from Lab to FabLearning how to manufacture polymer OLED microdisplays

Alastair Buckley

MicroEmissive Displays – 1999 to 2008

Outline OLED Microdisplays Requirements Device architecture Good and bad displays

Different processing methods in lab and fab Polymer deposition Electrode deposition Encapsulation Patterning methods

Manufacturing in the fab Toolset Metrology Test Yield

OLED Microdisplays Requirements Device architecture Good and bad displays

Different processing methods in lab and fab Polymer deposition Electrode deposition Encapsulation Patterning methods

Manufacturing in the fab Toolset Metrology Test Yield

OLED microdisplay

Light emitting display Si CMOS substrate Polymer OLED topping MED: 2000 - 2008

Advantages Nice to look at Cheap to make

Disadvantages Too small!

Light emitting display Si CMOS substrate Polymer OLED topping MED: 2000 - 2008

Advantages Nice to look at Cheap to make

Disadvantages Too small!

Microdisplay for “near to eye”

Lab to fab… we wanted to make this…

We wanted to make.. A display built on CMOS pixel drive electronics Metal anode – Al ideally Uniform white emitting polymer OLED RGB colour filters aligned with pixels (<1µm) No cross-talk between pixels Semi-transparent top electrode – cathode “Hermetic” encapsulation – zero defects

<5V total ~100 cd/m2

A display built on CMOS pixel drive electronics Metal anode – Al ideally Uniform white emitting polymer OLED RGB colour filters aligned with pixels (<1µm) No cross-talk between pixels Semi-transparent top electrode – cathode “Hermetic” encapsulation – zero defects

<5V total ~100 cd/m2

Zero defects… graininess

Type 1 - graininess

Zero defects …. cross talk artefact

Type 2 – cross talk

Type 3– ingress

Zero defects … differential degradation

Type 4 – differential aging

Zero defects …. dark spots

Type 5 – dark spots

Lab to fab – making microdisplays Different processing methods in lab and fab Polymer deposition Spin Inkjet others

Electrode and encapsulation deposition Thermal Sputtering Electron beam Etching

Manufacturing in the fab Toolset and process Metrology Defects and yield

Different processing methods in lab and fab Polymer deposition Spin Inkjet others

Electrode and encapsulation deposition Thermal Sputtering Electron beam Etching

Manufacturing in the fab Toolset and process Metrology Defects and yield

Methods – spin coating Mature process Semiconductor resist coating – $200 billion industry based on

spin coating

Ink factors – solvent vapour pressure – boiling point Film quality Thickness control

Mature process Semiconductor resist coating – $200 billion industry based on

spin coating

Ink factors – solvent vapour pressure – boiling point Film quality Thickness control

Laurell EVG

R&D

Production

Spin coating

1. Cover substrate with solution

2. Remove excess solution

3. Dry film – remove solvent

Spin coating – process trends Spin speed

Spin time

Exhaust rate

High vapour pressure solventThicker, less uniform film

Concentration ~ thickness

Spin coating can be used tocoat from <5 nm to >1 µm

High uniformity on roundsubstrates

More difficult to control on largesubstrates

Gyroset with rotating lidimproves uniformityLow coating speed also helps

Spin speed

Spin time

Exhaust rate

http://www.ossila.com/support/video.php

High vapour pressure solventThicker, less uniform film

Concentration ~ thickness

Spin coating can be used tocoat from <5 nm to >1 µm

High uniformity on roundsubstrates

More difficult to control on largesubstrates

Gyroset with rotating lidimproves uniformityLow coating speed also helps

Process control - Polymer deposition Precise thickness control of polymer layers <10% deviation in uniformity

Process control - Polymer deposition Thickness uniformity maps for bilayer polymer stack

Spin coating works very well – but its material inefficient and only uniformlayers can be coated – what are the other options?

Methods – inkjet printing More economic in terms of materials More flexible in terms of device fabrication The ultimate goal for all printed electronics

Diamatix

Litrex (ULVAC)

Preferred drop formation

R&D

Production

Inkjet printing – not so easy Ink formulation is

compromised againstcontrolling film thickness

Only certain viscosities ofsolution can be printed

Trying to achieve a uniform 40nm polymer film…

… requires a change in polymer Mw… requires a change in polymer Mw

10 mPa.s is the idealviscosity for IJP butsome polymers do notdissolve well enough, orare not extendedenough to make such asolution

Inkjet printing – not so easy

To uniformly fill each pixel a dualsolvent system is required. Onesolvent with a low bp and one with ahigh bp

The ratio of each solvent is varieduntil a uniform pixel is printed.

The high bp solvent pins the edge ofthe droplet to the substrate whilethe low bp solvent evaporates.

Ratio (A:B)(high bp:low bp)

90:10

To uniformly fill each pixel a dualsolvent system is required. Onesolvent with a low bp and one with ahigh bp

The ratio of each solvent is varieduntil a uniform pixel is printed.

The high bp solvent pins the edge ofthe droplet to the substrate whilethe low bp solvent evaporates.

