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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
Chapter 3. Thin-Film Evaporation Processes
1. Physical Vapor Deposition (PVD)
Atoms are removed from the source (target)Controllably transfer atoms from a source to a substrate wherefilm formation and growth proceed atomistically
- Evaporation by thermal means
- Sputtering by gaseous ions
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
I. 1. Evaporation Rate
Basic equation for the rate of evaporation from both liquid and solid surfaces, evaporation flux
( )TRMPPN heAe
e πα
2
−=Φ
αe : coefficient of evaporationPe : equilibrium pressurePh : hydrostatic pressureΦe : # of atoms/ unit area time
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
Maximum value of Φe ⇐ αe = 1 and Ph = 0
)sec/(10513.3 222 •×=Φ cmmoleculesTM
Pee
mass evaporation rate
( )sec/1084.5 22 •×= −Γ cmgPTM
ee
Pe in Torr
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
I.2 Vapor pressure of the elemetntClausius – Clapeyron equation : connection between temperature
and vapor pressure
For both solid-vapor and liquid-vapor equilibrium
( ) ( )2RTTHP
dTdP
VTTH
dTdP ∆
=→∆
∆=
PRTV VVVV vVcv
≈≈−=∆ ,
)constant(nevaporatioofheatmolarthe:)( eHTH
Since
∆≅∆
For practiceΦe = 10-5 g/cm2• sec10-3 Torr at MPis needed.
ITRH
P e +∆
−≈ln
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
I.3 Evaporation of CompoundsMetals : evaporate as atoms and occasionally as clusters of atomsCompounds : most inorganic compounds evaporate with molecularchange → stoichiometry of the film deposit will differ from thatof the source.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
I.4 Evaporation of Alloys- Electronic, magnetic, optical, decorative applications Al-Cu,
Permalloy (Fe-Ni), nichrome (Ni-Cr), Co-Cr, ……The constituent of the alloys evaporate nearly independent
of each other- EAB= EAA= EBB
- Ideal solution Raoult’s law
( )solution)in B offraction mole (
0
→
=
B
BBB
X
PXP
Metallic solutions usually are not ideal
( )B ofion concentrat ic thermodymeffective :activity
0
→
=
B
BBB
aPaP
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
tcoefficienactivity →
=
B
BBB Xaγ
γ
- The ratio of the fluxes of A and B atoms in the vapor streamabove the melt is given by
( )( ) M
MPXPX
A
B
BBB
AAA
B
A
0
0
γγ
=ΦΦ
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9
Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
<예제> For films (Al-Cu) with 2 wt % Cu, assuming
1010
4
3
2)0()0(
/2/98
−
−
×=
=ΦΦ
PP
MM
Cu
Al
Cu
Al
Cu
Al
1527
7.631022
98
10 3
4
=×
•= −
−
XX
cu
Al
∴ It is necessary to have 13.6 wt % Cu in source alloy at 1350K
- Melt composition usually changes as evaporation proceeds
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
I-5. Film Thickness Uniformity
I.5.1 Deposition GeometryThe Source ⋅ Substrate geometry influences the ultimate filmuniformity
Point source
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
area`unitperdeposited:4
cos4::
cos
:
2
2
0
massrr
MdAMd
rdAMMd
dAdA
dtdAM
massevaporatedtotalM
e
s
s
ces
sc
A ee
t
e
e
e
πθ
π
θ
=
=
=
⋅Γ= ∫∫
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
② From kinetic theory & experiment for surface source
lawondistributicosine
coscos2 ←=
rM
AdMd e
s
s
πθφ
More realistically ; cosnφ evaporation law
)0(2
coscos)1(2 ≥
+= n
rnM
dAsMd n
es
πθφ
- n is related to evaporation crucible qeometry and the scaleswith the ratio of the melt depth to the melt surface area.
