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`@WILEY SID Series in Display Technology
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`Copyright Q 2005
`
`.
`.
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`,.
`.
`John Wiley & Sons Ltd, lhe Atrium, Southem Gate, Chichestei,
`West Sussex P019 sso, England
`
`(+44) 1243 779777
`Telephone
`Email (for orders and customer service enquiries): cs—books@wiley.co.uk
`Visit our Home Page on www.wi1ey,com
`
`Reprinted with corrections September 2005
`
`T l<
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`7 5’ 8 5?
`_
`-—'— b
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`F: 5 :3
`a QC) 5
`
`All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in
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`Library of Congress Cataloging-in-Publication Data
`
`Flexible flat panel displays / edited by Gregory P. Crawford.
`p.
`cm.
`Includes bibliographical references and index.
`ISBN—13 978—0—470-87048-8 (alkpaper)
`ISBN—10 0-470—87048—6 (alk. paper)
`1.
`Information display systems. 2. Liquid crystal displays.
`3. Electroluminescent display systems.
`1. Crawford, Gregory Philip.
`TK7882.16F54
`2005
`621.3815’422—dc22
`
`British Library Cataloguing in Publication Data
`
`A catalogue record for this book is available from the British Library
`
`ISBN—13 973-0—470—87048-8 (HB)
`ISBN—10 0-470—87048—6 (HB)
`
`2005003238
`
`Typeset in 10/12pt Times by Thomson Press (India) Limited, New Delhi
`Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire
`This book is printed on acid»free paper responsibly manufactured from sustainable forestry
`in which at least two trees are planted for each one used for paper production.
`
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` 15
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`OLED Displays on Plastic
`
`Mark L. Hildner
`
`DuPont Displays
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`15. I
`
`Introduction
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`t
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`Organic light-emitting diode (OLED) technology has captured tremendous interest and has
`rapidly developed since the discovery of organic electroluminescence roughly 15 years ago.
`Commercial OLED displays on glass are now available and the industry is poised for
`substantial growth in the next few years. Much of the attention given to OLEDs is due to the
`performance advantages that it has over other types of flat panel display (FPD) technologies,
`including the industry dominant liquid crystal displays (LCDs). Recognized advantages
`include nearly Lambertian emission, which provides wider viewing angles than LCD; fast
`response times, which facilitate grayscale and video capabilities in active matrix applica—
`tiOns; and low—voltage operation, which leads to low-cost components and low-profile
`Packaging. Furthermore, the high efficiency of OLED materials makes OLED the lowest-
`Power emiSSive FPD technology and offers the potential for lower power consumption than
`backlit LCDs.
`An additional factor giving OLED technology impetus, perhaps to an extent equal to
`the Performance advantages. is the perception that OLEDs are a natural choice for flexible
`dismay; The very thin structure (the active layers are less than 1 pm), solid—state construc-
`tion (there is no cell gap as in an LCD), and active material composition of an OLED are
`'I
`'
`.3
`‘
`I
`I
`(L.
`t“
`
`a‘v‘teri slics
`f
`, have many thinking that OLED is the technology path to high—performance
`u”-coler flexible displays.
`
`.'
`.
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`'
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`l
`flexible Flat P
`S.- 2005 John cme Displays Edited by G P Crawford
`Wiley & Sons, Ltd
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`286
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`OLED DISPLAYS ON PLASTIC
`
`The two main types of OLED are based on small molecules and conjugated polymers.
`Small-molecule OLEDs (SMOLEDs) were first reported by Tang and VanSlyke (1987) and
`are typically thermally evaporated. Light cmission from polymer OLEDs (PLEDs) was first
`reported by Burroughes et a1. (1990), where a solution—processable precursor polymer was
`deposited by spin coating and then thermally converted at high temperature (2 250°C).
`Then Braun and Heeger (1991) were able to make a light-emitting device with a polymer
`that was soluble in its conjugated form, thus eliminating the need for high-temperature
`processing. Solution processing at low temperatures may revolutionize how displays are
`manufactured because it permits a number of process options (spin coating, inkjet printing,
`dipping, spraying, etc.) that are lower cost than vacuum deposition; it would replace much of
`the vacuum processing used in today’s FPD fab; it can cover large areas; and it is well suited
`for roll—to-roll manufacturing, which may lead to further cost reduction from the current
`batch process. That is why this chapter will focus on conjugated polymer OLEDs.
