thin film deposition - las positas collegelpc1.clpccd.cc.ca.us/lpc/tswain/lect9.pdf · thin film...

Thin Film Deposition

1

7 April 2003

Thin film Deposition!Evaporation

!Sputtering

Field Trip 6:00 PM, Monday April 21Do Not Come to Class!

Semicore

5027 Preston Ave.

Livermore, CA 94550

925-373-8201


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History of Thin Film Deposition

! thermally induced evaporation (by electrical resistance heating, induction heating, and electron beam heating),

! sputtering (diode, triode, magnetron)

! ion beam

! Chemical Vapor Deposition

! Molecular Beam Epitaxy

! laser ablation

PVD: Physical Vapor Depositon

source

evaporant

substrate

vacuum vessel

The three basic steps in any physical vapor deposition process: Evaporationfrom the source, Transport of evaporant, and Condensation of the evaporant.


3

Benefits of Vacuum Deposition

! High chemical purity.! Good adhesion between the thin

film and substrate.! Control over mechanical stress

in the film.! Deposition of very thin layers,

and multiple layers of different materials.

! Low gas entrapment.

Parameters You Can Influence

! Incident Kinetic energy.

! Substrate temperature.

! Deposition rate of the thin film.

! Gas scattering during transport of the evaporant

! Augmented energy applied to the film during growth.


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Strike Layers may be necessary

! Gold does not form a chemical bond with many substrate materials.

! depositing a thin (500Å thick) "Binder" layer of chromium or niobium is a remedy to dramatically improve adhesion.

! Chromium and niobium are reactive and will bond with both substrate Si02 and Gold.

Evaporation Temperatures

Material

Evaporation temperature,

°C

Comments

Zinc 325 High vapor pressure at RT Aluminum 1390 Copper 1516 Chromium 1612 Lead 1680 Toxic Iron 1829 Nickel 1848

All materials evaporate, even at room temperatures. Heat simply accelerates the process.

Equilibrium Vapor Pressure. Cadmium…Rhenium


5

Requirements for Filaments

"Electrically Conductive"High melting point."Good thermal shock resistance"Filament should be wettable by the charge materials."Low solubility for the charge materials.

helical filament

conical basket

flat boat with dimple

trough style boat

Induction Heated Thermal Evaporation! an electric current is

induced to flow through an electrically conductive charge material

! by the application of radio-frequency (RF) alternating current

! power may range from 1 to 50 kilowatts, depending on the size of the charge.

! The AC current is flowed through the copper coil which surrounds a refractory ceramic crucible.

induction coil

crucible


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Advantages and Disadvantages over Thermal Evaporation

Advantages! Low contamination of the deposited thin

films .! Improved control of deposition rate.! Larger charges can be loaded per

deposition run.

Disadvantages! RF power supplies are large and costly! Chemical interaction between the charge

and crucible can occur.

Work Accelerated Evaporator Concept

filament

focussing apreture

Cruciblecharge

electrons

filament heating power supply

accelerating voltage

vacuum vessel


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Cylindrical Focusing Electrode

filament

charge

evaporant

focussing electrode

electrons

water cooling circuit

Advantages: The evaporant may be directed at a substrate placed above the source without interference by a focusing aperture or filament. Additionally, the focusing aperture and filament do not become heavily overcoated.

Evaporator with steering coils


focussing electro- magnets

charge

substrates

evaporant

Advantages: source can be controlled by rastering the electron beam, improving the thickness uniformity and coverage of the substrate.

Note: in this design the electrons emitted from the filament impact the backside of the cathode, heating it so that it will in turn emit electrons. The cathode area which emits electrons is hemispherical,


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Self Accelerated e-beam evaporation

filament

focussing apreture

Cruciblecharge


accelerating voltage

vacuum vessel

anode

focussing coilsElectrons are

not accelerated into the charge.

Transverse Design is Commercialized

water inlet

water outlet

Top view

Side view

filamentcrucible liner charge

filament


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The Big Picture

TC 1

IG 2 TC 2

electric motorvacuum rotary feedthrough

substrate holding fixture

deposition shielding

Features (MDC)


10

Horizontal Source Assembly on 8” Conflat

Source Assembly (Courtesy MDC)


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Programmable Sweep Controller

~$5000 with some options

Plasma Enhanced DepositionCold Cathode Design

! the cathode is biased negatively from -5 to -20 kV,

! A partial pressure of Argon or Helium from 1 to 100 mTorr is dynamically maintained.

! Electrons emitted by the cathode ionize process gas atoms. These ions are accelerated to the cathode.

vacuum flange

grid lead

grid

cathode

electron beam exit aperture

shield


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Plasma Enhanced DepositionHot Cathode Design

! Much higher deposition rates are permitted with this design.

! Radio frequency AC electric current is supplied to the cathode from a low voltage, high current power supply.

