lecture 12.0
DESCRIPTION
Lecture 12.0. Deposition. Materials Deposited. Dielectrics SiO2, BSG Metals W, Cu, Al Semiconductors Poly silicon (doped) Barrier Layers Nitrides (TaN, TiN), Silicides (WSi 2 , TaSi 2 , CoSi, MoSi 2 ). Deposition Methods. Growth of an oxidation layer Spin on Layer - PowerPoint PPT PresentationTRANSCRIPT
Lecture 12.0Lecture 12.0
Deposition
Materials DepositedMaterials Deposited
Dielectrics– SiO2, BSG
Metals– W, Cu, Al
Semiconductors– Poly silicon (doped)
Barrier Layers– Nitrides (TaN, TiN), Silicides (WSi2, TaSi2, CoSi,
MoSi2)
Deposition MethodsDeposition 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 IssuesCritical Issues
Adherence of the layerChemical Compatibility
– Electro Migration– Inter diffusion during subsequent
processing • Strong function of Processing
Even Deposition at all wafer locations
CVD of SiCVD of Si33NN44 - Implantation mask - 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 conductorCVD 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 SiOCVD of SiO22 – Dielectric – 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 plugsCVD 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 EquilibriumChemical Equilibrium
CVD ReactorCVD Reactor
Wafers in Carriage (Quartz)
Gasses enterPumped out via
vacuum systemPlug Flow
Reactor
Vacuum
CVD ReactorCVD Reactor
Macroscopic Analysis– Plug flow reactor
Microscopic Analysis– Surface Reaction
• Film Growth Rate
Macroscopic AnalysisMacroscopic 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 EffectsCombined Effects
Contours = Concentration
Reactor LengthReactor Length Effects 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 RadiusDeposition 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 EffectsRadial 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 EffectsCombined 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 ReactorCVD Reactor
External Convective Diffusion– Either reactants or products
Internal Diffusion in Wafer Stack– Either reactants or products
AdsorptionSurface ReactionDesorption
Microscopic Analysis -Reaction StepsMicroscopic 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 StepsRate 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 ReactionsSurface Reactions
Deposition of GeDeposition of Ge
3"
22
22
1 HHGeClA
HGeClHAsDep
PKPK
PPKKkr
Ishii, H. and Takahashik Y., J. Electrochem. Soc. 135,1539(1988).
Silicon DepositionSilicon 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 SiSilicon 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 DiffusionSurface Diffusion
Monocrystal vs. PolycrystallineMonocrystal vs. Polycrystalline
PSiH4=? torr
Dislocation DensityDislocation Density
Epitaxial Film– Activation
Energy of Dislocation
• 3.5 eV
Physical Vapor DepositionPhysical Vapor Deposition
Evaporation from Crystal
Deposition of Wall
Physical Deposition - SputteringPhysical Deposition - Sputtering
Plasma is usedIon (Ar+) accelerated into a target
materialTarget material is vaporized
– Target Flux Ion Flux* Sputtering YieldDiffuses from target to waferDeposits on cold surface of wafer
DC PlasmaDC Plasma
Glow Discharge
RF Plasma Sputtering for RF Plasma Sputtering for Deposition and for EtchingDeposition and for Etching
RF + DC field
Sputtering ChemistriesSputtering Chemistries
Target– Al– Cu– TiW– TiN
Gas– Argon
Deposited Layer– Al– Cu– TiW– TiN
Poly Crystalline Columnar Structure
Deposition RateDeposition Rate
Sputtering Yield, S– S=α(E1/2-Eth
1/2)
Deposition Rate – Ion current into Target *Sputtering Yield– Fundamental Charge
gas(x) andtarget(t) ofnumbersatomic
)(
2.53/2
4/33/23/2
i
xt
x
xt
t
Z
energybindingsurfaceU
ZZ
Z
ZZ
Z
U
RF PlasmaRF Plasma
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
Plasma rfSheath
Sheath
Floating PotentialFloating Potential
Sheath surrounds objectFloating potential, Vf
kBTe=eV – due to the accelerating Voltage
eTemperaturelectronT
3.2ln
2q
Tk -VV
e
eBpf
e
i
m
M
Plasma ChemistryPlasma 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 Plasma Chemistry
Ionization leading to ion– e + CF4 CF3
- + F
– e + SiH4 SiH3+ + H + 2e
Reaction depend upon electron density
Plasma ChemistryPlasma Chemistry
Electrons have more energyConcentration of electrons is ~108 to
1012 1/ccIons and neutrals have 1/100 lower
energy than electronsConcentration of neutrals is 1000x
the concentration of ions
Oxygen PlasmaOxygen Plasma
Reactive Species– O2+eO2
+ + 2e
– O2+e2O + e
– O + e O-
– O2+ + e 2O
Plasma ChemistryPlasma Chemistry
Reactions occur at the Chip Surface– Catalytic Reaction Mechanisms
– Adsorption– Surface Reaction– Desorption
• e.g. Langmuir-Hinshelwood Mechanism
Plasma Transport EquationsPlasma Transport Equations
Flux, J
mobilityelectronμ
mobilityionμ
e
i
electronsforEndx
dnDJ
ionsforEndx
dnDJ
neutralsfordx
dnDJ
eee
ee
iii
ii
nnn