Lecture 10 Deposition Techniquesclasses.engr.oregonstate.edu/eecs/fall2020/ece617... · 2020. 10....

Lecture 10 – Monday October 26th 2020

ECE 617 – Fall 2020

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ECE 617 – Thin Film Electronics

Fall 2020 - John Labram

Lecture 10

Deposition Techniques

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Announcements

• Is due now.

• Please email it to me if you have not already done so.

Homework 4/10:

• I will return it one week from today later (November 2nd).

• Is online now.

• I will return it one week later (November 9th).

Homework 5/10:

• Due Monday November 2nd at the start of the lecture

(2:00pm).

• Homework 5 consists of content covered in Lectures 9 and 10.

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Lecture 10• Vapor Based Techniques.

• Thermal Evaporation.

• Sputtering.

• Chemical Vapor Deposition.

• Atomic Layer Deposition.

• Solution-Based Techniques.

• Spin Coating.

• Spray Coating.

• Printing.

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Large Area Deposition

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Large Area Electronics• As we discussed in

Lecture 1, the objective

of thin film electronics is

to have electronics over

large surfaces.IGNIS Innovation

• Eventually, we want this

to be possible at low

temperature, at low

cost, and on flexible

substrates.

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Large Area Electronics• This has forced us to rethink how we grow

semiconductors (we can no longer uses Si wafers).

https://www.youtube.com/watch?v=aWVywhzuHnQ Pi-kem

• So how to we get our semiconductor to form a

uniform film over a large area?

• There is no one answer to this question.

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Thermal Evaporation

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Thermal Evaporation• One of the crudest methods is to simply vaporize the

semiconductor, and direct the vapor towards the

substrate: Substrate holder

Deposition rate sensor

To vacuum

pump

Source

organic

Substrate

shutter

Substrate and

mask

Thermal

filament

• However for certain

materials the metal

source can be replaced

with a semiconductor.

• E.g. pentacene:

• This is conventionally

used for metals (e.g. Au,

Al) for contacts.

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Thermal Evaporation• Normally the material is held in a small “boat” made

of a resistive material.

• A large current is driven between the terminals,

causing the boat to heat up (can be ≫ 1000˚C for

certain materials).

https://angstromengineering.com/tech/resistive-thermal-evaporation/

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Thermal Evaporation• Because it is held in

a vacuum, the

material being

heated will vaporize.

• The material will

coat everything

inside the chamber.

• But it can be

partially directed

towards the

substrate. https://www.youtube.com/watch?v=qIn8vPIKV54

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Thermal Evaporation• We have thermal evaporators in the cleanroom in

Owen Hall.

• They are just used for metals, however.

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Thermal Evaporation• For organic semiconductors we need much lower

temperature (the materials will often decompose

above ~300 - 400˚C).

• Multiple sources can

be used so complex

structure can be

made, without

having to open the

chamber.

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Thermal Evaporation• It happens there are already many commercial

products based on thermally-evaporated organic

semiconductors.

• These all contain organic light-emitting diodes

(OLEDs).

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Thermal Evaporation• However at present there are no commercial

products that contain OTFTs. Yet there are plenty of

proof-of-principle demonstrations using OTFTs.

https://www.youtube.com/watch?v=TEey_Yvvc64

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Thermal Evaporation• The stacking of small molecules such as pentacene

are highly dependent on growth conditions.

Salzmann et. al. ACSNano. 6 (2012) 10874.https://www.youtube.com/watch?v=4E9K0W2lTK0

• The alignment of molecules has a strong effect on

electronic properties of TFTs.

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Thermal Evaporation• Many factors affect the growth of small organic

molecules on surfaces:

• Deposition Rate.

• Substrate

temperature.

• Chamber pressure.

• Surface energy of

substrate.

• Purity of material.

Oetritz et. al. Org. Electron. 14 (2013) 3070.

