advances_in_substrates

Latest advances in substrates for flexible electronics

W. A. MacDonaldM. K. LooneyD. MacKerronR. EvesonR. AdamK. HashimotoK. Rakos

Abstract — Recent advances in both organic- and inorganic-based electronics processed on flexiblesubstrates offer substantial rewards in terms of being able to develop displays that are thinner, lighter,robust, and conformable, and can be rolled away when not required. In addition, plastic-based sub-strates coupled with the recent developments in solution deposition and ink-jet printing for layingdown OLED materials and active-matrix thin-film-transistor (TFT) arrays open up the possibility ofcost-effective processing in high volumes using roll to roll (R2R) processing.1 To replace glass, how-ever, a plastic substrate needs to be able to offer some or all of the properties of glass, i.e., clarity,dimensional stability, thermal stability, barrier, solvent resistance, and low coefficient of thermalexpansion (CTE) coupled with a smooth surface. In addition, a conductive layer may be required. Noplastic film offers all these properties so any plastic-based substrate will almost certainly be a multilayercomposite structure.1,2 This paper will discuss the issues associated with selecting plastic materials,contrast the various options, and highlight how to gain optimum performance through process control.This will be illustrated with examples of film in use in flexible electronic applications.

Keywords — Flexible electronics, flexible displays, plastic film substrates, polyester, polyethylenenaphthalate.

1 Factors influencing film choice

1.1 Application areaThe requirements for the different applications envisagedfor printable electronics are very different and will requiresubstrates with different property sets.

This is summarized in Fig. 1. The applications aredivided into “simple” organic circuitry, for example, RFID,organic-based active-matrix backplanes, inorganic TFTs,and OLED displays. As one moves up the list the substraterequirements in terms of properties such as dimensional sta-bility, surface smoothness, low CTE, conductive and barrierproperties become increasingly complex and more demand-ing and this will be reflected in the cost of the substrate. Anon-heat-stabilized polyethylene terephthalate (PET) filmwith good surface quality is adequate for applications such

as RFID, but a film with high-temperature stability, withexcellent surface quality and excellent barrier propertieswill be required for OLED displays. Obviously, a substratewith the property set and price commensurate with theapplication should be chosen

1.2 Physical form/manufacturing processThe physical form of the display and whether it is

(i) flat but exploiting light weight and ruggedness;(ii) conformable, one time fit to non flat surface;(iii) flexible and ease of handling, e.g., electronic news-

paper;(iv) rollable/foldable

will also influence the film choice. As a generalization, arollable display will need to be thin to facilitate a balance ofrollability and compactness, whereas an electronic newspa-per will require some rigidity to render it handleable neces-sitating a thicker film. Similarly, whether the film ismanufactured by batch or by R2R will influence the filmselection. With R2R processing, thinner films allow longerprocessing runs for a given reel size before changeover. Thiscan influence the cost of manufacture of devices. With batchprocessing, a thicker and more rigid substrate can facilitatehandling though multistage processing.

1.3 Film property set

1.3.1 Polymer typePolyethylene terephthalate (PET), e.g., DuPont Teijin Films™Melinex® polyester film, and polyethylene naphthalate(PEN), e.g., DuPont Teijin Films™ Teonex® polyester film,

The authors are with DuPont Teijin Films, P.O. Box 2002, Wilton, Middlesbrough, Cleveland TS908JF, U.K.; telephone +44-164-257-2047,fax –2083, e-mail: [email protected].

© Copyright 2007 Society for Information Display 1071-0922/07/1512-1075$1.00

FIGURE 1 — Illustration of the changing property requirements ofdifferent applications.

Journal of the SID 15/12, 2007 1075

are biaxially oriented semicrystalline films.2–4 The differ-ence in chemical structure between PET and PEN is shownin Fig. 2.

