Lecture 13.0Lecture 13.0
Chemical Mechanical Polishing
What is CMP?What is CMP?
Polishing of Layer to Remove a Specific Material, e.g. Metal, dielectric
Planarization of IC Surface Topology
CMP ToolingCMP Tooling Rotating Multi-head
Wafer Carriage Rotating Pad Wafer Rests on Film
of Slurry Velocity= -
(WtRcc)–[Rh(Wh –Wt)] when Wh=Wt
Velocity = const.
SlurrySlurry
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 PropertiesPad Properties
Rodel Suba IV Polyurethane
– tough polymer• Hardness = 55
– Fiber Pile• Specific Gravity = 0.3• Compressibility=16%• rms Roughness =
30μm
– Conditioned
Heuristic Understanding of CMPHeuristic 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 ModelingCMP Pad Modeling Pad Mechanical Model - Planar Pad
• Warnock,J.,J. Electrochemical Soc.138(8)2398-402(1991).
Does not account for Pad Microstructure
CMP ModelingCMP 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).
PadU
Pappliedy
x
h(x)
Wafer
Slurry
D
Abrasive in 2D Flow ModelAbrasive in 2D Flow Model
Aw CAbrasiveoutwithRatePolishing
AbrasivewithRatePolishing 1
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).
Copper DissolutionCopper Dissolution
Solution Chemistry– Must Dissolve
Surface Slowly without Pitting
Supersaturation
Effect of Particles on CMP is Unknown.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 EffectsParticle Effects-Aggregated Particles are used-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 5085 40% 17.8 88 390 110
100 20% 15.1 87 370 NA
Layer Hardness EffectsLayer 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 ConditioningPad 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-Mass Transfer-Bohner, M. Lemaitre, J. and Bohner, M. Lemaitre, J. and Ring, T.ARing, T.A., "Kinetics of Dissolution of ., "Kinetics of Dissolution of --tricalcium phosphate," J. Colloid Interface Sci. 190,37-48(1997).tricalcium phosphate," J. Colloid Interface Sci. 190,37-48(1997).
Driving Force for dissolution, Ceq(1-S) S=C/Ceq 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-Solution Complexation-Chen, Y. and Chen, Y. and Ring, T.A.Ring, T.A., "Forced Hydrolysis of In(OH)3- Comparison of , "Forced Hydrolysis of In(OH)3- Comparison of Model with Experiments" J. Dispersion Sci. Tech., 19,229-247(1998).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 ComplexationSilica 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 ΔH3= -12 kJ/mole 4Si(OH)4 + 4 OH-1 <---> Si4O4(OH)4
-4 + 8 H2O pK4=~ -27
Solution ComplexationSolution Complexation
Si(OH)40
H3SiO4-1
Si(OH)3·H2O+1
Copper CMP uses a More Copper CMP uses a More Complex Solution ChemistryComplex Solution ChemistryK3Fe(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-ChemistryCopper Electro-ChemistryReaction-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
46233
36 )()(2)( CNFeNHCuNHCNFeCu EQK
j g
jproductsj
jtsreacjj TR
AakakakFluxJ jjj )1~
exp()( 2tan
21
Chemical PotentialChemical Potential
Mineral Dissolution
Metal Dissolution
ø=Electrode Potential=Faraday’s Constant
iigioigioi cTRaTR lnln
iiigioiigioi zcTRzaTR lnln
Fluid FlowFluid FlowMomentum BalanceMomentum Balance Newtonian
Lubrication Theory
Non-Newtonian Fluids
PadU
Pappliedy
x
h(x)
Wafer
Slurry
D
),(0 2 yxuP
),()(0 2 yxuP
CMP Flow Analogous to Tape CMP Flow Analogous to Tape CastingCasting--RING T.A.,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, Advances in Ceramics vol. 26", M.F. Yan, K. Niwa, H.M. O'Bryan and W. S. Young, editors ,p. 269-576, (1988).(1988).Newtonian Yc=0,
– Flow Profile depends upon PressureBingham Plastic, Yc0
Wall Shear Rate, Wall Shear Rate, ww
Product of– Viscosity at wall shear stress– Velocity Gradient at wall
Slurries are Non-Newtonian FluidsSlurries are Non-Newtonian Fluids
Crossian Fluid- Shear Thinning
Mass Transfer into SlurriesMass Transfer into Slurries
No Known Theories!
2-D CMP Model gives this Heuristic
Wall Shear Stress, w and Abrasive Concentration, CA are Important!
AwCAbrasiveatewithoutPolishingRasiveatewithAbrPolishingR 1
Mechanical PropertiesMechanical Properties
Elastic DeformationPlastic DamagePlastic Deformation
– Scratching
Abrasive Particles Cause Surface StressAbrasive Particles Cause Surface StressA. Evans “Mechanical Abrasion”A. Evans “Mechanical Abrasion”
Collisions with Wafer Surface Cause Hertzian Stress
Collision Rate ?
Stress Due To Collision P[ =(H tan2 )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
Hertzian Stress, sigma/Po
N
Mechanical Effects on Mass Mechanical Effects on Mass TransferTransferChemical Potential-Gutman, E.M., “Mechanochemistry at
Solid Surfaces,” World Scientific Publishing, Singapore, 1994.
– Mineral Dissolution
– Metal Dissolution
migioi VaTR ln
miigioi VzaTR ln
TRVVX
g
mii
T
i )ˆ()ln( ,
Effect of Stress on DissolutionEffect of Stress on DissolutionMetals Mineral-CaCOMetals Mineral-CaCO33
Mechano-Chemical EffectMechano-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 StressOxidation 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 Hertzian Shear StressShear Stress
Delatches the Oxide LayerWeak 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.
h
Lateral Cracks
b
CL
Hertzian Shear Stress, Tau/Po
M