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Surface coverage in atomic layer deposition - slides related to invited talk by Prof. Riikka Puurunen

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Invited talk given at the Workshop on Fundamentals of Atomic Layer Deposition (ALD): Modelling and Validation TU Delft, The Netherlands, July 3, 2019. Talk was recorded by TU Delft staff and is to be shared later. Website: https://www.tudelft.nl/en/faculty-of-applied-sciences/about-faculty/departments/chemical-engineering/scientific-staff/van-ommen-group/workshop-fundamentals-of-ald/. Twitter hashtag: #ALDfun

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Surface coverage in atomic layer deposition - slides related to invited talk by Prof. Riikka Puurunen

  1. 1. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Surface coverage in ALD Prof. Riikka Puurunen, Aalto University, Finland Workshop on Fundamentals of Atomic Layer Deposition (ALD): Modelling and Validation TU Delft, The Netherlands, July 3, 2019 aalto.fi/cmet/catalysis; @rlpuu
  2. 2. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 EuroCVD 22 - Baltic ALD 16, in Luxembourg, June 24-28, 2019. Abstract. Poster. 2002 Doctoral thesis, Helsinki University of Technology, 2002   Cartoon model of ALD GPC, Chem. Vap. Deposition 2005 ALD review: history, processes & Me3Al-H2O surface chemistry case study, J. Appl. Phys. (Appl. Phys. Rev.) >1400 time cited 2015 Microscopic conformality test structures  pillarhall.com; 2019 ALD conformality review App. Phys. Rev. 2013 ALD review, process update + crystallinity, J. Appl. Phys. (Appl. Phys. Rev.) 2013 Virtual Project on the History of ALD (VPHA),  vph-ald.com; review 2016 2018 Millennium Technology Prize 1974 Suntola ALE http://vph-ald.com Aldhistory.blogspot.fi 2
  3. 3. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Outline 1. ALD introduction + Why TMA + Al2O3? 2. Surface coverage -- (so) many definitions 3. Growth-Per-Cycle (GPC) in ALD: Cartoon model(s) 4. GPC in ALD: Model validation 5. Some terminology discussion 6. Conclusion Contains “Twitter portals” to click! 3
  4. 4. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 ALD introduction + Why TMA & Al2O3? 4
  5. 5. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Atomic layer deposition (ALD) ALD cycle Reactant A Reactant B By-product Substrate before ALD Step 2 /4 purge Step 4 /4 purge Step 1 /4 Reactant A Step 3 /4 Reactant B Reactant A Reactant B By-product (Scheme: Puurunen) George, Chem. Rev. 110 (2010) 111–131. DOI: 10.1021/cr900056b  Time  Most cited ALD review of all times 5
  6. 6. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 ALD materials by two- reactant processes > 700 processes ”Periodic table of ALD processes”: 1st ed.: Puurunen, J. Appl. Phys. 97 (2005) 121301. DOI: 10.1063/1.1940727 2nd ed.: Miikkulainen, Leskelä, Ritala, Puurunen, J. Appl. Phys. 113 (2013) 021301. DOI: 10.1063/1.4757907. Web-based voluntary updating in TU Eindhoven-led initiative in: https://www.atomiclimits.com/alddatabase/ https://www.atomiclimits.com/alddatabase/, 62005 Puurunen review2013 update+crystallinity
  7. 7. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 >700 ALD reactant - co-reactant pairs 7 H2O NH3 H2S Non-metal co-reactants, “thermal” ALD Energy-enhanced ALD O2 N2 H2 Metal precursor type Elements Halides Alkyls Cyclopentadienyls Alkoxides b-diketonates Alkylamides and silylamides Amidinates InorganicMetal-organic Organo- metallic Class N NM N M O M O O M M M M Cl M etc etc Puurunen, Appl. Phys. 97 (2005) 121301. https://doi.org/10.1063/1.1940727 Miikkulainen, Leskelä, Ritala, Puurunen, J. Appl. Phys. 113 (2013) 021301. http://doi.org/10.1063/1.4757907. O3 … Metal reactant type 2005 Puurunen review2013 update+crystallinity
  8. 8. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 In ALD, gas-solid reactions ideally: saturating & irreversible* amount adsorbed saturates amount adsorbed stays pulse purge Definition of ”ideal ALD”, especially for modelling purposes: Puurunen, J. Appl. Phys. 97 (2005) 121301. DOI:10.1063/1.1940727 *chemisorption non-saturationunsaturationdesorption NO: YES: ? partial reversibility? 82005 Puurunen review
  9. 9. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Concept of “ALD window”: where ALD conditions fulfilled 92005 Puurunen review Simplified George, Chem. Rev. 110 (2010) 111; DOI: 10.1021/cr900056b. Adapted from: Suntola, Atomic Layer Epitaxy. In Handbook of Crystal Growth, Vol. 3, Part B, Hurle, D. T. J., Ed.; Elsevier: Amsterdam, 1994; Chapter 14.
