Chemical Vapor Deposition (CVD)
Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer…or metal film growth : high vacuum, single element…
CVD: toxic, corrosive gas flowing through valves, T
… Contrast with up to 1000°C, multiple simultaneous reactions,
gas dynamics, dead layers… whose idea was it?
All layers above poly-Si made by CVD, except gate oxide and aluminum
Mon., Sept. 15, 2003 1
CVD
reactors
Controlmodule
Fourreactionchambers (similar to those for Si oxidation)
Control T,gas mixture,
pressure,flow rate
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CVD is film growth from vapor/gas phase via chemical reactions in gas and on substrate:
homogeneous nucleation),
e.g. SiH4 (g) Æ Si (s) + 2H2 (g)
Do not want Si to nucleate above substrate (but on substrate surface (heterogeneous nucleation).
Twall
Reactor
Transportof precursors
acrossdead layer to substrate
Pyrolysis: thermal
Susceptorfilm
T sub> TwallChemical reaction:Decomposed speciesbond to substrate
decomposition at substrate More details…
by-productsRemoval of
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CVD Processes
8
1 Bulk Bulk transporttransport of byproduct
Reactantmolecule 7 Diffusion of
TransportCarrier gas
2across bndry 4
(g) byproduct (Maintain hi p, layer Decompositionslow reaction) 6 Desorption
3 Adsorption 5J1 µDgDC Reaction with film
J2 ~ kiCi
Surface diffusionMon., Sept. 15, 2003
4
Gas transport
J1 µDgDC
Transportacrossboundarylayer
2
Knudsen NK ≡ lL
<1L
Viscous flow
Dgas ªlvx
2
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Revisit gas J1 = hg(Cg - Cs)dC D (Cg - Cs)J1 = D =dx d(x)dynamics:
Boundary layer Layer thickness, d(x)lv x (unlike solid)And we saw gas diffusivity D =2 z u
gas vel: u0boundary layer
Cg d (x)d (x) u = 0s Cs
wafer waferx x = L
hxFluid dynamics: d(x) = r = mass density, h = viscosityru0
L1 h 2 L Reynolds #: Re = ru0L
d = Ú d(x)dx =23
Lru0L
≡3 Re ease of gas flow h
L 0D 3 D
Æ ReSo: hg =d 2 L
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Several processes in series
Simplify CVD to 2 steps: Boundary
AB layerDgJ1 =d
DC J2 BA
J2 = k C s s Reaction rate constant, k
Sticking coefficient gAB, s
…as in oxidation, but no0 ≤ gAB ≤ 1 sold-state diffusion here,
reaction occurs at surface. AB bounces Good off surface adhesion
Let’s analyze, solve for J2…
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J1 = J2,
hg ( Cg - Cs ksCsJ2 = ksCs =
hgks
hg + ks
CgCs =hg
hg + ks
Cg,
Boundarylayer
J2 = ksCs
BA
AB
J2
J1 = hg Cg - Cs( ) process:
J1 =Dg
dDC
In steady state:
) =
Two main CVD
J1 = J2,
1+R2
1G2 /(G1+G2)
Electrical analogy:
R = R
G = 1/R= G
Two processes in series; slowest one limits film growth
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Boundarylayer
J2 = ksCs
BA
AB
J2
J1 = hg Cg - Cs( )
Two main CVDprocess:
J1 =Dg
dDC
J2 = ksCs =hgks
hg + ks
Cg
≡ v = J #area - t
Ê
ËÁ
ˆ
¯˜
1
N #vol
Ê
ËÁ
ˆ
¯˜
, v =hgks
hg + ks
Cg
N f
=Cg N f
1hg
+1ks
Film growth rate
Slower process controls growth
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Boundarylayer
J2 = ksCs
BA
AB
J2
J1 = hg Cg - Cs( )
Two main CVDprocess:
Examine these 2 limits of growth, h or k limited…g s
Transport limited growth, Reaction limited growth,
k << h :
v =Cg N f
1hg
+1ks
s gh << k :g s
gh C 3DC kTg x g3lv C Re v =ksCg =
Ck0e
-DG
v = g g Æ Re = N f 2LN f 4LN f
N f N f
ease of gas flow
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Transport limited growth : Reaction limited growth :
h C 3DC 3lv C Re k C C -DG