The substitution of the phenyl ring of PET by thenaphthalene double ring of PEN has very little effect on themelting point (Tm), which increases by only a few degreescentigrade. However, there is a significant effect on theglass-transition temperature (Tg), which increases from78°C for PET to 120°C for PEN.2 PET and PEN films areprepared by a process whereby the amorphous cast is drawnin both the machine direction and transverse direction. Thebiaxially oriented film is then heat set to crystallise thefilm.3,4

The success of polyester films in general application isderived from the basic polymer properties coupled with themanufacturing process of biaxial orientation and heat settingdescribed above. These properties include high mechanicalstrength, good resistance to a wide range of chemicals andsolvents, low water absorption, excellent dielectric proper-ties, good dimensional stability, and good thermal resistancein terms of shrinkage and degradation of the polymerchains. Fillers can be incorporated into the polymer tochange the surface topography and opacity of the film. Thefilm surface can also be altered by the use of pretreatmentsto give a further range of properties, including enhancedadhesion to a wide range of inks, lacquers and adhesives.These basic properties have resulted in PET films beingused in various applications, ranging from magnetic mediaand photographic applications, where optical properties andexcellent cleanliness are of paramount importance to elec-tronics applications such as flexible circuitry and touchswitches, where thermal stability is key. More-demandingpolyester-film markets, which exploit the higher performanceand benefits of PEN include magnetic media for high-den-sity data storage and electronic circuitry for hydrolysis-resis-tant automotive wiring.5 This property set provides the basison which one can now build to meet the demands of theorganic electronic market.

It is interesting to contrast these films with the otherfilms that are being considered for flexible electronics appli-

cations. The main candidates are shown in Fig. 3 which liststhe substrates in terms of increasing glass transition (Tg).1,2

The polymers can be further categorized into filmsthat are semicrystalline [PET, PEN, heat-stabilized PET(HS PET) and heat-stabilized PEN (HS PEN) [heat-stabi-lized films will be discussed in Section 1.3.4)], amorphousand thermoplastic, and amorphous, but solvent cast. Poly-mers with Tg‘s higher than 140°C that are semicrystallinetend generally to have melting points that are too high toallow the polymers to be melt processed without significantdegradation-PEN is the highest performance material avail-able as a biaxially oriented semicrystalline film. The nextcategory are polymers that are thermoplastic, but non-crys-talline and these range from polycarbonate (PC) for exam-ple Teijins PURE-ACE®6 and GE’s Lexan®7 with a Tg of~150°C to polyethersulphone (PES), e.g., Sumitomo Bake-lite’s Sumilite®8 with a Tg of ~220°C. Although thermoplas-tic these polymers may also be solvent cast to give highoptical clarity. The third category are high-Tg materials thatcannot be melt-processed and include aromatic fluorenecontaining polyarylates (PAR), e.g., Ferranias Arylite®,9 andpolyimide (PI), e.g., DuPont’s™ Kapton®.10

PET and PEN by virtue of being semicrystalline andbiaxially oriented have a different property set to the amor-phous polymers and for simplicity these two basic categorieswill be used when comparing and contrasting the propertiesof the film types and the importance to the property setrequired for flexible electronic application.

1.3.2 Optical clarityThe clarity of the film is important for bottom-emissive dis-plays where one is viewing through the film and a total lighttransmission (TLT) of >85% over 400–800 nm coupled witha haze of less than 0.7% are typical of what is required forthis application. The polymers listed above meet thisrequirement apart from polyimide which is yellow. Formany display applications such as top-emissive displays andfor electrophoretic-display applications, clarity in the basesubstrate is not essential.

FIGURE 2 — Chemical structure of PET and PEN films.

FIGURE 3 — Glass transition of film substrates of interest forflexible-electronics applications.

1076 MacDonald et al. / Latest advances in substrates for flexible electronics

1.3.3 BirefringenceBiaxially oriented films such as PET and PEN are birefrin-gent. For LCDs which depend on light of known polariza-tion, this means that birefringent films, which would changethe polarization state, are unlikely to be used as base sub-strates, although polyester films are used extensively toenhance LCD performance, e.g., brightness-enhancingfilms, etc. Films based on amorphous polymer are not bire-fringent and are more suitable for the base substrates forLCDs. Birefringence is not an issue with OLED, electro-phoretic displays and indeed some LCDs.

