FLCC Seminar

1. FLCC Seminar Your Photo Here Title: Plasma-Etch Limits: Molecular Dynamics Simulations and Vacuum Beam Measurements Faculty: David B. Graves Department: Chemical Engineering University: UC Berkeley Plasma Technology

2. Current and Future Challenges in Plasma Etch: Following Scaling and Beyond... R. Chau, Intel, 2005 Plasma Technology

3. Photoresist Roughness Challenges Plasma Technology

4. Etch Roughness Challenges: CD Control Following Ma, Levinson and Wallow, AMD; 2007 Plasma Technology

5. Etch Fundamentals Control Performance Metrics CD/Anisotropy Etch Fundamentals Ion impact: energy/angle Radical impact Electron impact Passivation layers Surface transport Etc. Roughness Selectivity Rate & Uniformity Following Tom Wallow, AMD, 2007 Plasma Technology

6. How to Attack Current and Future Plasma Etch Challenges? 1. Conduct studies of fundamentals of plasmas and plasma-surface interactions to develop intuition and insight into dominant mechanisms, usually on model systems. 2. Use fundamental studies to improve and extend empirical and statistical studies to address real, practical systems in a way that can directly affect process development. Plasma Technology

7. Ultimate Goals of Research Develop ‘theory’ about how plasmas alter/etch surfaces at atomic scale - what are important factors (species; energies; angles; surface temperature; types of mask; relevant length and time scales, etc.) 2. How do these (and other?) factors govern fidelity of mask-to-film pattern transfer? 3. Combine simulation and experiment to develop methods to usefully simulate ‘nanofeature’ profile evolution given information about plasma conditions (i.e. given factors above)* 4. Use these simulations/experiments to help develop and control practical plasma etch tools and processes *e.g. Professor Jane Chang, UCLA profile simulation, FLCC Plasma Technology

8. < 10’s nm feature scale atomic scale d ~ 1 nm Multi-Scale Plasma Etch* Pressure ~ 5 - 500 mTorr ; Gas temperature ~ 600 - 1000 K Electron ‘temperature’ ~ 2- 8 eV; Ion energy ~ 20-1000 eV Ar/C4F8/CHF3/... Etch Gas in plasma sheath d ~ 1 mm plasma Dielectric film 300 mm SiF4 COF2 Gas Flow Out VRF = V01sin (w1t) + V02sin (w2t) * e.g. Professor Mike Lieberman, FLCC collaborator Plasma Technology

9. Mechanisms of Plasma Etching: ‘Passivation’ or ‘Modification’ Layers All surfaces exposed to plasma are MODIFIED All surface processes occur through, and are affected by, a layer of surface modification Plasma-induced surface modification is a FIRST order effect and must be included in any serious model of plasma-surface interaction. Top surface modified layer Resist Film Feature sidewall modified layer Feature bottom modified layer Substrate Plasma Technology

10. Mechanisms of Plasma Etching: ‘Passivation’ or ‘Modification’ Layers Surface modification typically ~ nm thick Modification strongly influenced by ion bombardment-induced energy deposition, bond breaking and mixing Neutral species (typically radical fragments) play important roles as both etch precursors (e.g. F) and depositing precursors (e.g. CFx) Very few details understood, however Plasma Technology

11. 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) Plasma Technology

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

13. Molecular Dynamics Characteristics • Accessible time- and length-scales match part of the plasma-surface interaction problem • Energetic impacting species dissipate their energy within a picosecond among ~102 – 103 atoms • Tersoff-Brenner style REBO potentials for Si-C-F and C-H-F systems (Tanaka, Abrams, Humbird and Graves)* • Potentials are short-ranged, designed to simulate covalent bonds *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); D. Humbird and D.B. Graves, J. Chemical Physics, 120 (5), 2405-2412, 2004. Plasma Technology

14. initial configuration Molecular Dynamics (MD) Simulation Interatomic Potential Interatomic Forces typical MD time step: update positions 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 Plasma Technology

15. What are Mechanisms of Fluorocarbon Plasma Etching? • Known that etching generally takes place through a film of fluorocarbon material (various F/C ratios) • Generally assume that film reduces the rate of etching on the substrate • Substrates that react with C (e.g. O, but also N) will usually result in thinner steady state films, all things being equal • F atoms known to greatly reduce selectivity to PR, Si or nitride • Details very murky/ no self-consistent picture (descriptions, not predictive models) Plasma Technology

16. Surface Transport and Chemistry: Fluorocarbon Plasma Etching Steady state etching requires: • FC film material deposited and removed at equal rates. • Etchant (F) transported to substrate interface. • Etchant reacts with substrate to form etch product • Etch product transported to film surface. • Etch product leaves surface. FC film All processes must occur simultaneously! Substrate material Plasma Technology