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Thermal Evaporation• Plastic Logic (Cambridge spin-out) have a huge

facility in Dresden.

https://www.youtube.com/watch?v=OAzE-n6DeFM

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Sputtering

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Sputtering

Substrate

Gate Metal

• We need uniform deposition over large areas.

https://youtu.be/8mVK5dwyoEY

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Sputtering

Substrate

Gate Metal

• We need uniform deposition over large areas.

https://youtu.be/8mVK5dwyoEY

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Sputtering

A target material, such as

aluminum, is bombarded with

Argon ions. The displaced atoms of

the target material move across the

Plasma. They are then deposited

on the substrate.

Power Supply

Gas in Gas out

P

Substrate

Aluminum

Target

Argon ion

bombards the

aluminum surface

Sheath

Aluminum

Bulk

Displaced

aluminum

Atoms

Sheath

Bulk

Silicon Wafer

Aluminum atoms deposit on

the wafer and form a film

Aluminum

atom moving

around

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Plasmas• So called “4th State of Matter”.

• An ionized gas of electrons and ions:

• Commonly observed in

“neon” signs.

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Ionization• A high speed electron hits an atom hard enough to

knock out an electron.

• This forms an ion and another free electron.

• This is an elastic collision.

e- + Ar Ar+ + e- + e-

Neutral Atom (Ar) Ion (Ar+)

High speed

Electron

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Molecules: Dissociation• When a molecule undergoes dissociation we can get

two reactive atoms.

• These atoms are called free radicals.

e- + O2 O + O + e-

High speed

Electron

Stable

Molecule

Reactive

Free Radical

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Excitation and Relaxation• An electron hits and excites a neutral atom.

Neutral Atom (Ar) Excited Atom (Ar*)

High speed

Electron

en

erg

y

e-

Relaxed Atom (Ar)

Characteristic Light

hν

e- + Ar e- + Ar* e- + Ar + h𝜈

• Which then relaxes and gives light.

• This is what makes a plasma glow.

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Glow Discharge

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Sputtering• Sputtering can be used to deposit compounds as

well as elements, But the structure would not

necessarily be preserved.

• Hence it is better suited for fully amorphous systems.

Film

C

CC

C

C

C

CC

C

C

Zn

O

Zn

OZn

OZn

O

Zn

OZn

O

O

Zn

Zn

Zn ZnO

Zn

OZn

O

Zn

O

ZnO

ZnOO

Zn

ZnZn

Target

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Sputtering• At OSU we use sputtering to deposit metal oxides.

• In particular, metal oxides such as Indium Gallium

Zinc Oxide (IGZO).

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Chemical Vapor Deposition

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• Chemical vapor deposition (CVD) is another

deposition technique.


• As usual, more details can be found in ECE611.

• In its simplest incarnation, CVD involves flowing a precursor gas or gases into a chamber containing one or more heated objects to be coated.

• Chemical reactions occur on and near the hot surfaces, resulting in the deposition of a thin film on the surface.

• This is accompanied by the production of chemical by-products that are exhausted out of the chamber along with unreacted precursor gases.

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CVD vs PVDChemical Vapor DepositionPhysical Vapor Deposition

• Involves a chemical reaction.

• Source material is physically deposited.

https://www.youtube.com/watch?v=8mVK5dwyoEY https://www.youtube.com/watch?v=j80jsWFm8Lc

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Free radicals transport to the surface and grow a film


Transport to

surface

Film growth

Reactive species

move on surface

Energy

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Sources of Activation Energy

Inlet

Outlet

To vacuum

Reaction

chamber

WaferHeated

wall Inlet

Outlet

To vacuum

Reaction

chamber

Electrode

Plasma

Temperature Plasma

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Types of CVDAtmospheric Pressure CVD (APCVD)

• Advantages: High deposition rates, simple, high throughput.

• Disadvantages: Poor uniformity, purity is less than LPCVD

• Used mainly for thick oxides.

