Previous Lecture

Vacuum & Plasma systems for

Dry etching

Objectives

From this “evaporation” lecture you will learn:

Lecture 9: Evaporation & sputtering

• Evaporator system layout & parts

• Vapor pressure

• Crucible heating techniques

• Deposition rates

• Step coverage

• Multicomponent films

Evaporator system

WaferTilt / rotation stage

Wafer shutter

Charge in

crucible

Film thickness gauge

Film thickness monitor (FTM)

• FTM gauge is a piezo-electric

quartz crystal.

• The acoustic self oscillation

frequency f0 (~6 MHz) can easily

be measured electrically.

• f0 decreases with increasing

deposits.

• The FTM gauge should be

positioned to view the crucible

even when the wafer shutter is

closed.

Quartz crystal

Shutter

Dual head

Vapor pressure

The generation of vapor

pressure is called:

• Evaporation from a liquid

• Sublimation from a solid

E.g.

0.01 Torr - 1 Torr

Normal deposition rate

• Vapor pressure ~ 0.1 Torr

near the charge

C Cr Mg

Sb SiO SnO2

Ag

Vapor pressure

~ 0.1 Torr

The vapor pressure is local

and much higher than the

overall chamber pressure.

Too high vapor pressure

(deposition rates) may cause

the vapor to condense

prematurely, forming droplets

that could hit the wafer and/or

FTM.

Vapor pressure

High vapor pressure can

generate a virtual source

above the crucible.

• Resistive heating

Boats

Wire baskets

Wire heated crucibles

Plated rods

Box sources

• Inductive heating

• Electron beam heating

Focused electron beam

Low flux electron beam

Crucible heating techniques

Choice of crucible and heating

method depend on the material to

be deposited.

A poorly chosen crucible material

may alloy with the charge.

E.g. Al & Ni alloys with W

Resistive heating

I

Resistive heating

Wire basket

Chrome plated Tungsten rod

Resistive heating

Resistive heated crucible

Resistive heating

Baffled box sources

Some charge materials may jump

around during sublimation. Use a box

with chimney. (Baffled box source)

E.g. for SiO

Focused electron beam source

EmitterCrucible

Magnet poles

Direct heating of the

charge.

Temperature gradient

from center out.

Single crucible or

several crucibles in a

linear or revolving indexer.E-beam

Vapor

X-ray emission may

cause wafer damage.

x-rays

Focused electron beam source

~Beam current

control

Ibeam

~300mA

Uacc

~7kV

Ifil ~20A

Cathode

Anode

Beam

shaper

B

e-

Focused electron beam source

~

~

~

Long-sweep

Lat-sweep

Beam current

control

Lat-pos

Long-pos Ibeam

~300mA

Uacc

~7kV

Ifil ~20A

BB

B

e-

Deposition rates

Pe

Atomic (molecular) fluxatoms

time ∙ area

J =Pe

2 π k T M

RME = J ∙ M = Pe

M

2 π k T

Mass evaporation rate (Mass flux)mass

time ∙ area

Mass loss ratemass

time

RML = J ∙ M ∙ A = Pe ∙ AM

2 π k T

A

Equilibrium partial

vapor pressure

Deposition rates

Deposition rate thickness

time

Pe

A

R

Rd = J ∙ M ∙ A π R2

1ρ

1

Deposition rates

Deposition rate thickness

time

Pe

A

Φ

Θ

R

Rd = J ∙ M ∙ A =π R2

cosΘ cosΦ1ρ

=M

2 π k T

Pe ∙ A

ρ π R2

cosΘ cosΦ

Most vapor condenses on the chamber walls

Big material waste

Deposition rates

Deposition rate thickness

time

Pe

A

Φ

Θ

R

Rd =M

2 π k T

Pe ∙ A

ρ 4 π r 2

1

Spherical geometry

r

r

cosΘ = cosΦ =R2 r

Many wafers with equal deposition rate

Less material waste

Independent ofRΦΘ

Deposition rate

What is the deposition rate, in the center of the wafer, for gold at a charge

temperature of 1500°C and 3cm2 crucible area?

The distance from the

charge to the wafer

is 40cm.

0.04

40cm

Deposition rate

Step coverage

Except for near the source, the

vapor travels in straight paths.

Strong shadow effect.

Poor step coverage.

Tim

e

Wafer rotation and wafer heating

increase step coverage.

Wafer heating may also increase

film density and film adhesion.

Multicomponent films

Alloy evaporation Co-evaporation Multilayer evaporation

Conservation of

composition

requires identical

vapor pressure.

Correct

composition

requires well

controlled

deposition rates.

Layers can be heated and

alloyed.

Alloy film Alloy film

Example of co- and multilayer

evaporation is molecular beam

epitaxy. (MBE)

Objectives

From this “sputtering” lecture you will learn:

Lecture 9: Evaporation & sputtering

• DC & RF sputter systems

• Physics of sputtering

• Sputter yield

• DC & RF sputter sources

• Film quality

morphology

step coverage, stress, etc.

