Latest Developments In Polyester Film For Flexible Electronics

W MacDonald, K Rollins, D MacKerron, R Eveson, R Rustin, R Adam, K Looney, K Rakos and K Hashimoto- DuPont Teijin Films

2

Agenda

• Factors influencing film choice– Introduction to DTF family of films for flexible displays

• Characterisation of polyester films for flexible electronics– Surface quality– Mechanical properties of multilayer strucures– Control of dimensional reproducibility

• Influence of planarising coating on barrier performance• Examples of DTF film in flex electronic application

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Key Challenges for Engineered Substrates into Flexible Display applications

• Low Coefficient of Thermal Expansion• Low Shrinkage• Upper Temperature for Processing• Surface smoothness• Barrier• Solvent Resistance• Moisture Resistance• Clarity• Rigidity• Conductive layers• Commercial availability

• Substrates for the more demanding applications are likely to be multilayer structures containing both organic and inorganic layers

4

Factors Influencing Film Choice-Property Set

“Simple” organic circuitry

Organic AM backplanes

Inorganic AM backplanes

OLED displays Increasing complexity of substrate structure

More demanding property set

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Factors Influencing Film Choice-Physical Form/Manufacturing Route

• Physical form of display and type of usage will influence film choice particularly with respect to thickness– Flat but exploiting light weight, ruggedness– Conformable, one time fit to uneven surface– Flexible– Rollable

• Batch, fast sheet and R2R processing– Rigidity

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Rigidity

• Rigidity (D) of Teonex of different thickness calculated below and compared relative to 25 micron film

• Thickness has a significant effect on rigidity

Teonex Thickness Microns

Rigidity Nm x 10-4

Rigidity relative to 25 micron film

25 0.1 150 1 1075 3 30

125 15 140175 40 390200 60 580

D = E t3

12(1-ν)

E is the tensile or Youngs Modulut is the thickness, ν is Poissons ratio (0.3-0.4).

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Structure of PET and PEN Films

Biaxially oriented, semicrystalline films

Tetoron® and Melinex®

Polyethylene Terephthalate(PET)

Teonex®

Polyethylene Naphthalate(PEN)

C

O

O CH2CH2 O

C

O

n

CC

O O

n

PET Tm 255C Tg 78C

PEN Tm 263C Tg 120 C

O CH2CH2 O Tetoron® and Melinex®

Polyethylene Terephthalate(PET)

Teonex®

Polyethylene Naphthalate(PEN)

C

O

O CH2CH2 O

C

O

n

CC

O O

n

PET Tm 255C Tg 78C

PEN Tm 263C Tg 120 C

O CH2CH2 O

n

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DTF Grades for Flexible Electronics

• Teonex® Q65FA– One side pretreated, heat stabilised PEN film– “Thick” grade (>75 micron) with high clarity– Emerging as a leading material for OLED displays and AM

backplanes• Teonex® Q83

– “Thin” (25 and 50 micron), lightly filled,heat stabilised grade of PEN to give handleability

• Melinex® ST506– 2 side pretreat, heat stabilised PET film– “Thick” grade

• Melinex® ST504– 1 side pretreat, heat stabilised PET film– “Thick” grade

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Key properties of Teonex® Q65FA compared with heat stabilised PET (eg Melinex® ST506)

Glass transition, oC

Haze %

Moisture pickupat 20oC, 40%RH

Youngs Modulusat 20oC, GPa

Youngs Modulus at 150oC, GPa

Shrinkage in MD at 150o C after 30 mins (%)

Upper temperature for processing, oC

78oC

120oC

180-220oC

18-20ppm/oC

0.05%

1000ppm

0.7%

150oC 4GPa

0.1%

1000ppm

0.7%

5GPa

3GPa20-25ppm/oC 1GPa

Heat stabilised PET

Teonex® Q65A

CTE ppm/oC

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Surface Quality-Surface Smoothness

• Micro roughness which is dictated by whether film is unfilled, filled, pretreat coated

• Characterised by – AFM

▪ 1-50 micron field of view with lateral resolution down to nm’s

– White Light Interferometry▪ Micron to cm field of view with lateral resolution down to ca

0.2 micron

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Teonex Family

Teonex®Q83Sample size 600x400um

Teonex®Q65 “raw”Sample size 600x400micron

Teonex®Q65 pretreatSample size 2x2 micron(NB AFM-different scale)

50nm

-25nm

0nm

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Surface Quality-Surface Smoothness