1.3.4 The effect of thermal stress ondimensional reproducibilityPET and PEN films (Melinex® and Teonex®) are producedusing a sequential biaxial stretching technology, which iswidely used for semicrystalline thermoplastics.3,4 The processinvolves stretching film in machine and transverse direc-tions (MD and TD) and heat setting at elevated temperature.As a consequence, a complex semicrystalline microstructuredevelops in the material, which exhibits remarkablestrength, stiffness, and thermal stability. Various studieshave been made of the biaxial structure of polyester filmmanufactured in this way and many descriptions have beenwritten.11,12 The film comprises a mosaic of crystallites oraggregated crystallites accounting for nearly 50 wt.% of itsmaterial which tend to align along the directions of stretch.Adjacent crystallites may not however share similar orienta-tions. Crystallites show only a small irreversible response totemperature, which may take the form of growth or perfec-tion. The non-crystalline region also possesses some pre-ferred molecular orientation, which is a consequence of itsconnectivity to the crystalline phase. Importantly, themolecular chains residing in the non-crystalline region areon average slightly extended and therefore do not exist intheir equilibrium, Gaussian conformation. However, as aconsequence of this orientation process, PET and PENfilms undergo a variable and undesirable change in dimen-sions at the Tg, due to both molecular relaxation eventsassociated with the increased mobility of the polymer chainsand also “shrinkage” or “expansion” associated with the

relaxation of residual strain within the oriented parts of thefilm structure. To counterbalance this effect, PET and PENfilms can be further exposed to a thermal relaxation process,in which film is transported relatively unconstrainedthrough an additional heating zone. Some additional shrink-age is seen which signifies a relaxation of the molecular ori-entation in the material.1,2,12 Fundamental measurementsof fibers and films indicate that the relaxation occurs exclu-sively in the non-crystalline regions.13

Figure 4 illustrates how the shrinkage of heat-stabi-lized PET (Melinex®) and PEN (Teonex®Q65) change withtemperature. For applications requiring low shrinkageabove 140°C, Teonex®Q65 is the preferred option and thereis continuing work within DuPont Teijin Films™ to opti-mize the heat-stabilization process without having a detri-mental effect on final film properties.

In a batch-based process, it is also possible to annealthe film at temperatures around 200°C prior to processingon them. It has been shown that it is possible to achieve alevel of shrinkage down to 25 ppm at 150°C with Teonex®

Q65.14

The second factor that impacts dimensional repro-ducibility is the natural expansion of the film as the tem-perature is cycled as measured by the coefficient of thermalexpansion (CTE). A low CTE typically <20 ppm/°C is desir-able to match the thermal expansion of the base film to thelayers which are subsequently deposited. A mismatch inthermal expansion means that the deposited layers becomestrained and cracked under thermal cycling. In the tempera-ture range from room temperature up to the Tg, the typicalCTE of PEN is 18–20 ppm and PET 20–25 ppm (but notethat the Tg of PEN is 40°C higher than PET). Above the Tg,the CTE of PET and PEN films which have not been heatstabilized is dominated by shrinkage. This is a result ofrelaxation of the internal strains in the films as discussedabove. On the other hand, the heat stabilized films dis-cussed above show only a very small increase in CTE in thetemperature range from the Tg to the temperature at whichthey were heat stabilized.1,2 This contrasts favorably withthe quoted coefficient of expansion of amorphous polymerswhich is typically 50 ppm/°C2 below the Tg but can increaseby a factor of 3 times above the Tg.

In addition to dimensional stability another importantfactor to be considered is the upper processing temperature(Tmax) of the film. Although as outlined above the Tg does

FIGURE 4 — Shrinkage of heat-stabilized PET and PEN film vs.temperature.

FIGURE 5 — Upper processing temperature of film substrates of interestfor flexible-electronics applications.


not define Tmax with the semicrystalline polymers, it largelydoes with the amorphous polymers. Figure 5 shows theupper operating temperature if the effect of heat stabiliza-tion is taken into account.

