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Molecular Dynamics Simulations of Plasma-Surface Interactions and Etching

Molecular Dynamics Simulations of Plasma-Surface Interactions and Etching. David Graves and David Humbird University of California at Berkeley FLCC Research Seminar February 23, 2004. acknowledgements: Gottlieb Oehrlein and group at U. Maryland Cam Abrams

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Molecular Dynamics Simulations of Plasma-Surface Interactions and Etching

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  1. Molecular Dynamics Simulations of Plasma-Surface Interactions and Etching David Graves and David Humbird University of California at Berkeley FLCC Research Seminar February 23, 2004 • acknowledgements: • Gottlieb Oehrlein and group at U. Maryland • Cam Abrams • Harold Winters, John Coburn, Dave Fraser

  2. Broader Issues in Plasma-Surface Interactions 1. Modifications of surfaces by plasma exposure a highly advanced empirical art; for example... a.) etched structures controlled to within several nm over 300 mm wafer diameter b.) plasmas used to extend optical lithography (e.g. resist trim and spacer etch) 2. Models at plasma and feature scales require surface models 3. Complexity of plasma-surface processes and difficulties of direct observation (like all surface processes) challenges development of physically-based models

  3. What are Key Mechanisms in Plasma-Surface Interactions? plasma plasma creates positive ions and neutral radicals, both of which hit surface neutral species sputtered and desorbed neutral radical at ~ Tgas positive ion impact at ~ Vsheath Near-surface region altered; remains near Tgas zone of energy release

  4. Basic Challenge 1: Model Two Very Different Types of Species Ion bombardment and radical impact - ions impact surface with 10’s - 1000’s eV - energy rapidly released; profound surface effects - ions have chemical as well as physical effects - radicals may adsorb/react at room temperature with no barrier - coupled ion/radical effects at surfaces thought to explain most observed effects - experimental evidence for weakly bound neutral surface species: diffusion, reaction, desorption dynamics

  5. Basic Challenge 2: Model Effects with ‘Sufficient Accuracy’ Major goals are to use model to: a. interpret/understand experiments - elucidate mechanisms of etching b. develop surface rate expressions that can be used in feature and tool scale simulations

  6. Basic Challenge 3: Model Processes with Range of Time Scales For example, 1. Ion bombardment results in collision cascade that dissipates to heat in less than ~ 10-12 s. 2. Radical diffusion/reaction/desorption may take 10-12 - 10-3 s. 3. Experiments/processes conducted for ~ 102 s.

  7. Basic Challenge 4: Model Surface Processes in Which Surface Always Changes The Plasma Alters the Surface! The crucially important consequence: simulations must include enough impacts that the surface achieves steady state composition and structure. Methods that rely directly on ‘ab-initio’ electronic structure calculations will be too slow for practical purposes. Empirical potentials allow realistic fluences, but are they accurate enough?

  8. How to Model/Simulate Plasma-Surface Interactions? Molecular dynamics simulations: - classical, semi-empirical potentials - resolves vibrational timescales: ~ O(10-15 s) 1. Ion impact - crucially important energy input; ~ 10-13-10-12 s ‘collision cascade’ - MD time and length scales match physics of interactions - weakly bound species after collision cascade removed: simple TST for thermal desorption with Eb 0.8 eV. 2. Radical-surface chemistry - accuracy of interatomic potentials?? (cf. ab-initio) - time and length scales adequate?? (cf. experiment)

  9. typical MD time step: update positions initial configuration evaluate forces Ions assumed to neutralize before impact: fast neutral interacting with surface update velocities is assumed to model all reactive and non-reactive interactions Molecular Dynamics (MD) Simulation Interatomic Potential Interatomic Forces

  10. MD ‘Cell’ and Assumptions for Etch Simulation Top exposed to ion & neutral flux; impact location chosen randomly Impact events followed for ~ 1 ps; excess energy removed; statistics collected; new impact point chosen; repeat sequence ~ 103 times for steady state surface. Surface composition and structure must reach steady state. Lateral boundaries periodic; mimics semi-infinite surface Bottom boundary fixed; new Si added here ~ 2 nm

