Lecture 12.0
Deposition
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 NH3Si3N4 + 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 +O2SiO2 + 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
X
dX
– FAo= Reactant Molar Flow Vreactor  FAo
0 ' Awafer
Rate
 rA ( X )
Vreactor
– X = conversion
– rA=Reaction rate = f(CA)=kCA
 i   i X  P  To 
  
– Ci=Concentration of Species, i. Ci  Cio 
 1  X  Po  T 
– Θi= Initial molar ratio for
species i to reactant, A.
PAo
– νi= stoichiometeric coefficient C Ao 
RgT
– ε = change in number of moles
Combined Effects
Contours = Concentration
Reactor Length
Effects
SiH2Cl2(g) + 2 N2O(g) SiO2(s)+ 2 N2(g)+2 HCl(g)
nwafer VReactorPerWafer a
FAo
X



0
r'A ( X) 
rate( X) 
 SiO2
6000
Thickness(nm)
Deposition Rate, Wafer Number
Awafer
600
400
nm
m in
n ( X)
200
4000
rate ( X')  10 min
nm
2000
0
0
1
dX
r'A ( X)

2
Dwafer
4
MwSiO2
rate ( X)
X

n( X) 

 VReactorPerWafer a 
0
FAo
1
dX
r'A ( X)
0
0.5
X
Conversion
1
0
50
100
150
n ( X')
Wafer Number
How to solve? Higher T at exit!
Deposition Rate over the Radius
CAs
r
1 d 
d C A  " Aw

D
r

  rA
e
r dr 
dr 
V
De  DAB p
Boundary Conditions
C A  finite, r  0
C A  C As , r  Rw
Thiele Modulus
Φ1=(2kRw/DABx)1/2
Radial Effects
Pseudo First Order Results
CA    


1  s inh   1 

 
  s inh  1


5050
Thickness(nm)
Concentration
1
0.99
CA 
0.98
0.97
1
0.5

r/R.wafer
x  0.5
0


rate 1CA   10 min
5000
nm
4950
4900
1
0.5

r/R.wafer
This is bad!!!
0
Combined Length and Radial Effects
3600
Wafer 10
3400
3200
Thickness
Rate    10  10 min
nm
Rate    20  10 min
3000
nm
2800
2600
Wafer 20
2400
1
0. 5

r/R. wafer
0
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)+SA*S
– rAD=kAD (PACv-CA*S/KAD)

Surface Reaction-1
– A*S+SS*S + C*S
– rS=kS(CvCA*S - Cv CC*S/KS)

Surface Reaction-2
– A*S+B*SS*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.
Rate Limiting Steps

Adsorption
– rA=rAD= kADCt (PA- PC /Ke)/(1+KAPA+PC/KD+KIPI)

Surface Reaction
– (see next slide)

Desorption
– rA=rD=kDCt(PA - PC/Ke)/(1+KAPA+PC/KD+KIPI)
Surface Reactions
Deposition of Ge
"
Dep
r

k s K A K H PGeCl2 PH 2
1  K P
A GeCl2
 K H PH 2

3
Ishii, H. and Takahashik Y., J. Electrochem. Soc. 135,1539(1988).
Silicon Deposition

Overall Reaction
– SiH4  Si(s) + 2H2

Two Step Reaction Mechanism
– SiH4  SiH2(ads) + H2
– SiH2 (ads)  Si(s) + H2

Rate=kadsCt PSiH4/(1+Ks PSiH4)
– Kads Ct = 2.7 x 10-12 mol/(cm2 s Pa)
– Ks=0.73 Pa-1
Silicon Epitaxy vs. Poly Si


Substrate has Similar Crystal Structure and
lattice spacing
– Homo epitaxy Si on Si
– Hetero epitaxy GaAs on Si
Must have latice match
– Substrate cut as specific angle to assure latice match

Probability of adatoms getting together to form stable
nuclei or islands is lower that the probability of adatoms
migrating to a step for incorporation into crystal lattice.
– Decrease temp.
– Low PSiH4
– Miss Orientation angle
Surface Diffusion
Monocrystal vs. Polycrystalline
PSiH4=? torr
Dislocation Density

Epitaxial Film
– Activation
Energy of
Dislocation
• 3.5 eV
Physical Vapor Deposition
Evaporation
from Crystal
 Deposition of
Wall

Physical Deposition - Sputtering
Plasma is used
 Ion (Ar+) accelerated into a target
material
 Target material is vaporized

– Target Flux  Ion Flux* Sputtering Yield
Diffuses from target to wafer
 Deposits on cold surface of wafer

DC Plasma

Glow Discharge
RF Plasma Sputtering for
Deposition and for Etching
RF + DC field
Sputtering Chemistries

Target
–
–
–
–


–
–
–
–
Al
Cu
TiW
TiN
Gas
– Argon
Deposited Layer

Al
Cu
TiW
TiN
Poly Crystalline
Columnar
Structure
Deposition Rate

Sputtering Yield, S
– S=α(E1/2-Eth1/2)

2/3
 Zx 
Z  Z 
x
 t
U  surface binding energy
Zi  atomic numbers of (t) target and (x) gas
5.2
Zt

U ( Z t2 / 3  Z x2 / 3 )3 / 4
Deposition Rate 
– Ion current into Target *Sputtering Yield
–
Fundamental Charge
Sheath
RF Plasma

Plasma
Sheath
Electrons dominate in the Plasma
– Plasma Potential, Vp=0.5(Va+Vdc)
– Va = applied voltage amplitude (rf)

Ions Dominate in the Sheath
– Sheath Potential, Vsp=Vp-Vdc

Reference Voltage is ground such
that Vdc is negative
rf
Floating Potential
Sheath surrounds object
 Floating potential, Vf

k BTe  M i 

Vf  Vp ln 
2q  2.3me 
Te  electron Temperatur e

kBTe=eV
– due to the accelerating Voltage
Plasma Chemistry

Dissociation leading to reactive neutrals
– e + H2  H + H + e
– e + SiH4  SiH2 + H2 + e
– e + CF4  CF3 + F + e
– Reaction rate depends upon electron
density
– Most Probable reaction depends on
lowest dissociation energy.
Plasma Chemistry

Ionization leading to ion
– e + CF4  CF3- + F
– e + SiH4  SiH3+ + H + 2e

Reaction depend upon electron
density
Plasma Chemistry
Electrons have more energy
 Concentration of electrons is ~108 to
1012 1/cc
 Ions and neutrals have 1/100 lower
energy than electrons
 Concentration of neutrals is 1000x
the concentration of ions

Oxygen Plasma

Reactive Species
– O2+eO2+ + 2e
– O2+e2O + e
– O + e  O– O2+ + e  2O
Plasma Chemistry

Reactions occur at the Chip Surface
– Catalytic Reaction Mechanisms
– Adsorption
– Surface Reaction
– Desorption
• e.g. Langmuir-Hinshelwood Mechanism
Plasma Transport Equations

Flux, J
dnn
J n  Dn
for neutrals
dx
dni
J i  Di
 i ni E for ions
dx
dne
J e  De
 e ne E for electrons
dx
μ i  ion mobility
μ e  electron mobility