  10. 10. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 ALD: Cycle dependency Puurunen & Vandervorst, JAP 2004, https://doi.org/10.1063/1.1810193 Some growth modes Steady regime: GPC ~constant 102005 Puurunen review
  11. 11. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Steady regime: GPC ~constant “Monolayer” / (“atomic layer”) growth? R. L. Puurunen, ” A Short History of Atomic Layer Deposition: Tuomo Suntola's Atomic Layer Epitaxy”, Chem. Vap. Deposition 20 (2014) 332-344; DOI:cvde.201402012 (open access) Zn + S  ZnS Me3Al/H2O  Al2O3 112005 Puurunen review ~1/3 ML
  12. 12. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Prom Puurunen, Can. Chem. Conf., Ottawa, June 14, 2015 (in SlideShare) 2015 12
  13. 13. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 y = 0.37x + 1.68 R2 = 0.97 0 2 4 6 0 2 4 6 8 10 OH concentration (nm-2 ) Aladsorbed(nm -2 ) Why TMA-water? Archetype & benchmark • Near-ideal; surface chemistry comprehensively reviewed (2005) • Wide ALD window: r.t. to ~ 300C • Recent development e.g. saturation profile 2005 Puurunen review2013 update+crystallinity2019 conformality review 13 Example image: Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908; DOI:10.1116/1.5093620 Puurunen, Appl. Surf. Sci. 245 (2005) 6. y = ax + b, a ≠ 1, b > 0 1:1
  14. 14. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Surface coverage - (so) many definitions 14
  15. 15. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Adsorption Physisorption • non-specific • minimal electronic interaction • chemical nature of the adsorbate not altered • adsorption energy similar to the energy of condensation (exothermic) • non-activated • equilibrium is established • multilayers may form Chemisorption • chemical specificity • changes in electronic state • reversible/irreversible • chemisorption energy as for a chemical reaction (exothermic/endothermic) • often involves an activation energy • for “large” activation energies (“activated adsorption”), true equilibrium may be achieved slowly • monolayer adsorption http://old.iupac.org/reports/2001/colloid_2001/manual_of_s_and_t/node16.html, accessed 2.7.2019 Ideal ALD: sequential use of self-terminating gas–solid reactions (i.e., chemisorption) 152005 Puurunen review
  16. 16. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Coverage & monolayer: so many definitions! Chemisorbed ML, Q Physisorbed ML,  ML of the ALD-grown material,  *, , Q, … * units? none; nm-2 162005 Puurunen review
  17. 17. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Different uses for the definitions Chemisorption: coverage Q at saturation unity by definition Physisorption: Estimate steric hindrance Material monolayer: ALD sanity check (GPC < 1 ML); process throughput 172005 Puurunen review Ideal ALD: irreversible chemisorption (Model III)
  18. 18. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Coverage - one more definition: GPC in saturation profile modelling • Rose & Bartha 2009  “The density of reactive sites equals the density of deposited <metal> atoms divided by the number of deposition cycles.” • Ylilammi et al. 2018  Adsorption density q defined via gpcsat • Arts et al. 2019: “… the surface coverage θ is defined as the reacted fraction of available reaction sites, in such a way that θ = 1 in saturation”… “… average area … of an adsorption site … can be calculated from the growth per cycle.” Rose & Bartha, Appl. Surf. Sci. 255 (2009) 6620; DOI:10.1016/j.apsusc.2009.02.055 Ylilammi et al. J. Appl. Phys. 123 (2018) 205301; DOI:10.1063/1.5028178 Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908; DOI:10.1116/1.5093620 18 pillarhall.com