`A number of flexible materials are being explored as OLED substrates. The first flexible
`OLED display demonstralion was on a lransparenl plastic substrate (Gustafsson et a1. 1992).
`Plastic is a logical choice because ils transparency allows much of the architecture of an
`OLED on glass to be used. Plastic is rugged, more so than regular glass: able to be accurately
`cut with a laser, allowing for irregular shapes with the only downside being some
`discoloration at
`the cutting site; and is already incorporated into roll—to-roll process
`technology, both in its own manufacture and current applications.
`there are
`While flexibility may be the ultimate goal for OLED displays on plastic,
`significant opportunities that are less technologically demanding than a display that can be
`flexed or rolled up multiple times. A flat plastic OLED display is thin, lightweight, and
`rugged. These are significant attributes that may be taken advantage of in mobile applica—
`tions. Plastic displays can be easily cut into a wide variety of nonrectangular shapes, and can
`be bent into a curved, but rigid, format. These characteristics allow greater freedom in
`product design. Even for these nonflexible display manifestations,
`there are significant
`development challenges to bringing a plastic OLED display to the marketplace.
`After a brief introduction describing how a PLED display works, this chapter will present
`the challenges associated with two key technology developments that must take place. The
`first is to obtain a plastic substrate that can withstand processing and lead to a reliable and
`long-lived device. The second is to obtain an understanding of the manufacturing issues
`associated with a plastic substrate, and then to incorporate this understanding into device
`processing. The issues associated with making a passive matrix (PM) OLED will
`then
`be discussed, and finally, there will be a review of thin film transistor technologies that are
`appropriate for plastic active matrix (AM) backplanes.
`
`
`
`15.2.1 Conjugated Polymers
`
`Conjugated polymers are characterized by alternating single and double or single and triPle
`bonds (Heeger 20m ). Overlapping of the pZ orbitals from the double or triple bonds along
`the polymer backbone loads to the formation c-fa dolocalized rr‘honding system. This gives
`rise to energy bands similar to those in an inorganic semiconductor. The occupied rr—liand-
`analogous to the valence band,
`is comprised of hole-transport states. and the highe’“
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`PLED BASICS
`
`287
`
`occupied molecular orbital (HOMO) is analogous to the valence band edge. The unoccupied
`T[.band. analogous to the conduction band, is comprised of electron-transport states, and the
`how!“ unoccupied molecular orbital (LUMO) is analogous to the conduction band edge.
`Despite this analogy. charge transport in conjugated polymers differs in a number of ways
`from that in inorganic semiconductors (Patel et al. 2002): intrinsic and extrinsic carriers are
`generally negligible and conduction is dominated by injected carriers; the polymer chains
`distort around the charge carrier so that the charged excitation is best described as a polaron
`tthe charge plus the distortion): and the energy bands are inhomogeneously broadened due to
`the amorphous polymer structure and.
`therefore, transport is through hopping along or
`between polymer chains.
`
`15.2.2
`
`Light-Emitting Diodes
`
`A conjugated polymer can emit light because it has an energy gap. Figure 15.1 shows three
`common light-emitting polymers: polyQJ-phenylenevinylene) or PPV; poly[2—methoxy,
`
`.
`.
`He
`--t-th tint
`
`to).
