thin film deposition techniques

Thin Film Deposition techniques

Thermal EvaporationThermal Evaporation

1SCPY663 Sem2/2009 (Physics-MUSC)

An evaporator

EEvaporator

2SCPY663 Sem2/2009 (Physics‐MUSC)

Evaporation rate

• all materials have an equilibrium vapor pressure, pe(T) (T: source temperature)

t ffi i tl hi h t t th i i t t Φ Φ ( )• at sufficiently high temperature, the gas impingement rate Φ = Φ (pe)can be high enough to cause deposition of material (thin-film growth)on a cold substrate (TS<<T, TS: substrate temperature)

“cold” substrate, Ts

T>> Tvapor

T>> Ts

“hot” source, T


Evaporation rate

Gas impingement rate for thermal evaporation (Knudsen equation, Ch.2 – p.8)

p : equlibrium vapor pressure of the sourcepe: equlibrium vapor pressure of the source

ph: hydeostatic pressure acting on the evaporantMRTppN heA

e π2)( −

=Φ

Usually: ph = 0 (vacuum)

pp

1 ML≈ 3 ÅsML

MT

gKtorrp

scmmolecules

MT

gKtorrp ee

⋅⋅

⋅≈⋅

⋅⋅

⋅=Φ 72

22 105.3105.3

Γe: mass evaporation rate

scmg

torrp

gK

TM

e ⋅⋅⋅⋅⋅⋅=Γ −

22108.5

typically 10-4 g/(cm2s)at 10-2 torr

( )

scmtorrgT ⋅

!! pe = pe(T) !!4SCPY663 Sem2/2009 (Physics-

MUSC)

Clausius – Clapeyron equation (solid – liquid or liquid - vapor equlibrium)

ΔH: molar enthalpy differenceΔV: molar volume differenceVT

HdTdpe

Δ⋅Δ

=

e pTHdp⋅

Δ=

)(

pRTVV gas /≈≈Δ

epRTdT

⋅= 2

ΔH(T) ≈ He molar heat of evaporation ⎟

⎠⎞

⎜⎝⎛−⋅=

RTHpTp e

e exp)( 0

Good approximation, but not exact (non – perfect gas behavior, ΔH(T) ≠ const.)Water: H = 40 6 kJ/molWater: He = 40.6 kJ/mol

p(373K) = 105 Pa - p(273K) = 103 Pa - p(77K) = 10-17 Pa5SCPY663 Sem2/2009 (Physics-

MUSC)

Clausius-Clapeyron equation only applicable if system is in equilibrium(molar evaporation and condensation rates, Qv and Qc, are balanced)( p , v c, )

ifi di t λ

pe

Q

orifice diameter << λ

q

Qc Qv

q

Qvevaporation ratecan be calculated

evaporation ratemust be measured

q q

Knudsen cell

vacuum evaporator

Rule of thumb: if pe(TM)≤10-3 torr then T >TM is required


Vapor pressure of selected elements

AlCuCu

1350K

• melting temperature Tm 7SCPY663 Sem2/2009 (Physics-MUSC)

Angular distribution of evaporants: cosine law

Emitting flux from any point on the surface; JvTotal evaporation rate from a source of area A; Q = JvA

KQJdifhFdAr

ФJJrQJ

0

20

coson distributiflux cosine

r; radiusofsphereFor

θθ =

=

r

ө

Ф

dA

rr dAr

JJdA 20 coscos;on Flux Receiving φθ

=

dAs

K cellsourcepoint ;

41

factorgeometry

π=

=K

K-cell

J

boat e.g. source shape disc ; 14

π

π

=

Jo

өJocos ө

Deposition rate is determined in J┴=J cos2ФIn the typical case of disc-shaped source Ф=ө ;

J┴ = Jocos4ө /r2


Evaporation of Compounds and Alloys

only very few compounds evaporate as molecules→ vapor composition and film stoichiometry do not differ from that of h SiO B O G O S O AlN C F M F )the source e.g. SiO, B2O3, GeO, SnO, AlN, CaF2, MgF2)

→ most compounds decompose,e.g. (1) Ag2Se(s) → 2Ag(g)+½Se2(g)

(2) SiO2(s) → SiO(g)+½O2(g)( ) 2( ) (g) 2(g)

