Thin-Film Evaporation Process
Introduction
Evaporation – by thermal energy
Sputtering – at room temperature, through the impact of gaseous ions
– stoichiometry
PVD v.s. CVD
•
•
•
•
Reliance on solid or molten sources, as apposed to generally gaseous
precursors in CVD
The physical mechanisms (evaporation or collisional impact) by which source
atoms enter the gas phase
A reduced pressure environment through which the gaseous species are
transported
The general absence of chemical reactions in the gas phase and at the
substrate surface (reactive PVD processes are exceptions)
Evaporation Rate
Hertz Observation
1. Not limited by insufficient heat supplied to the surface of the molten evaporant.
2. Proportional to the difference between the equilibrium pressure, Pe, of Hg at
the given temperature and the hydrostatic pressure, Ph, acting on the evaporant.
He concluded that a liquid has a specific ability to evaporate at a given
temperature.
 e N A ( Pe  Ph )
e 
2MRT
Joule
m ole K
N m
 8.314
m ole K
m
Kg  2  m
s
 8.314
m ole K
R  8.314
N
m2
m
Kg  2
s
 1.01 105 
2
m
1 atm  1.01 105 
cm
2
1
5
s
 1.01 10 1000

2
m
100
cm
Kg  2  m
s
cm
 8.31410001002 
g

m ole K
 1.01 106  2 s
2
cm
cm  s
g 2
cm
7
s
g

 8.31410 
1
m ole K
1 torr 
atm  1329 2 s
760
cm  s

P

NA
2MRT
M:
g
mole
g
cm
1
23
6.0210
 P(in torr ) 1329 2 s
m ole
cm  s

cm2
g 2
g
7
s ) T (K )
2M (
)  (8.31410
m ole
m ole K
g
Evaporation Rate
• in number of atoms (or molecules) per unit area, per
unit time
– Φe = NAαe (Pe –Ph) (2πMRT)-1/2
– Φe = 3.513x1022 Pe (MT)-1/2 molecules/cm2-s
 (M/N )
A
–
– Γe
–
–
–
–
–
–
e 
e
M
NA
= 5.84x10-2 Pe (M/T)1/2 g/cm2-s (mass evap. rate)
αe is the evaporation coef., generally taken to be unity
Pe is the vapor pressure of the evaporant (in torr)
Ph is the hydrostatic pressure surrounding the evaporant
NA is Avogadro’s number
M is the molecular weight (g/mole)
Γe /D = cm/s
D: film density
Typical Γe = 10-4 g/cm2-s at 10-2 torr.
Vapor Pressure of the Elements
Vapor Pressure of the Elements
Clausius-Clapyeron equation
dG  VdP  SdT  0
dP S
H
H PH




dT V TV TVv RT 2
H
H
ln P  
 I , P  Po exp(
) if H  f (T )
RT
RT
a
ln P   b ln T  c if H  f (T )  H o  c p (T  T o )
T
Vapor Pressures of Metals
Vapor Pressures of Semiconductor Materials
10-3 torr at Melting Point
Most metals evaporate using liquid phase.
Cr, Ti, Mo, Fe, and Si evaporate using solid phase.
Evaporation of Multielement Materials
Ionic compound
Compound semiconductor
alloys
Evaporation of Multielement Materials
• Evaporation of compounds and alloys often yields films with
different composition (See Table 3-1, Ohring)
Compounds:
– Many compounds evaporate dissociatively and noncongruently (e.g. dioxides of Si, Ge, Ti, Zr)
– III-V compounds, such as GaAs, are also good examples
– Materials that evaporate non-dissociatively, e. g. CaF2,
AlN, SiO, can be evaporated to form stoichiometric films
– Some II-VI compounds, such as CdTe, evaporate
dissociatively but congruently (with equal rates), such that
compounds can be formed.
GaAs Phase Diagram at Low Pressures
1. Growth window must be As-riched
 What will happen if Ga rich?
2. At 10-6 torr, the growth temperature must be between 630
and 1000 K.
 What will happen if temperature fall out of this region?
3. Operation at a lower pressure narrows the usable deposition
range.
106 torr
109 torr
2500
2500
2000
2000
v
1500
1500
v
l v
T (K )
1000
l c
500
 v
cv
c 
0
1000
l v
500
l c
 v
0
Ga
As
Ga
cv
c 
As
Two-phase c(InSb) + v field is contracted compared with
that of GaAs
• Vapor pressure of Sb is less than that for As
– Solidus line at lower pressure
• Vapor pressure of In exceeds that for Ga
– Vaporous line at higher pressure
850 K
1000 K
c 
103
100
103
l c
100
l c
P(Torr)
c 
l
cv
3
3
cv
10
10
106
106
9
9
l v
v
10
10
v
Ga
As
Ga
As
Alloys:
– evaporated flux equals source composition only if solution
is ideal (i.e. Raoultian) -- seldom true
– Roaultian law: vapor pressure of component B in solution
is reduced relative to the vapor pressure of pure B (PB(0))
in proportional to its mole fraction XB. PB = XB PB(0)
– deviations from ideality are common
PB = aB PB(0) where aB = B XB
– while evaporation rates can, in principle, be calculated if
activities are known, the source composition changes
1/ 2
continually 