`
`PPV
`
`HOMO and LUMO, respectively; this defines the need for a high work function anode and a
`
`OCHa
`MEH-PPV
`
`Polyfluorene
`
`Figure 15.] Example light—emitting conjugated polymers
`
`5-(2’-ethyl-hexyloxy)—p—phenylenevinylene] or MEH—PPV; and polyfluorine. The basic
`polymer light—emitting device (a diode) consists of a light-emitting polymer (LEP) film of
`~100 nm sandwiched between an optically transparent anode, which sits on an optical
`quality glass or plastic substrate, and a metallic cathode. The anode is usually indium tin
`oxide (ITO), which has a high work function, whereas the cathode is typically a low work
`function metal such as Ca or Mg. When a bias greater than the difference between the anode
`and cathode work functions (the built—in potential) is applied as illustrated in the band
`diagram of Figure 15.2(a), electrons are injected from the cathode into the ail-band, and
`holes are injected from the anode into the 7r—band. The injected charges (electron and hole
`type polarons) form bound polaron-excitons, i.e. neutral bipolarons bound by their Coulomb
`attraction and their shared distortion (Heeger 2001). Electroluminescence (EL) results from
`the radiative decay (electron—hole recombination) of these excitons. The device is a diode
`because application of a reverse bias prevents charge flow (there is no light emission). The
`energy gap and thus the emission color of the diode can be tuned by changing the length of
`the polymer molecule, by changing the structure of the polymer repeat unit, by making
`copolymers, and/or by making polymer blends (Braun er al. 1992; Berggren et al. 1994;
`Akcelrud 2003).
`A number of factors influence the efficiency of this EL process (Patel et al. 2002). Barriers
`to injection result from the mismatches of the anode and cathode work functions with the
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`OLED DISPLAYS ON PLASTIC
`
`Electrons
`
`"
`
`V—(ra— $0)
`l
`f
`IIIJMOI___
`rag
`¢
`Holes
`a
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`i/W’i” ’
`/27/
`f/
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`a.
`'EUMol"
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`Holes
`WWW no;
`. ‘
`WW...
`--,
`
`HITL '
`
`Anode
`ITO
`
`Electrons
`/
`
`
`
`HOMO
`
`l
`
`J, l
`r;
`l
`
`LEP
`
`Cathode
`Ca or Mg
`
` 288
`
`Anode
`ITO
`
`LEP
`
`Cathode
`C3 or Mg
`
`(a)
`(b)
`Illustrative band diagrams ofta] single—layer and (b) two—layer PLEI) diodes under a bias
`Figure 15.2
`V with LEP energy gap Eg, cathode work function aim and anode work function to...
`
`singlet state for every three triplet states, and the maximum efficiency obtainable is 25%.
`However. there are some polymer systems where exciton formation is spin—dependent and
`singlet fractions are as high as 60% (Wilson et a]. 200]) — the efficiency could potentially be
`increased further by doping with phosphorescent dyes to Significantly enhance the spm-fllp
`
`force interactions. To prevent this, it has become customary to insert a hole injection and
`transport layer (HITL) between the anode and thc LEP, as shown in the band diagram 0f
`Figure 15.2 b
`(You
`= )-‘
`L
`’
`'-
`-
`- -
`.
`O
`" ses:
`increases the likelihood of recombination in the LEP with the proper choice of HOMO and
`LUMO by blocking and localizing holes in the active layers of the device: (2) it can increase
`hole injection at the anode by lowering the barrier; (3) and it can reduce leakage current and
`the prospects for shorting by planariziug the rough. potentially spiked ITO surface lBI'flWl1
`r?! of.
`[992: Hceger at of.
`l994; Shoals oi oi.
`I996; Patel at a]. 2002). Polyethylene
`dioxythiophene/polystyrenesullbnatc (PEDOT:PSS) and polyaniline (PANI) are two DUI)"
`mers: commonly used for the HlTL.
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`PLASTIC SUBSTRATES FOR OLED
`
`289
`
`1513 OLED Display Types
`
`The pixels of a passive matrix OLED display (PMOLED) are created by patterning the
`anode into columns and the cathode into rows. Light is emitted from an individual pixel by
`addressing its corresponding row and column and applying a current-controlled forward bias
`that will provide the desired luminance. To create an image, each line is sequenu'ally
`addressed and briefly illuminated (Sempel and Buchel 2002). The illumination must be
`extremely bright in order to obtain the desired overall display luminance and increases with
`i the number of lines in the display (the peak luminance required is proportional to the product
`of the number of lines and the average luminance). This means that high currents and
`Voltages are needed, which has a number of consequences. (1) The transparent anode (ITO)
`lines need to be bussed with metal to reduce significant power losses in the electrode lines.
