lecture 6 thin film deposition,physical vapour deposition

8/9/2019 Lecture 6 Thin film deposition,physical vapour deposition

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Lecture 6

Thin Film Deposition


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Physical vapor deposition (PVD) – Thermal evaporation

– Sputtering

– Others

Example

Thin Film Deposition


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Physical vapor deposition (PVD):

Thermal evaporation Physical evaporation is one of the oldest methods of

depositing metal films. Aluminum and gold are heated to

the point of vaporization, and then evaporate to form to athin film covering the surface of the silicon wafer. All film

deposition takes place under vacuum or very carefully

controlled atmosphere. The degrees of vacuum and

units is shown below:

1 atm = 760 mm = 760 torr = 760,000 µm Hg= 29.92 in Hg = 14.7 lb/in2 = 14.7 p.s.i

= 1,013,250 dynes/cm2

1 torr = 1mm Hg

1 millitorr = 1 µm Hglow vacuum 760 to 25 mm Hg

Rough vacuum 760 to 1 mm Hg

High vacuum 10-3 to 10

-6 mm Hg

Very high vacuum 10-6 to 10

-9 mm Hg

Ultra-high vacuum < 10-9

mm Hg = vacuum in space


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Thermal evaporation

Heat Sources Advantages Disadvantages

Resistance No radiation Contamination

e-beam Low contamination Radiation

RF No radiation Contamination

Laser No radiation, lowcontamination

Expensive

6


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Thermal evaporation

Typical vacuum

system used forevaporation includes: – vacuum chamber

– roughing pump. – high vacuum pump

– valves

– gauges


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Thermal evaporation Vacuum Processing Equipment

Most thin films are deposited under reduced pressure conditions. Mechanical pumps

and absorption pumps are used in the rough pressure range.

Mechanical Pumps

Rotary piston type - The pump compresses the air and removes it. This type of system

requires oil and oil often times contaminates the chamber.

Getters Pumps

Getters are materials included in a vacuum system or device for removing gas by

sorption. Various metal evaporated - Au, Al, Cr, Ni, Ti, In, Zn, AuGe, SiAl

The Diffusion Pump

A low-boiling point silicone based oil provides a jet of super sonic oil molecules. These

oil molecules capture vapor molecules and condense. The reduces the pressure locally

and we then have a diffusion of molecules from inside the vessel to be evacuated.

Baffles must be added to reduce back streaming of the hot oil molecules. This type of

system is only effective of the original pressure in the vessel is only @ 1 to 50µ m.


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Thermal evaporation

Filament evaporation

– loops of a metal (such as Al) are hung

from a filament (W)

– evaporation is accomplished by

increasing the temperature of the

filament until the metal loops are

melted and vaporized.

Electron-beam evaporation

– an electron beam instead of filament is

used

– The electron beam with energy up to

15keV is focused on the source target

containing the materials to be

evaporated.


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Thermal evaporation

Heat Sources Advantages Disadvantages

Resistance No radiation Contamination

e-beam Low contamination Radiation

RF No radiation Contamination

Laser No radiation, lowcontamination

Expensive

N = No exp-ΦekT

6


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Thermal evaporation

Kinetic Gas Theory

The ideal gas law can describe the behavior of gases under vacuum. Pressure P,

volume V, and temperature T of one mole of a gas are related by

PV = Nav kT Nav=Avogadro's #, k=Boltzman constant

– The concentration of gas molecules is given byn = Nav/V = kT/P

– The rate of formation of a surface layer is determined by the impinging molecules if100% stick

Φ = P/(2! mkT)1/2 (molecules/cm2 - sec) where m is the mass of the molecule.

– This can be reduced to

Φ = 2.63 x 1020

P/√(MT)P = pressure in Pa, M = molecular weight

– The time required to form a monolayer on the surface is given by

T = NS/Φ = NS(2! mkT)1/2 /P Ns is the number of molecules/cm2 in the layer .