g x g g kTv = g g Æ Re = v = s g = k0eN f 2LN f 4LN f N f N f
Most CVD is done in this limit where gas dynamics, ∆G = free energy change in reaction
reactor design are important. (∆G @ ∆H for gas
becasue gas reaction no ∆S)
3o -10o
BA
J2
Remedy for boundary layer
Susceptor,
More uniform ug, C fi Choice of reactants and g
uniform film growth rate , v temperature are critical
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CVD FILM GROWTH
GAS TRANSPORT-LIMITED REACTION-RATE LIMITED 3lv C
DGv = x g k C C -g kT4N f L Nv = s g = k0e
f N f
2kBTv = , ∆G = free energy change in reaction x pmkBT
Re
l = ,2pd 2Pg (∆G @ ∆H for gas Æ no ∆S for
gas reaction)Re ~ u01
=kBT
v ~ e- DHkT
v µT 12 u0
g
Pg
C
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Transport ln (v) high Tlimited 1
2v µT u0low T
Reactionlimited
gas - vel , u0- DH
Rate: v ~ e kT
Most CVD is transport- limited. Slow, layer-by-layer ln (v)
growth, epitaxy. Requires T 1/2high T, low pressure, low gas
viscosity. Chamber design, fi DHgas dynamics control process. Arrhenius-likeTo reduce nucleation of 1 / Tproducts in gas phase, use T
1000K 400Klow partial pressure (LPCVD).
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Review CVD We saw…
CVD is film growth from vapor/gas phase via chemical reactions
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4 (g) Æ 2 (g)
Pyrolysis:
at substrate
filmSusceptor
Reactor
Twall
T sub> Twall
Transport
across
substrate
by-products
:
bond to substrate
in gas and at substrate: e.g. SiH Si (s) + 2H
thermal decomposition
of precursors
dead layer to
Removal of
Chemical reactionDecomposed species
ln (v
v µT u01/ 2
Transport-limited CVD.Chamber design, gas dynamics
control film growth. Non uniform film growth. ln (v)
Slow, layer-by-layer growth, epitaxy, require high T,
low pressure, l/L = NK >> 1.
That puts you in the
limited
Rate: v ~ e- DHkT
high T
low T
u0
)
Reactionlimited
Arrhenius-likeH
T 1/2
Gas transport
fi D
Reaction-limited regime 1 / TT
1000K 400K
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Some CVD reactions
Silane pyrolysis
(heat induced reaction)
SiH4 (g) Æ Si (s) + 2H2 (g) ( 650°C)
This fi poor Si at 1 atm, so use low pressure
Silane oxidation (450°C) SiH4 (g) + O2(g) Æ SiO2 (s) + 2H2 (g)
(by LPCVD for gate oxide)
vSi - tetrachloride reduction
SiCl4 (g) + 2H2 (g) Æ
Si (s) + 4HCl (g) (1200°C) CrystallinePSiCl4
PH 2
Poly Si
(Si-tetra…actually much more complex than this; etch
8 different compounds are formed, detected by RGA)
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Some CVD reactions (cont.)Doping
Phosphine Diborane
2PH3 (g) Æ2P (s) + 3H2 (g) B2H6 (g) Æ2B (s) + 3H2 (g)
GaAs growthTrimethyl Ga (TMG) reduction
(CH3)3 Ga + H2 Æ Ga (s) + 3CH4 Least abundant element on surface
Arsene 2AsH3 Æ 2As (s) + 3H2 limits growth velocity
750°Cæ Æææ 6 GaAs (s) + 6 HCl gOr As4 (g) + As2 (g) + 6 GaCl (g) + 3 H2 (g)¨ æææ850°C
Si-nitride compound formation 3 SiCl2H2 (g) + 4NH3 (g) Æ Si3N4 (s) + 6H2(g) + 6HCl (g) (750° C)
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How can you select process parameters to get desired product and growth characteristics?
Consider: SiH4 (g) Æ SiH2 (g) + H2 (g) TThree unknown pressures
1) Total pressure = Â partial Ps Ptot = PSiH4+ PH2
+ PSiH2
…still have 22 unknown Ps
Si2) Conservation of ratio => H
PSiH2+ PSiH4
4PSiH4+ 2PSiH 2
+ 2PH 2
= const
…still have 11 unknown P
3) “Equilibrium constant”, K (cf. Law of mass action)
K ≡PH 2
⋅PSiH2
PSiH4
= K0e-
DGkT
= ∆H for gas
And similarly for each reaction.