• Reactive sputtering

• Ion beam sputter deposition

• Evaporation / sputtering comparison

DC sputter system

Ar gas

inletWafer

Shutter

Source

DC source works for conducting targets only.

Difficult to fit a film

thickness monitor.

RF sputter system

Ar gas

inletWafer

Shutter

Source

RF source works also for non conducting targets.

~

z-match

Difficult to fit a film

thickness monitor.

Physics of DC sputtering

Sputtered target atoms Deposit on the wafer

Electrons & ions Create new ions through collisions

Target (Electrode)

Wafer

E

Sputtered target

atom

Secondary

electron emission

Ion impactAr

IonizationAr

Ar

Target dependence of sputter yield

Threshold energy

Sputter yield increases with

ion energy but starts to

saturate near 1keV.

Argon

Sputter yield for some target

materials depend strongly on the

angle of the incident ion.

Projectile dependence of sputter yield

Highest sputter yield for:

• Heavy ions

• Ions with full valence shell

45keV !!!!

Much higher than magnetron

sputtering.

Ar

Ne

Kr

Xe

Magnetically confined plasma

Magnetron, commonly used for sputter deposition sources.

E

Review from Lecture 6.

Magnetron sputter source

• DC and RF sources have targets with negative potential.

• Free electrons try to get away from the target proximity.

• The escape is less successful where the magnetic field is perpendicular

to the escape route.

e-

B

Target erosion

• The target of a typical magnetron source erodes

in a ring pattern.

Eroded target area

• Uneven erosion reduces the lifetime of the target.

• Uneven erosion may change film thickness

uniformity over time.

Target erosion

A new gold sputter target

will be expen$ive!

Eroded target area

W. Disney

Alternative magnetron orientation

Horizontal sputtering Sputter-up

T

T1

2

3

Morphology

Zone 1: Amorphous, porous film.

Zone T: Small grains, smooth and relatively dense film.

Zone 2: Tall narrow columnar grains. Moderately rough surface.

Zone 3: Large 3-D grains. Moderately rough surface.

Au evaporated onto

wafer @85°C

3-zone model

Movchan &

Demchishin

Example:

T/TM = 358K / 1335K =

Evaporation

= 0.27

Morphology

Zone 1: Amorphous, porous film.

Zone T: Small grains, smooth and relatively dense film.

Zone 2: Tall narrow columnar grains. Moderately rough surface.

Zone 3: Large 3-D grains. Moderately rough surface.

3-zone model

1Pa = 7.5mTorr

T

Ti sputtered on wafer

@ 250°C, 7mTorr

Example:Thornton

T/TM = 523K / 1948K =

= 0.27

Directionality

Magnetron sputter deposition has low directionality.

Table

Wafer

Target

The size of the target is about the same as the distance between the

target and the wafer. Approx. 10cm – 15cm

Most material deposits on the wafer.

Step coverage

Deposition progress of sputtered film

All surfaces covered

Low directionality Good step coverage

Directionality

The deposition directionality can be increased by the

use of a collimator.

Table

Wafer

Target

Collimator

A substantial fraction of target material is wasted.

Film stress

Compressive film stress

The film expands if detached

from the wafer.

Tensile film stress

The film contracts if detached

from the wafer.

Film stress causes

Thermal mismatch Tensile or Compressive

The film and the substrate have different thermal expansion

coefficients. The deposition is not done at room temperature.

Atomic attraction Tensile

Atomic attraction seeks to bridge microvoids during deposition.

Mechanical deformation Compressive

Ion impacts during deposition may cause permanent deformation of

the film.

Examples

Measuring film stress

= Film stress

= Bow change

t = Film thickness

E = Young’s modulus of wafer

= Poisson’s ratio of wafer

T = Wafer thickness

R = Wafer radius

2

2

31 R

TE

t

Wafer with

stressed film

R

Reactive sputter system

Ar gas

inlet

~z-match

Process

gas inlet

Why reactive sputtering? Examples of reactive sputtering:

SiO2O2

Si Si3N4N2

Ti TiNN2

In/Sn In2O3 / SnO2O2

Si

Target Gas Film

(ITO)

• Variety of films from few targets.

• Often higher deposition rates.

• DC sputter deposition of oxides possible.

Ion beam sputter deposition system

Ion gun

Ar gas

inlet

Ar gas

inlet

Ion gun

Targets

Wafer

Widely used for dense, high-quality optical coatings.

Evaporation-Sputtering comparison

E-beam

evaporationMagnetron

sputtering

High film density

Good adhesion

Good step coverage

High lift-off compatibility

High film thickness uniformity

Low material consumption (waste)

Low material investments

Low x-ray emission

In-situ thickness & rate control

Good compound deposition control

High system layout flexibility

X

X

X

X

X

X

X

X

X

X

X

Next Lecture

Thin film deposition

using

Pulsed Laser Deposition