– An important film-deposition parameter-----the mean free path.

λ = kT/(√2)!

Pd

2

d is the diameter of the gas molecule at room temperature


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Thermal evaporation

Growth rate

For batch fabrication, a

planetary substrate holder

consisting of rotating sections

of a sphere is used

Independent of substrate position


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Thermal evaporation

E-beam evaporation system with a planetary substrate

holder which rotates simultaneously around two axes


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Thermal evaporation Wafers are rotated around source to

ensure uniform coverage

Wafers are also often radiantly heatedto improve adhesion and uniformity

of thin films.

Deposition rate controlled by

changing the current and energy ofelectron beam.

Deposition rate monitoring by using a

quartz crystal. The resonant frequency

of the crystal shifts in proportion to

the thickness of the deposited film

Evaporation techniques tend to be

directional---shadowing and poor step

coverage


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Thermal

evaporation

ββββ2222 = 70= 70= 70= 70 0000ββββ1111 = 0= 0= 0= 0 0000

t2

t1

Substrate

t 1

t 2 =

cos ββββ1cos ββββ2

≈≈≈≈ 3

Surface feature

Source

Source

Shadow

t1/t2=cosββββ1111/cosββββ2222

λλλλ = (ππππRT/2M)1/2 ηηηη/PT

Shadowing and poor step

coverage


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Thermal evaporation

Si

Resist

d

ββββ

θθθθEvaporant containerwith orifice diameter DD

Arbitrary

surface element

1-exp (-d/λλλλ)

Kn = λλλλ/D>1

A ~ cosββββ cos θθθθ/d2

N (molecules/unit area/unit time) =3.513.1022Pv(T)/ (MT)

1/2

The cosine law


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Sputtering

A DC sputtering system: the target material acts as

the cathode of a diode and the wafers are mounted

on the system anode


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Sputtering

Sputtering yield vs. ion energy for a dc

sputtering system using Ar

W= kV iPTd

-V working voltage

- i discharge current

- d, anode-cathode distance

- PT, gas pressure

- k proportionality constant

Momentum transfer


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Sputtering

Deposition of conductive materials such as Al, W, Au, Pt,

Ti, and alloys can use a dc power source in which thetarget acts as the cathode in the diode system

Sputtering of dielectrics such as SiO2, Al2O3, ZnO, and

PZT, etc. requires an RF power source to supply energy to

the argon atoms

Sputtering results in the incorporation of some Ar into the

film

Sputtering provides excellent coverage


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Deposition of metal films:Thermal Evaporation vs. Sputtering


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Evaporation vs. sputtering: comparisonEv apo ratio n S puttering

Rate Thousand atomic layers per second(e.g. 0.5 µm/min for Al)

One atomic layer per second

Choice of materials Limited Almost unlimited

Purity Better (no gas inclusions, very highvacuum)

Possibility of incorporating

impurities (low-medium vacuum

range)

Substrate heating Very low Unless magnetron is used substrateheating can be substantial

Surface damage Very low, with e-beam x-raydamage is possible

Ionic bombardment damage

In-situ cleaning Not an option Easily done wi th a sputter etch

Alloy composi t ions,stochiometry

Little or no control Alloy composition can be tightly

controlled

X -ray damage Only with e-beam evaporation Radiation and particle damage is poss ible

Changes in sourcematerial

Easy Expensive

Decomposi t ion of material

High Low

Scal ing-up Difficult Good

Uniformity Difficult Easy over large areas

Capital Equipment Low cost More expensive

Number of deposi t ions

Only one deposition per charge Many depositions can be carried

out per target

Thickness control Not easy to control Several controls poss ible

Adhesion Often poor Excellent

Shadow ing e f fec t Large Small

Film properties (e .g.grain s ize and s tep

coverage)

Difficult to control Control by bias, pressure,

substrate heat


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-

MBE

– Epitaxy:

» homo-epitaxy

» hetero-epitaxy – Very slow: 1µm/hr

– Very low pressure:

10-11 Torr

Physical vapor

deposition (PVD):MBE


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-

Laser sputter deposition

– Complex compounds

(e.g.HTSC,

biocompatible

ceramics)

Physical vapor

deposition (PVD):Laser Ablation


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Ion plating

Ion plating

– Combines evaporationwith a plasma

» faster than sputtering

» complex compositions

» good adhesion


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Cluster beam Cluster beam

– From 100 mbar (heater cell) to 10-5 to 10-7 mbar (vacuum)--suddencooling

– Deposits nano-particles


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ExampleDC reactive magnetron sputtering for on-

chip AlN thin film resonators


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Wireless networks are growing rapidly in the spectrumfrom 500 MHz to 6 GHz – Applications

» Wireless communication devices

» Consumer electronics

» Specialized scientific and military equipment

– Quartz resonators and SAW devices are widely used as Oscillators andfilters for signal processing, RF and microwave frequency control. However,However,

Why thin film bulk acoustic

wave resonator (TFBR)?

Higher frequency rangeHigher frequency range

Low insertion lowLow insertion low

Good out of band rejectionGood out of band rejectionHigher frequencyHigher frequency

Higher performanceHigher performance

OnOn--chip integrationchip integration

ResonatorsResonatorsFiltersFiltersARE NEEDED !!ARE NEEDED !!


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Thin film Bulk Acoustic WaveResonators

– PZT, ZnO, AlN thin films

Principle of operation:

– A longitudinal standing acousticwave is excited electrically in a thin

piezoelectric film.

– The layer thickness of the piezoelectric film and of theelectrodes determines the resonance

frequency of the BAW resonator.

Frequency can be up to 15, even 30 GHzFrequency can be up to 15, even 30 GHz




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The resonance frequency depends on the

thickness of the thin film.



Since the acoustic wave velocity in AlN is about10400 m/s

t v f

a

2=

For a resonator with f = 1 GHz

AlN film thickness t= 5.20 µ m

t=2.5 µ µµ µ m f = 2.1 GHz

t=0.5 µ µµ µ m f = 10.4 GHz

t=0.2 µ µµ µ m f = 26 GHz


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Thin film bulk acoustic resonators

– Air gap resonator: the acoustic wave will

be oscillating within the piezoelectriclayer

» Mechanical reliability problem

» Low yield, difficult for packaging

– Solidly mounted resonators: acoustic

reflector layers are used under the activelayer to substantially reduces energy lossinto the substrate

» Difficulty in the thickness control ofthe reflector layers

» Multiple thin film processing stepsneeded,

» COST ISSUE is a concern

Air gap and solidly mounted thin film BAW

resonator configurations.

Thin film resonator design

K.M. Lakin et al.


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Resonator designResonator design

– Suspended thin film resonator,similar to quartz but on-chip

– Structural layers SiO2 or Si3 N4will be used

» Enhance the mechanical strength

» Improve the frequency-

temperature stability


SiOSiO22 is positive TC material, anis positive TC material, anappropriate AlN/SiOappropriate AlN/SiO22 thickness ratio willthickness ratio will

improve theimprove the f f - - T T stability of the resonatorstability of the resonator


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AlN and PZT thin film resonator structures

AlN

resonator

PZT

resonator

AlNAlN

PZTPZT



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AlN Thin Film Deposition – DC Reactive magnetron sputtering

– High purity Al target – Deposition gases: Ar/N2 – Substrate: Pt/Ti/SiO2/Si(100)

– Processing parameters:

» Temperature: 500-600oC

» Pressure: 3-5 mTorr

» Gas mixture ratio: N2:Ar = 1:1.2

» Target-substrate distance: 30-60mm

» Quality of bottom electrode (Pt/Ti):

