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