1.3.5 Cyclic oligomersPET film, after 200 days in R134/ester oil at 150°C has beenfound to contain 1.1 wt.% cyclic oligomer. These cyclic oli-gomers can migrate to the surface if the film is held at tem-peratures >100°C for tens of minutes. The presence ofthese oligomers on the surface can interfere with the TFTmanufacture process. PEN with 0.3 wt.% cyclic content hassignificantly lower cyclic oligomer content compared withPET and there is significantly less migration to the surfaceof the film (Fig. 6). The cyclic oligomer crystals form on thesurface and can be removed by washing the film withmethylethylketone solvent. Cyclic oligomer migration canbe reduced by minimizing the time the film is held at tem-perature and reducing processing temperatures. Where thisis not possible, changing from PET to PEN offers an alter-native option to minimizing the presence of cyclics on thesurface. A further option is to exploit planarizing coatings toact as a barrier to cyclic oligomer migration. This is dis-cussed in Section 4.

1.3.6 Solvent and hydrolysis resistanceA wide range of solvents and chemicals can potentially beused when laying down the various layers in displays depend-ing on the processing steps involved. Amorphous polymersin general have poor solvent resistance compared to semi-crystalline polymers (Table 1).

This deficiency is overcome by the application of ahard coat to the amorphous resins which significantly improvesthe solvent and chemical resistance to solvents such as NMP,IPA, acetone, methanol, THF, ethyl acetate, 98% sulphuricacid, glacial acetic acid, 30% hydrogen peroxide, and satu-rated bases such as sodium hydroxide.15 With polyethyleneterephthalate and polyethylene naphthalate films, a hard-coat is not required to give solvent resistance.

The residual shrinkage of Teonex®Q65 after 30 min-utes at 150°C is of the order 500 ppm but can be reduced to

below 100 ppm or better by careful process control – thiswill be discussed later. In addition to this residual shrinkage,the processing environment, in effect the prevailing humid-ity, must be taken into account as even crystal l ineTeonex®Q65 will absorb up to 1500 ppm of moisture underambient conditions, yet readily loose this at higher process-ing temperatures.

In a previous publication it has been shown16 that forevery 100 ppm of moisture absorbed, the film expands byapproximately 45 ppm isotropically in all three film dimen-sions.

With the knowledge of the solubility level of moisturein PEN polymer and its diffusivity as a function of tempera-ture, it is possible to model the impact of various environ-mental conditions on moisture content changes and hencevolumetric changes in the different thicknesses of film. PETand PEN will absorb about 1000 ppm water at an RH of 40%@ 20°C and it can take up to 12 hours to reach equilibrium,depending on thickness. Although PET and PEN will reachthe same equilibrium level, PET picks up moisture at aboutthree times the rate of PEN as shown in Fig. 7.

FIGURE 6 — Growth of cyclic oligomers on PET and PEN film with timeat 100, 120, and 140°C.

TABLE 1 — Table of solvent resistance.

FIGURE 7 — Moisture pickup vs. time for PET and PEN.


Films can typically take over 12 hours to reach equi-librium. Figure 8 which shows the impact of RH on thisprocess and indicates that the final (equilibrium) moisturelevels will change significantly with varying RH. At an RHof 20%, the film reaches an equilibrium level of approxi-mately 500 ppm, but at 60% RH an equilibrium level ofapproximately 1500 ppm.

PET and PEN films will lose moisture upon sub-sequent heating as shown in Fig. 9, but at a much faster rate.At 150°C, the film takes 6 min to reach an equilibrium mois-ture level of 5 ppm, but at 90°C the film takes 30 min toreach an equilibrium level of 40 ppm.

Upon removal from the heated environment back intoambient conditions, the film initially picks up to 50% of it’sequilibrium moisture level while cooling in less than aminute or within a few seconds , but the remaining moistureto reach equilibrium is picked up at a much lower rate.

