Thermodynamics in Chip Processing II

Terry A. Ring

CVD

Materials Deposited

• Dielectrics– SiO2, BSG

• Metals– W, Cu, Al

• Semiconductors– Poly silicon (doped)

• Barrier Layers– Nitrides (TaN, TiN), Silicides (WSi2, TaSi2, CoSi,

MoSi2)

Deposition Methods

• Growth of an oxidation layer• Spin on Layer• Chemical Vapor Deposition (CVD)

– Heat = decomposition T of gasses– Plasma enhanced CVD (lower T process)

• Physical Deposition– Vapor Deposition– Sputtering

Critical Issues

• Adherence of the layer• Chemical Compatibility

– Electro Migration– Inter diffusion during subsequent processing

• Strong function of Processing

• Even Deposition at all wafer locations

CVD of Si3N4 - Implantation mask

• 3 SiH2Cl2 + 4 NH3Si3N4 + 6 HCl + 6 H2

– 780C, vacuum

– Carrier gas with NH3 / SiH2Cl2 >>1

• Stack of wafer into furnace– Higher temperature at exit to compensate for gas

conversion losses

• Add gases• Stop after layer is thick enough

CVD of Poly Si – Gate conductor• SiH4 Si + 2 H2

– 620C, vacuum

– N2 Carrier gas with SiH4 and dopant precursor

• Stack of wafer into furnace– Higher temperature at exit to compensate for gas

conversion losses

• Add gases• Stop after layer is thick enough

CVD of SiO2 – Dielectric• Si0C2H5 +O2SiO2 + 2 H2

– 400C, vacuum– He carrier gas with vaporized(or atomized) Si0C2H5

and O2 and B(CH3)3 and/or P(CH3)3 dopants for BSG and BPSG

• Stack of wafer into furnace– Higher temperature at exit to compensate for gas

conversion losses

• Add gases• Stop after layer is thick enough

CVD of W – Metal plugs

• 3H2+WF6 W + 6HF– T>800C, vacuum– He carrier gas with WF6

– Side Reactions at lower temperatures• Oxide etching reactions• 2H2+2WF6+3SiO2 3SiF4 + 2WO2 + 2H2O• SiO2 + 4HF 2H2O +SiF4

• Stack of wafer into furnace– Higher temperature at exit to compensate for gas conversion

losses

• Add gases• Stop after layer is thick enough

Chemical Equilibrium

CVD Reactor

• Wafers in Carriage (Quartz)

• Gasses enter

• Pumped out via vacuum system

• Plug Flow Reactor

Vacuum

CVD Reactor

• Macroscopic Analysis– Plug flow reactor

• Microscopic Analysis– Surface Reaction

• Film Growth Rate

Macroscopic Analysis

• Plug Flow Reactor (PFR)– Like a Catalytic PFR Reactor– FAo= Reactant Molar Flow

Rate– X = conversion– rA=Reaction rate = f(CA)=kCA

– Ci=Concentration of Species, i.– Θi= Initial molar ratio for species i to

reactant, A.– νi= stoichiometeric coefficient– ε = change in number of moles

TR

PC

T

T

P

P

X

XCC

V

AXr

dXFV

g

AoAo

o

o

iiioi

X

reactor

waferA

Aoreactor

1

)(0

'

Combined Effects

Contours = Concentration

Reactor Length Effects

SiH2Cl2(g) + 2 N2O(g) SiO2(s)+ 2 N2(g)+2 HCl(g)

nwafer VReactorPerWafer a

FAo0

X

X1

r'A X( )

d n X( )FAo

VReactorPerWafer a 0

X

X1

r'A X( )

d

rate X( )

r'A X( )4

Dwafer2

SiO2

MwSiO2Awafer

0 50 100 1500

2000

4000

6000

Wafer Number

Th

ick

ness

(nm

)

rate X'( ) 10 minnm

n X'( )0 0.5 10

200

400

600

Conversion

Dep

osi

tio

n R

ate

, W

afe

r N

um

ber

rate X( )

nm

min

n X( )

X

How to solve? Higher T at exit!

Deposition Rate over the Radius

r

wAsA

A

pABe

wA

Ae

RrCC

rfiniteC

ConditionsBoundary

DD

V

Ar

dr

CdrD

dr

d

r

,

0,

1 "

CAs

Thiele Modulus Φ1=(2kRw/DABx)1/2

Radial Effects

This is bad!!!

Pseudo First Order Results

CA 1

sinh 1 sinh 1

00.510.97

0.98

0.99

1

r/R.wafer

Con

cent

rati

on

CA

00.51

4900

4950

5000

5050

r/R.wafer

Thi

ckne

ss(n

m)

rate 1 CA 10 min

nm

x 0.5

Combined Length and Radial Effects

00.512400

2600

2800

3000

3200

3400

3600

r/R.wafer

Th

ick

ness

Rate 10 10 minnm

Rate 20 10 minnm

Wafer 20

Wafer 10

CVD Reactor

• External Convective Diffusion– Either reactants or products

• Internal Diffusion in Wafer Stack– Either reactants or products

• Adsorption

• Surface Reaction

• Desorption

Microscopic Analysis -Reaction Steps

• Adsorption – A(g)+SA*S– rAD=kAD (PACv-CA*S/KAD)

• Surface Reaction-1 – A*S+SS*S + C*S

– rS=kS(CvCA*S - Cv CC*S/KS)

• Surface Reaction-2– A*S+B*SS*S+C*S+P(g)– rS=kS(CA*SCB*S - Cv CC*SPP/KS)

• Desorption: C*S<----> C(g) +S– rD=kD(CC*S-PCCv/KD)

• Any can be rate determining! Others in Equilib.• Write in terms of gas pressures, total site conc.

CMP

What is CMP?

• Polishing of Layer to Remove a Specific Material, e.g. Metal, dielectric

• Planarization of IC Surface Topology

Scratching Cases

• Rolling Indenter• Line Scratches

– Copper Only– Copper & ILD

• Chatter Scratches• Uncovery of Pores

120 microns

CMP Tooling

• Rotating Multi-head Wafer Carriage

• Rotating Pad• Wafer Rests on Film of

Slurry • Velocity= -

(WtRcc)–[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)2398-402(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),761-766(1999).

PadU

Pappliedy

x

h(x)

Wafer

Slurry

D

Copper Dissolution

• Solution Chemistry– Must Dissolve Surface

Slowly without Pitting

• Supersaturation

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