17. Si Etch Yield vs. Average FC Film Thickness Typical Experimental Results Plasma Technology

18. Model Study of Fluorocarbon Plasma Etching (Si) • Si etch analogous to other non-O containing films (e.g. silicon nitride, photoresist) • Role of FC film in etch similar for all materials • Popular chemistry: F-deficient (e.g. C4F8; C4F6; C5F8, etc.), heavily diluted in Ar • Model chemistries: • xCF2 + yF + Ar+ (200 eV) • xC4F4 + yF + Ar+ (100, 200 eV) • xCF + yF + Ar+ (100, 200 eV) Plasma Technology

19. ‘Sticky’ Precursors Yield Desired Result (Ar+ 200 eV; Normal incidence) Flux ratio (CxFy/F/Ar+) Etch Yield (Si/Ar+) Film Thickness (nm) Case Neutral specie (avg. KE) Plasma Technology

20. Si Etch Yield vs. Average FC Film Thickness Varying C4F4/F/Ar+ or CF/F/Ar+ ratios ExperimentalResults MD Simulation Results Plasma Technology

21. Typical Snapshots Showing Fluctuations Note layering of near-surface region, fluctuating FC layer; surface modification ~ 4-5 nm. Plasma Technology

22. Incoming Ion Si-White, Red C-Black,Yellow F- Grey, Green Ar-Purple Bottom 2 layers are fixed Top is open Periodic BC in lateral dimensions Colored atoms will be etched ~22Å Relatively Large Products Leave Surface Role of FC clusters in plasma, emitted by surface? Re-deposition of clusters/heavy species? Plasma Technology

23. (a) (b) (c) FC Films are NOT All Alike! Comparison of films deposited by CF on initially bare Si with CF at (a) 300K, (b) 5eV, and (c) 100eV. Plasma Technology

24. Conclusions: FC Film Formed During Etch • ‘Stickiness’ of FC precursor important • precursor C/F=1; creates porous, ‘fluffy,’ open FC film • weakly cross-linked and low density FC film can be sputtered in clusters: causes film thickness fluctuations • F transports to substrate and products are removed through pores and film thickness fluctuations • FC film thickness fluctuates as impacting ions occasionally remove clusters of FC; assists etch product removal and enhances overall transport • Ion impact and ion mixing still play central role in FC plasma etch with FC film present Plasma Technology

25. Questions Inspired by Simulations • Do surface fluctuations represent a future fundamental limitation to plasma etch pattern transfer fidelity? • Random, nanometer-scale fluctuations can lead to differences between otherwise identical (even adjacent) features • Electrostatic charging of fluctuating surface topography/composition might amplify fluctations, altering ion trajectories over larger distances*. * Suggested by R.A. Gottscho, Lam Research Plasma Technology

26. Thoughts Inspired by Simulations, continued • Are surface fluctuations from point to point on the surface related to roughness? • Similar ideas came up with polymer/organic film etch simulations • No thermal diffusion of any species included in simulation: could this be important for time-scales and length-scales of importance in etching? (Very likely...) Plasma Technology

27. Plasma-Organic Film Interaction Mechanisms • E.g. photoresist etch &roughening mechanisms • Organic masking layers for novel pattern transfer • Nano-imprint lithography • Block co-polymers • Other applications involving organic films Plasma Technology

28. How Do Organic Polymers Resist Etching? • Organic polymers are soft and easily sputtered • not obvious how they act as etch masks! • Plasma dramatically modifies top surface layer • First step in understanding etch/roughening mechanisms for organic films is to understand near-surface modifications due to plasma MD study begun with simulated beam experiments: polystyrene/Ar+; then polystyrene/Ar+/F Plasma Technology

29. Ion Gun Faraday Cup Substrate Turbo Pump Experimental Technique • UHV Chamber: • Base Pressure: ~5x10-8 Torr pumped with a 2000 L•s-1 turbo pump • PHI Model 04-191 Ion Gun: • Chamber pressure rises to ~1x10-6 Torr • Ions: He+, Ar+ and Xe+ • Energies: 0.2 - 1 keV • Beam Size: ~0.5 cm • Substrate temperature control • Neutralizing filament to prevent surface charging Plasma Technology

30. 2 Formation of carbon-rich surface layer 1 Steady-state sputtering / sputtering of already implanted material 2 1 Evolution of sputter yield with fluence: Ar+ Rohm and Haas 193 nm photoresist Mass Loss Etch yield from slope Plasma Technology

31. MD Simulation of Model Polystyrene Impacted with 100 eV Ar+ dehydrogenated surface layer ion penetration depth (~nm) transition region: large changes in materials properties over a very small thickness } undisturbed polymer Plasma Technology

32. Comparison of steady-state sputtering yields • Empirical formula: etch yield is proportional to Ohnishi parameter N: total number of atoms in monomer NC: number of carbon atoms in monomer NO: number of oxygen atoms in monomer Plasma Technology