→ evaporate from separate sources (1) or introduce O2 partial pressure (2- reactive evaporation)introduce O2 partial pressure (2 reactive evaporation)

evaporated metal alloy films are widely used:evaporated metal alloy films are widely used:• Al-Cu metallization in integrated circuits• Fe-Ni magnetic data storage

t• etc.9SCPY663 Sem2/2009 (Physics-

MUSC)

metal atoms in an alloy are less tightly bound than atoms in an inorganic compoundconstituents nearly evaporate independently of each other,

enter vapor phase as single atomsenter vapor phase as single atomsmetallic melts are solutions and follow thermodynamics

binary alloy AB:interaction energy A-B usually differs from energies A-A, B-B (real solution)interaction energy A B usually differs from energies A A, B B (real solution)

partial pressure of A in AB at T, pA≠ partial pressure of pure A at T, pA(0)

pA=γA.XA.pA(0)γA: activity coefficient, XA: mole fractionγA y , AKnudsen equation → flux ratio

BAAAA M)0(X pγ=

Φ

γi, Mi, and T determine the vapor and film compositionABABB M

.)0()X-(1 pγ

=Φ


M/98 ⎫Φ

Is it feasible to evaporate an Al-2wt%Cu alloy at T=1350K?

)( 15MM

)0()0(

10)0(

M/2M/98

CuAlCuCuAlCuAl

3Cu

Al

Cu

Al

γγγγ

=≈ΦΦ

=⎪

⎪⎪⎬

⎫=ΦΦ

− pp

XX

p M)0(102

10)0()0( AlAlCuAlCu

4Cu

Al γ Φ⎪⎪⎭×

= −

pXpp

the melt should have the composition 13wt-% Cu in order to compensatefor the preferential evaporation of Al

melt volume should be large to avoid composition changesdue to preferential evaporation

better: evaporation from dual sources maintained at different temperaturesIf keeping Al at 1350K, what should be Cu temperature to have Al-2wt%Cu?p g p


Evaporation Source

resistance-heated evaporation sources(few V, 10-50A)( )

tungsten wire sourcesevaporant wets W &

is retained by surface tensionis retained by surface tension

refractory metal sheet sources(Ta W Mo)(Ta, W, Mo)

for poor wetting evaporants or powders

cr cible so rcescrucible sources(Al2O3, BN, graphite, WC, indirectly

heated by W wires or sheets)for evaporants that alloy

with Ta, W, Mo)


Knudsen Cell

Heater Crucible


disadvantages:disadvantages:contamination, alloy formation, chemical

reaction of source material and evaporant possible outgassing of hot source materialpossible outgassing of hot source materialsmall evaporation rates, low input power

small evaporant volume


Electron Beam Evaporator

suitable for almost all evaporantsheated filament: thermionic electron emissionno direct lines of sight filament evaporant andno direct lines of sight filament-evaporant andfilament-substrate: no contamination of thefilm/coating of the filament

electron acceleration (1...10 kV)electron deflection by magnetic field (Lorentz force)high power (kW): can evaporate high meltingg p ( ) p g gpoint materialswater cooled crucible: material only melts atthe surface, no alloying with crucible etc.

1 W/cm2 for evaporation0.1 W/cm2 for kinetic energy of vapor atoms10 W/ 2 f di i h l10 W/cm2 for radiation heat losskW/cm2 for heat conduction into the cruciblepower consumption



Crucible rotation

water coolingwater cooling flanges



qΦ−

The electron current density je leaving the hot filament is due to thermionic emission, as expressed by Richardson's equation:

kTqATjeΦ−

= exp2

where A is Richardson’s constant (1.20×106 A/m2), q is elementary electronic charge, and Φ is the ( ), q y g ,work function.

The e-beam evaporation source is a relatively high intensity source, as it is not necessary to raise aThe e beam evaporation source is a relatively high intensity source, as it is not necessary to raise a furnace or other enclosure to the temperature of the evaporant. The evaporant flux can be so dense near the evaporant surface that it is in laminar flow. The beam intensity of e-beam evaporators has been described with a cosn(θ) law.

( ) ( )ϑϑ nn JCCJ coscos)1(~ ⋅∝+−Ω

with the exponent n varying from 2 to at least 6, as J increases. Crucibles are typically made of materials with a high melt point, like Al2O3, graphite, TiN, BN, etc., and often cooled by waterand often cooled by water.


