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.