Lecture 13 0 Chemical Mechanical Polishing What

Lecture 13. 0 Chemical Mechanical Polishing

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

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

Slurry l 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 l Abrasive – 5 -20% wgt of ~200± 50 nm particles • Narrow PSD, high purity(<100 ppm) • Fumed particle = fractal aggregates of spherical primary particles (15 -30 nm)

Pad Properties Rodel Suba IV l Polyurethane l – tough polymer • Hardness = 55 – Fiber Pile • Specific Gravity = 0. 3 • Compressibility=16% • rms Roughness = 30μm – Conditioned

Heuristic Understanding of CMP l 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 l Pad Mechanical Model - Planar Pad • Warnock, J. Electrochemical Soc. 138(8)2398402(1991). l Does not account for Pad Microstructure

CMP Modeling l Numerical Model of Flow under Wafer – 3 D-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 2 D Flow Model l 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 l 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 l Solution Chemistry – Must Dissolve Surface Slowly without Pitting l Supersaturation

Effect of Particles on CMP is Unknown. l 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

Indentation Elastic Behavior Plastic Damage Brittle Damage

Layer Hardness Effects l Effect of Mechanical Properties of Materials to be polished l Relationship of pad, abrasive and slurry chemistry needed for the materials being polished.

Pad Conditioning l Effect of Pad on CMP • Roughness increases Polishing Rate – Effect of Pad Hardness &Mechanical Properties – Effect of Conditioning – Reason for Wear-out Rate

Mass Transfer- Bohner, M. Lemaitre, J. and Ring, T. A. , "Kinetics of Dissolution of tricalcium phosphate, " J. Colloid Interface Sci. 190, 37 -48(1997). l Driving Force for dissolution, – C-Ceq=Ceq(1 -S) – S=C/Ceq l Different Rate Determining Steps – Diffusion - J(Flux) = kc. Ceq (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 l Navier Stokes Equation (Newtonian Fluid) l – Rotation of Wafer (flat) – Rotation of Pad (flat) • Sohn, I. -S. , Moudgil, B. , Singh, R. and Park, C. -W. , Mat. Res. Soc. Symp. Proc. Near Wafer Surface Pad Surface Tufts University Expt. Results

Pseudo-2 D Macro Flow Model x = Rw - r l Velocity field in the gap near edge of wafer Velocity Field Shear Rate Across Gap

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 l Complexation Equilibria l – 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. NO 3 -, OH– Multiple Metals - M, e. g. M+m, NH 4+, H+ l Complexation Needed to Determine the Equilibrium and Species Activity, {}i=ai

Silica Dissolution - Solution Complexation

Solution Complexation H 3 Si. O 4 -1 Si(OH)3·H 2 O+1 Si(OH)40

Copper CMP uses a More Complex Solution Chemistry l K 3 Fe(CN)6 + NH 4 OH – Cu+2 Complexes • • • OH- - i: j= 1: 1, 1: 2, 1: 3, 1: 4, 2: 2, 3: 4 NO 3 - -weak NH 3 - 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 l Reaction-Sainio, C. A. , Duquette, D. J. , Steigerwald, J. M. , Murarka, J. Electron. Mater. , 25, 1593(1996). l 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

Chemical Potential l Mineral l Metal Dissolution l ø=Electrode Potential l =Faraday’s Constant

Fluid Flow Momentum Balance l Newtonian Lubrication Theory l Non-Newtonian Fluids

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, Yan, Niwa, (1988). l Newtonian Yc=0, – Flow Profile depends upon Pressure l Bingham Plastic, Yc 0

Wall Shear Rate, w l Product of – Viscosity at wall shear stress – Velocity Gradient at wall

Slurries are Non-Newtonian Fluids l Crossian Fluid- Shear Thinning

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

Mechanical Properties l Elastic Deformation l Plastic Damage l Plastic Deformation – Scratching

Abrasive Particles Cause Surface Stress A. Evans “Mechanical Abrasion” l Collisions with Wafer Surface Cause Hertzian Stress l Collision Rate ? l Stress l Due To Collision P[ =(H tan 2 )1/3 Uk 2/3] is the peak load (N) due to the incident kinetic energy of the particles, Uk, The load is spread over the contact area

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

Effect of Stress on Dissolution Metals Mineral-Ca. CO 3

Mechano-Chemical Effect – Effect on Chemical Potential of solid – Effect of Activity of Solid l As a result, Dissolution Rate of Metal and Mineral are Enhanced by Stress.

Oxidation of Metal Causes Stress l Stress, i = E i (P-B i – 1)/(1 - i) • P-Bi is the Pilling-Bedworth ratio for the oxide

Hertzian Shear Stress l Delatches the Oxide Layer l Weak Interface Bond l CL=0. 096 (E/H)2/5 Kc-1/2 H-1/8 [ 1 - (Po/P)1/4]1/2 P 5/8 • A. Evans, UC Berkeley.

CMP Problems l Defectivity – WIWNU – Dishing and Erosion – Line Erosion – Scratching

Scratching Cases Rolling Indenter l Line Scratches l – Copper Only – Copper & ILD Chatter Scratches l Uncovery of Pores l