50:50

Courtesy of CDT

Other deposition methods

Gravure

Doctor bladingSpray coating

Gravure

Ultrasonic spray coating

Summary of formulation requirementsfor different coating methods

F.C. Krebs, Sol. Energy Mater. Sol. Cells (2008), V93 P394 (2009)doi:10.1016/j.solmat.2008.10.004

Methods – Electrode and encapsulation Physical vapour deposition Thermal evaporation Sputtering Electron beam evaporation

Ion beam etching to pattern polymer layers

Physical vapour deposition Thermal evaporation Sputtering Electron beam evaporation

Ion beam etching to pattern polymer layers

Display die topograpy

Device structure

Thermal evaporation

Heated crucibles can be used to evaporate many low melting pointmaterials

Greater service interval than sputtering or ebeam so not ideal choice forproduction

Typically used in research setting Many useful materials evaporate well using thermal evaporation: Ca, LiF,

Ag, Ba, BaF2

Others like Al require more care since they wet and react with certaincrucible materials

Heated crucibles can be used to evaporate many low melting pointmaterials

Greater service interval than sputtering or ebeam so not ideal choice forproduction

Typically used in research setting Many useful materials evaporate well using thermal evaporation: Ca, LiF,

Ag, Ba, BaF2

Others like Al require more care since they wet and react with certaincrucible materials

Knudsen cell by TectraKurt J Lesker

Electron beam evaporation Electrons are accelerated into material to be deposited High temperatures and high deposition rates possible Industry standard for some optical coatings

• OK for organic electronics so long assecondary electrons are controlled

• Certain materials are not stable inelectron beam: Ca, LiF no good

• Others like Al, Ag, Ba work well

• OK for organic electronics so long assecondary electrons are controlled

• Certain materials are not stable inelectron beam: Ca, LiF no good

• Others like Al, Ag, Ba work well

Sputtering Industry standard for thin film deposition of metals Ions are accelerated from a confined plasma towards a target and “sputter”

atoms of metal towards substrate. Sputtered particles have high energy and therefore good adhesion Typically damage organic electronic materials – need passivation layer to

prevent damage!

Substrate

Ions

Vacuum pump

ArN2

O2

Target Material

Substrate

NS

SN

NS MFC

Plasma

Magnetic field

GassupplyMagnetic ring

RF power supply

Water cooling

In the fab… Prep and anode deposition Polymer deposition Polymer etching Cathode deposition Encapsulation Lamination Singulation and assembly

Prep and anode deposition Polymer deposition Polymer etching Cathode deposition Encapsulation Lamination Singulation and assembly

Anode deposition Sputter clean Mo/MoOx deposition (3nm) Thickness determined by cross

talk limit and layer conduction

Anode deposition

Sputter etch

Mo reactive sputter

Robot handler

Polymer coating Coat Anneal Etch

Spin coating

CoaterN2 Hotplate

Cassette in Cassette out

Polymer annealing Cross linking hole transport layer 180 degree C in N2 oven Batch process of up to 12 wafers

Cross linking hole transport layer 180 degree C in N2 oven Batch process of up to 12 wafers

Ion beam etching

Veeco source

Ion beam etching

Two steps needed tocreate islands of polymer

Careful temperature controlneeded – cooling plate

Two steps needed tocreate islands of polymer

Careful temperature controlneeded – cooling plate

Cathode & encapsulate LiF – 2nm Ca – 20nm Encapsulation – multilayer (Vitex)

In the fab… cathode deposition

Etch 1Etch 2 LiF

Ca AlOxEtch 1

LiFCa AlOx

Wafers in

Mask store

To encapsulation

Vitex – multilayer encapsulation

Spray deposited acrylate monomerUV polymerisation~ 500 nm thickplanarising

Al2O3 sputter deposited layers50 nm thick

Pinholes non-coincident3-4 “dyads” needed

Al2O3 sputter deposited layers50 nm thick

Pinholes non-coincident3-4 “dyads” needed

In the fab… encapsulation

Oxide sputter

Acrylate deposition and cure

Lamination, package and assembly

In the fab … Lamination tool

Pre-align – vacuum

Fine alignLight cure

Epoxy spray

Pre-align – vacuumLight cure

EVG

Packaging Singlulation – wafer saw – high pressure water cooling Assembly to flexible circuit – thermal limits <80 deg C Wire bonding – polymer residue Encapsulation – thermal limits < 80 deg C

Singlulation – wafer saw – high pressure water cooling Assembly to flexible circuit – thermal limits <80 deg C Wire bonding – polymer residue Encapsulation – thermal limits < 80 deg C

Metrology, test and yield Thin film metrology Elipsometry

Defect metrology Surfscan KLA

Defect measurement and classification Ca particles Swamp defect

Thin film metrology Elipsometry

Defect metrology Surfscan KLA

Defect measurement and classification Ca particles Swamp defect

Yield model Thin film technologies Eliminate defects for high yield

Fab metrology

Elipsometer

Wafer probe station

Surfscan

Elipsometer

Spectroscopic ElipsometryHow to measure sub nm film thicknesses

Very precise (+/- <1nm) Ability to determine optical constants Non-intrusive

Expensive, complex

How to measure sub nm film thicknesses

Very precise (+/- <1nm) Ability to determine optical constants Non-intrusive

Expensive, complex

J A Woolam

Elipsometer

Rotating analyserPolariser

White light sourceDetector

Rotating analyserPolariser

Wafer

Polymer, Mo, MoOx, Epoxy, Al2O3, Acrylate, Ca/Al2O3, LiF, etc..

Defects result in dark spots

Defect types

Surfscan

Scattering of laser from defects allows defect counting and in somecases classification

Tencor KLA Particle detector combined with SEM Can identify defect and then take image Additional EDX elemental analysis

Ca defectsBaffle boats eliminates them

Swamp defect – remains a puzzle…

Thankyou to.. Staff of MicroEmissive Displays Ossila Ltd. For images and illustrations Core MED supply chain CDT, Toppan, Sumation, Varitronix, Dai Nippon, Advanced

Neotech, EV Group, Scotech

EPMM research group at University of Sheffield

Staff of MicroEmissive Displays Ossila Ltd. For images and illustrations Core MED supply chain CDT, Toppan, Sumation, Varitronix, Dai Nippon, Advanced

Neotech, EV Group, Scotech

EPMM research group at University of Sheffield