Laser-beam evaporation

High energy laser beam is used to evaporate the target material. It heats the source surface only and can be operated at a high pressure.

A laser beam evaporation system:p y1 – CO2 laser, 2 – ZnSe window, 3 – crucible, 4 – target material, 5 – pump, 6 – gauge, 7 mass flow controller 8 mirror7 – mass flow controller, 8 – mirror, 9 – substrat, 10 – substrate heater


Pulsed Laser depositionl d l d i i (PLD) "fl h i " h d

•condensible vapor is produced after target absorbing a

•pulsed laser deposition (PLD) = "flash evaporation" method. •Special deposition technique for specialized thin films of unusual stoichiometries or super-lattices. •pulsed excimer laser for evaporation directed through a viewport at a target mounted in a vacuum chamber.

•condensible vapor is produced after target absorbing a powerful laser beam strikes a target and vaporizes a thin surface region. • vaporized region of the target ~ several hundred to 1000 angstroms thick. • conical plume of evaporant caused by ablation of the material is created. •The axis of the vapor plume normal to the target's surface;•The axis of the vapor plume normal to the target s surface;•follows a cosine distribution rule. Such a visible plume appears when the emitted vapor is ionized by the laser, forming a plasma. •characteristic speed of the evaporant particles (which can be both neutrals and ions) - 3×105 cm/sec ~ kinetic energy of - 3 eV. •The film growth rates can approach 0 5 μm/minThe film growth rates can approach 0.5 μm/min.•KrF excimer laser most frequently used lasers •operating at wavelength=248 nm. Thus the energy carried by a photon is hν=hc/λ, which should be over the energy y p gyrequired for an atom escaping from the solid target, hν >Econdense. 19SCPY663 Sem2/2009 (Physics-

MUSC)

Excimer laser used for PLD:

Laser medium ArF KrF XeCl XeF

Wavelength (nm) 193 248 308 351

Pulse energy (mJ) 400 600 400 320gy

Average power (W) 10 16 11 8

Gas lifetime (106 pulse) 0.4 1 10 2

Some representative laser parameters are as follows:

( p ) 4

Some representative laser parameters are as follows:• i) The wave length is in the UV range ii) A pulse on the order of 25 ns in duration (thepulse duration, δt) p , )iii) At a power density j of 2.4×108 W/cm2 at the target• The illuminating an area of the target (δA) of typically 0.1 cm2

• At a repetition rate (f) of 50 Hz.The fluence of this typical pulse (j δt) is thus 6 J/cm2. The incident energy per pulse is 300-600 mJ. The instantaneous power is 2.4×107 W, and the average power is 30W.20SCPY663 Sem2/2009 (Physics-

MUSC)

The fluence of this typical pulse (jδt) is thus 6 J/cm2. The incident energy per pulse is 300-600 mJ. The instantaneous power is 2.4×107 W, and the average power is 30 W.One of the most successful PLD applications has been the preparation of high temperature superconducting thin films. PLD seems unusually effective in recreating in the thin film the stoichiometric composition of the complex multi-component target materials; thestoichiometric composition of the complex, multi-component target materials; the vaporization is so fast that segregation is nearly impossible. Sometimes preserving stoichiometry is assisted by performing PLD with a high partial pressure (in the mtorr range) of reactive gas, such as oxygen, due to the absence of hot filaments or other hot components. The drawbacks of PLD are the relatively small deposition area, poor thickness uniformity, and the surface outgrowths that lead to a rough film surface.


Example I

Determine the rate of loss of MgO by evaporation when exposed to a temperature of 2000K. MgO(s)=Mg(g)+O(g) Given the Gibbs Free Energy g ( ) g(g) (g) gy

J 2038.2826.10082290 TG −=Δ


Sources for Evaporating multi-component film

Figure 12.10 Methods for evaporating multicompoment films include (a) g p g p ( )single source evaporation, (b) multisource simultaneous evaporation, and (c) multisource sequential evaporation


Example II

Consider a drop of water inside the room temperature vacuum chamber. If the drop forms a hemisphere of radius r0, and if the drop remains at room temperature, calculate the time it will take to evaporate the drop. Assume r0 = 1 mm.


