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Lecture 13.0

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

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Lecture 13.0

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  1. Lecture 13.0 Chemical Mechanical Polishing

  2. What is CMP? • Polishing of Layer to Remove a Specific Material, e.g. Metal, dielectric • Planarization of IC Surface Topology

  3. CMP Tooling • Rotating Multi-head Wafer Carriage • Rotating Pad • Wafer Rests on Film of Slurry • Velocity= -(WtRcc)–[Rh(Wh –Wt)] • when Wh=Wt Velocity = const.

  4. 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)

  5. Pad Properties • Rodel Suba IV • Polyurethane • tough polymer • Hardness = 55 • Fiber Pile • Specific Gravity = 0.3 • Compressibility=16% • rms Roughness = 30μm • Conditioned

  6. 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. Sun,S.C., Yeh, F.L. and Tien, H.Z., Mat. Res. Cos. Symp. Proc. 337,139(1994)

  7. CMP Pad Modeling • Pad Mechanical Model - Planar Pad • Warnock,J.,J. Electrochemical Soc.138(8)2398-402(1991). • Does not account for Pad Microstructure

  8. CMP Modeling • 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).

  9. 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 • 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).

  10. Copper Dissolution • Solution Chemistry • Must Dissolve Surface Slowly without Pitting • Supersaturation Corrosion Immunity Johnson, H.E.and Leja, J., J. Electrochem. Soc. 112,638(1965).

  11. 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 P*V Jairath, R., Desai, M., Stell, M., Toles, R. and Scherver-Brewer, D., Mat. Res. Soc. Symp. Proc. 337,121(1994).

  12. Particle Effects-Aggregated Particles are used Data from Cabot Patent 5,527,423 Are rate results due to fractal shape or % alpha?

  13. Indentation Plastic Damage Brittle Damage Elastic Behavior

  14. 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. VARIOUS GLASSES Izumitani, T. in Treatis on Materials Science and Techn., Academic Press, 1979, p.115.

  15. 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 Jairath, R., Desai, M., Stell, M., Toles, R. and Scherver-Brewer, D., Mat. Res. Soc. Symp. Proc. 337,121(1994).

  16. Mass Transfer-Bohner, 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

  17. 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) Near Wafer Surface Pad Surface Wafer Surface Tufts University Expt. Results

  18. Pseudo-2D Macro Flow Model x = Rw - r • Velocity field in the gap near edge of wafer x=5 micron x=50 micron x=500 micron x=5 mm Velocity Field Shear Rate Across Gap

  19. Solution Complexation-Chen, 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

  20. Silica Dissolution - Solution Complexation

  21. Solution Complexation H3SiO4-1 Si(OH)3·H2O+1 Si(OH)40

  22. 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

  23. Copper Electro-Chemistry • Reaction-Sainio, C.A., Duquette, D.J., Steigerwald, J.M., Murarka, J. Electron. Mater., 25,1593(1996). • Activity Based Reaction Rate-Gutman, E.M., “Mechanochemistry at Solid Surfaces,” World Scientific Publishing, Singapore, 1994. • k”=reaction rate constant 1=forward,2=reverse • aj=activity, j=stociometry, μj =chemical potential • Ã =Σνjμj =Overall Reaction Affinity

  24. Chemical Potential • Mineral Dissolution • Metal Dissolution • ø=Electrode Potential • =Faraday’s Constant

  25. Fluid FlowMomentum Balance • Newtonian Lubrication Theory • Non-Newtonian Fluids g

  26. 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, Yc0 Increase P

  27. Wall Shear Rate, w • Product of • Viscosity at wall shear stress • Velocity Gradient at wall

  28. Slurries are Non-Newtonian Fluids • Crossian Fluid- Shear Thinning Increasing Particle Rotation Rate of Strain Diffusion altered rotation

  29. Mass Transfer into Slurries • No Known Theories! • 2-D CMP Model gives this Heuristic • Wall Shear Stress, w and Abrasive Concentration, CA are Important!

  30. Mechanical Properties • Elastic Deformation • Plastic Damage • Plastic Deformation • Scratching

  31. Abrasive Particles Cause Surface StressA. Evans “Mechanical Abrasion” • Collisions with Wafer Surface Cause Hertzian Stress • Collision Rate ? • Stress Due To Collision • P[ =(H tan2)1/3 Uk2/3] is the peak load (N) due to the incident kinetic energy of the particles, Uk,The load is spread over the contact area

  32. Mechanical Effects on Mass Transfer • Chemical Potential-Gutman, E.M., “Mechanochemistry at Solid Surfaces,” World Scientific Publishing, Singapore, 1994. • Mineral Dissolution • Metal Dissolution

  33. Effect of Stress on DissolutionMetals Mineral-CaCO3 Slope of Curve=Rate 2xRate Stress=7 MPa Inhibitor=Caprylic Acid C7H15COOH Reaction in 3% HCl

  34. 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.

  35. 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 P-B 3.4 2.1

  36. Hertzian Shear Stress • Delatches the Oxide Layer • Weak Interface Bond • 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.

  37. CMP Problems • Defectivity • WIWNU • Dishing and Erosion • Line Erosion • Scratching

  38. Scratching Cases • Rolling Indenter • Line Scratches • Copper Only • Copper & ILD • Chatter Scratches • Uncovery of Pores

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