Transcript Lecture13.0 CMP.ppt

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 = 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
 
Rww  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  Rg T ln ai   io  Rg T ln  i ci
Metal Dissolution
 i   io  Rg T ln ai  zi    io  Rg T 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, Yc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
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