`(2) Highly efficient polymers are needed to mitigate the other power dissipation factors in
`the display; this requirement is an order of magnitude greater for full—color displays because,
`compared to a pixel from a monochrome display of the same resolution, each color subpixel
`has one—third the area and will be addressed one—third of the time. (3) High-resolution and
`large—area PM displays are impractical because the power dissipation will be too large and
`the polymer emitters will degrade faster with the increased peak current demands. Compared
`to glass displays, the power issues are more acute for plastic displays: resistive losses are
`generally greater due to a greater resistance in the anode lines, and plastic substrates have a
`lower tolerance for the Joule heating from resistive losses. For these reasons, PMOLED
`displays on plastic are limited to a display size of ~15 in (Innocenzo 2002).
`Active matrix OLED displays (AMOLED), in which a thin film transistor (TFT) circuit is
`placed at each pixel, overcome many of the PMOLED problems. The TFT circuit provides a
`controlled current source with storage so that each pixel emits continuously. This drastically
`reduces the peak currents, which in turn significantly reduces power dissipation and the
`efficiency and lifetime demands on the polymer emitters. Current control also makes
`accurate grayscale achievable, which is difficult in PMOLED. A blanket cathode can also
`be used in an AMOLED, which eliminates many of the cathode patterning challenges
`associated with PMOLEDs.
`
` 15.3 Plastic Substrates for OLED
`
`75.3.1 Substrate Requirements
`
`The plastic substrate suitable for an OLED display must satisfy numerous requirements: high
`optical quality, which means few defects and high transmission (> 85%) across the visible
`spectrum to let
`the light out; substrate smoothness in the nanometer range to prevent
`protrusions into subsequent barrier and device layers and to provide a surface that will
`promote high—quality deposition of subsequent films;
`the ability to withstand processing
`temperatures, which are expected to be at least 150 °C and possibly as highas 300 °C for
`AM displays; a good barrier to moisture and oxygen; good dimensional stability so that the
`various patterned device layers can be aligned; good resistance to any chemicals used in
`processing; and low water absorption to minimize the consequent dimensional changes and
`the exposure of the device to moisture.
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`OLED DISPLAYS ON PLASTIC
`Because there is no plastic iilm that meets or is likely to meet all ofthese requirements,
`there is a great deal of t'levclopment w
`ork focused on obtaining a multil
`substrate that will. The approach is to start with an optical quality base it
`critical specifications L such as high transparency. high working temperature, and good
`dimensional stability — and add surface treatments For smoothing: coatings t'or scratch
`resistance and. if needed. for chemical resistance: and barrier layers.
`75.3.2
`Plastic Base Film
`
`'
`:
`J
`..
`'5
`jr
`'
`
`Plastic film candidate properties
`Table 15.]
`Pl
`PCO
`PAR
`PEN
`PC
`PES
`PET
`Base polymer
`Teonex
`Lexan
`Melinex
`A commercial name
`340
`330
`355
`120
`150
`220
`78
`7,"g (°C)
`Arylite Appear Kapton
`Sumalite
`Transmission at
`90
`91.6
`orange
`85
`>90
`90
`>85
`400—7’00run (374;)
`53
`74
`30—60
`13
`60—70
`54
`15
`CTE lpptnf”C]
`10
`100
`CHE (Pom/‘56 RH)
`0.4
`0.03
`1.8
`0.14
`0.4
`1.4
`0.14
`Waterabsorption (99)
`2.9
`1.9
`2.5
`6.1
`1.7
`2.2
`5.3
`Young's modulus tGPa)
`100
`50
`231
`275
`83
`225
`Tensile strength (MPH)
`84
`220
`54
`1.8
`62
`40
`wvrs tweperday}
`1500
`580
`388
`5.5
`100
`160
`ore (CC/m2 per day
`issues
`issues
`good
`good
`issues
`issues
`good
`Chemical resistance
`suitable plastic films with their material properties that are relevant to making plastic OLED
`displays. The glass transition temperature. 73 is used as a rulenl thumb to: the temperatutt.