X
P
(
0
)
M

NA
3.5131022 
A
A
A A
B
  e 
Pe 
Pe 

1/ 2
2MRT
MT


 B  B X B PB (0) M A
• Solutions to the above problems, involving multiple
evaporation sources
Al-Cu Alloy Deposition
2wt% Cu from single crucible heated to 1350 K
 A  A X A PA (0) M B1/ 2

 B  B X B PB (0) M 1A/ 2
PA (0), PB (0)

X A  A  B PB (0) M 1A/ 2


X B  B  A PA (0) M B1/ 2
X Al 98 / 27.0 2 104 (27.0)1/ 2


 15
3
1/ 2
X Cu
2 / 63.7 110 (63.7)
Maintaining Melt Stoichiometry
1. Evaporate from independent sources
2. Continuous addition of external mass
A1-YBY Deposition
 A  A X A PA (0) M B1/ 2

 B  B X B PB (0) M 1A/ 2
 A 1  Y (1  X S ( B))PA (0) M B1/ 2


B
Y
X S ( B) PB (0) M 1A/ 2
X S ( B) 
1
(1  Y ) PB (0) M 1A/ 2
1
YPA (0) M B1/ 2
X S ( B) : steady state composition of B
Maintaining Melt Stoichiometry
The number of B atoms
added per second
 Vr
The number of B atoms
lost by evaporation
The number of B atoms
accumulate in the melt
VrY VrY X B
V dX B

[
]

 X S ( B)  dt
X B  X S ( B)
VrYt
 exp(
)
Y  X S ( B)
VX S ( B)
 : atom icvolum e (cm3 / atom)
Vr : volum etric feed rate of alloy A1Y BY
V : m elt volum e (cm3 )
(cm3 / s )
Film Thickness Uniformity and PurityC
Deposition geometry
Thickness control
Evaporation Source
dAc
Point source
Surface source
Point Source n = 0
dAc
dAc  dAs cos
dM s : M e  dAc : 4r 2
dM s M e cos

dAs
4r 2
dM s : m ass falls on the substrate of dAs
M e : total evaporatedm ass
Knudsen Cell or Effusion Cell n=1
Cosine distribution flow through a hole
dM s M e cos cos

dAs
r 2
Supplements
http://www2.ece.jhu.edu/faculty/andreou/495/2000/LectureNotes/PhysicalVaporDeposition.pdf
http://www2.ece.jhu.edu/faculty/andreou/495/2000/LectureNotes/PhysicalVaporDeposition.pdf
http://www2.ece.jhu.edu/faculty/andreou/495/2000/LectureNotes/PhysicalVaporDeposition.pdf
(? d 2 M e  md2 Ne  dAe dt)
http://www2.ece.jhu.edu/faculty/andreou/495/2000/LectureNotes/PhysicalVaporDeposition.pdf
w =A/ r
2
Real Cell n
n0
dM s M e cos

dAs
4r 2
n 1
dM s M e cos cos

dAs
r 2

nn

dM s M e cos cosn 

, n0
2
dAs
2r /(n  1)
Real Cell n
Generally, the mass of material emitted from an evaporation source
at a fixed angle is: m (w) = m cosn 
(n is related to source geometry)
dM s M e cos cosn 

, n0
2
dAs
2r /(n  1)
Film Thickness d
Thickness
Point source
dM s M e cos

dAs
4r 2
d
M e cos
Me h
M eh


4r 2
4r 2 r 4 (h 2  l 2 )3 / 2
d
1

d o (1  (l / h) 2 )3 / 2
Surface source dM s  M e cos cos
dAs
r 2
dM s
d
dAs
do : thicknessat l  0
M e cos cos
Me h h
M eh2
d