Pt with (111) orientation – – Identify the optimal conditions for AlNIdentify the optimal conditions for AlN

film depositionfilm deposition

Thin film resonator fabrication

Highly cHighly c--axis orientationaxis orientation

Thickness uniformityThickness uniformity

Precise thickness controlPrecise thickness control


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X-ray diffraction for phase and crystalline

orientation identification

SEM for surface morphology and cross-sectionmicrostructure characterization

SPM for surface roughness characterization

Electromechanical Property measurement

– Elastic property

– Piezoelectric coefficient

– Mechanical quality factor

– Effective electromechanical coupling coefficient

– The relationship of frequency-film thickness

AlN thin film characterization

Materials properties are critical for theMaterials properties are critical for the

fabrication of thin film resonatorsfabrication of thin film resonators


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Thin film patterning and etching

– Wet chemical etching – Reactive ion etching (RIE)

AlN thin film deposition and

characterization

The PlasmaTherm 790

RIE Etching SystemWet Chemical Bench

Suss MA 6

Mask Aligner

AlN etching using RIE

Cl2, BCl3, Ar, and O2 gases

Etch selectivity:AlN film/Al top electrode/

photoresistHF and BHF etching

Si3O4, AlN, SiO2,

photoresist


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AlN thin film deposition rate is determined bymany factors such as sputtering pressure, power,

target-substrate distance and so on.

Deposition rate vs. powerDeposition rate vs. power

Deposition Rate

0

200

400

600

800

1000

1200

0 50 100 150 200

time (min)

t h i c k n e s s

( n m

40 W80 W

120 W


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1

2

3

4

5

6

20 30 40 50 60 70 80 90

l o g 1 0

( C o u n t s / s )

2θ ( o )

Pt (111)

AlN (002)

AlN (100)

Si (400)

The orientation dependence of theThe orientation dependence of the

deposited AlN thin films on the substratedeposited AlN thin films on the substrate

A: on commercially purchased Pt/Ti/SiO2/Si(100); B: on in-situ deposited Pt/Ti/SiO2/Si(100) with AlN

The results indicate that the AlN thin film deposited in-situ with Pt/Ti/SiO2/Si(100) substrates shows high (0001)

orientation.

1

2

3

4

5

6

20 30 40 50 60 70 80 90

l o g 1 0

( C o u n t s / s )

2θ ( o )

AlN (100)

AlN (002)

Pt (111)

Si (400)

A B


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C-axis orientation of AlN film

The orientation dependence of theThe orientation dependence of the

deposited AlN thin films on the substratedeposited AlN thin films on the substrate

Hyun Ho Kim et al.

in Microelectronics

Reliability


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Growth of c-axis oriented AlN films

on Pt(111) electrodes

Side view of a dense and smooth

AlN film

Orientation of the AlN film

AlNAlN


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The following factor can influence the c-

axis orientation growth of the AlN film: – Substrate temperature

– Ar:N2 ratio

– DC power

– Sputtering pressure

– Sputtering temperature

– Target-substrate distance

Orientation of the AlN film


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Thickness Measurement of AlN film by SEMThickness Measurement of AlN film by SEM

The thickness measurement along the radial direction of 3 inch wafer


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Thickness uniformity of theThickness uniformity of the

AlN filmAlN film

0

100

200

300

400

500

1 2 3 4

t h

i c k n e s s ( n m )

position

measurement point

average thickness

The measurement results indicate that the maximum deviation is less than 3.5%


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AlN Thin Film Acoustic Wave Resonator

250250 µµµµµµµµm x 250m x 250 µµµµµµµµmm suspended AlN thin film membrane (top view)suspended AlN thin film membrane (top view)


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Characterization of thin film resonator

AlN Film


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Thin Film Resonator AlN and PZT thin film resonator structures

AlN

resonator

PZT

resonator

AlN

PZT

lecture 6 thin film deposition,physical vapour deposition

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