Chemical Vapor Deposition (CVD)
Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer
clean and simple
or metal lm growth :
high vacuum, single element clean and simple
Contrast with CVD: toxic, corrosive gas owing through valves, T up to 1000C, multiple, simultaneous chemical reactions, gas dynamics, dead layers whose idea was it?
(Dr. Estis Zmaart?)
CVD is the single most widely used deposition method in IC manufacture
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CVD reactors
Four reaction chambers (similar to those for Si oxidation) Control T, gas mixture, pressure, ow rate
Control module
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CVD is lm growth from vapor/gas phase via chemical reactions in gas and on substrate: e.g. SiH4 (g) Si (s) + 2H2 (g) Do not want Si to nucleate above substrate (homogeneous nucleation), but on substrate surface (heterogeneous nucleation).
Twall
Reactor
Transport of precursors across dead layer to substrate
Removal of by-products
Susceptor Pyrolysis: thermal decomposition at substrate
sub>
Chemical reaction: Twall Decomposed species bond to substrate
lm
More details
3
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CVD Processes
8 1
Bulk transport
Bulk transport of by-product
Reactant molecule Carrier gas
(Maintain hi p, slow r eaction)
2 Transport
across bndry layer
3
Diffusion of (g) by-product
Decomposition
5 6 Desorption
J1 " Dg #C
Adsorption
Reaction with lm
J 2 ~ k iCi
Surface diffusion
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Hi vel, low P
Gas transport
2
Low vel, hi P Laminar ow across plate:
Transport across boundary layer
Hi vel, low py Low vel, hi py shorter
J1 " Dg #C
" <1 L
"=
Knudsen NK
kB T 2#d 2 P
Viscous ow
Dgas "
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#v x 2
Laminar ow pipe.
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Conductance A/L
5
dC D Revisit J1 = "D =" (Cg " Cs ) J1 = "hg (Cg " Cs ) gas dx # (x) Boundary layer Layer thickness, (x) dynamics: "v u D = x (unlike solid) z And we saw gas diffusivity ! 2 !
gas vel: u0
boundary layer
Cg
wafer
(x) x=L
(x)
wafer
us = 0
Cs
Fluid dynamics:
"( x ) =
#x $u0
= mass density, = viscosity
1 " = L
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$ # " (x )dx = 2 L %u L & 2 L 3 3 Re 0 0
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uL Reynolds #: Re = " 0 # ease of gas ow
D 3D So: hg = " # 2 L Re
6
Several processes in series Simplify CVD to 2 steps:
AB Boundary layer
J1 =
Dg #C "
J2
A
J 2 = ksCs
! Sticking coefcient AB, 0 AB 1
AB bounces off surface Good adhesion Reaction rate constant, ks no sold-state diffusion here, reaction occurs at surface.
!
Lets analyze, solve for J2
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Two main CVD process: AB
J1 =
Dg #C "
Boundary layer
J1 = hg (Cg " Cs)
J2
B A
J 2 = k sCs
!
In steady state:
J1 = J2,
hg ( Cg - Cs ) = ksCs
Electrical analogy:
Cs =
hg C , hg + k s g
J 2 = k sCs =
hg k s C hg + k s g
J1 = J2,
R = R1+R2 G = 1/R= G1 G2 /(G1+G2)
Two processes in series; slowest one limits lm growth
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Two main CVD process: AB
J1 =
Dg #C "
Boundary layer
J1 = hg (Cg " Cs)
J2
B A
J 2 = k sCs
!
J 2 = k sCs =
$ # ' ) Film growth rate " v = J& % area # t (
v (thickness/time)
hg k s C hg + k s g
v= hg k s C g C g N f = 1 1 hg + k s N f + hg k s
1 , $ # ' Nf& ) % vol (
Slower process controls growth
!
