Lecture 13.0 Chemical Mechanical Polishing What is CMP? Polishing of Layer to Remove a Specific Material, e.g. Metal, dielectric Planarization of IC Surface Topology CMP Tooling Rotating Multi-head Wafer Carriage Rotating Pad Wafer Rests on Film of Slurry Velocity= (WtRcc)–[Rh(Wh –Wt)] when Wh=Wt Velocity = const. - Slurry Aqueous Chemical Mixture – Material to be removed is soluble in liquid – Material to be removed reacts to form an oxide layer which is abraded by abrasive Abrasive – 5-20% wgt of ~200±50nm particles • Narrow PSD, high purity(<100ppm) • Fumed particle = fractal aggregates of spherical primary particles (15-30nm) Pad Properties Rodel Suba IV Polyurethane – tough polymer • Hardness = 55 – Fiber Pile • Specific Gravity = 0.3 • Compressibility=16% • rms Roughness = 30μm – Conditioned Heuristic Understanding of CMP Preston Equation(Preston, F., J. Soc. Glass Technol., 11,247,(1927). – Removal Rate = Kp*V*P • V = Velocity, P = pressure and Kp is the proportionality constant. CMP Pad Modeling Pad Mechanical Model - Planar Pad • Warnock,J.,J. Electrochemical Soc.138(8)2398402(1991). Does not account for Pad Microstructure CMP Modeling Papplied y D Wafer h(x) Slurry x U Pad Numerical Model of Flow under Wafer – 3D-Runnels, S.R. and Eyman, L.M., J. Electrochemical Soc. 141,1698(1994). – 2-D-Sundararajan, S., Thakurta, D.G., Schwendeman, D.W., Muraraka, S.P. and Gill, W.N., J. Electrochemical Soc. 146(2),761-766(1999). Abrasive in 2D Flow Model In the 2-D approach the effect of the slurry and specifically the particles in the slurry is reduced to that of an unknown constant, , determined by experimental measurements Polishing Rate with Abrasive 1 w CA Polishing Rate without Abrasive where w is the shear stress at the wafer surface and CA is the concentration of abrasive. Sundararajan, Thakurta, Schwendeman, Mararka and Gill, J. Electro Chemical Soc. 146(2),761-766(1999). Copper Dissolution Solution Chemistry – Must Dissolve Surface Slowly without Pitting Supersaturation Effect of Particles on CMP is Unknown. Effect of Particles on CMP – Particle Density – Particle Shape & Morphology – Crystal Phase – Particle Hardness & Mechanical Properties – Particle Size Distribution – Particle Concentration – Colloid Stability Particle Effects -Aggregated Particles are used SSA(m2/gm) Phase(%alpha)Primary Diameter(nm)Agg. Diameter(nm)W Rate(nm/min.) Selectivity(W/SiO2) 55 80% 27.5 86 485 50 85 40% 17.8 88 390 110 100 20% 15.1 87 370 NA Indentation CL CR Elastic Behavior Plastic Damage Brittle Damage Layer Hardness Effects Effect of Mechanical Properties of Materials to be polished Relationship of pad, abrasive and slurry chemistry needed for the materials being polished. Pad Conditioning Effect of Pad on CMP • Roughness increases Polishing Rate – Effect of Pad Hardness &Mechanical Properties – Effect of Conditioning – Reason for Wear-out Rate Mass TransferBohner, M. Lemaitre, J. and Ring, T.A., "Kinetics of Dissolution of tricalcium phosphate," J. Colloid Interface Sci. 190,37-48(1997). Driving Force for dissolution, – C-Ceq=Ceq(1-S) – S=C/Ceq Different Rate Determining Steps – Diffusion - J(Flux) = kcCeq (1-S) – Surface Nucleation • Mono - J exp(1-S) • Poly - J (1-S)2/3 exp(1-S) – Spiral(Screw Dislocation) - J (1-S)2 Macro Fluid Flow Continuity Equation Navier Stokes Equation (Newtonian Fluid) – Rotation of Wafer (flat) – Rotation of Pad (flat) • Sohn, I.-S., Moudgil, B., Singh, R. and Park, C.