• E.g.:

𝑆𝑖 𝑂𝐶2𝐻5 4 𝐿 + 𝐶𝐻3 3𝑃𝑂3 𝐿 + 𝐶𝐻3𝑂3 3𝐵(𝐿) →

TEOS TMPOAmmonium

chloride

𝑆𝑖𝑂2: 𝐵2𝑂3, 𝑃2𝑂5 𝑆 + gas products

BPSG (Borophosphosilicate

glass)

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Types of CVDLow Pressure CVD (LPCVD at ~0.2 to 20 torr)

• Advantages: Excellent uniformity, purity

• Disadvantages: Lower (but reasonable) deposition rates than APCVD

• Used for polysilicon deposition, dielectric layer deposition, and doped dielectric deposition.

• E.g.:

3𝑆𝑖𝐶𝑙2𝐻2 𝐺 + 10𝑁𝐻3 𝐺 → 𝑆𝑖3𝑁4 𝑆 + 6𝑁𝐻4𝐶𝑙 𝐺 + 6𝐻2(𝐺)

Dichloro-silane

(DCS)Ammonia Silicon nitride Ammonium

chlorideHydrogen

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Types of CVDPlasma Enhanced CVD

• Plasmas are used to force reactions that would not be possible at low temperature.

• Advantages: Uses low temperatures necessary for back end processing.

• Disadvantages: Plasma damage typically results.

• Used for dielectrics coatings.

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CVD MechanismRequirements for CVD Growth

• Good thickness uniformity.

• High purity and density.

• Controlled composition and stoichiometries.

• High degree of structural perfection.

• Good electrical properties.

• Excellent adhesion.

• Good step-coverage / conformal coverage.

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CVD MechanismMore Detail:

Reactant

molecule

Carrier gas

(to maintain high

pressure & slow

reaction rate)

1). Bulk

transport

2). Transport

across

boundary layer

𝐽1 ∝ 𝐷𝑔∆𝐶

3). Adsorption

4). Surface diffusion

5). Decomposition

6). Reaction with

film

𝐽2 ∝ 𝑘𝑖𝐶𝑖

7). Diffusion of

gaseous by-

product

8). Bulk transport

of by product

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Atomic Layer Deposition

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• Sometimes you need to deposit films with monolayer

precision.


• This doesn’t necessarily mean you need a single layer

of a material, but you do need strong control over

the number of layers.

• E.g. you want 10 monolayers, not between 8 and

10.

• Atomic layer deposition (ALD) is a self-limiting

process that enables the user to have monolayer

control over films.

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• The reaction is designed that occurs in two steps.


• A good example is the growth of Al2O3 (an insulator)

using Al(CH3)3 and H2O:

• Cycle 1:

• Cycle 2:

Al-OH* + Al(CH3)3 → Al-O-Al(CH3)2* +CH4

https://beneq.com/en/technology/ald/

Al-CH3* + CH4+H2O →Al-OH*+CH4

Asterisk =

surface species

https://pubs.acs.org/doi/pdf/10.1021/cm0304546

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• The key point with ALD is that because the growth

must take place via two steps, we cannot get more

than a monolayer every two cycles.


• If the half-monolayer is saturated, then the excess

precursor has no sites left to react with, and hence

will be removed when the chamber is purged.

• This allows us to grow materials layer by layer, albeit

at slow rates.

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• This process is best visualized with video:


https://www.youtube.com/watch?v=HUsOMnV65jk

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• In general, ALD is of interests for conventional CMOS

microelectronics.


• This is because it is highly effective for conformal

coatings.

https://www.youtube.com/watch?v=4G8wXQGEBrA

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• However, deposition over large areas is possible, and

is an active area of research.

Large-Area ALD

https://www.sciencedirect.com/science/article/pii/S1631070517300567#fg0020

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Spin Coating

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Solution Deposition• The great thing about some semiconductors (e.g.

organics) is that many are soluble in certain solvents.

• A bit like polystyrene in acetone:

• Often we have to use other organic solvents.

https://www.youtube.com/watch?v=f4-K5p8ZnWA

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Spin-Coating

https://www.youtube.com/watch?v=Ea6gykoPrvk https://www.youtube.com/watch?v=hJZK5b_Uobc

https://www.youtube.com/watch?v=9TjCTXcU8qU https://www.youtube.com/watch?v=53W_ZWwhnTQ

• Because many organic semiconductors are soluble,

one of the most widely-studied deposition

techniques in laboratories is spin-coating.