`this temperature thepolymer molecules start H
`‘
`' ally rearrange themselve-‘i
`However, there are heat stabilization techniques that enable films to here a Working tern-
`'l‘al-ting polyethylene terephthalatc (PET) as an exam le. wit;
`-
`- ."
`‘"
`,
`'
`I
`t
`'
`
`good candidate for AM.
`“I
`- .
`,
`' Lice-l1 used in :1 great deal ol'resenrch because
`
`‘ltlatl expansion, good chemical
`o ter properties (clarity.
`low coel'fiCi
`'
`that it is the most readily available all"!
`resistance, and low water absorption} and the tact
`economically reasonable film. This could be a viable candidate For PMOIED were it not for
`the unacceptable number of defects and the surface roughness of commercially available
`films (the payback for addressing these issues appears small given the availability oftither
`films. most notably PEN).
`Plate [5.] shows some
`.
`.
`.
`.
`-
`,1
`e
`images 01 detects observed on commerenil grade PET and ”1
`consequences they have on
`. eff:
`.
`.
`.
`.
`.
`‘
`a single diode dewce: surracc roughness can teat-t to pool HIM:-
`
`'
`
`'
`
`extreme that
`
`the film can withst“
`
`a-
`
`a
`
`’
`
`.
`
`‘
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`.
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`ranges (above
`
`pcrature above T...
`
`quilibrium below Ts)
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`
`(C)
`
`(d)
`
`(b) magnification of defect: (c)
`(a) Defects on eounnercially available optical grade PE’I‘;
`Flute 15.1
`ale having poor surface quality shows low output. grainy appearance. and
`display made with PET stthstr
`on PET substrate with improved surface quality has improved
`tdi display made
`tine defect;
`characteristics
`
`quality in addition to creating a risk for device shorting. This poittts out that optical quality
`is not any easier to achieve titan other plastic film requirements. The importance of the
`additional material properties will become clear in subsequent sections.
`
`15.3.3 Barrier
`
`OLED devices are extremely sensitive to moisture and oxygen. Emission can be easily
`quenched when organic light—emitting materials are exposed to water, and the highly reactive
`low work function cathodes can be easily corroded by moisture and oxygen. This means that
`the substrate must protect the device from the ingress of these materials. Glass substrates are
`essentially impermeable to moisture and oxygen. but plastic films provide little protection. A
`barrier structure must
`therefore be deposited onto the base film. There is currently no
`instrumental method to measure the ultra low permeation rates required for OLEDs, but an
`estimate for the water vapor transmission rate (WVTR) has been made by calculating the
`amount of water needed to oxidize the reactive cathode (Burrows et al. 2001). For operating
`lifetimes in excess of 10 000 h, it was determined that the barrier structure should limit the
`WVTR to about 10”6 g/m2 per day. For similar lifetimes, oxygen transmission rate (OTR)
`requirements have been estimated to be somewhere between 10‘3 and 10’5 cc/m2 per day.
`These transmission rate requirements have made barrier development one of the biggest
`challenges in making OLED displays on plastic because the transmission rates of bare
`plastic films are six or more orders of magnitude greater than these values, and the most
`demanding requirements outside this arena are in the packaging industry. where the
`barrier structures provide only two to three orders of magnitude improvement at best
`(Chatham 1996).
