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Chemical Mechanical Polishing for Manufacturing of Smooth Nb Surfaces

Chemical Mechanical Polishing for Manufacturing of Smooth Nb Surfaces George Calota 1 , Natalia Maximova 2 , Katherine Ziemer 2 and Sinan Muftu 1,3 1 Department of Mechanical Engineering 2 Department of Chemical Engineering 3 NSF-NSEC-Center for High-rate Nanomanufacturing

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Chemical Mechanical Polishing for Manufacturing of Smooth Nb Surfaces

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  1. Chemical Mechanical Polishing • for Manufacturing of Smooth Nb Surfaces • George Calota1, Natalia Maximova2, Katherine Ziemer2 and Sinan Muftu1,3 • 1Department of Mechanical Engineering • 2Department of Chemical Engineering • 3NSF-NSEC-Center for High-rate Nanomanufacturing • Northeastern University • Boston, MA 02115 • s.muftu@neu.edu, (617) 373-4743 • Acknowledgement: • H.C. Starck, Inc, Newton, MA • NSF-Center for High-rate Nanomanufacturing (Award # NSF-0425826)

  2. CMP in Semiconductor Manufacturing Chemical Mechanical Polishing (CMP) is a critical step in integrated circuit (IC) manufacturing, typically used for planarizing: • Dielectric materials: SiO2 • Conductors: copper (Cu) and tungsten (W) • Diffusion barrier: Tantalum (Ta) In between processing steps. Evans, D.R. “Metal Polishing Process,” in Chemical-Mechanical Planarization of Semiconductor Materials, ed. M.R. Oliver, Springer, 2003, p 41.

  3. Planarizing a SiO2 wafer using CMP Experimental results: Our polishing experiments on SiO2 show: • Large wavelength roughness is reduced to 1 μm level • Short wavelength roughness is reduced to 1 nm level. • Sub nanometer roughness is typical for Si wafers

  4. Goal and Outline The goal of this presentation is to demonstrate the potential of CMP as an alternative method to manufacture very (nearly atomically) smooth Niobium surfaces. • Outline • Brief description of chemical mechanical polishing (CMP) • Studies of Oxidation of Niobium • Proposed two-step process for polishing of Niobium • Initial results • Summary and conclusions

  5. CMP Process Schematic of CMP operation • Process description: • A wafer is pushed against a polymeric polishing pad • (Pa = 1-10 psi) • Pad and wafer rotate independently (~60 rpm). • Slurry, containingoxidizing chemicalsandabrasive particlesis supplied into the interface. • Material removal occurs due to particle abrasion of the chemically passivated wafer surface. Wafer Slurry Designed to create (low-hardness) oxides. • Chemicals: • Oxidizers • Buffers • Surfactants • Particles: • Material: • Silica (SiO2), alumina (Al2O3, Ceria (CeO2) • Size: 50-150 nm • Shape: Spherical

  6. Contact at wafer-pad interface Pad wafer interfacial contact wafer pad particles Polishing pad (IC1000) Polyurethane • The mechanical component of material removal is primarily dominated by particle-wafer contact. • But, particle-to-wafer contact forces depend on many variables: • Applied pressure • Particle size • Particle concentration • Pad elasticity • Pad thickness • Pad roughness

  7. Material removal rate Material Removal: • Material removal is governed by an abrasive removal process: k: removal rate constant F Normal force LS: Sliding distance H: Hardness (material property) V(‘): Worn volume Archard’s Law In CMP literature material removal rate (MRR) is used: k: removal rate constant Pc: Applied push down pressure V: Sliding speed Preston’s Law • Removal rate constant k represents the effects of: • Abrasive particle size, concentration, hardness, morphology • Wafer hardness, surface roughness • Slurry chemistry • Pad roughness, elasticity

  8. XPS Studies on Niobium oxidation • In general a passivated metal-oxide is softer and easier to remove mechanically. • CMP strives to find a delicate balance between oxide formation and mechanical abrasion • The first step of our investigation was to understand oxidation of Niobium for conditions relevant to CMP • Characterization using X-ray Photoelectron Spectroscopy (XPS) under various processing conditions carried out • Characteristics of Niobium Oxide • Effect of base and acid on oxidation • Effect of buffered chemical polish (BCP) on Niobium • Hardness test on Niobium oxide underway.