33. 0.5 keV 1 keV 20°C 10 nm 1.49 nm 0.57 nm 5 nm 0 nm 45°C 2.65 nm 1.68 nm Surface roughening of 193 nm photoresist:ion energy and substrate temperature • Xe+ bombardment: ion energy and substrate temperature effect (fluence ~1.3x1017 ions•cm-2 for all samples) 1 mm Plasma Technology

34. Xe+ Ar+ He+ 0.5 keV 5 nm 1.49 nm 0.52 nm 0.29 nm 2.5 nm 1 keV 0 nm 0.33 nm 0.21 nm 0.57 nm 0.325 nm Surface roughening of 193 nm photoresist:comparison of Xe+, Ar+, and He+ bombardment (fluence ~1.3x1017 ions•cm-2 for all samples) 1 mm Plasma Technology

35. 50 eV or more 1 eV or less MD: 100 eV Ion Penetration and KE Deposition 2 nm Ar+ Ar+ (S.S.) Xe+ He+ Polystyrene layers. 500 impact trajectories on virgin PS surface. Trajectories shifted to the same initial lateral location; each trajectory is color coded to the KE remaining in the ion at a given position. ‘Ar+ (S.S.)’ shows impacts on the steady state (dehydrogenated) surface, indicating greater scattering and shallower penetration for a given ion. Plasma Technology

36. Summary: sputtering of polymers by rare gas ions, normal incidence • Polymer sputtering characterized by an initial high yield. A lower steady-state yield, similar to pure carbon, is reached after fluences of ~5x1016 ions•cm-2. • Steady-state yields of Ar+ bombardment follow the empirical Ohnishi parameter, taking into account inherent chemical effects of the polymer. Ohnishi parameter does not necessarily hold true in the presence of more complex etch chemistry. • The amount of material removed prior to reaching steady-state is polymer dependent. • more mass removed prior to reaching steady-state for 193 nm photoresist compared to 248 nm photoresist • Ion beam etch yields consistent with argon plasma experiments. Plasma Technology

37. Summary: roughening of polymers by rare gas ions, normal incidence • Ion energy effect: small increase in roughening from ~ 200 eV to 1000 eV • Ion mass effect on roughening: Xe+ > Ar+ > He+ • Substrate temperature effect: increased roughness with increased substrate temperature (45°C > 20°C) Plasma Technology

38. Current Vision: Competing Mechanisms in Sputtering Incoming Ions CxHy Products Ion Scattering Dehydrogenation Transient period—competition between dehydrogenation/crosslinking and HC removal Large HC cluster ejection can remove components from the initial crosslink Once significant crosslinking/dehydrogenation occurs, large cluster ejection is hampered, dehydrogenation dominates Crosslinking Modified Layer Undisturbed Polymer Plasma Technology

39. Summary and Concluding Remarks: Organic Film Sputtering and Roughening ‘Virgin’ organic films are soft and sputter readily. Rare gas ion bombardment (e.g. Ar+) can create protective C-rich ‘skin’ at surface, greatly reducing etch yield. Plasma-generated reactive radicals (e.g. F) can attack and thin or remove ‘skin,’ resulting in great increase in etch yield. ‘Scissioning’ polymer behaves differently in transient than ‘cross-linking’ polymer; ion bombardment decomposes scissioning polymer into monomer more than cross-linking polymer. Relationship with observed greater roughening for scissioning polymer still speculative: greater cluster ejection due to monomer decomposition? Greater MD cell-to-cell variation linked with more roughness? Cluster desorption related to observed greater roughness at higher surface temperature? Why do higher mass ions result in rougher sputtered films? Why does lower ion energy (between 1000 eV and ~100 eV) result in rougher sputtered films? Plasma Technology

40. Challenge to Connect Length Scales Experiments show roughness and structure on ~10’s nm – ~100’s nm length scales. MD suggests smoothing on ~ nm length scales. Need to extend theories to energies, materials and chemistries of interest to low temperature plasma processing studies, such as plasma etching. Nm-scales becoming important for plasma processing of semiconductor devices and related thin film products – very helpful to improve understanding of fundamental phenomena. Couple nm-scale phenomena to feature scale through feature scale simulations Plasma Technology

41. Acknowledgements Joe Vegh (PhD student, UC Berkeley) Dave Humbird (currently at Lam Research) Erwine Pargon (currently at LETI, Grenoble) Dustin Nest (PhD student, UC Berkeley) Harold Winters, John Coburn and Dave Fraser, UCB G. Oehrlein, et al. University of Maryland SRC CAIST NSF GOALI (DMR 0406120 ) NSF NIRT (CTS-0506988) FLCC: UC Discovery Grant from the Industry-University Cooperative Research Program (IUCRP) Plasma Technology