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PHYSICAL VAPOR DEPOSITION (PVD)PVD II: Evaporation
We saw CVD Gas phase reactants:pg 1 mTorr to 1 atm.
Good step coverage, T> 350 K
We saw sputtering Noble (+ reactive gas)p 10 mTorr; ionized particles
Industrial process, high rate, reasonable step coverage
Extensively used in electrical, optical, magnetic devices.
Now see evaporation: Source material heated,peq.vap. =~ 10-3 Torr, pg < 10
-6 Torr
Generally no chemical reaction (except in reactive deposn),
= 10s of meters, Knudsen numberNK>> 1
Poor step coverage, alloy fractionation: pvapor
Historical (optical, electrical)
Campbell, Ch. 12 is more extensive than Plummer on evaporation
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Standard vacuum chambers
pi 10-6 Torr (1.3 10-4N / m2)
Mostly H2O, hydrocarbons, N2 He byresidual gas analysis (RGA = mass spec.)
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p < 10-8 Torr demands:
Stainless steel chamber
Bakeable to 150oC
Turbo, ion, cryo pumps
Ultra-high vacuum chambers
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-6 -4 -2 0 2 4
Log[P(N/m2)]
25
23
Log[n (#/m3)]
21
19
17
15
= 10 0.01 cm
p = 10-10 10-8 10-6 10-4 10-2 100 Torr
1 Atm=0.1 MPa
760 mm
14 lb/in2
Generally /L < 1
Films less pure
Epitaxy is rare
Evaporation
Ballistic, molecular flow, /L >> 1
High purity films
Epitaxy
Sputtering
CVD
Knudson number 1
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Atomic flux on surface due to residual gas
J atomsarea t
= nvx
2= p
2kBT2kBTm
= p2mkBT
= J
Given 10-6 Torr of water vapor @ room temp, find flux
p =106 Torr 1atm760 T
105Pa
atm, kBT RT( )= 0.025eV = 4 1021J
p =1.3104
m
2mH 2O =
18
NA
= 31026kg
What is atomic density in 1 monolayer (ML) of Si?
N= 5 x 1022 cm-3 => 1.3 x 1015 cm-2.
So at 10-6 Torr, 1 ML of residual gas hits surface every 3 seconds!
Epitaxy requires slow deposition, surface mobility,So you must keep pressure low to maintain pure film
J= 4.8 1014atoms/molecules
cm2sec
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Now add evaporation source
Equilibrium vapor pressure:
H= heat of vaporization
pv = p0 exp H
kBT
Strong Tdependence
e-
+
_
e-beam
B field
Resistive heater
I
RF-induction
heater
Work
function
e-
Heat of
vaporization
solid
V(x)
free
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Vapor pressure of elements employed in
semiconductor materials. Dots
correspond to melting points
Rely on tables, attached:pvapor> > pvac,
Elemental metals easy to evaporate, but
alloys
compounds
Differentialpvapor
so use 2 crucibles
or deposit multilayers
and diffuse
Oxides, nitrides deposit in oxygen(or other) partialp
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(note W, Mo
can be used
as crucibles.)
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The source also flux
J=
pvap
2msourcekBTsource
For Al at 1000 K, pvap = 10-7 Torr (from figure)
m = species to be evaporated, = 27 amu for Al
JAl 2 x 1013 Al/cm2-s just above crucible
Mass flow out of crucible ~J Ac m ( mass / t)
We expressed flux of residual gas:
J 5 1014 moleculescm2s(Chamberp = 10-6 TorrJ=
p
2mkB
T
Net flow out of crucible ~J Ac (# / t)Ac
Therefore heat Al to T> 800 C
but thats not all
Note: 3 different temperatures: Tsource Tevaporant >> Tsubstrate > Tchamber = Tresid gas RT.
System not in thermal equilibrium; only thermal interaction among them is by radiation
and/or conduction through solid connects (weak contact). No convection when NK
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How much evaporant strikes substrate? At 10-6 Torr, trajectories are uninterrupted.
While a point source deposits uniformly on a sphere about it, a planar source does not:
Geometric factor =
c
2R 2 cos1 cos2cos
1 =cos
2 = 2r
Deposition rate = JmAc
4r2
m
area t
or J
Ac
4r2
#
area t
substrate
1
2R
R1
2
r
r
substrate
Convenientgeometry
Geometric factor = c
4r2
Film growth rate = JmAc
4r2
1
f
thick
t
cos1J
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vox =H pg N
1
h
+tox
D
+1
ks
oxide
In PVD growth, strike balance
R =deposition rate
Surface diffusion rate
R > 1 stochastic growth, rough
R < 1 layer by layer, smooth (can heat substrate)
Film growth rate
for evaporation
= JmAc
4r2
1
f
thick
t
v =pvap
2msourcekBTsource
m
m
Ac
4r2=
pvap
m
Ac
4r2msource
2kBTsource
Cf. CVDvf =
CgN
1
ng+
1
k
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Exercise
Deposit Al (2.7 g/cm3) at r= 40 cm from 5 cm diam.
crucible heated to 1100C (cfTmelt) pAl vap 10-3 Torr,
pH2O =106 Torr
Compare arrival rate of Al and H2O at substrateand calculate film growth
rateJH2O =
106 760( )105
2 0.025eV e( ) 18 NA( )= 4.8 1018
molecules
m2s
JAl =10
3
105
760( )2 1373kB( ) 27 NA( )
Ac
4r2
=1.76 1018 atoms
m2s
Ac = 5
2
2
Leave shutter closed so initial Al deposition can getter O2 and H2O.
Also, better done at lower orhigher deposition rate.pH2O
(this is not good)
v = pvapm
Ac4r2
msource2kBTsource
1011 m /s slow!
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Step coverage is poor in evaporation (ballistic) - Shadow effects
Heat substrate to increase
surface diffusion.
DS =D0S exp Ea
Surf
kT
Ea
S better step coverage
W
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Other methods of heating charge.
Resistive heater
I
RF-induction
heater e-
+
_
e-beam
B field
Can you suggest other methods?
Laser: Pulsed Laser Deposition (PLD), laser ablation
Ion beam deposition (IB)D:
keep substrate chamber at low P, bring in ion beam
through differentially pumped path.
Source,
target
Substrate,
film
Ion beam
Ion beamCan also use ion beam on film to add energy
(ion beam assisted deposition, IBAD)
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Surface energy in a growing film depends on the number of bonds
the adsorbed atom forms with the substrate (or number unsatisfied).
This depends on the crystallography of the surface faceand on the type of site occupied (face, edge, corner, crevice).
Macroscopically, a curved surface has higher surface energy
(more dangling bonds) than a flat surface.
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Microstructure types observed in sputtered films
with increasing substrate temperature
normalized to melting temperature of deposited species.
Quenched growth Thermally activated growth
< a < a Ts/Tm > 0.3 Ts/Tm > 0.5
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Interfaces between dissimilar materials
Four characteristic equilibrium, binary phase diagrams, above,
and the types of interface structures they may lead to, below (non-equilibrium).
The first-column figures would apply to Ga-As, the second to Si-Ge. Third case,
B diffuses into A causing swelling; A is forced by swelling into B as a second phase.
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Schematic stress strain curveshowing plastic deformation
beyond the yield point.
Upon thermal cycling,a film deposited under conditions
that leave it in tensile stress
may evolve through compression
then even greater tension .
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