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CLEANING EFFICIENCY OF CARBON FILMS BY OXYGEN PLASMAS IN THE PRESENCE OF METALLIC GETTERS

CLEANING EFFICIENCY OF CARBON FILMS BY OXYGEN PLASMAS IN THE PRESENCE OF METALLIC GETTERS. Francisco L. Tabarés, J.A. Ferreira, D. Tafalla 1 , I. Tanarro, V. Herrero 2 , C. Gómez-Aleixandre and J.M. Albella 3. 1 Laboratorio Nacional de Fusión (CIEMAT) 2 IEM. CSIC 3 ICMM. CSIC.

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CLEANING EFFICIENCY OF CARBON FILMS BY OXYGEN PLASMAS IN THE PRESENCE OF METALLIC GETTERS

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  1. 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. CSIC 3 ICMM. CSIC

  2. 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..)

  3. 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)

  4. C/B layers in He/O2 Glow Discharge

  5. Experimental set-up

  6. In situ thickness monitoring • Laser Interferometry: He/Ne orDiode laser (633,670 nm) • Cross-checked with: • Profilometry • XPS • C balance

  7. Carbon deposition Deposition in He Deposition in Ar • He/CH4 followed by Ar/CH4 (80:20) 1Pa, 100 mA • Total deposited …270 nm

  8. 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??

  9. Sequential deposition with Li Li layer? • 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!!

  10. C/Li mixing by simultaneous deposition 10% at. Li O2 CO Constant erosion rate, ≈ pure C film. 50% of cracked O2 missing!!

  11. 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 C 9% at. Mg

  12. Particle balance

  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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