Thin-Film Evaporation Process
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Thin-Film Evaporation Process
Introduction
Evaporation – by thermal energySputtering – 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.
MRT
PPN heAee
2
)(
MRT
P
N A 2
Kmolescm
g
Kmole
msm
Kg
Kmole
msm
Kg
Kmole
mNKmole
JouleR
2
2
7
22
2
10314.8
1001000314.8
314.8
314.8
314.8
scmscm
g
cmscm
g
msm
Kg
m
Natm
26
2
25
2
25
25
1001.1
100
110001001.1
1001.1
1001.11
mole
gM :
)()10314.8()(2
13291
1002.6
2
2
7
2)(23
KTKmole
scm
g
moleg
M
scmscm
gP
mole torrin
scmscm
gatmtorr
21329
760
11
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/NA
)
– Γe = 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
MN A
ee
Typical Γe = 10-4 g/cm2-s at 10-2 torr.
Vapor Pressure of the Elements
Vapor Pressure of the Elements
Clausius-Clapyeron equation
)()(lnln
)()exp(,ln
0
2
op
o
o
v
TTcHTfHifcTbT
aP
TfHifRT
HPPI
RT
HP
RT
HP
TV
H
VT
H
V
S
dT
dP
SdTVdPdG
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 MaterialsIonic compoundCompound semiconductoralloys
• Evaporation of compounds and alloys often yields films with different composition (See Table 3-1, Ohring)
Compounds:
– Many compounds evaporate dissociatively and non-congruently (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.
Evaporation of Multielement Materials
0
500
1000
1500
2000
2500
v
vl
cl
v
vc
c
Ga As
0
500
1000
1500
2000
2500
v
vl
cl
v
vc
c
Ga As
)(KT
torr610 torr910
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.
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
910 v
cl
vc
c
Ga As
610
310
010
310
vl
910v
cl
vc
c
Ga As
610
310
010
310 l
)(TorrP
K850 K1000
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 continually
• Solutions to the above problems, involving multiple evaporation sources
2/1
2/1
)0(
)0(
ABBB
BAAA
B
A
MPX
MPX
ee
Ae P
MTP
MRT
N 2210513.3
2
Al-Cu Alloy Deposition
2wt% Cu from single crucible heated to 1350 K
)0(),0( BA PP
2/1
2/1
)0(
)0(
ABBB
BAAA
B
A
MPX
MPX
2/1
2/1
)0(
)0(
BAA
ABB
B
A
B
A
MP
MP
X
X
15)7.63(101
)0.27(102
7.63/2
0.27/982/13
2/14
Cu
Al
X
X
Maintaining Melt Stoichiometry
1. Evaporate from independent sources2. Continuous addition of external mass
2/1
2/1
2/1
2/1
)0()0()1(
1
1)(
)0()(
)0())(1(1
BA
ABS
ABS
BAS
B
A
MYPMPY
BX
MPBX
MPBX
Y
Y
A1-YBY Deposition
2/1
2/1
)0(
)0(
ABBB
BAAA
B
A
MPX
MPX
BofncompositiostatesteadyBX S :)(
))(
exp()(
)(
])(
[
BVX
YtV
BXY
BXX
dt
dXV
BX
XYVYV
S
r
S
SB
B
S
Brr
The number of B atoms lost by evaporation
The number of B atoms added per second
The number of B atoms accumulate in the melt
)(:
)/(:
)/(:
3
31
3
cmvolumemeltV
scmBAalloyofratefeedvolumetricV
atomcmvolumeatomic
YYr
rV
Maintaining Melt Stoichiometry
Film Thickness Uniformity and PurityC
Deposition geometryThickness control
Point source Surface source
dAc
Evaporation Source
24
cos
r
M
dA
Md e
s
s
cossc dAdA 24:: rdAMMd ces
massevaporatedtotalM
dAofsubstratetheonfallsmassMd
e
ss
:
:
dAc
Point Source n = 0
Knudsen Cell or Effusion Cell n=1
Cosine distribution flow through a hole
2
coscos
r
M
dA
Md e
s
s
http://www2.ece.jhu.edu/faculty/andreou/495/2000/LectureNotes/PhysicalVaporDeposition.pdf
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
)(? 22 dtdANmdMd eee
= A / r2
24
cos
r
M
dA
Md e
s
s
2
coscos
r
M
dA
Md e
s
s
0,)1/(2
coscos2
nnr
M
dA
Md ne
s
s
Real Cell n
0n
1n
nn
Real Cell n
Generally, the mass of material emitted from an evaporation source at a fixed angle is: m () = m cosn (n is related to source geometry)
0,)1/(2
coscos2
nnr
M
dA
Md ne
s
s
Film Thickness d
s
s
dA
Mdd
24
cos
r
M
dA
Md e
s
s
2
coscos
r
M
dA
Md e
s
s
2/32222 )(444
cos
lh
hM
r
h
r
M
r
Md eee
Point source
2/32 ))/(1(
1
hld
d
o
222
2
22 )(
coscos
lh
hM
r
h
r
h
r
M
r
Md eee
Surface source
22 ))/(1(
1
hld
d
o
Thickness
0: latthicknessdo
0
2.0
4.0
6.0
8.0
0.1
0 5.0 0.1 5.1 0.2
hl /
odd /
SourcePointSourceSurface
Film Thickness d
Two Point Sources
Example 1It 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?
cmD
cmhcmr
4.1032.866.022
2.8687.0/75752/150 v
1.19.0 od
d6.0/ v hD 87.0/ v hr
It is obvious that the uniformity tolerance can always be realized by extending the source-substrate distance, but this is wasteful of evaporant.
v/ hD
Example 2
R = 20 cm, tolerance = 1% hv/R = 1.33, r/R = 0.6, hv = 1.3320 = 26.6 cm
How high above any given source should a 25 cm diameter substrate be rotated to maintain the desired film tolerance of 1% in thickness?
Example 3
constr
M
r
r
r
r
r
M
r
M
dA
Md
o
e
oo
ee
s
s 222 422
coscos
For Knudsen source only
A clever way to achieve thickness uniformity
1. Physically, deposition thickness uniformity is achieved because short source-substrate distances are offset by unfavorably large vapor emission and deposition angles.
2. Uniformity of columnar grain microstructure, e.g., tilt, is not preserved, however, because of variable flux incidence angle.
3. 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.
4. 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.
5. Similarly, cylindrical, wire, and ring evaporation source geometries have been treated.
More about Thickness Uniformity
Conformal Coverage and Filling of Steps and Trenches
1. A “breadloaf” film topography evolves that tends to choke off further deposition in the trench.
2. As a consequence, a void may be trapped within, leading to a defective “keyhole” structure.
3. 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.
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. In generating the simulated film profiles surface migration of atoms was neglected, which is valid assumption at low substrate temperatures.
2. Heating the substrate increases surface diffusion of depositing atoms, thus promoting coverage by filling potential voids as they form.
Computer Modeling of Step CoverageLine-of-sight motion of evaporant atoms and sticking coefficients of unity can be assumed in estimating the extent of coverage.
Evaporant vapor impingement rate
aA MdN /
Gas molecule impingement rate
scmMT
P
MRT
PN A 22/1
22 /#)(
10513.32
Impurity concentration Ci
dTM
PMC
g
ai
21082.5
)(:
:
:
)/(:
:
torrpressurevaporgasresidualP
weightmolecularevaporantM
weightmolecularevaporantM
scmratedepositiond
densityfilm
g
a
Film Purity
aAi MdNC
/
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. 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.
2. 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.)
3. 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.