Taking into account the variation in moisture contentthat can result depending upon temperature, RH and filmthickness, which can be of the order of several 100s of ppm,it can be seen that uncontrolled moisture pickup will have asignificant influence on dimensional reproducibility. Thiswill be particularly significant at lower processing tempera-tures where there is minimal film shrinkage. If processingsteps requiring registration of layers are carried out before

the film has reached its equilibrium moisture level at a givenprocessing temperature, the layers will become misaligned.By carrying out modelling studies of the type describedabove and thereby understanding the time required to allowa film to reach its equilibrium moisture level at a given proc-essing temperature, then the detrimental impact of thismoisture change on registration for multilayer processingcan be minimized.

1.3.7 The effect of mechanical stress ondimensional reproducibilityOne of the main drivers in moving to plastic substrates isthat it opens up the possibility of roll-to-roll processing(R2R) and the process and economic advantages that thisbrings. Under these conditions, a winding tension willclearly be present and polymer film substrates with lowmoduli will be susceptible to internal deformation, particu-larly at elevated process temperatures. Figure 10 shows acomparison between polyethylene terephthalate and poly-ethylene naphthalate films.

The storage modulus, E′ is recorded using DynamicMechanical Thermal Analysis. As the temperature is increased,the stiffness of both materials decreases. However, in theregion between 120–160°C, PEN is significantly stiffer andstronger, with a modulus almost twice that of PET.

Any small tension or load applied to film at that tem-perature will therefore impose strain in the material, whichwill be “frozen-in” upon cooling and reappear as shrinkageupon reheating. The implication of this is that if the film isconstrained through a thermal processing cycle, e.g., in aR2R process or by lamination to a rigid carrier, the strainintroduced into the film may result in subsequent shrinkageon reheating.

Measurements of stiffness parameters such as theYoungs Modulus are thickness independent and do not indi-cate how rigidity will change with thickness. Rigidity (D) canbe defined by16,17

D = Et3/12(1 – ν),

FIGURE 8 — Effect of RH on moisture pickup at 20°C.

FIGURE 9 — Moisture loss vs. time on heating to different temperaturesafter exposure of the film to an RH of 40% at 20°C.

FIGURE 10 — Change in storage modulus of PET and PEN withtemperature.


where E is the elastic modulus (6.1 GPa), t is the thickness,and ν is the Poisson Ratio (0.33).

Assuming a Youngs Modulus of 2 GPa for amorphousfilms and 6 GPa for PEN film, it can be seen in Table 2 that200-µm polyethylene naphthalate film is four times morerigid than 125-µm polyethylene naphthalate film and 12times more rigid than a 125-µm amorphous film. This stiff-ness may prove to be an advantage in a batch-based display-manufacturing process.

This extra rigidity can be exploited to reduce the stressand resulting distortion as evidenced by the difference indistortion measured through a-Si TFT process on 125 µm(5 mil) PEN compared to 200 µm (8 mil) PEN where thedistortion through the process is reduced from approxi-mately 175 to 100 ppm (Fig. 11).

1.3.8 Surface qualityThe surface quality of the film is essential for the more-demand-ing display applications to remove any surface defects ordebris that could protrude through subsequent conductiveor barrier layers. Surface quality can be divided into

(i) inherent surface smoothness: PET and PEN havean inherent surface smoothness of less than 1 nm, in termsof both roughness average (Ra) and root-mean-squareroughness (Rq), as measured using white-light interferomet-ric methods. However, the use of Ra and Rq can be mislead-ing as they tend to be measured over a small sampling area

and do not capture the occasional peak that occurs on thesample size of a display. The surface smoothness is largelydefined by the internal cleanliness of the film substrate andtypified by the presence of peaks of size tens to hundreds ofnanometres. Typically, over a 1-cm sample area these inter-nal contaminants exhibit a range of surface peaks less than0.2 µm high. The frequency of such surface peaks (Rp val-ues) can be shown in the graph below in Fig. 12.

The graph comprises numerous measurements, whichin total makes up an 1 × 1 cm sampling area. For each meas-urement, the highest point (Rp value) is recorded and thefollowing distribution observed. The particles are predomi-nantly below 140 nm. The predominance of these particlecan be influenced by the polymer recipe and by process con-trol.

