Chemical Mechanical Polishing

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 WhWt 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 20050nm particles
• Narrow PSD, high purity(lt100ppm)
• 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
• Compressibility16
• rms Roughness 30µm
• Conditioned

6
Heuristic Understanding of CMP

• Preston Equation(Preston, F., J. Soc. Glass
  Technol., 11,247,(1927).
• Removal Rate KpVP
• 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(1
  991).
• 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

PV
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-CeqCeq(1-S)
• SC/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

x5 micron x50 micron x500 micron x5 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 Mm j A-a ? Mi Aj(im-ja)
• Kij Mi Aj(im-ja)/Mmi A-a j
  Activity
• Multiple Anions - A, e.g. NO3-, OH-
• Multiple Metals - M, e.g. Mm, NH4, H
• Complexation Needed to Determine the Equilibrium
  and Species Activity,iai

20
Silica Dissolution - Solution Complexation
21
Solution Complexation
H3SiO4-1
Si(OH)3H2O1
Si(OH)40
22
Copper CMP uses a More Complex Solution Chemistry

• K3Fe(CN)6 NH4OH
• Cu2 Complexes
• OH- - ij 11, 12, 13, 14, 22, 34
• NO3- -weak
• NH3 - ij 11, 12, 13, 14, 22, 24
• Fe(CN)6-3 - ij11(weak)
• Fe(CN)6-4 - ij11(weak)
• Cu1 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.
• kreaction rate constant 1forward,2reverse
• ajactivity, ?jstociometry, µj chemical
  potential
• Ã S?jµj Overall Reaction Affinity

24
Chemical Potential

• Mineral Dissolution
• Metal Dissolution
• øElectrode Potential
• ?Faradays 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 Yc0,
• 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 Stress A. 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 CurveRate
2xRate
Stress7 MPa
InhibitorCaprylic 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
• CL0.096 (E/H)2/5 Kc-1/2 H-1/8 1- (Po/P)1/41/2
  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