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The Utility of Zeta Potential Measurements in the Characterization of CMP Slurries

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Guest speaker Dr. David Fairhurst joins HORIBA Scientific to discuss the important role zeta potential (ZP) plays in the performance of CMP slurries. In this webinar we will first review and discuss …

Guest speaker Dr. David Fairhurst joins HORIBA Scientific to discuss the important role zeta potential (ZP) plays in the performance of CMP slurries. In this webinar we will first review and discuss the importance of the iso-electric point of metal-oxide polishing agents and the role of slurry pH and then we will examine the effect of slurry fluid chemistry using ZP measurements to characterize the chemically-modulated development of surface charge of such polishing agents during aqueous polishing.

While particle size has been long recognized an important metric, the most fundamental and critical aspect of the CMP slurry system is the molecular interaction between the suspended particles and the medium at the interface. It is such an interaction, along with the particle–particle interactions, that determines the dynamic properties of the dispersion and strongly affect the performance of the slurry. In polishing, it is generally accepted that the removal rate is directly related to the availability of the surface hydroxyl groups on the abrasive particles. Thus, understanding the role of surface charge effects is important in optical finishing. The zeta potential is a parameter (symbol ζ), which is related to the surface charge (vide the interfacial chemistry), a property that all materials possess, or acquire, when suspended in a fluid; ZP measurements have long been recognized as a surrogate function of particle surface chemistry.

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  • 1. David Fairhurst, Ph.D.Colloid Consultants, LtdThe Utility of Zeta Potential Measurements inthe Characterization of CMP SlurriesHORIBA ScientificWebinarJune 12th 2013
  • 2. Dispersed SystemsExtent of InterfaceInterfacial ChemistryStability and Structure System PropertiesAn interface always exists when one component (phase I) isdispersed in a state of subdivision in a medium (phase II)The interfacial extent and interfacial chemistry are not independent parametersTwo fundamental parameters control the natureand behavior all dispersed systems
  • 3. Measurement Techniques used toCharacterize Particulate SuspensionsINTERFACIAL EXTENT INTERFACIAL CHEMISTRYParticle Size and Distribution*Particle Shape and Morphology*Surface Area* (external/internal)PorositySurface Charge*Nature/type of group(s)Number and distributionDissociation/ionizationPreferential adsorptionHydrophobic/hydrophilic balanceSurface(interfacial)TensionContact Angle*
  • 4. The Nature of Surface Charge in WaterMineral oxide crystal lattices are anisotropic charge development because of n and p defects in crystal structure results in hydroxyl groups (-OH)  can undergo reaction with either H+ or OH- difficult to control surface chemistry in manufacture and processing reactant impuritiesMetal Oxide Surfaces in WaterAdsorption of protons and hydroxyl ionsH+ OH-MOH2+  MOH  MO- + H2OFormation of hydroxylated speciesMn+ + xOH-  M(OH)x(n-x)+M(OH)x(n-x)+ + OH-  M(OH)x+1(n-x-1)+M(OH)x+1(n-x-1)+ + H+  M(OH)x-1(n-x+1)+Dissociation driven by pH avoid swings in pHEquilibria driven byparticle concentration constant %solids
  • 5. Nanoparticle Surface Chemistry10nm particle size:ca 30% of molecules/atoms thatcomprise the molecular structurebecome “surface moieties”1nm particle size:80% of the atoms are on the surfaceA typical micellar solution containing0.1M amphiphile has ~4x104 m2 ofmicellar-water interfacial area per literof solution!For nano-size systems surface charge effects play a dominant rolein determining the physicochemical properties (surface chemicalactivity) of the system as a whole
  • 6. The Electric Double Layerψ = ψd exp [- κx]κ is termed the Debye-Hückel parameter.Its value is dependent upon the concentration andvalence of electrolytic species in solution
  • 7. ψ0 - cannot be measuredψd - mathematical conceptζ - experimental parameterζ ≈ ψdψ = ζ exp [- κx]The electric potential depends (through κ) on the ioniccomposition of the medium.If κ is increased (i.e. the electric double layer is “compressed”)then the potential must decrease
  • 8. Zeta Potential of Corundum (Al2O3) inSolution of Various Electrolytes
  • 9. Depends upon:• Fundamental “surface” sites – howmany, what type• Solution conditions – temperature,pH, electrolyte concentrationUseless to quote a zeta potential valuewithout specifying suspension conditionsZeta Potential: The “Effective” SurfaceCharge in solution
  • 10. Most common measurement technique:microelectrophoresis (ELS/PALS)Electrophoretic mobility,UE = Vp/ExVp is the particle velocity (μm/s) and Ex is the applied electric field (Volt/cm)Relation between ζ and UE is non-linear: UE = 2εε0 ζ F(κa)/3ηThe Henry coefficient, F(κa), is a complex function of ζCalculation of the zeta potentialζ is not determined directlyAll modern commercial instruments provide both the measuredelectrophoretic mobility and an estimation of the zeta potential
  • 11. The Effect of pH on Zeta PotentialISOELECTRIC pHAcidicSurfaceBasicSurfaceMaximum dissociation/ionizationof surface groupsMaximum dissociation/ionizationof surface groupsImperative to determine ZP vs pH profile for any CMP agent prior to useZetapotentialpHpositivenegative7 1400
  • 12. Aqueous Isoelectric Points ofSilica and AluminapHZetaPotential(mV)Silica: iep <pH2Titanium DioxideAlumina: iep ca. pH 9Calcium CarbonateThe IEP can vary batch-to-batch and lot-to-lotM. Kosmulski, Surface Charging and Points of Zero Charge, Boca Raton, FL, CRC Press; 2009
  • 13. Zeta Potential of Two CommercialColloidal SilicasAlways check ZP/pH profile when substituting material fromdifferent suppliersManufacturer 1Manufacturer 2Treated Untreated ▲
  • 14. Slurry Zeta Potential and thePolishing of Optical GlassGlass: Corning 7940 Fused Silica (IEP: 3.5)Polishing agents: CeO2, ZrO2 (monoclinic) and Al2O3 (nanocrystalline)PolishingAgent IEP Surface Chargeat pH 4Surface Chargeat pH 7Surface Chargeat pH 10ZrO26.3 ++ - - - -CeO28.8 ++++ ++ - -Al2O39.3 +++++ +++ -The ZP of the polishing slurries varies with solution pH positive or negative depending upon the metal oxide IEP
  • 15. Slurry Zeta Potential and Glass SurfaceRoughnessThe glass surfaceroughness depends uponthe difference in fluid pHand the IEP of the polishingagentThe more positive thedifference the smootherthe surfaceSolution pH must be > the polishing agent IEPChoose CMP agent whose IEP < Fluid pH
  • 16. Zeta Potential of Non-oxidesThe ZP value affected by presence of surface impurities andcontamination
  • 17. In ConclusionSurface Charge is a fundamental parameter that impacts theperformance of CMP agentsThe polishing efficiency of CMP slurries is dependent upon not just themetal oxide particle size but also the hydoxyl [OH] concentration andstate of dissociationThe Zeta Potential of aqueous suspensions of metal oxide polishingagents critically impacts polishing efficiency. It should be a basic QCmeasurement for all incoming materialIn practice, either the CMP IEP must be < fluid pH or the solution pHmust be adjusted to a value > CMP IEP
  • 18. Q&AAsk a question at labinfo@horiba.comKeep reading the monthly HORIBA Particlee-mail newsletter!Visit the Download Center to find the videoand slides from this webinar.Jeff Bodycomb, Ph.D.P: 866-562-4698E: jeff.bodycomb@horiba.com

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