(ii) Surface cleanliness: this is dominated by dust andsurface scratching. These defects can range in size up totens of microns both laterally and vertically and are unavoid-able in film that is handled and cut to size in a non-clean-room environment. A proportion of the dust particles can beremoved by some form of surface cleaning, but to eliminatecompletely the influence of these particles of subsequentlayers it is necessary to apply a planarizing coating which iscoated in a clean-room environment.2,13 These coatingshave the dual function of providing both surface smoothnessover wide area , but also an improved surface hardness toprevent scratching of the surface during film processing.This subject area of minimizing the defects on the surfaceand at the same time being able to measure defects duringactual film line processing is of considerable interest at thepresent

2 Summary of key properties of basesubstratesThe main properties of heat-stabilized PET (Melinex®

ST504/506) and PEN (Teonex®Q65) relevant to flexibleelectronics are summarized in Fig. 13.

Unstabilized PET and PEN films exhibit the sameproperty set apart from the shrinkage and upper processingtemperature which for these films is largely dictated by theTg.

FIGURE 11 — Distortion through a-Si TFT deposition (slide courtesy ofFlexible Display Centre, ASU).

TABLE 2 — Relative rigidity of amorphous and semicrystallinefilms.

FIGURE 12 — Frequency distribution of particles for PEN film(Teonex®Q65FA).


Table 3 contrasts the property set of the different sub-strates under consideration for flexible electronics.

These properties have been discussed in the text andalso in previous papers.2,16 This table shows that both heat-stabilized PET (e.g., Melinex® ST504) and heat-stabilizedPEN (Teonex®Q65A) have an excellent balance of the keyproperties required for flexible electronics and it is thisproperty set that is leading to Teonex®Q65A emerging as aleading polymer-based films for the base substrate of bothorganic and inorganic active-matrix backplanes.

The ideal substrate for high-performance displayapplications would have the key attributes obtained withsemicrystalline and biaxially oriented polymers, i.e., lowCTE, excellent solvent resistance, coupled with the thermalstability of amorphous polymers such as polyimide, but withhigh clarity. In addition to this the film should be commer-cially available with a film surface quality that is only reallyachieved by manufacture at volume scale. Although researchis being carried out on new materials, the development andproduction costs of developing a new film based on a newpolymer possibly requiring novel raw materials coupledwith access to both polymer and film making facilities, for

what is a relatively small volume market, are prohibitivelyexpensive. The time periods to commercialize a new filmare also lengthy. In the authors opinion, this is not an attrac-tive proposition for a substrate supplier and a more likelyroute forward is to push existing films to their limits andmarry this with the advances that are being made in low-temperature processing of inorganic materials.

Prototype displays and backplanes have been pro-duced on flexible stainless-steel substrates. Stainless steeloffers some obvious advantages over plastic substrates,namely, high thermal stability, good dimensional stability,excellent barrier properties, and good UV stability. How-ever, commercially available stainless steel has poor surfacequality and for active-matrix backplanes the surface must beplanarized. In addition, an insulating layer may also have tobe applied to provide electrical passivation and this layeralso acts as a barrier to inorganic ions which can migratefrom the stainless steel at elevated processing temperatureand contaminate the active components of organic active-matrix backplanes. The presence of these coatings can alsolead to warpage due to a mismatch in CTE. Stainless steel isnot transparent which will limit its use in applicationsrequiring clarity. The minimum radius of curvature of stain-less steel is higher than plastic film and stainless steel isprone to “kinking” on impact due to its lower yield strainrelative to plastic film. Stainless steel with a density of 7.9g/cm3 is considerably heavier than, e.g., PEN with a densityof 1.36 g/cm3.

3 Planarizing coatingsThe surface quality of the film can be controlled to an extentvia recipe control, plant hygiene, web-cleaning techniques,and film line-processing conditions. However, the film stillhas sporadic surface peaks from internal contamination andis typically not manufactured in a environment which isclean enough to give a level of extrinsic contamination suit-able for the more-demanding applications in flexible elec-tronics.

