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

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 3

Factors Influencing Film Choice-Property Set “Simple” organic circuitry

Organic AM backplanes

Inorganic AM backplanes

Increasing complexity of substrate structure More demanding property set

OLED displays

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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 Rigidity Nm x Rigidity relative Thickness 10-4 to 25 micron Microns film 25 0.1 1 50 1 10 75 3 30 125 15 140 175 40 390 200 60 580

D=

E t3 12(1-ν)

E is the tensile or Youngs Modulu t 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 O

O

C

C

PET Tm 255C Tg 78C O

CH2CH2

O

Tetoron® and Melinex® Polyethylene Terephthalate (PET)

n n

O O C

C

O

CH2CH2

PEN Tm 263C Tg 120 C

O

Teonex® Polyethylene Naphthalate (PEN) n 7

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 8

Key properties of Teonex® Q65FA compared with heat stabilised PET (eg Melinex® ST506) Upper temperature for processing, oC 180-220oC Shrinkage in MD at 150o C after 30 mins (%)

Youngs Modulus at 20oC, GPa

0.05% 150oC

5GPa

4GPa

0.1% Youngs Modulus at 150oC, GPa

18-20ppm/oC 20-25ppm/oC

CTE ppm/oC

1GPa

3GPa

Teonex® Q65A Heat stabilised PET

78oC

1000ppm 1000ppm Moisture pickup at 20oC, 40%RH

o 120oC Glass transition, C

0.7% 0.7% Haze %

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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 50nm

Teonex®Q83 Sample size 600x400um 0nm -25nm

Teonex®Q65 “raw” Sample size 600x400micron

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

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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 long 10 microns high

Scratch 150 microns long 0.5 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 coatings But at expense of surface roughness Ra 1.53nm Sample size 594micron

Planarised Melinex ® ST506 Very smooth surface-on a par with Polished glass mirror Ra 0.6nm Sample size 608 microns 15

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. 1000 900

Rp count

Peak 800 Count700 600 500 400

MELINEX ST504

300

HARDCOAT 200 100 0 40

60

80

100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 < Rp height (nm)

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 18

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 Thicknes Young’s Poisson’s Material s Modulus Ratio GPa µm Acrylic 0.5 5 0.38 ITO 0.03 145 0.2 Active 0.1 0.1* 0.4* ITO 0.03 145 0.2 PET 75 4 0.3

Outer stress MPa 4.3 110 0.09 109 3.2

Inner stress MPa 4.3 110 0.09 109 -3.4 19

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 20

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 reproducibility Effect of RH on Moisture Pickup at 20C 1600 RH 20%

1400

1440 ppm

RH 40% RH 60%

Moisture(ppm)

1200 1000

957 ppm 800 600

486 ppm

400 200 0 0

2

4

6

8

10

12

14

16

Time(hrs)

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Hypothetical Processing Examples Moisture Pick-Up in Humid Air at Elevated Temperatures 35

Moisture Content (ppm)

30

100C & 0.6% AH

25 20 15 150C & 1% AH

10 5

150C & 0.6% AH

0 0

3

6

9

12

15

Time (min)

0.6% absolute humidity equivalent to 41% RH at 20°C 1% absolute humidity equivalent to 68% RH at 20°C 23

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

Moisture Permeability (g/m2/day/atm)

PEN is a factor of 5 better barrier than PET but additional barrier technology will be required to meet OLED Display requirements (water vapour transmission rates of