• Within micro roughness possible to also see sporadic surface peaks up to 10’s microns lateral dimensions, 100’s nm height-illustrative examples below

• Due to internal particulate burden both organic and inorganic

• Largely controlled via polymer recipe, plant hygiene

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Surface Quality- Surface Cleanliness

• Surface Cleanliness, extent of which depends upon the 'external'contaminants such as air-borne debris, scratches, etc. Up to 10micron high, 10’s of microns long-illustrative examples shown below

• Control through– surface cleaning eg tacky roller– planarising coating in clean room

Dust-40 microns long10 microns high

Scratch 150 microns long0.5 microns high at ridge

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Planarised Films

• DTF is developing a family of planarising coatings that– Give glass smooth surfaces– Meet product requirements

▪ hardness vs smoothness vs ability to withstand stress/strain▪ adhesion, solvent resistance, environmental resistance etc

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Melinex ® ST506-Benefit of Planarisation

Pretreat on Melinex ® ST506 gives good adhesion to subsequent coatingsBut at expense of surface roughnessRa 1.53nmSample size 594micron

Planarised Melinex ® ST506Very smooth surface-on a par withPolished glass mirrorRa 0.6nmSample size 608 microns

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Effect of Planarising Coating on Reducing Surface Peaks

Extreme Surface Peak 'Rp' (All high points > 25nm) - Frequency Distribution comparison, for Melinex ST504 non pre treated surface and hardcoat upon it's pre treated surface.

0

100

200

300

400

500

600

700

800

900

1000

40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400

< Rp height (nm)

Rp

coun

t

MELINEX ST504

HARDCOAT

PeakCount

Peak Size

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Surface Quality

• DTF at leading edge of surface metrology • DTF’s surface metrology allows characterisation of

nanometer to centimeters (lateral scale) and heights from nm to 10’s of microns

• DTF are developing techniques to characterise surface cleanliness

• Currently using a combination of techniques to – understand what surface defects dominate electronic product

manufacture and performance– develop film grades that meet product requirements

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Mechanical Behaviour

• Component layers in flexible displays embrace a wide range of mechanical behaviour– Polymeric layers are flexible and tough– Conductive and barrier layers are stiff and brittle inorganics

• Structures may be subjected to residual stresses– Manufacture– Differential thermal expansion– Bending during handling

• Spreadsheet model has been developed to apply beam theory to laminate structures

• Inputs-material properties, stack geometry, mode of mechanical stress

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Hypothetical Example

• 5 layer laminate based on – PET (substrate)– ITO (mimic inorganic layer)– Acrylic (organic coating)– Active layer eg PEDOT

• Laminate bent to a radius of 50mm

Layer Material

Thickness

Young’s Modulus

Poisson’s Ratio

Outer stress

Inner stress

µm GPa MPa MPaAcrylic 0.5 5 0.38 4.3 4.3ITO 0.03 145 0.2 110 110Active 0.1 0.1* 0.4* 0.09 0.09ITO 0.03 145 0.2 109 109PET 75 4 0.3 3.2 -3.4

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Modelling Studies

• Tensile or compressive strength through the thickness of the 5 layer laminate

– High modulus ITO layers carry high stress and displace the neutral axis for bending from 37.5 to >40micron

– Neutral axis is still far removed from layers developing high stress– Control via base layer thickness / modulus or different product structure

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Mechancial Behaviour

• Further work is required to build up data on “active”layers and to further validate the model

• Model is useful for predicting and rationalising failure behaviour

• Models can be used as design tools to optimise structure to minimise risk of failure

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Dimensional reproducibilityEffect of RH on Moisture Pickup at 20C

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14 16Time(hrs)

Moi

stur

e(pp

m)

RH 20%

RH 40%

RH 60%

486 ppm

957 ppm

1440 ppm

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Hypothetical Processing Examples

Moisture Pick-Up in Humid Air at Elevated Temperatures

0

5

10

15

20

25

30

35

0 3 6 9 12 15Time (min)

Moi

stur

e C

onte

nt (p

pm)

100C & 0.6% AH

150C & 0.6% AH

150C & 1% AH

0.6% absolute humidity equivalent to 41% RH at 20°C1% absolute humidity equivalent to 68% RH at 20°C

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Conclusions

• Moisture pickup will have a significant effect on dimensional change-ca 45ppm in a given direction per 100ppm moisture