  19. 19. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Saturation profile modelling:  lumped sticking probability 19 https://en.wikipedia.org/wiki/Langmuir_adsorption_ model#/media/File:Langmuir_Adsorption_Model.jpg Ylilammi, Ylivaara, Puurunen, J. Appl. Phys. 123, 205301 (2018); DOI: 10.1063/1.5028178 Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908; DOI:10.1116/1.5093620 https://en.wikipedia.org/wiki/Langmuir_adsorption_model, accessed 13.9.2018 • Flat surface & isothermal conditions • Surface sites are equal • Adsorbed species do not interact • Adsorption & desorption are elementary processes Cremers, Puurunen, Dendooven, Appl. Phys. Rev. (2019) in press, DOI: 10.1063/1.5060967 AssociationAssumed: Langmuir adsorption
  20. 20. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Chemisorbed ML Physisorbed ML ML of the ALD- grown material (d) “Sticking coefficient and GPC-related monolayer & coverage” NEW! Coverage & monolayer: so many definitions! … and there are more … 20(a)-(c): 2005 Puurunen review
  21. 21. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Growth-Per-Cycle (GPC) in ALD: Cartoon model(s) 21
  22. 22. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 ALD GPC cartoons FIG. 13. Schematic illustration for analyzing sterically hindered chemisorption • on the basis of the size of the MLn reactant Model I by Ritala et al. Refs. 462 and 468 and Morozov et al. Ref. 133, • the size and geometry of the chemisorbed MLz species Model II by Ylilammi Ref. 1127, and • the size and number of ligands L Model III by Siimon and Aarik Ref. 432 and Puurunen Ref. 1128. Left: side view, right: top view. 222005 Puurunen review Model III: Ligands L
  23. 23. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 The ALD Model: background Fig. 11, Puurunen, J. Appl. Phys. 97 (2005) 121301. https://doi.org/10.1063/1.19 40727 open access pdf Dissociation L Reaction: MLn + surface L/M < n L/M = n L/M = n 2005 Puurunen reviewPuurunen, CVD 9 (2003) 249; DOI:10.1002/cvde.200306265
  24. 24. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 The ALD Model (part 1 of 2) 1. Mass balance for L for MLn chemisorption • ligands can be lost via ligand exchange with “-a”: 24 2. Ligand size (nm2) •  Maximum packing density Puurunen, Chem. Vap. Deposition 9 (2003) 249; DOI:10.1002/cvde.200306265  max
  25. 25. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 3. Know / assume the reaction mechanism / cause of saturation • Fixed or flexible L/M ratio? • Ligand exchange / association-dissociation? 4. Know / assume number of “-a” sites (nm-2) • Which fraction reacts? The ALD model (part 2 of 2) Puurunen, CVD 2003; https://doi.org/10.1002/cvde.200306265 25
  26. 26. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Example 1: ML4 + –a  -ML3 + gas (L/M = 3) • Ligand exchange with one -a • Steric hindrance or –a concentrati on limits Puurunen, Chem. Vap. Deposition 9 (2003) 249; https://doi.org/10.1002/cvde.200306265 GPC in ML 26
  27. 27. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Example 4: ML4 + –a  -MLx + gas (L/M varies) • All –a consumed in ligand exchange • Further association/ dissociation • Steric hindrance limits Puurunen, Chem. Vap. Deposition 9 (2003) 249; https://doi.org/10.1002/cvde.200306265 GPC in ML 27
  28. 28. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 GPC in ALD: Model validation 28
  29. 29. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Example: HfCl4 • Amount of Hf adsorbed is used to measure (“low”) surface –OH group concentration • Soethoudt/IMEC, at #CVDALD2019 • Reaction with one -OH • cL maxthr = 8.8 Cl/nm2 (Cl ionic radius used?) 29 Nyns et al., J. Electrochem. Soc. 155 (12) (2008) G269; DOI: 10.1149/1.2980427 Analogy: Example 1
  30. 30. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Example: Me3Al + H2O  Al2O3 • ~All OH groups react • Steric hindrance cL maxthr = 7.2 Me/nm2 (van der Waals radius),  max = 0.7 L/M Ligand exchange Dissociation/ association 302005 Puurunen review Puurunen, Appl. Surf. Sci. 245 (2005) 6-10. DOI:10.1016/j.apsusc.2004.10.003 Analogy: Example 4
  31. 31. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Some terminology discussion 31