  11. energy Ignore events between ion (& neutral) impacts (> 10-3 s) time Follow events during ion impacts (~ 10-12 s) Mimic Experimental Time Scales by Accumulating Effects of Many 10-12 s Impacts Simulation strategy: accumulate effects of repeated impacts; ignore time between impacts except to remove weakly bound species

  12. How to Simulate Radical-Surface Interactions? ‘Simplest’ problem: simulate spontaneous etching of un-doped room temperature silicon by F impacting at room temperature. Previous studies using Stillinger-Weber and related potentials failed to predict ANY Si etching from F impact at 0.03 eV (~ 300 K)!1 Recent results using modified Brenner/Tersoff-style potential for Si-F are more encouraging. 2 1F.H. Stillinger and T.A. Weber, Phys. Rev. Lett., 62, 2144, (1989); P.C. Wiekliem, C.J. Wu, and E.A. Carter, Phys. Rev. Lett., 69, 200, (1992); T. A. Schoolcraft and B. J. Garrison, J. Am. Chem. Soc. 113, 8221, (1991). 2 C.F. Abrams and D.B. Graves, J. Appl. Phys., 86, 5938, (1999); D. Humbird and D.B. Graves, J. Chem. Phys., in print, (2004).

  13. Stillinger-Weber/ Carter Si-F Potentials: Spurious Barriers? Accurate representation of thermal Si-F chemistry is needed Stillinger-Weber/Carter Si-F potentials do not predict spontaneous etching Si-F potential of Cam Abrams does, but with product distribution in disagreement with experiment* * H.F. Winters and J.W. Coburn, Surf. Sci. Rep., 14 (4-6): 161-269, (1992)

  14. Improving the Si-F potential (Abrams) • Added a correction function to Abrams Si-F that accounts for changes in bond energy as Si becomes more fluorinated • Parameterized against DFT* (Humbird) * S. Walch, Surf. Sci. , 496, 271, (2002)

  15. More Results: Si-F Spontaneous Etch • Results at 300 K are reasonable wrt reaction probability and product distribution • Product shift at higher surface temperature matches experiment; rate does not • Etch kinetics above ~ 400K dominated by spontaneous decomposition • Simulation misses ‘long-time scale’ events: KMC/TST needed?

  16. Spontaneous Etch Reaction Probabilities (Si atoms etched per incident F atom) Author Value Flamm et al.a 0.00672 Ninomiya et al.b 0.025 Vasile and Steviec 0.064 H. F. Wintersd 0.00325—0.0075 This work 0.03 a D. L. Flamm, V. M. Donnelly, and J. A. Mucha, J. Appl. Phys. 52, 3633 (1981). b K. Ninomiya, K. Suzuki, S. Nishimatsu and O. Okada, J. Appl. Phys. 58, 1177 (1985). c M. J. Vasile and F. A. Stevie, J. Appl. Phys. 52, 3799 (1982). d H. F. Winters, private communication (2003). Note: Evidence that measured F coverage (7-10 ML or more) may be due to roughness. This ‘texture’ question likely to become more important in future as features/films become smaller. One current issue is LER.

  17. Ion-Assisted Etching: Comparison to Experiment 200 eV Ar+/Si + F

  18. Ion-Assisted Etching: Weakly Bound Products 200 eV Ar+/Si + F

  19. Fluorocarbon Plasma Etching of Si • Important issue: depositing species play role in selectivity and CD control • FC plasmas readily etch SiOx; Si and other materials etch more slowly • ‘Model’ case for studying mechanisms of etch selectivity • Popular chemistry: F-deficient (e.g. C4F8; C4F6; C5F8, etc.) heavily diluted in Ar • Model chemistry: xCF2 + yF + Ar+ (20 eV and 200 eV) • Potentials: Si-C-F* (with recent Si-F revisions) * C.F. Abrams and D.B. Graves, J. Appl. Phys., 86, 5938, (1999); J. Tanaka, C.F. Abrams and D.B. Graves, JVST A 18(3), 938 , (2000)

