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)

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.

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

Goal and Outline 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

Designed to create (low-hardness) oxides. CMP Process Process description: A wafer is pushed against a polymeric polishing pad (Pa = 1-10 psi) Pad and wafer rotate independently (~60 rpm). Slurry, containing oxidizing chemicals and abrasive particles is supplied into the interface. Material removal occurs due to particle abrasion of the chemically passivated wafer surface. Schematic of CMP operation 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

Contact at wafer-pad interface Polishing pad (IC1000) Polyurethane Pad wafer interfacial contact wafer pad particles 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

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

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.

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)

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

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

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

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.

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))

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.

Two step process Diluted 0.5 micron alumina polish SiO2 Slurry

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

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

Backup Slides

Model predictions of material removal 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) Die scale Wafer scale Feature scale Modeling used to uncover fundamentals of the mechanisms enabling macro- and nano scale material removal. x y Contact pressure Slurry pressure Slurry pressure distribution under wafer Contact pressure distribution on the wafer 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

BCP treatment

XPS Analysis Reveals Nb-O Bonding Arbitrary Units Binding Energy (eV) Nb metal 3d 5/2 3d 3/2 Nb oxide Δ eV ~ 5 eV Δ eV is Characteristic of Nb2O5 Oxide Thickness ~4.5 nm Sampling Depth: ~ 7 nm Single oxidation state (Nb2O5) even though other oxides possible (NbO2, NbO). Distance between metal peak and oxide peak is characteristic of the Nb2O5 bonding state Oxide thickness is estimated by the attenuation of the Nb metal peak. (Note that this assumes a layered structure of oxide on Nb metal) Note: this example is from the Si Slurry CMP

Nb 3d Binding Energy (eV) Metal Peaks Active Oxidizer: H2O2 Active Etchant: HF Active Oxidizer: H2O2 Binding Energy (eV) Nb 3d Metal Peaks Cannot remove all of the oxide – oxidizes in air and seems limited to ~4.5 nm Acid = 5 ml HF, 17 ml Nitric, 51 ml Methanol Base = 5 ml ammonium hydroxide, 10 ml H2O2, 50 ml DI water Limited claim comes from leaving in air and not seeing an increase – and this is the same amount of oxide that we seem to see after CMP as well – therefore or thought is self-limiting….

BCP Dip Study; Step 1 of 2-Step Process Buffered Chemical Polishing (BCP) Formula: 10 mL HF (49%), 10 mL HNO3 (65%), and 20 mL H3PO4 (85%) Time in minutes Grams removed Mass Removal Rate Nb 3d 6-minute BCP dip BCP dip removes oxide from as received state Compared to as received, narrower metal peak (more order in Nb atoms – or etched away disordered surface) as-received Binding Energy (eV)

BCP Dip Study; Step 1 of 2-Step Process Time in minutes Grams removed Mass Removal Rate Binding Energy (eV) Nb 3d 6-minute BCP dip as-received BCP dip removes oxide from as received state Compared to as received, narrower metal peak (more order in Nb atoms – or etched away disordered surface)

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 Lower FWHM = more ordered environment in Nb metal – perhaps removed all disordered surface layers – so greater material removal Less oxide with BCP?? Perhaps smoother surface overall??? Ratio of 3d5/2 metal:oxide With BCP: 7:10 Without BCP: 5:10 1 minute BCP + Cu Slurry CMP FWHM = 1.7 eV Cu Slurry CMP Binding Energy (eV)

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.

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.

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

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

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

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

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