`
`
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`292
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`OLED DISPLAYS ON PLASTIC
`
`The fact that the most sensitive commercially available systems that can measure WVTR
`and OTR have detection limits at 5 X 10’3 g/m2 per day and 5 X 10‘3 cc/m2 per day,
`respectively, has presented the additional challenge of developing techniques to obtain the
`needed sensitivity. A method that is commonly used is the Ca test (Nisato er a]. 2001). There
`are variations of this test, but they all involve observing the optical changes that a reactive
`metal layer undergoes as it oxidizes. An effort has been made to quantify this test and water
`vapor transmission rates for barrier films have been reported to be as low as 4 X 10' 7 g/m2
`per day using this technique. The Ca test, however, cannot distinguish between moisture and
`oxygen permeation. Techniques that can make this distinction and that are truly quantitative
`are currently being pursued, and if they are successful, should accelerate the understanding
`and development of barrier structures (Dunkel 2004; Vogt 2004). In contrast, the Ca test has
`the benefit of being able to distinguish between bulk permeation and permeation through
`defects, which is evidenced by spots in the Ca film. This is important because the reduction
`in permeation through barrier films developed in the packaging industry — typically single-
`layer thin film oxides — has been shown to be limited by transport through defects such as
`pinholes, grain boundaries, and microcracks (Chatham 1996). For this reason, the current
`barrier development efforts are either directed at creating dense defect-free films (Pakbaz
`2004; Snow 2004) or multilayer films that decouple the defects and create a long circuitous
`path for the diffusing species (Burrows et al. 2001; Rutherford 2004; Yan 2004).
`
`
`
`15.3.4 Composite Substrate
`
`Finally, the base film, appropriately chosen coatings, and barrier structure must be integrated
`into a mechanically stable composite substrate. While flexibility may ultimately place the
`strongest demands on mechanical integrity, the demands are still great even if the substrate is
`not incorporated into a flexed display. This is because the substrate must be able to withstand
`the temperature, humidity, chemical, and process conditions that it will be exposed to during
`device fabrication and device lifetime. Adhesion of the various layers is of particular concern
`and can be tested for each layer after being subjected to either ambient 0r accelerated
`conditions that simulate the anticipated exposure (O’Regan 2003).
`Adhesion or peel tests can be performed using a standard tensile tester. A minimum
`adhesion or bond strength specification can be established based on an understanding of the
`process conditions and product environments the substrate is expected to experience.
`Samples can be tested as fabricated, after exposure to either thermal cycling or combined
`high temperature and humidity, or after outgassing in vacuum. Submersion in boiling water
`is a truly accelerated condition for testing survivability to moisture that provides quiCk
`.
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`‘ pee
`es s n e ermine l
`a senes of sur ace
`treatments on top of Teonex Q65 PEN would provide a good surface for various barrifir
`layers to adhere to (Hildner 2004). The tests are for samples as fabricated (or received) and
`for samples after being immersed in boiling water for 2h. Each sample was cut into a
`2.5 in X 4 in strip. Two pieces of 4.5 in X 0.5 in testing tape (3M 4905/Foil) were attached
`onto the center of the strip and the excess length of tape was folded over to make a tab. Care
`was taken to avoid wrinkles and excessive air pockets during tape attachment and then thf”
`strip was put through the heated rollers (75 °C) of a dry film laminator to remove all.alr
`pockets and set the tape. The back side of the strip was mounted onto a German rotatlng
`wheel fixture of an Instron peel tester with 1 in double—sided tape (1 in Permacel double’
`
`
`
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` uzre WVTR
`
`SUBSTRATE PROCESSING ISSUES
`
`
`293
`
`Figure 15.3 Adhesion of barrier layers on 'l‘couex Q65 with various suifaee treatments; tests were
`performed on samples as received and after immersion in boiling water (BW)
`sided adhesive tape from Anderson Distributors), and the tape tab was clamped to the
`constant 90° angle peel fixture of the Instron. The tape was then peeled a minimum length of
`1.5 in at a speed of 2in/n1in while the force required to peel the tape was monitored by the
`load weighting system of the lnstron. For this test to be meaningful, the bond strength of the
`tape to the sample surface, SIS, must be greater than the minimum specification. If this
`criterion is met, then the tape will either peel off when the load exceeds S[S or earlier if one of
`the films or coatings in the sample has a weaker bond strength than Sts. An examination of
`the tape surface is required to see if any sample coatings peeled off, and adhesion failure is
`noted only if peel-off is observed and that it occurred at a load below specification. Figure
`15.3 shows that the surface treatments under investigation provide the needed adhesion
`properties. Interestingly. some samples required a greater load after boiling water; this is
`probably due to a change in the surface character of the sample, which in turn changed the
`tape adhesion properties to increase Sm.