  9. XPS Studies on Niobium oxidation-I • Oxides formed by Nb: • Conditions • Exposed to air, as received • After CMP using Cu and SiO2 slurry • (small amount of material removal) • Results • The XPS study shows that • The oxide thickness is ~ 4.5 nm • The majority of the oxide is Nb2O5 • The oxide layer thickness self limiting type • (long term exposure to ambient or to an oxidizer doesn’t change oxide depth)

  10. XPS Studies on Niobium oxidation-II • Effect of Base (H2O2) and Acid (HF) • Conditions • Acid = 5 ml HF, 17 ml Nitric, 51 ml Methanol • Base = 5 ml ammonium hydroxide, • 10 ml H2O2, 50 ml DI water • Results • The XPS study shows that • Base (H2O2) forms oxide • Acid (HF) removes oxide

  11. XPS Studies on Niobium oxidation-III • Buffered Chemical Polish (BCP) • BCP Formula: • 10 mL HF (49%), • 10 mL HNO3 (65%), • 20 mL H3PO4 (85%) • Results • 18 min of BCP removed up to 200 um • Surface roughness changed from 8 um-10 um (PV) • The XPS study shows that • BCP treatment exposes a highly ordered Nb

  12. Niobium CMP using SiO2 slurry Surface roughness vs polishing time Pad and wafer rotational speeds: 60 rpm Supply pressure : 500 g/cm2 Carrier head linear oscillations : on Slurry : Microplanar CMP 1150 by EKC Technology, Inc. Polishing Pad : IC1000 A2, by Rohm and Haas Electronic Materials

  13. XPS Studies on Niobium oxidation - Conclusion • Characterization of Niobium oxide formation under various processing conditions: • Nb2O5, a stable oxide, is the dominant form when Nb is exposed to oxidants • This oxide is ~4.5 nm thick and its thickness is self-limiting. • a. It does not appear that passivation, in the CMP sense, will be helpful in polishing a relatively rough surface. • b. Passivation may be useful in planarazing a smooth Niobium surface.

  14. Two step polishing process • Two step process : • Step-1: Use large, hard abrasive particles to remove large PV roughness • Slurry-1: 0.5 um Alumina (Al2O3) 11 weight percent, dispersed in H20 (calcinated) • Slurry-2: 1.0 um Alumina (Al2O3) 11 weight percent, dispersed in H20 (polycrystalline) • Pace Technologies, Tucson, AZ. • Step-2: Use CMP approach to planarize the surface using available slurries • Pourbaix diagrams give guidance in the selection. • W-slurry • Cu-slurry • SiO2 (particle size O(50 - 100 nm))

  15. Abrasive wear Stachowiak, G.W., Batchelor A.W. Engineering Tribology, 2nd edition, BH Publishing, 2001. Two body abrasive mode: arises when a hard rough surface slides against a softer surface, digs into it and plows a series of grooves. Three body abrasive mode: arises when abrasive particles are introduced between sliding surfaces. 3-body wear produces lower wear rates, and more randomized wear marks.

  16. Two step process Diluted 0.5 micron alumina polish 0.5 micron alumina polish 0.5 micron alumina polish 1 micron alumina polish SiO2 Slurry

  17. Evolution of a smooth Nb surface by 2-Step Process t = 0, PV=7.2 um t = 28 min, PV = 3.3 um t = 17 min, PV = 4.6 um t = 42 min, PV=1.8 um t = 58 min, PV = 0.4 um t = 52 min, PV = 1.5 um t = 71 min, PV =0.2 um t = 67 min, PV = 0.3 um t = 60 min, PV = 0.5 um

  18. Summary and conclusions • Chemical mechanical polishing of Niobium is investigated • Niobium forms a stable Ni2O5 oxide, ~ 4.5 nm thick, and self limiting. • BCP treatment exposes ordered Niobium metal, but prolonged treatment does not improve surface roughness. • A two step procedure is proposed to first polish and then to planarize. • Preliminary experiments show substantial improvements in surface finish. • Peak-to-Valley roughness reduced from ~7 um to 0.2 um • Process parameters need to be optimized • These include pressure, particle size and polishing time, final CMP slurry type • Implementation inside the cavities are not considered in this short term investigation • Considering the potential surface quality obtainable implementation of this approach inside cavities should be explored further