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! Mon., Feb. 14, 2005
Two main CVD process: AB
Boundary layer
J1 = hg (Cg " Cs)
J2
B A
Examine these 2 limits of growth: hg limited or ks limited Reaction limited growth, Transport limited growth, ks<< hg: hg << ks:
C N v= g f 1 1 + hg ks
J 2 = k sCs
hC 3DCg 3#v xCg Re v= g g " Re = Nf 2LN f 4LN f
ease of gas ow
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v=
#G " k sCg Cg = k 0e kT Nf Nf
10
Transport limited growth :
Reaction limited growth :
v=
hg Cg 3DCg 3#v xCg Re " Re = Nf 2LN f 4LN f
v=
#G " k sCg C = g k 0e kT Nf Nf
Most CVD is done in this limit where gas dynamics, reactor design are important. Remedy for boundary layer
G = free energy change in reaction (G H for gas becasue gas reaction no S)
J2
A
Susceptor, 3o -10o
More uniform ug, Cg uniform lm growth rate , v
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Choice of reactants and temperature are critical
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CVD FILM GROWTH
GAS TRANSPORT-LIMITED REACTION-RATE LIMITED
3"vx Cg v= Re 4N f L
v=
#G " k sCg Cg = k 0e kT Nf Nf
vx =
kBT "= , 2#d 2 Pg Cg 1 = Pg k B T
2k B T , "m
G = free energy change in reaction G = H - TS (G H for gas no S for gas reaction)
Re ~ u0
v "T
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u0
Mon., Feb. 14, 2005
v~e
" #H
kT
12
Transport limited
ln (v)
1 2
high T
low T
v "T
u0
Reaction limited
gas " vel ,
u0
Rate:
v~e
" #H
kT
Most CVD is transportln (v) limited. Slow, layer-by-layer growth, epitaxy. Requires high T, low pressure, low gas viscosity. Chamber design, gas dynamics control process. To reduce nucleation of products in gas phase, use low partial pressure (LPCVD).
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T 1/2
H Arrhenius-like
1/T
1000K 400K
T
13
Mon., Feb. 14, 2005
We saw
Review CVD
CVD is lm growth from vapor/gas phase via chemical reactions in gas and at substrate: e.g. SiH4 (g) Si (s) + 2H2 (g)
!
Twall
Reactor
Transport of precursors across dead layer to substrate
Susceptor
lm
Removal of by-products
sub> Twall
Pyrolysis: thermal decomposition at substrate
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Chemical reaction: Decomposed species bond to substrate
14
Gas transport ln (v) limited
Reaction limited high T
low T
v "T
1/ 2
u0
Transport-limited CVD. Chamber design, gas dynamics control lm growth. Non uniform lm growth. ln (v) Slow, layer-by-layer growth, epitaxy, require high T, low pressure, /L = NK >> 1. That puts you in the Reaction-limited regime
u0
Rate:
v~e
" #H
kT
T 1/2
Arrhenius-like H
1/T
1000K 400K
T
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Some CVD reactions
Silane pyrolysis
(heat induced reaction)
Silane oxidation
(450C)
SiH4 (g) + O2(g) SiO2 (s) + 2H2 (g)
(by LPCVD for gate oxide)
SiH4 (g) Si (s) + 2H2 (g) ( 650C)
This poor Si at 1 atm, so use low pressure
Si - tetrachloride reduction
rate
c1 c2 SiCl4 H2
Poly Si Crystalline
etch
SiCl4 (g) + 2H2 (g) Si (s) + 4HCl (g) (1200C)
PSiCl4 PH 2
(Si-tetrachloride actually much more complex than this; 8 different compounds are formed, detected by RGA)
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Some CVD reactions (cont.)
Doping Phosphine 2PH3 (g) 2P (s) + 3H2 (g) Si-nitride compound formation 3 SiCl2H2 (g) + 4NH3 (g) Si3N4 (s) + 6H2(g) + 6HCl (g) (750 C) Diborane B2H6 (g) 2B (s) + 3H2 (g)
GaAs growth
Trimethyl Ga (TMG) reduction (CH3)3 Ga + H2 Ga (s) + 3CH4 Arsene Or
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2AsH3 2As (s) + 3H2
750 C
850 C
Least abundant element on surface limits growth velocity
"" # " As4 (g) + As2 (g) + 6 GaCl (g) + 3 H2 (g)"# # 6 GaAs (s) + 6 HCl g #
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How can you select process parameters to get desired product and growth characteristics?
Consider: 1) SiH4 (g) SiH2 (g) + H2 (g) Three unknown pressures Total pressure = partial Ps
Ptot = PSiH4 + PH2 + PSiH2
PSiH2 + PSiH4 = const 4PSiH4 + 2PSiH 2 + 2PH 2
still have 2 unknown Ps 2) Conservation of ratio
Si => H
still have 1 unknown P 3) Equilibrium constant, K (cf. Law of mass action) rate
K"
%G $ PH 2 # PSiH2 = K0 e kT PSiH4
c1 c2 SiCl4 H2
And similarly for each reaction.
