Plasma-Surface Interactions, Erosion, and Impact on Plasma Distribution Functions

Transcription

1 Plasma-Surface Interactions, Erosion, and Impact on Plasma Distribution Functions N. Fox-Lyon, G.S. Oehrlein Department of Material Science and Engineering, Department of Physics, and Institute for Research in Electronics and Applied Physics University of Maryland, College Park D.B. Graves Department of Chemical Engineering, University of California, Berkeley S.Y. Moon, V.M. Donnelly, D.J. Economou Department of Chemical and Biomolecular Engineering, University of Houston

2 Importance of plasma-wall interactions Important influence of surface on plasma properties plasma flow Low temperature plasma: Plasma process endpoint detection Large optical/electrical effects observed with different materials exposed Hot plasma: Fusion plasma first wall materials Material transport in plasma causes unintentional electrical/thermal effects 2

3 Measuring surface and plasma in real-time Ar/H 2 /D 2 Questions: What is the effect of reactive gas (H 2 /D 2 /CH 4 ) impurities on inert (Ar) plasma What is the effect of surface derived carbon on the plasma properties and how does this compare to CH 4? What dependence does the surface erosion/modification and plasma properties have on impurity isotope mass?

4 Ar plasma is sensitive to impurities University of Houston 4

5 Ar plasma is sensitive to impurities pure Ar electron density H 2 /D 2 effect CH 4 effect CH x +H x University of Houston 5

6 Additional Questions What is the effect of different reactive gases/isotopes on inert plasmas? What effect do isotopes have on changes to surface properties and erosion rates of graphitic films? How does surface derived C/CH x /CD x compare to CH 4 addition to inert plasmas? 6

7 Outline Introduction Reactive gas addition to inert plasma Surface erosion of plasma facing materials Surface derived species feedback on plasma Summary and conclusions 7

8 Experimental setup and parameters Inductively coupled plasma chamber mtorr W source power Deposition of soft/hard a-c:h films Erosion of a-c:h films: Ar/H 2 /D 2 plasmas Real-time characterization: In-situ single wavelength ellipsometry Langmuir probe HIDEN EQP ion energy analyzer Optical Emission spectroscopy HIDEN EQP ion sampling Langmuir probe Ellipsometer Laser Ellipsometer detector 8

9 Reactive/inert plasma characterization Small differences in H 2 /D 2 addition to Ar plasma Large differences in surface feedback effect Why is this? 9

10 Erosion rate differences by plasma impurity Dissociation of reactive atoms saturates surfaces in both H 2 and D 2 situations Main difference is ion composition and mass Ar/D 2 plasma has faster transition to ArD + 2 and D + 3 from Ar + than for H2 situation at 10% addition Average mass of ions significantly higher in Ar/D 2 than in Ar/H 2 10

11 Ar/H 2 ion-graphitic surface interactions Erosion differences in pure plasmas reflect ion mass differences Predominantly triatomic D + 3 and H + 3 ions Modification/rate measured using ellipsometry Peak erosion rate ~10% at these conditions 11

12 D 2 vs. H 2 ion-graphitic surface interactions Erosion differences in pure plasmas reflect ion mass differences Predominantly triatomic D + 3 and H + 3 ions Modification/rate measured using ellipsometry Strong isotope dependence in chemical sputtering 12

13 H 2 or D 2 in Ar: Mass of ions 13

14 H 2 or D 2 in Ar: Mass of ions 14

15 H 2 or D 2 in Ar: Mass of ions Ar/D 2 has higher % of reactive atoms and higher mass 15

16 Surface processes on graphitic films Ar ions Ar ions C and H/D atoms and molecules low erosion yields due to purely physical sputtering Ar plasma a-c:h H deficient layer a-c:h Initial Surface Ar/H/D ions Steady State H/D ions C and H/D atoms and molecules high erosion yields due to chemical sputtering Ar/H 2 Ar/D 2 plasmas a-c:h Initial Surface H/D-rich layer a-c:h Steady State Inert plasmas cause densification of surface (physical sputtering) Reactive plasmas cause softening of surface (chemical sputtering) Isotope dependence seen in chemical sputtering of graphitic materials 16

17 Surface feedback on plasma pure Ar electron density H 2 /D 2 effect CH 4 effect CH x +H x Increased C atom yield from gas composition and substrate bias C on Ar/H 2 /D 2 plasma from surface C derived from surface was shown to behave similarly to CH 4 injected into plasma lower electron density Isotopic dependence on ER reflected in change in plasma properties 17

18 Comparison of surface to CH 4 flow into plasma CH 4 and surface derived C behave similarly Ar/D 2 erosion of graphitic film caused loss of density close to 1% addition of CH 4 to plasma CH 4 addition to plasma also contributes H atoms, which causes greater loss in density 18

19 Effect shown as changes in electron density Ar/X 2 19

20 Effect shown as changes in electron density H 2 on graphite Ar/X 2 20

21 Effect shown as changes in electron density D 2 on graphite Ar/X 2 21

22 Effect shown as changes in electron density D 2 on graphite 1% CH 4 in H 2 Ar/X 2 CH 4 in Ar 22

23 Effect shown as changes in electron density D 2 on graphite 5% CH 4 in H 2 Ar/X 2 CH 4 in Ar 23

24 Summary Reactive impurity addition to inert plasma Loss of electron density with addition of CH 4, H 2 and D 2 to Ar First experimental measurement of Ar/H 2 and Ar/D 2 complete ion compositions ArH + and ArD + are the predominant ions, even at high impurity concentrations flowed into plasma Ar/H 2 /D 2 plasmas on graphite Isopic and reactive dependence of surface modification of graphitic films in plasma In pure reactive plasmas (H 2 and D 2 ) strong erosion rate differences are be observed and attributed to large differences in reactive ion masses (D 3 + vs. H 3+ ) C addition to Ar/H 2 /D 2 plasmas CH 4 added to inert plasmas causes a decrease in plasma density Surface derived carbon acts like CH 4 flow into plasma Isotope effects from surfaces directly effect plasma properties 24

25 Acknowledgements We thank D. Metzler, E. Bartis and K. Mecadon for helpful discussions and contributions to this project. We gratefully acknowledge support of this work by the US Department of Energy Office of Fusion Energy Sciences (DE- SC ) 25 16