  19. Backup Slides

  20. Model predictions of material removal Wafer scale Feature scale Slurry pressure Contact pressure y x y x Slurry pressure distribution under wafer Contact pressure distribution on the wafer Die scale Uniformity of material removal depends primarily on local contact pressure, but also affected by slurry-chemistry, abrasive size, wafer speed, pad properties/roughness • Polishing uniformity is important at three scales: • Wafer (affects wafer bow) • Die (affects die-scale bow) • Feature (affects nano-wire flatness) Modeling used to uncover fundamentals of the mechanisms enabling macro- and nano scale material removal. Non-uniform contact pressure on the wafer will cause non-uniform material removal and wafer bow at wafer-scale More material removal predicted on wafer’s edges

  21. BCP treatment

  22. XPS Analysis Reveals Nb-O Bonding Nb oxide 3d 5/2 Oxide Thickness ~4.5 nm Nb oxide 3d 3/2 Nb metal 3d 5/2 Nb metal 3d 3/2 Arbitrary Units Binding Energy (eV) Δ eV is Characteristic of Nb2O5 Δ eV ~ 5 eV Sampling Depth: ~ 7 nm

  23. Nb 3d Metal Peaks Active Oxidizer: H2O2 Cannot remove all of the oxide – oxidizes in air and seems limited to ~4.5 nm Active Etchant: HF Binding Energy (eV)

  24. BCP Dip Study; Step 1 of 2-Step Process Mass Removal Rate Grams removed Time in minutes Buffered Chemical Polishing (BCP) Formula: 10 mL HF (49%), 10 mL HNO3 (65%), and 20 mL H3PO4 (85%) Nb 3d 6-minute BCP dip as-received Binding Energy (eV)

  25. BCP Dip Study; Step 1 of 2-Step Process Mass Removal Rate Nb 3d 6-minute BCP dip Grams removed as-received Time in minutes Binding Energy (eV)

  26. Impact of 1-minute BCP Dip 1 minute BCP produced significantly narrower linewidth (more ordered matrix) with potentially more surface removal than Cu Slurry alone….. Nb 3d FWHM = 1.3 eV 1 minute BCP + Cu Slurry CMP FWHM = 1.7 eV Cu Slurry CMP Binding Energy (eV)

  27. Pourbaix Diagram for Nb-H2O System Asselin, E., Ahmed, T.M., and Alfantazi, A., “Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 DegC” Corrosion Science, 49(2): p. 694-710, 2007.

  28. Pourbaix Diagram for Nb-H2O System Cu Slurry + H2O2 Cu Slurry Si Slurry Asselin, E., Ahmed, T.M., and Alfantazi, A., “Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 DegC” Corrosion Science, 49(2): p. 694-710, 2007.

  29. CMP using High-Pressure & Copper slurry Surface roughness vs polishing time Pad and wafer rotational speeds: 60 rpm Supply pressure : 1000 g/cm2 Carrier head linear oscillations : on Slurry : Microplanar CMP 1150 by EKC Technology, Inc. Polishing Pad : IC1000 A2, by Rohm and Haas Electronic Materials

  30. CMP using High-Pressure & Copper slurry on BCP treated Nb Surface roughness vs polishing time Pad and wafer rotational speeds: 60 rpm Supply pressure : 1000 g/cm2 Carrier head linear oscillations : on Slurry : Microplanar CMP 1150 by EKC Technology, Inc. Polishing Pad : IC1000 A2, by Rohm and Haas Electronic Materials

  31. CMP-Physics-I: Continuum effects Williams, J.A. Engineering Tribology, Oxford, 2000. Liquid slurry lubrication: Reynolds eqn. Multiasperity Contact: Greenwood et al. (1966, 1967) Pad deflections: Elasticity Pad wafer clearance

  32. CMP Physics-II: Force Balance • Forces acting on the pad need to be in balance: • Slurry pressure, p • Normal and tangential contact tractions, µpc, pc

  33. CMP-Physics:-III Material Removal: • Material removal is governed by an abrasive removal process: k: removal rate constant F Normal force LS: Sliding distance H: Hardness (material property) V(‘): Worn volume Archard’s Law In CMP literature material removal rate (MRR) is used: k: removal rate constant Pc: Applied push down pressure V: Sliding speed Preston’s Law • Removal rate constant k represents the effects of: • Abrasive particle size, concentration, hardness, morphology • Wafer hardness, surface roughness • Slurry chemistry • Pad roughness, elasticity

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