Chip Modeling - University of Utah
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Transcript Chip Modeling - University of Utah
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 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
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
Rg T
– 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 VReacto rPerWafer a
FAo
X
0
1
r'A ( X)
dX
r'A ( X)
rate( X)
4
SiO2
4 00
nm
2 00
dX
2
Awafer
4 00 0
rate ( X') 1 0 min
nm
2 00 0
0
0
r'A ( X)
6 00 0
min
n ( X)
1
Dwafer
6 00
Thickness(nm)
Deposition Rate, Wafer Number
MwSiO2
rate ( X)
X
n( X)
VReacto rPerWafer a
0
FAo
0
0 .5
X
Co n versio n
1
0
50
1 00
1 50
n ( X')
Wafer Nu mb er
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 D AB p
BoundaryConditions
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 sinh 1
sinh 1
5 05 0
0 .9 9
Thickness(nm)
Concentration
1
CA
0 .9 8
0 .9 7
1
0 .5
r/R.wafer
x 0.5
0
rate 1CA 1 0 min
5 00 0
nm
4 95 0
4 90 0
1
0 .5
r/R.wafer
This is bad!!!
0
Combined Length and Radial Effects
3 60 0
Wafer 10
3 40 0
3 20 0
Thickness
Rate 1 0 1 0 min
nm
Rate 2 0 1 0 min
3 00 0
nm
2 80 0
2 60 0
Wafer 20
2 40 0
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)+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.
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
ks K A K H PGeCl2 PH 2
1 K P
A GeCl 2
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)
Zx
Z Z
x
t
U surface bindingenergy
5.2
Zt
U ( Z t2 / 3 Z x2 / 3 )3 / 4
2/3
Z i atomicnumbers of (t) target and (x) gas
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 B Te M i
Vf Vp ln
2q 2.3me
Te elect ronT emperat ure
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+eO2+ + 2e
– O2+e2O + 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
ene E for electrons
dx
μ i ion mobility
μ e electronmobility