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= - (Wt  Rcc)–[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 = K p *V*P V = Velocity, P = pressure and K p is the proportionality constant.

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

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), (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 where  w is the shear stress at the wafer surface and C A is the concentration of abrasive. Sundararajan, Thakurta, Schwendeman, Mararka and Gill, J. Electro Chemical Soc. 146(2), (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

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 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 eq (1-S) S=C/C eq Different Rate Determining Steps –Diffusion - J(Flux)  (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

Solution Complexation- Chen, Y. and Ring, T.A., "Forced Hydrolysis of In(OH)3- Comparison of Model with Experiments" J. Dispersion Sci. Tech., 19, (1998). Solutions are Not Simple but Complex Complexation Equilibria –i M +m + j A -a  [M i A j ] (im-ja) –K ij ={[M i A j ] (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 + Complexation Needed to Determine the Equilibrium and Species Activity,{} i =a i

Silica Dissolution - Solution Complexation

Solution Complexation Si(OH) 4 0 H 3 SiO 4 -1 Si(OH) 3 ·H 2 O +1

Copper CMP uses a More Complex Solution Chemistry 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) i:j=1:1(weak) Fe(CN) 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). Activity Based Reaction Rate -Gutman, E.M., “Mechanochemistry at Solid Surfaces,” World Scientific Publishing, Singapore, –k”=reaction rate constant 1=forward,2=reverse –a j =activity,  j =stociometry, μ j =chemical potential –Ã =Σν j μ j =Overall Reaction Affinity

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

Fluid Flow Momentum Balance Newtonian Lubrication Theory 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 , (1988). Newtonian Y c =0, –Flow Profile depends upon Pressure Bingham Plastic, Y c  0

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 Wall Shear Stress,  w and Abrasive Concentration, C A 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 ? Stress Due To Collision P[ =(H tan 2  ) 1/3 U k 2/3 ] is the peak load (N) due to the incident kinetic energy of the particles, U k,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, –Mineral Dissolution –Metal Dissolution

Effect of Stress on Dissolution Metals Mineral-CaCO 3

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-B i is the Pilling-Bedworth ratio for the oxide

Hertzian Shear Stress Delatches the Oxide Layer Weak Interface Bond C L =0.096 (E/H) 2/5 K c -1/2 H -1/8 [ 1- (P o /P) 1/4 ] 1/2 P 5/8 A. Evans, UC Berkeley.