半导体工艺之CVD

T 1/2
fi DH Arrhenius-like
1/T
1000K 400K
T
13
Mon., Sept. 15, 2003
Review CVD
We saw… CVD is film growth from vapor/gas phase via chemical reactions in gas and at substrate: e.g. SiH4 (g) Æ Si (s) + 2H2 (g)
Twall
Reactor
Transport of precursors across dead layer to substrate
Susceptor Pyrolysis:
film
Removal of by-products
T
sub>
Twall
Chemical reaction: Decomposed species bond to substrate
Mon., Sept. 15, 2003 7
Two main CVD process: AB
J1 =
Dg
Boundary layer
d
DC
J1 = hg (Cg - Cs)
J2
B A
J 2 = k sCs
In steady state:
J 1 = J2 ,
hg ( Cg - C s ) = k sC s
Mon., Sept. 15, 2003 4
Gas transport
2 Transport
across boundary layer
J1 µ Dg DC
l <1 L
Knudsen NK ≡
Viscous flow
L
lv x Dgas ª 2
Mon., Sept. 15, 2003
5
Revisit gas J = h C - C 1 g( g s) dynamics:
CVD:
T 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
T
sub>
Twall
More details…
3
Mon., Sept. 15, 2003
CVD Processes
8 1
Bulk transport
Bulk transport of byproduct
Reactant molecule Carrier gas
(Maintain hi p, slow reaction)
Electrical analogy:
Cs =
hg hg + k s
Cg
,
J 2 = k sCs =
hg k s hg + k s
Cg
J1 = J2,
R = R1+R2 G = 1/R= G1G2 /(G1+G2)
Two processes in series; slowest one limits film growth
wafer
us = 0
waferd Biblioteka x) x=LCsx
Fluid dynamics:
d( x ) =
hx ru0
r = mass density, h = viscosity
Reynolds #: Re = r 0 h ease of gas flow
1 d = L
L
h 2 L 2 Ú d (x )dx = 3 L ru L ≡ 3 Re 0 0
Mon., Sept. 15, 2003 1
CVD reactors
Four reaction chambers (similar to those for Si oxidation) Control T, gas mixture, pressure, flow rate
Control module
Cg N f v= 1 1 + hg ks
J 2 = k sCs
v=
hg Cg Nf
3DCg 3lv xCg Re Æ Re = 2LN f 4LN f
ease of gas flow
Mon., Sept. 15, 2003
DG k sCg Cg v= = k 0e kT Nf Nf
10
Transport limited growth :
v=
k sCg Nf
=
Cg Nf
k 0e
-
DG kT
l=
Cg Pg
kBT , 2 2pd Pg = 1 kBT
2k B T vx = , pm
∆G = free energy change in reaction (∆G @ ∆H for gas Æ no ∆S for gas reaction)
Re ~ u0
14
thermal decomposition at substrate
Mon., Sept. 15, 2003
Gas transport limited ln (v)
Reaction limited high T
low T
v µ T 1/ 2 u0
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 Reaction-limited regime
Twall
Reactor
Transport of precursors across dead layer to substrate
Susceptor Pyrolysis: thermal decomposition at substrate
film
Removal of by-products Chemical reaction: Decomposed species bond to substrate
∆G = free energy change in reaction (∆G @ ∆H for gas becasue gas reaction no ∆S)
J2
A
Susceptor, 3o -10o
B
More uniform ug, Cg fi uniform film growth rate , v
Mon., Sept. 15, 2003 8
Two main CVD process: AB
J1 =
Dg
Boundary layer
d
DC
J1 = hg (Cg - Cs)
J2
B A
J 2 = k sCs
J 2 = k sCs = hg k s hg + k s
Cg
Cg N f = v= 1 1 hg + k s N f + hg k s
hg k s C g
Ê # ˆ 1 ˜ , Film growth rate ≡ v = J Á Ë area - t ¯ Ê # ˆ NÁ ˜ Ë vol ¯
Slower process controls growth
Mon., Sept. 15, 2003 9
Two main CVD process: AB
Mon., Sept. 15, 2003
uL
D 3D So: hg = d Æ 2 L Re
6
Several processes in series Simplify CVD to 2 steps:
AB Boundary layer
Dg J1 = DC d
J2
A
B
J 2 = ksCs
Sticking coefficient gAB, 0 ≤ gAB ≤ 1 AB bounces off surface Good adhesion Reaction rate constant, ks …as in oxidation, but no sold-state diffusion here, reaction occurs at surface. Let’s analyze, solve for J2…
Reaction limited growth :
DG k sCg Cg v= = k 0e kT Nf Nf
v=
hg Cg Nf
Æ
3DCg 2LN f
Re =
3lv xCg Re 4LN f
Most CVD is done in this limit where gas dynamics, reactor design are important. Remedy for boundary layer
Chemical Vapor Deposition (CVD)
Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : … Contrast with high vacuum, single element… toxic, corrosive gas flowing through valves,
Choice of reactants and temperature are critical
Mon., Sept. 15, 2003 11
CVD FILM GROWTH
GAS TRANSPORT-LIMITED REACTION-RATE LIMITED
3lvx Cg v= Re 4N f L
v µT
1
2
u0
Mon., Sept. 15, 2003
v~e
- DH
kT
12
Transport limited
ln (v)
1 2
high T
low T
v µT
u0
Reaction limited
gas - vel ,
u0
Rate:
v~e
- DH
kT
Most CVD is transportlimited. Slow, layer-by-layer ln (v) 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).
u0
Rate:
v~e
- DH
kT
T 1/2
Arrhenius-like fi DH
1/T
1000K Mon., Sept. 15, 2003 400K
T
15
Some CVD reactions
Boundary layer
J1 = hg (Cg - Cs)
J2
B A
Examine these 2 limits of growth, hg or ks limited… Reaction limited growth, Transport limited growth, ks<< hg: hg << ks:
And we saw gas diffusivity gas vel: u0
Boundary layer
dC D J1 = D = (Cg - Cs) dx d(x)
Layer thickness, d(x) (unlike solid)
l vx D= 2
z
u
Cg
boundary layer
d (x)
2 Transport
7
across bndry layer
3
Diffusion of (g) byproduct
Decomposition
5
4
6 Desorption
J1 µ Dg DC
Adsorption
Reaction with film
J 2 ~ k iCi
Surface diffusion
Mon., Sept. 15, 2003
2
CVD is film growth from vapor/gas phase via chemical reactions in gas and on substrate: e.g. SiH4 (g) Æ Si (s) + 2H2 (g) Do not want Si to nucleate above substrate (homogeneous nucleation), but on substrate surface (heterogeneous nucleation).