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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

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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

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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

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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

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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

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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

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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

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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

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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)