- when n is large, the vapor flux is highly directed.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
1.5.2 Thickness Distribution
depositsofdensity, == ρρ s
s
dAMddFilm thickness
For the point source ;
0at deposit thickest,})/(1{
1)(444
cos
2/32
2/32232
=+
=
+⋅
=⋅
==
ldhld
dlhhM
rhM
rMd
o
o
eee
πρπρπρθ
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
For the surface source ;
22
222
2
22
})(1{
1)(
coscos
hld
dlh
hMrh
rh
rM
rMd
o
eee
+=
+=⋅⋅==πρπρπρ
φθ
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16
Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
- less thickness uniformity with the surface source- uniformity ↑ as h↑ but waste of evaporant
Rotating Disk
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17
Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
A clever way to achieve thickness uniformity
- surface source and substrates locate on the surface of a sphere
22 4
cos cos
o
ee
S
S
rM
rM
dAMd
ππθφ==
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
Conformal coverage- semiconductor contact- interconnection metallization
→ source of failure in device
Computer modeling of step coverage
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
1.6 Film purity
From background gasImpurity concentration in the film
) sec (atoms
105.3
12
22
−−•
×=Φ
cm
TMP
b
bb
rate deposition :
•
•∝
d
d
MTM
PC a
b
bi
ρFrom source
12 sec atoms −−
•
•=Φ cmM
dN
a
AS
ρ
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
Traditionally “evaporation” producescleaner film than “sputtering” because of
bPlower andhigher •
d
Greatly improved in magnetron sputtering these days and gives similar results in cases of evaporation and sputtering of Al
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
3.4 Evaporation Hardware
3.4.1 Electrically heated evaporation sources
- Tungsten wire sources
- Refractory metal sheet sources
- Sublimation furnaces
- Crucible sources
- Estimating the temperature of resistance heaters
cn ALTTiRip /)]0(/)[0(22 ρ== 1≈n
Stefan-Boltzmann law for radiated power
Emissivity
Stefan’s constant))0(( 44 TTAp sr −= εσ
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
3.4.2 Electron-beam evaporation
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
Disadvantages of resistively heated evaporation sources :
- Contamination by crucibles, heaters, and support materials
- Limitation of relatively low input power levels
Therefore, e-beam evaportion becomes the preferred vacuum
evaporation technique for depositing films.
The evaporant charge is placed in the depression of a water-
cooled copper hearth.
Accelerate the electrons by 4KV-20KV, deflected 270oC by a
transverse magnetic field.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
3.5.2 Pulsed laser deposition(PLD)
- Gas excimer lasers : ArF(193nm), KrF(248nm), XeCl(308nm)
- Laser output power of ~ 500 mJ/pulse
- Pulse repetition rates of several hundred Hz
- The absorbed beam energy is converted into thermal, chemical, and mechanical energy, causing electronic excitation of target atoms, ablation and exfoliation of the surface, and plasma formation.
- Evaporants form a plume above target, consiting of a motley collection of energetic neutral atoms, molecules, ions, electrons, atom clusters, mincron sized particulates, and molten droplets. The plume is highly directional, i.e., 128,cos << nwheren φ
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
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33
Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
The thickness of material evaporated per laser pulse:
( )sec/1084.5 22 •×= −Γ cmgPTM
ee
( )sec1084.51 2 •×=• − cmPTM
eBd ρ
For Al, TB=2793K, Pe=760torr, evaporation rate=0.0436 m/s
for a 10ns laser pulse, evaporation rate is 0.4nm/pulse
Typical values are 1-10 nm/pulse
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.
3.5.4 Ion Beam Assisted Deposition(IBAD)
To improve the film properties,- Alternate substrate temperature- Employ ion bombardment of the substrate
- Use of broad-beam (Kaufman) ion source- Unlike plasma, independently control ion flux and energy- 1mA/cm2 ≈ 6.25 × 1015 ions/cm2⋅sec, low energy (10ev-1KeV)
→ enhancement of the density and index of refraction ofoptical coating
- control stress, hardness, adhesion, refractive index, step coverage
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Thin Film Technology
Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab.