Chapter 3. Thin-Film Evaporation Processes 1. …master/lecture/TFT2010/Lecture...1 Thin Film Technology Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab. Chapter 3. Thin-Film

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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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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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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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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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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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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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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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<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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I-5. Film Thickness Uniformity

I.5.1 Deposition GeometryThe Source ⋅ Substrate geometry influences the ultimate filmuniformity

Point source

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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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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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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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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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- less thickness uniformity with the surface source- uniformity ↑ as h↑ but waste of evaporant

Rotating Disk

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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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Conformal coverage- semiconductor contact- interconnection metallization

→ source of failure in device

Computer modeling of step coverage

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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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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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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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3.4.2 Electron-beam evaporation

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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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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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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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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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Chapter 3. Thin-Film Evaporation Processes 1. …master/lecture/TFT2010/Lecture...1 Thin Film Technology Dept. of Mats. Sci. & Eng. | Nanophotonic Semiconductors Lab. Chapter 3. Thin-Film

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