These equations provide a starting place for growth parameters. (Many eqs. for real systems; done on computer) Do a run, analyze results,
tweak process. Mon., Sept. 15, 2003 18
-Where does K ≡
PH 2⋅PSiH2 = K0e
D
kTG
come from? PSiH4 Consider “mass action” for class groups…
Consider “mass action” for electrons and holes:
Intrinsic semiconductor N-type semiconductor
Conduction nin
band EFDonor levelsEF
pi pValence band
nRecombination 2 2nprobability set ni = ni pi i = npby energy gap and More free electronsnumber of each species p => more recombination,
fewer holes (Eg same)PH2
⋅ PSiH2= K PSiH4 K indicates a bbias at equilibrium in the reaction
toward the products(different molecular species)
Mon., Sept. 15, 2003 19
ExerciseAssume reaction: AB Æ A + B Ptot = 1 atm, T = 1000 K,¨
K = 1.8 ¥ 109 Torr ¥ exp ( - 2 eV / kB T )
Assume PA ≈ PB find PAB
Solution: K = PAPB and at 1000 Kelvin, K = 0.153 Torr,PAB
and Ptot = PA + PB + PAB , PA ≈ PB. \ 760 T = 2PA + PAB
PA2 = 0.153 PAB = 0.153 (760 - 2 PA) Æ PA = PB = 10.9 Torr, PAB = 738 Torr
Small value of K, 0.153 Torr, implies that at equilibrium,
the product of the right-hand side partial pressures
Is but 15% of the reactant (left-hand-side) partial pressure;
the reaction may not produce much in equilibrium. What if lower T?
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Atmospheric Pressure CVD: APCVD (little used today, but illustrative)
High P, small l => slow mass transport, large reaction rates; film growth limited by mass transfer, boundary layer; (quality of APCVD Si from silane is poor, better for dielectrics).
Example: SiH4 + 2O2 Æ SiO2 + 2H2O T = 240 - 450°C
Done in N2 ambient (llow partial pressure of active gas, reduces reaction rate)
Mon., Sept. 15, 2003
add 4 - 12% PH3
low T,
limited
ln v Transprt ltd APCVD
T
to make silica flow, planarize.
reaction rate
1/21
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
Equilibrium not achieved at low P where l= K >1nL(molecular flow, few collisions).
l =kBT2pd 2P
lower P => higher Dg, hg improves transport
reduces boundary layer,
Mon., Sept. 15, 2003 22
F.9.13
LPCVDhg
ln v
Transportlimited Reaction limited
T
hg
ks term extends reaction-limited regime
at 1 Torr
1/
at 760 Torr
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
Hot wall reactor fi uniform T distribution but surface of reactor gets coated. So system must be dedicated to 1 species to avoid contamination.
Cold wall reactor Reduce reaction rate,
deposition on surfaces. For epi Si.
All poly-Si is done by hot-walled LPCVD; good for low pin-hole SiO2, conformality
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23
Low Pressure CVD (LPCVD) for dielectrics and semiconductors In such non-equilibrium, large l cases,
growth rate is reaction limited,
Low P LPCVD kinetically controlled,reaction-rate limited.
Silane pyrolysis SiH4 (g) Æ Si (s) + 2H2 (g) ( 575 - 650°C)
10 - 100 nm/min
(Atm. P APCVD equilibrium, transport ltd.)
LPCVD+ requires no carrier gas + fewer gas-phase reactions, fewer particulates + eliminates boundary layer problem + lower P => higher Dg, extends reaction-limited regime + good conformal growth (unlike sputtering or other PVD methods
which are directional) - strong temperature dependence to growth rate + easier to control T with hot-wall furnace
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R.F. Plasma-enhanced CVD (PECVD) for dielectric
MOS metallization: avoid contact interaction betw. Al & Si, SiO2, T < 450°C
At low T, surface diffusion is slow,
must supply kinetic energy for surface diffusion.
Plasma provides that energy…and enhances step coverage.
What is a plasma? Ionized noble gas, accelerated by AC (RF) or DC voltage, collides with active species in gas and at surface, importing Ekin
Metal CVD
Step coverage is important for electric contacts.
oxideWF6 + 3H2 Æ W + 6 HF oxide
DG ≈ 70 kJ / mole (0.73 eV/atom) semi
below 400°C
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