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,