lecture 6 thin film deposition,physical vapour deposition
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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
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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
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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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Physical vapor deposition (PVD):
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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Physical vapor deposition (PVD):
Ion plating
Ion plating
– Combines evaporationwith a plasma
» faster than sputtering
» complex compositions
» good adhesion
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Physical vapor deposition (PVD):
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
Why thin film bulk acoustic
wave resonator (TFBR)?
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The resonance frequency depends on the
thickness of the thin film.
Why thin film bulk acoustic
wave resonator (TFBR)?
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
Thin film resonator design
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
Thin film resonator design
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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
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