  20. Thermal CF2 / Ar+:9/1 (Si Impacts) Surface C, F, Si etch (ML) vs. CF2 Fluence Si etch yield (Si/ion) vs. CF2 Fluence Ar+ 20 eV Uptake / Etch (ML) (Steady Deposition) CF2 Fluence (1015 cm-2) Ar+ 200 eV Uptake / Etch (ML) Etch Yield Si/Ion CF2 Fluence (1015 cm-2) CF2 Fluence (1015 cm-2)

  21. Thermal F & CF2 / 200 eV Ar+ Surface C, F, Si etch (ML) vs. CF2 Fluence Si etch yield (Si/ion) vs. CF2 Fluence CF2/F/Ar+ Uptake / Etch (ML) Etch Yield Si/Ion 8/1/1 (10% F) CF2 Fluence (1015 cm-2) CF2 Fluence (1015 cm-2) Uptake / Etch (ML) Etch Yield Si/Ion 7/2/1 (20% F) CF2 Fluence (1015 cm-2) CF2 Fluence (1015 cm-2)

  22. Simulation & Experiment Agreement Si-C/C-C CF C-CFx CF2 CF3 Si(2p) XPS C(1s) XPS Experiment* (C4F8) Simulated Experiment* Simulated Ar+/CF2 C4F8 / 90% Ar • Increasing self-bias forms SiFx bonds • Low energy: passive deposition • High energy: CFx bonds reduced, Si-C, C-C, Si-F form; Si etch observed * Measurements courtesy G.S. Oehrlein et al.

  23. Stratified Layers Close inspection reveals superficially fluorinated Si-C network.

  24. Silicon Transport and Surface Loss Mechanism (as deduced from simulation results after steady state reached) Subsurface F attacks Si Si joins Si-C mixing layer Si appears on surface Si stripped of F Mixing Mixing Etching F “recycle” Deep mixing required Shallow mixing required

  25. Single-Impact Movies(available from website) Deep impact From below Si-C layer Shallow impact with product From overhead Green: Argon Blue: Silicon Yellow: Carbon White: Fluorine Red: Fluorines of interest 0.6 ps duration 0.5 ps duration

  26. Movies of Si Layer Etch: F and Si Evolution(available from website) • Elapsed time: ~ 10 s (per mA/cm2 ion flux) • 7 CF2, 2 F (each at 0.03 eV) per Ar+ (at 200 eV) • Simultaneous images: left F (red, green); right Si (blue) • Note the ‘front advance’ down into silicon

  27. Ion Energy Deposition Through SiC Layer top view side view 0% F 10% F 20% F

  28. Observations About FC/Ar+ Etching of Si 1. Remarkable layer segregation induced by strong Ar+bombardment. - SiCx and SiFx layers seen in TEM images? (ASET) - ‘leading front’ of SiF, followed by SiC, then top layer F 2. Under conditions examined (e.g. low Gn/G+), SiC layer plays key role in reduction of silicon etch rate rather than CFx layer. 3. F adsorbed at surface is transported to Si layer by ion bombardment. - form of incident F less relevant than total adsorbed C/F ratio - simulation shows that F is most effective in thinning SiC layer and in creating ‘dynamic porosity’ in SiC layer 4. Ion energy remaining at SiFx layer appears to correlate to overall yield. 5. Etch mechanism driven by ion mixing and assumes all neutral species adsorb initially into strongly bound states. 6. Recent results hint that surface roughness could play role in FC observed experimentally.

  29. Concluding Remarks 1. Encouraging evidence that empirical potentials with parameters fit to DFT cluster calculations can capture spontaneous Si etching via F. - may be due to strongly exothermic, barrier-less process and ‘prompt’ reactions - suggestion that etch at T> 400K requires KMC/TST 2. Simulations of Ar+ with FC radicals on Si shows general agreement with measurements and explains complex process. 3. Current simulation assumes that all processes driven by ion mixing and that thermal neutrals first chemisorb at surface. Ignores weakly bound reactants diffusing into sub-surface: are these important? 4. Weakly bound etch products commonly observed (in agreement with experiment). 5. Field requires closer coupling between beam/plasma experiments and simulations to test assumptions and extend interpretations.

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