C. Hopf et al. JNM(2007)
Laboratorio Nacional de Fusión
4/13
C/B layers in He/O2 Glow Discharge
Time (s)
A.U
.
a-C:H
600 700 800 900 1000 1100 1200 1300 1400 150010
-9
10-8
10-7
10-6
10-5
15442818113224
H2
He
O2
CO2
CO
Time (s)
A.U
.
a-C/B:H
0 500 1000 150010
-9
10-8
10-7
10-6
10-5
442818113224
He
H2
CO2
CO
O2
Laboratorio Nacional de Fusión
5/13
Experimental set-up
Laboratorio Nacional de Fusión
6/13
In situ thickness monitoring
Laser Interferometry: He/Ne or Diode laser (633,670 nm)
Cross-checked with: – Profilometry– XPS– C balance
Time(s)
Inte
nsity
(A
.U.)
0 500 1000 15000
2
4
6
8
10
12
1 Fringe = 2/2n ... ~ 144 nmn
C hard=2,2
Laboratorio Nacional de Fusión
7/13
Carbon deposition
Time (s)
Par
tial P
ress
ure
(A.U
.)
1400 1600 1800 2000 2200 2400 2600 2800 30000
0.5
1
1.5
2
2.5
3
3.5
4x 10
-5
42151626
Time (s)
Par
tial P
ress
ure
(A.U
.)
0 500 1000 1500 20000
0.5
1
1.5
2
2.5x 10
-5
215162640
He/CH4 followed by Ar/CH4 (80:20) 1Pa, 100 mA– Total deposited …270 nm
Deposition in He Deposition in Ar
Laboratorio Nacional de Fusión
8/13
a:C/H erosion
Good balance of O atoms: O removed = O in CO,CO2
CO/CO2=10 High erosion rate >10
nm/min Strong H2 release End point by RGA=IF Higher rate over He
produced films, but O balance?
Lower erosion in the presence of released H??
Time (s)
Par
tial P
ress
ure
(A.U
.)
0 500 1000 1500 2000 25000
0.5
1
1.5
2
2.5
3
3.5
4x 10
-6
2283244
Laser
Laboratorio Nacional de Fusión
9/13
Sequential deposition with Li
C deposition in He/CH4 +Li evaporation+Ar/CH4 deposition:270 nm (C)
He/O2 GD removal of the film: decaying RGA signals– 50% of cracked O2 missing!!
Time(s)
Pa
rtia
l Pe
ssu
re (
A.U
.)
0 1000 2000 3000 4000 5000 60000
0.2
0.4
0.6
0.8
1x 10
-6
28272625232181644
Time(s)
Inte
nsity
(A.U
.)1000 2000 3000 4000 5000 6000
0
2
4
6
8
Li layer?
Laboratorio Nacional de Fusión
10/13
C/Li mixing by simultaneous deposition
Time (s)
Par
tial p
ress
ure
(A.U
.)
0 1000 2000 3000 40000
0.5
1
1.5
2x 10
-5
282625182444161532
Time (s)
Par
tial p
ress
ure
(A.U
.)
0 1000 2000 3000 40000
0.5
1
1.5
2x 10
-5
282625182444161532
Time (s)
Par
tial p
ress
ure
(A.U
.)
0 1000 2000 3000
0.5
1
1.5
2
2.5
3
3.5
4x 10
-6
2826251844161532
O2
CO
Constant erosion rate, ≈ pure C film. 50% of cracked O2 missing!!
10% at. Li
Laboratorio Nacional de Fusión
11/13
Mg/C mixed films
Why Mg?.. BeO MgO
Hform(kJ/mol) -609 -601
Density (g/cm3) 3.01 3.58
M.P. (K) 2200 3073
Etching by He/O2 Plasma:– Constant erosion rate– O balance: Not matched– Similar rate as in pure CTime (s)
Par
tial p
ress
ure
(A.U
.)
0 1000 2000 3000 40000
0.5
1
1.5x 10
-5
282625182444161532
9% at. Mg
Laboratorio Nacional de Fusión
12/13
Particle balance
Film
RGAC C/Li layer C/Li mixed C/Mg mixed
O2 1.8e-6 1.1e-6 3.3e-6 1.2e-6
O2 1.06e-6 3.8e-7 2.7e-6 9.13e-7
CO 9.06e-7 1.5e-7 1.1e-6 6.08e-7
CO2 1.09e-7 2.9e-8 2.0e-7 8.64e-8
H2 3.5e-6 1.4e-7 7.0e-6 4.07e-6
Erosion rate
(nm/s)0.18 0.06 0.23 0.19
CO+2CO2/ O2 1 0.55 0.55 0.85
Laboratorio Nacional de Fusión
13/13
Conclusions
Erosion of a-(C+M)/H films (M = Li, Mg) by He/O2 GD monitored by laser IF and RGA:
High erosion rates achieved, but not yet optimized. R>10 nm/min, similar to deposition rates in He/CH4 plasmas, seen in pure C and mixed with Li and Mg
No major changes in product distribution by M doping (but CO/CO2 ?)
Layered deposition (Li) slows down erosion, but recovers later.
Results stress the impact of film structure on oxidation behavior
More structural characterization required (in progress)
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