2
2
r
r r r  (h 2  l 2 ) 2
d
1

d o (1  (l / h) 2 ) 2
Film Thickness d
1 .0
0 .8
0 .6
d / do
0 .4
PointSource
SurfaceSource
0 .2
0
0
0 .5
1 .0
l/h
1 .5
2 .0
Two Point Sources
Example 1
It is desired to coat a 150-cm-wide strip utilizing two evaporation sources oriented as
shown. If a thickness tolerance of 10% is required, what should the distance between
sources be and how far should they be located from the substrate?
D / hv 
d
 1.1  D / hv  0.6  r / hv  0.87
do
 r  150/ 2  75 cm  hv  75 / 0.87  86.2 cm
0.9 
 2D  2  0.6  86.2  103.4 cm
It is obvious that the uniformity tolerance can always be realized by extending the
source-substrate distance, but this is wasteful of evaporant.
Example 2
How high above any given source should a 25 cm diameter substrate be rotated to
maintain the desired film tolerance of 1% in thickness?
R = 20 cm, tolerance =  1%  hv/R = 1.33, r/R = 0.6,
hv = 1.3320 = 26.6 cm
Example 3
A clever way to achieve thickness uniformity
For Knudsen source only
dM s M e cos cos M e r r
Me

 2

 const
2
2
dAs
r
r 2ro 2ro 4ro
More about Thickness Uniformity
1.
2.
3.
4.
5.
Physically, deposition thickness uniformity is achieved because short sourcesubstrate distances are offset by unfavorably large vapor emission and deposition
angles.
Uniformity of columnar grain microstructure, e.g., tilt, is not preserved, however,
because of variable flux incidence angle.
Two principal methods for optimizing film uniformity over large areas involve
varying the geometric location of the source and interposing static as well as
rotating shutters between evaporation sources and substrates.
In addition to the parallel source-substrate configuration, calculations of thickness
distributions have also been made for spherical as well as conical, parabolic, and
hyperbolic substrate surfaces.
Similarly, cylindrical, wire, and ring evaporation source geometries have been
treated.
Conformal Coverage and Filling of
Steps and Trenches
When a film of the same thickness coats the
horizontal as well as vertical surfaces of substrates,
we speak of conformal coverage.
Step-coverage problems have been shown to be
related to the profile of the substrate step as well as to
the evaporation source-substrate geometry.
1.
2.
3.
A “breadloaf” film topography evolves that
tends to choke off further deposition in the
trench.
As a consequence, a void may be trapped within,
leading to a defective “keyhole” structure.
Collimation of the arriving atomic flux and
heated substrates favor deeper and more
conformal trench penetration, the former by
minimizing shadowing and the latter by
promoting surface and bulk diffusion of atoms.
Computer Modeling of Step Coverage
Line-of-sight motion of evaporant atoms and sticking coefficients of unity can be
assumed in estimating the extent of coverage.
1.
2.
In generating the simulated film profiles surface migration of atoms was neglected,
which is valid assumption at low substrate temperatures.
Heating the substrate increases surface diffusion of depositing atoms, thus
promoting coverage by filling potential voids as they form.
Film Purity
Evaporant vapor impingement rate
N Ad / M a
Gas molecule impingement rate

N AP
P
 3.5131022
( MT )1/ 2
2MRT
# / cm2 s
Impurity concentration Ci

Ci 
N Ad / M a
Ci 
2
5.82  10 PM a
M g T  d
 : film density
d : depositionrate (cm / s )
M a : evaporantm olecularweight
M g : evaporantm olecularweight
P : residual gas vapor pressure (torr)
Vacuum Requirements
• The chamber pressure during evaporation must be
sufficiently low to minimize:
– Scattering of evaporated species in the region between the
evaporate source and the substrate
• Minimized for pressures < 10-4 Torr, where the mean free path in
air is ~45 cm.
– background gas impurity incorporation into the film
• depends upon the incorporation probability of the impurity into the
film and the growth rate.
• typical background species present in vacuum systems.
• increasing the growth rate decreases the impurity content of
evaporated films.
• UHV systems are preferred when high purity films are required.
Contamination
1.
2.
3.
In order to produce very pure films, it is important to deposit at very high rates
while maintaining very low background pressures. Typical deposition rates from
electron beam sources can reach 1000Å/s at chamber pressures of ~10-8 torr.
In sputtering processes, deposition rates are typically about two orders of
magnitude lower and chamber pressures four orders of magnitude higher than for
evaporation. Therefore, the potential exists for producing films containing high gas
concentrations. (Not as “clean” a process as evaporation.)
Very high oxygen incorporation occurs at residual gas pressures of 10-3 torr.
Advantage of this fact is taken in reactive evaporation processes where
intentionally introduced oxygen serves to promote reactions with the evaporant
metal in the deposition of oxide films.