`Adhesion remains a mechanical integrity concern when the substrate is flexed or bent. An
`additional concern during bending is the development of cracks in the coatings, particularly
`in the barrier as this will lead to increased permeability. Contributing to these failures are the
`internal stresses of the various layers, which will be introduced as the various materials —
`which have different thermal and mechanical properties i are processed together. One of the
`objectives in developing these composite substrates will
`therefore be to control
`these
`stresses. The critical strain — the strain above which a film will form a crack — of individual
`layers is another factor that will determine the mechanical reliability of the composite
`substrate and the extent to which it Can bend. A considerable amount of study is being
`directed at developing testing methods for understanding these issues (Nisato 2004; Bouten
`2002; Gorkhali er al. 2003).
`
`15.4 Substrate Processing Issues
`15.4.1
`Processing Issues
`
`_
`
`Conventional photolithography is used to pattern many of the layers of PM and AM OLED
`displays on glass, and can be used on plastic as well, but the glass processes cannot be
`
`Minimum _
`requirement
`
`Barrier
`Surface treatment
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`Apple EX1018 Page 11
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`
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`294
`
`OLED DISPLAYS ON PLASTIC
`
`directly transferred to plastic. Internal stresses will be present in the deposited films, and
`they will probably be of a different character from those on glass substrates because during
`deposition, which is usually at elevated temperature, the plastic will experience significant
`dimensional change. These stresses can have a sizable influence on the flatness of a plastic
`substrate and may also lead to film adhesion issues. Furthermore,
`in the remaining
`photolithography steps — these include coating, baking, exposing, developing, and stripping
`of photoresist, and etching of deposited films — the substrate will be exposed to heat and
`solvents, which will have a significant impact on the flatness and dimensional stability of the
`substrate. Another nontrivial issue is that current flat panel display tooling is designed for
`glass and is therefore not equipped to handle substrates that flex.
`
`15.4.2
`
`Film Stress
`
`There will be stress in deposited films from the differences in thermal expansion between the
`substrate and film (thermal stress) and from the microstructure of the deposited film
`(intrinsic stress) (Thornton and Hoffman 1977; Leten'ier 2003). When a deposition is
`performed at elevated temperature,
`the difference between the coefficient of thermal
`expansion (CTE) of the film and the CTE of the substrate will result in a different amount
`of contraction during cooling, imposing a compressive or tensile stress in the film. On plastic
`substrates, which have higher CTEs than most films of interest,
`this thermal stress is
`typically compressive. The intrinsic stress, on the other hand,
`is determined by the
`deposition process and is thus largely independent of substrate type and film thickness.
`The physical and chemical vapor deposition processes typically used for deposition are
`nonequilibrium in nature and will lead to a quenched disordered state in the film with an
`intrinsic internal stress that can be either tensile (resulting from attractive interactions across
`nanovoids) or compressive (as a result of high atomic density).
`Internal stresses (both thermal and intrinsic) lead to curling of the substrate, a phenom-
`enon that has been modeled extensively. According to the classic one-dimensional model of
`Stoney (1909), the radius of curvature of the coated film, R, is related to its internal stress, 0;,
`by the expression
`
`
`
`
`
`
`
`
`
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`the substrate and film, respectively. This curvature will be noticeable (very small R) if the
`internal stress is large enough (for compressive stress, the film is on the convex side of the
`curl). For a display substrate, such curling will make completion of subsequent processing
`steps difficult if not impossible. This illustrates, in part, the value of selecting a substrate
`with a low CTE and a high Young’s modulus. The thermal stress, and thus the imposed
`amount of curling, is smaller for a small-CTE substrate because it is roughly proportional t0
`the CTE mismatch between the substrate and film (Leterrier 2003). Furthermore, the radius
`of curvature resulting from all internal stresses will be smaller if the Young’s modulus of th6
`substrate is large. Once the substrate has been selected, however,
`the only option for
`reducing the radius of curvature is to control the intrinsic stress by tuning the film depositiorl
`parameters.
`
`The internal str
`of a film. The coal
`
`it depends on grov
`to the surface mo:
`In addition to
`
`substrate will outg
`and/