• Critical to understand how equilibrium level of moisture will change through device manufacturing process to obtain registration and to maximise dimensional reproducibility– this will vary depending upon a given set of processing

conditions and film type• Optimise dimensional reproducibility via control of

– inherent shrinkage of base film – processing environment

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Barrier

101

100

10-1

10-2

10-3

10-4

10-5

10-6

Moi

stur

e Pe

rmea

bilit

y(g

/m2 /d

ay/a

tm)

Several Plastic Films

Target for OLED

Substrate for Plastic LCDPCTFELimit of Mocon Test

10-12 Glass

PEN is a factor of 5 better barrier than PET butadditional barrier technologywill be required to meet OLED Display requirements(water vapour transmission rates of <10-6 g/m2/day and oxygen transmission rates of <10-5 mL/m2/day).

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Approach to Barrier Films for OLED Displays

• Multilayer organic/inorganic coatings eg Vitex Systems inc– Polymer layer planarises and fills defects in inorganic layers– Provide tortuous path for molecules

Barrier“Stack”

SEM Photo courtesy of Vitex Systems

Teonex®PEN Substrate

Barrier“Stack”

SEM Photo courtesy of Vitex Systems

Teonex®PEN Substrate

• Single layer of dense inorganic coating eg Symmorphix– Requires very low defect area

• Several other companies actively developing barrier technology based on variations of the above

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Barrier

• In principle a perfect layer of SiOx of only a few nm would give adequate water and oxygen barrier

• Reality– Surface defects on substrate lead to pinhole damage– Vacuum deposited thin films often show columnar growth and have

densities less than bulk material

1000nm

20nm

RF sputtered multilayer Pulsed DC sputtered multilayer

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Barrier

• Investigating the impact of substrate on barrier performance– The deposition of thin films via microwave assisted pulsed DC

reactive magnetron sputtering (high energy) on planarised film (minimised defects)

• 120nm SiOx has been deposited on – Teonex® Q65– Planarised Teonex ® Q65

• Comparison sample of lower energy RF process on planarised® Teonex Q65 also run

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Ca Button Test Results-Change in OD with Time

0

20

40

60

80

100

120

0 200 400 600 800 1000

Hours

% o

f Orig

ina

Teonex Q65 withplanariser coating(microwaveassisteddeposition)Teonex Q65 withplanariser (RF)

Teonex Q65(microwaveassisteddeposition)

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Density of SiOx coating

• The density of thin silica films on different substrates was calculated from refractive index measurements

• SiOx via microwave assisted pulsed DC reactive magnetron sputtering on planarised Teonex Q65 is unusually dense

• Correlates with good Ca button test performance

Density g/cm3 Crystalline Quartz 2.65Bulk fused silica 2.2SiO2 on planarised Teonex Q65 2.52

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R2R Coater

• DTF is currently commissioning R2R sputter coater in clean room

• Designed for flexibility• Conductive / inorganic

coatings on prototype scale

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Polymer Vision

• Approach- organic based TFTs• Combine AM polymer driving

electronics with a reflective “electronic ink” front plane on extremely thin sheet of plastic

• Only 100 mµ thick• Bending radius: ~0.75 cm• Weight: 1.5g• Using engineered substrates

supplied by DTF

Slide courtesy of Polymer Visionhttp://polymervision.nl/

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Plastic Logic

• The worlds largest flexible organic AM display

• 10" diagonal SVGA (600 by 800) with 100ppi resolution and 4 levels of greyscale

• Thickness less than 0.4mm• Using engineered substrates

supplied by DTF

Slide courtesy of Plastic Logichttp://www.plasticlogic.com/

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A-Si TFT Backplanes

• Exciting progress reported by PARC, Honeywell on successfully building a-Si TFTs on Teonex Q65 using low temperature process

• Dimensional reproducibility of <100ppm at 150C reported

• By careful control of processing environment it appears that one can push Teonex beyond DTF data sheet spec

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Summary

• Choice of film type and thickness is crucial– Important to pick the right film for the application

• Control of surface quality is critical– DTF at leading edge of surface metrology– Critical to understand what type of surface defect impacts on

device manufacture and performance

• Modelling studies are important for– Optimising device architecture– Minimising the effects of environment on processing

• Preliminary results indicate improvement in barrier performance achieved on planarised film

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Summary

• DTF are developing a family of engineered substrates for different flexible electronic applications

• DTF substrates evaluated in a wide set of application spaces including– e-paper– Organic TFTs– a-Si flexible TFTs– Flexible OLEDs

• Positive feedback from the market• DTF can advise on which films are suitable for a given

application/process