These 3 equations provide a starting place for growth parameters.
(Many equations for real systems; done on computer) Do a run, analyze results, tweak process.
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Where does
%G $ PH 2 # PSiH2 K" = K0 e kT come from? PSiH4 Consider mass action for class groups
Consider mass action for electrons and holes: Intrinsic semiconductor Conduction band EF Valence band
Recombination 2 probability set i i by energy gap and number of each species
N-type semiconductor n EF p n
2
ni
Donor levels
pi
n = n pi
p
ni = np
More free electrons => more recombination, fewer holes ( Eg same)
K indicates a bias at equilibrium in the reaction toward the products(different molecular species)
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K PSiH4 = PH 2 " PSiH 2
19
!
When a system undergoes a process in which only heat energy, dQ, and pV work occur at constant T,
then dG = Vdp
dQ = TdS
Vdp
dG = NkB T
!
dp dp or RT p p
B B
" dG = RT " d ln( p) # G
A A
$ GA = RT ln( pB / pA ) !
# dG & pB exp%" () $ RT ' pA
Dene this ratio pB/pA, as K
For multiple reagents:
K"
%G $ PH 2 # P! 2 SiH = K0 e kT PSiH4
Entropy of mixing, dSmix = "RN i ln(N i ) , leads to similar result
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10
Exercise Assume reaction: AB A + B Ptot = 1 atm, T = 1000 K, K = 1.8 109 Torr exp ( - 2 eV / kB T ) = 0.153 Assume PA PB nd PAB Solution:
K=
PAPB PAB
= 0.153
and Ptot = PA + PB + PAB ,
PA PB. 760 Torr = 2PA + PAB
PA2 = 0.153 PAB = 0.153 (760 - 2 PA), quadratic PA = 10.8 Torr = PB , so PAB = 738 Torr 2
PA + 0.306PA " 0.153 # 760 = 0
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 you change T?
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Atmospheric Pressure CVD: APCVD
(little used today, but illustrative)
High P, small => slow mass transport, large reaction rates; lm growth limited by mass transfer, boundary layer;
(quality of APCVD Si from silane is poor, better for dielectrics).
Example: SiH4 + 2O2 SiO2 + 2H2 O
T = 240 - 450C
Done in N2 ambient (low partial pressure of active gas, reduces lm growth rate) add 4 - 12% PH3 to make silica ow, planarize. ln v
Transport ltd APCVD low T, reaction rate limited
1/T
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11
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
Equilibrium not achieved at low P where (molecular ow, few collisions).
" = Kn > 1 L
"=
kB T 2# d 2 P
lower P => higher Dg, hg; this improves transport reduces boundary layer, extends reaction-controlled regime (which is where you operate)
ks term
ln v
hg at 1 Torr
LPCVD
hg at 760 Torr
Transport limited
Reaction limited
1/T
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Low Pressure CVD (LPCVD) for dielectrics and semiconductors
If hot-wall reactor If cold-wall reactor uniform T distribution but Reduce reaction rate, surface of reactor gets coated. reduce deposition on So system must be dedicated to surfaces. For epi Si. 1 species to avoid contamination. All poly-Si is done by hot-walled LPCVD; good for low pin-hole SiO2, conformality
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12
Low Pressure CVD (LPCVD) for dielectrics and semiconductors
In such non-equilibrium, large , reactant-starved cases, growth rate is controlled by gas kinetics LPCVD is kinetically throttled; transport-rate controlled Silane pyrolysis SiH4 (g) Si(s) + 2H2 (g)
T = 575 - 650C
10 - 100 nm/min
(APCVD is at equilibrium; transport limited) LPCVD
Requires no carrier gas Fewer gas-phase reactions, fewer particulates Eliminates boundary-layer problem Lower p => larger Dg, extends reaction limited regime Good conformal growth (unlike sputtering or other PVD methods which
are more directional)
Strong T dependence to reaction growth rate.
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Easier to control T with hot-walled furnace.
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R.F. Plasma-enhanced CVD (PECVD) for dielectric
MOS metallization: avoid contact interaction betw. Al & Si, SiO2, T < 450C At low T, surface diffusion is slow, must supply kinetic energy for surface diffusion. Plasma provides that energyand 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. WF6 + 3H2 W + 6 HF
G 70 kJ / mole (0.73 eV/atom)
below 400C
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oxide
oxide semi
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