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Spin-Coating1). Organic semiconductor (in

solvent) is deposited in

center of the substrate.

Substrate is held in place

with vacuum chuck.

2). Substrate is rotated slowly

(200 rpm) to distribute

material.

3). Accelerate the substrate to

final speed (~5000 rpm).

Spin the substrate at

constant speed for 30 -

60s. Forms a uniform film

and evaporates solvent.

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Film Thickness• It can be shown (we won’t) that the final film thickness

depends on the spinning speed via:

𝑡 ∝1

𝜔Film

thickness

Angular

velocity

• Actual thickness will also

depend on:

• Concentration.

• Solvent evaporation

rate

• Viscosity.

• Local temperature.

• Local humidity.

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Surface Wetting• Surface energy (Jm-2) is very important in growth.

• Energy required to create one unit of surface area.

• Surface energy exists because bonds are broken

to create/increase the surface.

• Surface stress: bonds are elastically strained.

https://www.nature.com/articles/srep44213#f3

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Spray Coating

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Spray Coating• Spin coating is fine for laboratory testing, but is

clearly not suited for large-area deposition.

• Researchers are interested in developing scalable

techniques such as spray-coating.

Sonotec

• This is still an emerging area of research.

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Spray Coating• The motivation is that spray-coating is already

industrially-established.

• Therefore it is potentially easy to scale up for other

applications

https://www.youtube.com/watch?v=K3rusvc_c7E https://www.youtube.com/watch?v=wJnJxZfSfKM

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Metal Oxide Deposition• As with organics, many metal

oxide semiconductors are

solution-processable.

• One challenge is conversion.

• Most metal oxide solutions are

actually formed of precursors.

• After deposition, they

typically need treatment

(normally thermal) to convert

from a precursor to a

semiconductor.https://www.youtube.com/watch?v=crAugJXG3QA

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Spray Coating• Currently surface roughness is a lot higher for spray-

coated vs spin-coated films.

Faber et. al., ACS Appl. Mater. & Int. 7 (2015) 782. Labram et. al., Adv. Funct. Mater. 26 (2016) 1656.

Spray Cast In2O3 Spin Cast In2O3

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Printing

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Printing• Spin-coating is a very simple technique for use in the

laboratory.

• But the goal of thin-film electronics is to one day be

highly-scalable and incredibly cheap.

• For a lot of us, the end goal is printed electronics.

• This is a difficult engineering challenge to solve.

• There are a few different printing technologies:

• Gravure printing.

• Inkjet printing.

• Roll-to-roll printing.

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• Gravure printing is a

simple, physical

deposition method.

https://www.youtube.com/watch?v=wsX6uaKNZ2k

• There are indentations

in a roll, which define

the material pattern.

• Clearly for such an approach the definition of

features is going to be limited by physical features on

the gravure roll.

• The deposition can be

pretty inhomogeneous.Thiburce et. al. Adv. Electon. Mater. (2017) 1600421.

Gravure Printing

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Gravure Printing• As can the

performance:

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Inkjet Printing• Traditional inkjet printing should be familiar.

https://www.youtube.com/watch?v=5iiJMv-jh7U

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Inkjet Printing• Inkjet printing for organic electronics has the same

goals: desktop-printable flexible electronics.

• The research is in early

stages.

• The “coffee-ring” effect

is a notable issue:

Teichler et. al. JMCC. 1 (2013) 1910.

Teichler et. al. JMCC. 1 (2013) 1910.

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Roll-to-Roll Printing• But one of the ultimate goals of printed organic

electronics is to be able to create products using roll-

to-roll printing methods.

https://www.youtube.com/watch?v=zPhDUN5IBZ8

• This remains a long way off commercialization.

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Next Time…• Gate Dielectrics:

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