Plasma-Surface Interactions and Impact on Electron Energy Distribution Function N. Fox-Lyon(a), N. Ning(b), D.B. Graves(b), V. Godyak(c) and G.S. Oehrlein (a) (a) University of Maryland, College Park (b) University of California, Berkeley (c) RF Plasma Consulting, Brookline, MA Motivation Control of plasma distribution functions (DF): Clarify issues when plasma-surface interactions at plasma boundaries change strongly, e.g. non -reactive vs. reactive discharges and/or surfaces 2 Laboratory for Plasma Processing of Materials Motivation OES MS MS Ion MS Ion Wall Ar/HH22 Plasma Probe Erosion Transport C, CHn Redeposition Si, SiHn’ CHm or SiHm’ Different Surface Carbon or Silicon Surface Modified Surface Real-time Layer ellipsometry (intensity, extent) • Ar/H 2 plasma interacting w/ a-C:H • Characterize impact of surface-generated species on f(v,r,t) • Probe, plasma sampling and OES measurements of f(v,r,t) for modified situations • Characterize surface processes using ellipsometry, and compare results with MD simulations of surface processes • Interpret data by developing overall model 3 Laboratory for Plasma Processing of Materials Wrinkling-induced Surface Roughness Formation Wrinkle wavelength and amplitude calculated using measured using damaged properties vs. AFM derived properties R. L. Bruce, et al., J. Appl. Phys. 107, 084310 (2010) 4 Helium Plasma Pre-Treatment Possible approach: Plasma pretreatment (PPT) UV plasma radiation reduces plane strain modulus E s (chain-scissioning) and densifies without stress before actual PE Helium PPT: No ion crust establishment More photons at low wavelengths (58.4 nm) 5 5 Outline Ar/H 2 plasmas Langmuir probe measurements Comparison with existing studies Ar/H 2 plasma interaction with hydrocarbons Surface effects on H 2 plasma Hydrocarbon erosion into H 2 plasma 6 Laboratory for Plasma Processing of Materials Experimental setup and parameters Inductively coupled plasma chamber • 10-30 mTorr • 300-600 W source power Deposition of Soft/Hard a-C:H films: Erosion of a-C:H films: Characterization: In-situ single wavelength ellipsometry HIDEN EQP Atomic force microscopy Ion sampling Langmuir Probe Ion Sampling system Modeling: TRIM.SP simulations MD simulations Multilayer ellipsometric modeling 7 Laboratory for Plasma Processing of Materials H2 with CH 4 Addition Langmuir probe measurements Probe at high temperature and sputtering conditions between sweeps Increase in plasma density seen with CH4 added Compare to surface derived C xHy 8 Laboratory for Plasma Processing of Materials H2 plasma density when eroding a-C:H Ellipsometrically derived equivalent flows from erosion rates of a-C:H on substrate electrode Shows increase in plasma density similar to adding methane 9 Laboratory for Plasma Processing of Materials Real-time ellipsometry hard PECVD deposition data ~46% H n = 1.6 soft soft ~30% H n= 2.1 hard Real-time ellipsometry to determine properties of a-C:H films Relationship between optical index (n), density and % H is well established (Jacob et al. ) Allows for determining level and depth of modification in real-time Surface topography, chemical composition, density Hard/soft films have very different degree/type of modification 10 Laboratory for Plasma Processing of Materials Ar on graphitic a-C:H Ar -100 V bias 10 mTorr 300 W Ar plasma causes surface densification Modification increases with ion energy 11 Laboratory for Plasma Processing of Materials Ar on Hard a-C:H – MD vs. Experimental After 3000 200 eV Ar+ impacts Experimental plasmas: Ar 10 mTorr 300 W Initial film Steady state Molecular Dynamics simulations Tersoff–Brenner style reactive empirical bond order potential Molière potential to describe Ar interaction with other species Described in detail in D. B. Graves and P. Brault, Journal of Physics D: Applied Physics 42 (19), 194011 (2009) Ar surface evolution under 100 eV impacts Loss of H due to physical sputtering Densification of a-C:H up to ~ 2nm into the film Comparable fluence to achieve steady-state 12 Laboratory for Plasma Processing of Materials H2 on graphitic a-C:H H2 -100 V bias 30 mTorr, 600 W H2 plasma causes surface hydrogenation At low energies modified layer is very thick 13 Laboratory for Plasma Processing of Materials H saturated layer formation on graphitic a-C:H Using TRIM.SP graphite parameters Hopf, Jacob 2005 D+ H+ Substrate biasing Yields calculated using ion fluence by substrate bias current + + Yield not normalized for molecular H 2 /H 3 ions Hydrogenated layer ↑ bias voltages lead to ↓ in modified layer Decreases due to increase in erosion rate (yield) and physical sputtering – inhibits H incorporation/saturation D2 plasma shows similar effect of decreasing modification with increasing bias 14 Laboratory for Plasma Processing of Materials Ar/H 2 measurements (10 mTorr, 300W) Plasma density is seen to decrease and electron temperature increases 15 Laboratory for Plasma Processing of Materials Ar/H 2 measurements (10 mTorr, 300W) Electron temperature is seen to increase with addition At lower densities, molecular interactions do not sap electron energies (as seen for measurements done at higher pressure and densities) 16 Laboratory for Plasma Processing of Materials Ar/H 2 plasmas on graphitic a-C:H Surface modification At low hydrogen contents (<5%) the surface densifies like pure Ar At 5% H 2, the surface first densifies then hydrogenates to about 2 nm Higher %H 2 begin to approach pure H 2 in terms of H saturation at the surface 17 Laboratory for Plasma Processing of Materials Ar/H 2 plasmas on graphitic a-C:H - 33%H a-C:H - - Hard to soft layer modification Hard a-C:H (30%-35% H 2) hydrogenates at low H x ion energies in H 2 plasmas At low energies, a modified layer is formed At even higher energies the film surface may deplete Soft a-C:H (47%-42% H 2) depletes surface H in H 2 plasmas Within our range of operational ion energies 35% a-C:H can be switched from soft hard surface modified layer under H 2 plasma 18 Laboratory for Plasma Processing of Materials Conclusions H2/Ar plasmas profoundly influence hard a-C:H surface properties during interaction Low levels of surface carbon can change plasma distribution functions Change in electron temperature small Increase in plasma density (impact on secondary electron emission coefficient?) Change in chemical nature of discharges may be most significant e.g. small fluxes of chemically active species, nature of VUV photons, … 19 Acknowledgements We gratefully acknowledge support of this work by the US Department of Energy Office of Fusion Energy Sciences (DE-SC0001939) 20 Laboratory for Plasma Processing of Materials.