  32. 32. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Some “ALD window” discussion • Barry, 2018: “I belive that some processes show an #ALDep window, I just don't believe it is a necessary condition for #ALDep to occur” • Yanguas-Gil, 2018: “I object to the idea that stability with temperature of the growth per cycle is a property of self-limiting processes.” • Elliott, 2018: “… the max coverage is 100%, so above a sufficiently high temperature, GPC=const... ignoring non-ALD reactions of course.” • Puurunen, 2019: “... The sources … which presented #ALDwindow as a region of constant GPC, pointed as reference to https://doi.org/10.1021/cr900056b … - now the most cited #ALDep review of all times … George 2010, Chem. Rev., DOI:10.1021/cr900056b2010 Over- simplified? 32
  33. 33. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 ALD window discussion: already since 2005 RLP’s shared Google file: ALD-window-historical-evolution 332005 Puurunen review
  34. 34. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Surface chemistry of TMA-water process Puurunen: • Different works/reviews present substantially different conclusions on the surface chemistry. See e.g. TSF 2014, DOI:10.1016/j.tsf.2013.11.112 98th CCC 2015, SlideShare Vandalon & Kessels, JVSTA 2017, DOI:10.1116/1.4993597: • ” … the self-limiting behavior in the TMA half-cycle cannot be caused by steric hindrance.” “The absolute –CH3 densities reached after the TMA half-cycle, <6 nm-2 …, are not high enough for steric hindrance”” Adomaitis, at EuroCVD-BalticALD (#CVDALD2019), unpublished: 34
  35. 35. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Saturation profile (TMA-water) “ALD measurable” (Yanguas-Gil, 2017)  Fingerprint saturation profile (?) * Contains maximum information Ylivaara, Yim, Ylilammi, Utriainen, Puurunen, unpublished material, manuscript in preparation Example image: Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908; DOI:10.1116/1.5093620 Normalized saturation profile ( coverage) * Enables extraction of lumped sticking coefficient, but: * Effect of e.g. temperature on GPC lost pillarhall.com 35
  36. 36. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Conclusion 36
  37. 37. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Conclusion 1. Surface coverage(s) in ALD central for describing surface chemistry. Many definitions may cause confusion 2. “Cartoon” / “ball” models help to see the big picture 3. TMA-water ALD process needs better understanding of surface chemistry 37
  38. 38. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Thank you! Discussion? • Thanks to Dr. Fatemeh Hashemi for organizing the workshop & Prof. Ruud van Ommen for inviting me to visit TU Delft ”All men by nature desire to know” Tuomo Suntola Aristotle #SuntolaQuote 38 2018 Millennium Technology Prize
  39. 39. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Additional materials Web of Knowledge Puurunen RL citation analysis with papers indicated 39 2005 Puurunen review 2013 update+crystallinity 2019 conformality review 2003 Cartoon model
  40. 40. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019 Web of Knowledge citation analysis, 30.6.2019, for author: Puurunen RL 2003 Cartoon model of ALD GPC Island growth model 40 2005 Puurunen review (APR) 2013 update + crystallinity (APR) 2019 conformality review (APR) New! Appl. Phys. Rev. : https://doi.org/10.1063/1.50 60967
  41. 41. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019 Island growth model: cases Cartoon model GPC: cases Cartoon model GPC: case 41
  42. 42. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019 Cartoon model GPC: case Surface chemistry case TMA on alumina TMA on silica “non-growth ligand exchange” 42
  43. 43. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019 Random deposition model 43
  44. 44. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 44 • Puurunen @rlpuu, 2017: “Haukka's ending slide: great #ALDep cartoon from years ago. But who is the original artist? Can Twitter find out? #ALDALE2017 #RealTimeChem” • Twitter found out!  By: Nick Kim, http://www.lab-initio.com ALD cartoons! & The power of Twitter!

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