-W., Mat. Res. Soc. Symp. Proc. v 566, p.181-86(2000) Velocity Vector Field Velocity Vector Field Near Wafer Surface ( Ux, Uy ) Wafer Surface ( Ux, Uy ) Pad Surface Tufts University Expt. Results Pseudo-2D Macro Flow Model x = Rw - r Velocity field in the gap near edge of wafer Velocity Field y y y ) cerf ( ) cerf ( ) cerf ( x x x x x 2 2 2 V V V V V Vx V y 2 y cerf ( 2 ) cerf ( ) ... x x x x 2 2 V V V V y and 1 x x 2 2 V V Shear Rate 1/ 2 2 2 p L Rww r p r L ( cos ) sin Rw w Rw Rw w Rw Across Gap Solution ComplexationChen, Y. and Ring, T.A., "Forced Hydrolysis of In(OH)3- Comparison of Model with Experiments" J. Dispersion Sci. Tech., 19,229-247(1998). Solutions are Not Simple but Complex Complexation Equilibria – i M+m + j A-a [Mi Aj](im-ja) – Kij ={[Mi Aj](im-ja)}/{M+m}i {A-a }j {}=Activity – Multiple Anions - A, e.g. NO3-, OH– Multiple Metals - M, e.g. M+m, NH4+, H+ Complexation Needed to Determine the Equilibrium and Species Activity,{}i=ai Silica Dissolution - Solution Complexation SiO2(c) + H2O <---> Si(OH)4 Amorphous SiO2 dissolution Si(OH)4 + H+1 <---> Si(OH)3·H2O+1 pKo= -2.44 ΔHo= -16.9 kJ/mole Si(OH)4 + OH-1 <---> H3SiO4-1 + H2O pK1= -4.2 ΔH1= -5.6 kJ/mole Si(OH)4 + 2 OH-1 <---> H2SiO4-2 + 2 H2O pK2= -7.1 ΔH2= -6.3 kJ/mole 4Si(OH)4 + 2 OH-1 <---> Si4O6(OH)6-2 + 6 H2O pK3= -12.0 4Si(OH)4 + 4 OH-1 <---> Si4O4(OH)4-4 + 8 H2O pK4=~ -27 ΔH3= -12 kJ/mole Solution Complexation H3SiO4-1 Si(OH)3·H2O+1 Si(OH)40 Copper CMP uses a More Complex Solution Chemistry K3Fe(CN)6 + NH4OH – Cu+2 Complexes • • • • • OH- - i:j= 1:1, 1:2, 1:3, 1:4, 2:2, 3:4 NO3- -weak NH3 - i:j= 1:1, 1:2, 1:3, 1:4, 2:2, 2:4 Fe(CN)6-3 - i:j=1:1(weak) Fe(CN)6-4 - i:j=1:1(weak) – Cu+1 Complexes Copper Electro-Chemistry Reaction-Sainio, C.A., Duquette, D.J., Steigerwald, J.M., Murarka, J. Electron. Mater., 25,1593(1996). EQ Cu Fe(CN )36 2 NH3 Cu( NH3 )2 Fe(CN )64 K Activity Based Reaction Rate-Gutman, E.M., “Mechanochemistry at Solid Surfaces,” World Scientific Publishing, Singapore, 1994. J ( Flux ) k1 a j j k2 j reac tan ts aj j products j k 2 a j j j ~ A exp( 1) Rg T – k”=reaction rate constant 1=forward,2=reverse – aj=activity, j=stociometry, μj =chemical potential – Ã =Σνjμj =Overall Reaction Affinity Chemical Potential Mineral Dissolution i io RgT ln ai io Rg T ln i ci Metal Dissolution i io RgT ln ai zi io RgT ln i ci zi ø=Electrode Potential =Faraday’s Constant Fluid Flow Papplied y D Wafer Momentum Balance h(x) Slurry Newtonian Lubrication Theory 0 P u ( x, y) 2 Non-Newtonian Fluids 0 P ( )u ( x, y) 2 x U Pad CMP Flow Analogous to Tape Casting -RING T.A., Advances in Ceramics vol. 26", M.F. Yan, K. Niwa, H.M. O'Bryan and W. S. Young, editors ,p. 269-576, (1988). Newtonian Yc=0, – Flow Profile depends upon Pressure Bingham Plastic, Yc0 Wall Shear Rate, w Product of – Viscosity at wall shear stress – Velocity Gradient at wall Slurries are Non-Newtonian Fluids Crossian Fluid- Shear Thinning Mass Transfer into Slurries No Known Theories! 2-D CMP Model gives this Heuristic PolishingR atewithAbrasive 1 wC A PolishingR atewithoutAbrasive Wall Shear Stress, w and Abrasive Concentration, CA are Important! Mechanical Properties Elastic Deformation Plastic Damage Plastic Deformation – Scratching Abrasive Particles Cause Surface Stress A. Evans “Mechanical Abrasion” Collisions with Wafer Surface Cause Hertzian Stress Collision Rate ? Hertzian Stress, sigma/Po Stress Due To Collision P[ =(H tan2 )1/3 Uk2/3] is the peak load (N) due to the N incident kinetic energy of the particles, Uk,The load is spread over the contact area Mechanical Effects on Mass Transfer Chemical Potential-Gutman, E.M., “Mechanochemistry at Solid Surfaces,” World Scientific Publishing, Singapore, 1994. – Mineral Dissolution i io Rg T ln ai Vm (Vˆi Vi ,m ) ln( X i ) Rg T T – Metal Dissolution i io Rg T ln ai zi Vm Effect of Stress on Dissolution Metals Mineral-CaCO3 Mechano-Chemical Effect – Effect on Chemical Potential of solid – Effect of Activity of Solid As a result, Dissolution Rate of Metal and Mineral are Enhanced by Stress. Oxidation of Metal Causes Stress Stress, i = E i (P-B i – 1)/(1 - i) • P-Bi is the Pilling-Bedworth ratio for the oxide Hertzian Shear Stress Hertzian Shear Stres s , T au/Po Delatches the Oxide Layer Weak Interface Bond M CL h b Lateral Cracks CL=0.096 (E/H)2/5 Kc-1/2 H-1/8 [ 1- (Po/P)1/4]1/2 P5/8 • A. Evans, UC Berkeley. CMP Problems Defectivity – WIWNU – Dishing and Erosion – Line Erosion – Scratching Scratching Cases Rolling Indenter Line Scratches – Copper Only – Copper & ILD Chatter Scratches Uncovery of Pores 120 microns