! Ionization of the process gas occurs as a result of the applied electrical power.

! The cathode needs to be water cooled.

Power supply 1000 A, 50 V

process gas

electrons

electro- magnets

process gas

evaporant

water cooling jacket

detail of cathode

Sputtering

"The verb to SPUTTER originates from Latin SPUTARE (To emit saliva with noise).

"Phenomenon first described 150 years ago… Grove (1852) and plücker (1858) first reported vaporization and film formation of metal films by sputtering.

"Key for understanding discovery of electrons and positive ions in low pressure gas discharges and atom structure (J.J. Thomson, Rutherford), 1897

"Other names for SPUTTERING were SPLUTTERING and CATHODE DESINTEGRATION.


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Penning 1935 (Post Magnetrons)

B

-E

Plasma

Cold Cathode Invention

Clarke 1971

Inversed cylindrical magnetron.Sputter inside a cylinder.Target also cylindrical.

Peter Clarke Founded Sputter Films in Santa Barbara on this Patent


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Materials Res. Co. 1991

DVD(Balzers)

Process Specific Magnetrons

Rotatable cylindrical magnetron (BOC, 1994).

Web coatings and glass coating.

Target materials sometimes difficult to find in cylindrical shape.

Alternative Magnetron Designs


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Diode Sputtering Basics

working distance

cathode

anode

G

e-

G+

C

vacuum vessel wall

mounting flange

electrical power feedthrough

insulator

power supply

- V

+V

Fundamentals of Sputare

! Following evacuation of the vessel to a low base pressure, a process gas (Typ. Argon) is admitted and maintained at a user-selectable pressure between 1-100 mTorr

! An electric bias of from 500 to 5000 V DC is applied to the target. Electrons emitted by the target strike process gas molecules in the vicinity of the target, and may cause the gas to become ionized.

! The positive ions thus created are accelerated towards the cathode by the applied negative bias. When the positive ions collide with the cathode, the kinetic energy transferred is sufficient to eject atoms of the cathode material.

! Secondary electrons, ions, and light (IR, visible, UV and X-rays) are also emitted during this collision.


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Downstream Pressure ControlThrottling Gate Valves permit switching between

100mT and 1E-07 T pressures with a turbopump

Why Argon?

In general, the sputter yield is greatest for the following set of conditions:

! High atomic weight process gas.! Low atomic weight cathode material.! Low concentration of reactive gas species

in the vessel.Argon is the most commonly employed process

gas for sputter deposition processes, as it has a high sputter yield for most metals, is chemically inert and non-toxic, and is relatively inexpensive (compared with the other noble gases (Krypton and Xenon).


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Characteristics of Sputtering

! Emissions from cathode include neutral atoms, ions (both positive and negative), electrons, neutral clusters of atoms and charged clusters of atoms. Of these, the vast majority are neutral atoms. These atoms have kinetic energies approximately 50 to 100 times that of neutral atoms generated from thermal evaporation sources.

! This additional energy is thought to be the reason for the greater adhesion often observed for sputter deposited films over thermally evaporated films.

! Due to the relatively high pressure in an operating sputter deposition chamber, the mean-free path of sputtered species is short. The numerous gas-phase collisions which the sputtered material suffers between the target and substrate tend to reduce the amount of kinetic energy the depositing species have upon arrival. the density and crystal structure of the thin film are affected.

Thermalization

! When sputtered atoms lose energy by gas collisions, they are said to be "thermalized“

! their kinetic energy is reduced to equal that expected for similar atoms at the ambient temperature. 100101.1

.1

1

10

100

Argon Pressure [mTorr]

Dis

tanc

e to

Rea

ch T

herm

al E

nerg

y [c

m]

Ta

Al


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‘Gas’ of charged particles Electrons (negative)Ions (positive)

Plasma ParametersDensity (ne, ni)Electron temperature (Te)Usually measured in electron volts (1eV = 12000°c)Plasma potential (Vp)

“Basic” Concepts

Collision type Mean free path

Sputtered neutral argon 5cm

Sputtered neutral – electron (ionisation) 400m

For 5 mtorr of argon (~300K) with 5ev electrons

The mean free path of the sputtered neutrals depends

on the target material and the background gas

The probability of a sputtered neutral being ionised by electron

collision on the way to the substrate (say 10 cm) is 0.025%

Mean Free Path Issues


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Collision type Mean Free Path

Electron – electron 40 m

Electron – argon (Momentum loss) 50 cm

Electron – argon (Ionisation) 5 m

Electron – argon (Double ionisation) 100 m

Argon – argon 2 cm

For 5 mtorr of argon (~300K) with 5eV electrons

The mean free path is the average distance a particlewill travel before undergoing a collision

Mean Free Path influenced by Particle Size

Triode Sputtering

cathode

anode

vacuum vessel wall

mounting flange

electrical power feedthrough

insulator

cathode power supply

- V

Filament power supply

e-e-e-

e-

anode power supply

substrate


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Magnetron Sputtering

N S N

DC Power supply

target (cathode)

plasma ringmagnetic field lines

Oblique view of planar magnetron cathode.