Evaporation rate

Gas impingement rate for thermal evaporation (Knudsen equation, Ch.2 – p.8)

pe: equlibrium vapor pressure of the sourceph: hydeostatic pressure acting on the evaporant

MRTppN heA

e π2)( −

=Φ

Usually: ph = 0 (vacuum)

1 ML≈ 3 Ås

MLMT

gKtorrp

scmmolecules

MT

gKtorrp ee

⋅⋅

⋅≈⋅

⋅⋅

⋅=Φ 72

22 105.3105.3

Γe: mass evaporation rategpKM

e ⋅⋅⋅⋅⋅=Γ −2

2108.5typically 10-4 g/(cm2s)at 10-2 torr

!! p = p (T) !!

scmtorrgTe ⋅2

!! pe = pe(T) !!


Example III

An evaporator is used to deposit aluminum. The aluminum boat is maintained at a uniform temperature of 1100 °C. If the evaporator planetary has a radius of 40 cm yand the diameter of the crucible is 5 cm, what is the deposition rate of aluminum? If the chamber also has a background pressure of 10-6 torr of water vapor and the water vapor is assumed to be at room temperature, determine the ratio of the arrival rated of the aluminum atoms and the water molecules, the mass density of solid aluminum is 2.7 g/cm3.


Planetary

http://www.edmundoptics.com/TechSupport/DisplayArticle.cfm?articleid=298http://www.ifp.tuwien.ac.at/forschung/duenne_schichten/english/equipment.htm


Vapor pressure of selected elements

Al

1373K=1100C

• melting temperature Tm 28SCPY663 Sem2/2009 (Physics-MUSC)

Step Coverage

Standard evaporation :

Aspect Ratio (AR) = step height/step diameter

Discontinuous film for 1< AR Marginal for 0.5< AR<1Continuous for AR <0.5

Improvements :i) Substrate Heating

hi h d iti thigh deposition rateii) Substrate Rotating


Angular distribution of evaporants: cosine law

Emitting flux from any point on the surface; JvTotal evaporation rate from a source of area A; Q = JvA

KQJdifhFdAr

ФJJrQJ

0

20

coson distributiflux cosine

r; radiusofsphereFor

θθ =

=

r

ө

Ф

dA

rr dAr

JJdA 20 coscos;on Flux Receiving φθ

=

dAs

K cellsourcepoint ;

41

factorgeometry

π=

=K

K-cell

J J cos ө

boat e.g. source shape disc ; 14

π

π

=

Jo

ө

Jocos ө

Deposition rate is determined in J┴=J cos2ФIn the typical case of disc-shaped source Ф=ө ;

J┴ = Jocos4ө /r2


Deposition Geometry

Figure 3-4 Evaporation from (a) point source, (b) surface source.Figure 12.3 The geometry of deposition for a wafer (A) in an arbitrary position and ( ) y p(B) on the surface of a sphere.


Cosn θ distribution

Figure 3-5 Calculated lobe-shaped vapor clouds with various cosine exponents. (From Ref.9.)



qΦ−

The electron current density je leaving the hot filament is due to thermionic emission, as expressed by Richardson's equation:

kTqATjeΦ−

= exp2

where A is Richardson’s constant (1.20×106 A/m2), q is elementary electronic charge, and Φ is the ( ), q y g ,work function.

The e-beam evaporation source is a relatively high intensity source, as it is not necessary to raise aThe e beam evaporation source is a relatively high intensity source, as it is not necessary to raise a furnace or other enclosure to the temperature of the evaporant. The evaporant flux can be so dense near the evaporant surface that it is in laminar flow. The beam intensity of e-beam evaporators has been described with a cosn(θ) law.

( ) ( )ϑϑ nn JCCJ coscos)1(~ ⋅∝+−Ω

with the exponent n varying from 2 to at least 6, as J increases. Crucibles are typically made of materials with a high melt point, like Al2O3, graphite, TiN, BN, etc., and often cooled by waterand often cooled by water.


Thickness uniformity

20 densitybulk material ; 4

sourcepoint for

hd =

Φ= ρ

πρ

2/320

sourcesurfacefor

;})/(1{

1hld

d+

=

20

1

densitybulk material ;

dh

d =Φ

= ρπρ

220 })/(1{

1hld

d+

=


thin film deposition techniques

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