Side view of planar magnetron cathode showing magnetic structure.

Confinement between a negatively biased target and ‘closed’ magnetic field produces a dense plasma.

High densities of ions are generated within the confined plasma, and these ions are subsequently attracted to negative target, producing sputtering at high rates.

+ +-

Neg

ativ

ely

bias

ed ta

rget

-V

High density plasmagenerated by the combineelectrical and magnetic field

Resulting erosionof the sputter target

Magnetron Sputtering


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Plasma is a fluid of positive ions and electrons in a quasi-neutral electrical state

The vessel that contains this fluid is formed by electric and magnetic fields.

In many plasma coating applications positive ions are generatedby collisions between neutral particles and energetic electrons.

The electrons in a plasma are highly mobile,especially compared to the larger ions (typically argon for sputtering)

Control of these highly mobile plasma electrons is the key to all formsof plasma control

0+ e-1+

e-

Conversion of a neutral atom into an ion by electron collision

in a plasma

What is a Plasma

e-

xB

B

e-

E

B

ExB

S N N S

e-

Electron motion in a combined electric & magnetic field


22

Rotating arrays (Garrett ,1983- Fujitsu, 1985 -Varian,Applied Materials 1990)# Semiconductor industry

Planar Rotating Magnetron

Basic Rectangular Magnetron


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Material will be sputtered at a rate that depends on the current of ions to the target, and the sputter yield:

But the energy of the incoming ions is just the voltage on the cathode, and the yield is approximately linear with energy:

So we can say:

( )EYIR ⋅∝

PowerVIR =⋅∝( ) VEEY ∝∝

Planar Magnetrons allow high Deposition rates

Target Uniformity also drives cost


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Magnetron Structure Cooling

N

S

N

SS

N

S S

water cooling circuit

magnetic field lines

sputtered material

ground plane shield

Typical Operating Parameters of a Magnetron

! Current density at the cathode: 10-50 mA/sq-cm

! Process gas pressure (Argon): 3 to 50 mTorr

! Cathode bias: -400 to -2000 VDC

! Working distance: 2 to 20 cm


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In-situ Film Metrics

! Quartz Crystal Microbalance

! Optical Monitoring

! Mechanical Stress Measurment

Ar+

Electrons lost from the discharge can be replaced by secondary emission - electrons liberated from the target due to ion impact.

This is characterised by the parameter γ. For every ion impacting on the target γ electrons are released by secondary emission.

For most metals, γ ~ 0.1, and is insensitive to ion impact energy (for ion energies < 1keV).

Secondary Ion Emission


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e-

ArAr+

e-

e-

Ions and electrons lost from the discharge can be replaced by ionising collisions

e- + Ar ⇒ e- + e- + Ar+

Main Menu

Ionizing Collisions

If an electron collides with an Argon ion, there is a possibility of the Argon losing another electron,

becoming doubly ionised

e- + Ar+ ⇒ e- + e- + Ar++

e-

Ar+

Ar++

e-

e-

Double Ionization


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B B

e- Ar+

Magnetic field traps electrons in the discharge,resulting in an electron having more ionising collisions

before being lost from the discharge.Ions, being much heavier, have a much larger gyro-radius,

and are relatively unaffected by the field.

Increasing Trajectory through Magnetic Fields

Electrons trapped by magnetic field, so lower pressures can be used.

Lower pressure means that sputtered atoms are less likely

to have a collision on the way to the substrate

High Pressure Discharge Low Pressure Discharge (idealised)

Magnetic Files allow a lower sustainable plasma pressure


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Sheath

Distance

Pote

ntia

l

Quasineutrality and the Wall Boundary Condition

He Ne Ar Kr Xe

Si

Ti

Al

Cu

0

0.5

1

1.5

2

2.5

3

Plasma Gas

TargetMaterial

Sputter YieldsFor Selected Plasmas Sputtering Different Target Materials (Ion Energy = 600 eV)

Ions impacting on the target can liberateneutrals from the target

Relative Sputter Yields


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The energy distribution of the sputtered neutrals is given by:

( ) ( )3BEE

EEf+

∝

E = energy of sputtered neutral

EB = surface binding energy of target ~ 5 eV

F(e) = probability of a sputtered neutral

being emitted with energy E

Thompson Sigmund Distribution

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12 14 16 18 20

Energy Of Sputtered Neutral (eV)

f(E) /

Arb

itary

Uni

ts

EB/2

Thompson Sigmund Distribution

thin film deposition - las positas collegelpc1.clpccd.cc.ca.us/lpc/tswain/lect9.pdf · thin film...

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