While a number of cleaning techniques can substan-tially reduce extrinsic contamination, it is difficult to removethem all. To achieve the surface quality required for mostelectronic devices, a planarizing coating is necessary. Thiscoating covers all surface defects and comprises a low vis-cosity liquid which flows easily over the film imperfectionsto create a smooth surface with low defects over wide area.The planarizer coating is coated in a cleanroom to ensurethe surface has low intrinsic and extrinsic contamination.The planarizer coating also has anti-scratch properties pre-venting subsequent scratching of the film during sub-sequent processing and handling.

The choice of chemistry and coating thickness of theplanarizing coating has an impact on the physical propertiesof the planarized film. Factors to consider include maxi-mum processing temperature during film processing andthe chemistry of layers that will be deposited on the planar-

FIGURE 13 — Summary of PET and PEN properties relevant to flexibleelectronics.

TABLE 3 — Comparison of the key properties relevant to flexibleelectronics.


izer surface. Suitable chemistry types which offer pencilhardnesses >2H range from predominantly inorganic mate-rials, e.g., sol gel derived coatings based on siloxane chemis-try to organic materials (for example, polyfunctional acrylatecoatings).

A thermal mechanical analysis was carried out on heat-stabilized PEN and an inorganic–organic hybrid planarizedcoated heat-stabilized PEN using a Perkin Elmer TMA-7thermomechanical analyzer. The thermograph in Fig. 14shows that the dimensional response to temperature cyclingof planarized PEN film is very similar to that of uncoatedPEN, i.e., the planarized film remains a dimensionally repro-ducible substrate up to 180°C. This is within the perform-ance requirements of a flexible substrate for a display basedon a-Si-H TFT.

As discussed in Section 1.3.5, cyclic oligomers canform on the surface of PET and PEN on processing the filmat elevated temperatures for extended periods of time. Oneof the benefits of the planarizing coatings is that they act asa barrier to oligomer migration to the surface. This is illus-trated in Fig. 15 which shows planarized and unplanarizedTeonex®Q65 annealed at 200°C for 30 hours. Planarized

coatings on PET film also prevent migration of cyclics to thesurface.

Figure 16 shows a white-light interferometry image ofa 600 × 400 µm sample of a planarized coated PET film. Themeasured Ra is 0.4 nm. This film has a pencil hardness of3H.

The planarized-coated PET and PEN films showexcellent chemical resistance to common solvents used inphotolithography processes and solution-based organicsemiconductors.

4 Examples of film in useAt the time of writing this paper, both Polymer Vision andPlastic Logic have announced scale-up plans for developingelectronic-paper-type applications based on electrophoreticdisplays driven by organic active-matrix backplanes. Othercompanies are also developing a range of electronic prod-ucts based on organic thin-film transistors, for example, inthe area of RFID tags. The key film features required forthese applications are

(i) dimensional stability at processing temperatures toobtain dimensional reproducibility through multi-layer processing (discussed in Section 1.3.4).

(ii) excellent surface smoothness to prevent surfaceartefacts from interfering with the organic TFTs(discussed in Sections 1.3.8 and 3).

(iii) solvent resistance – the films must be able to with-stand the solvents used in TFT manufacture (dis-cussed in Section 1.3.6).

Both PET and PEN films are being exploited in thisapplication area, the actual film choice being dictated by theupper processing temperature. Similarly significant advancesare being made on reducing a-Si-H deposition tempera-tures to allow fabrication of a-Si TFTs on plastic film at tem-peratures below 200°C.18 In this case, the property set

FIGURE 14 — Thermal mechanical analysis of heat-stabilized PEN andplanarized heat-stabilized PEN.

FIGURE 15 — The difference of the surface of planarized heat-stabilizedPEN compared to uncoated heat-stabilized PEN when heated at 200°Cfor 30 hours.

FIGURE 16 — White-light interferometry image of planarized PET(Melinex ST506).


required of the base substrate is more demanding in particularwith respect to dimensional reproducibility at elevated tem-peratures and the higher temperature performance of PENis required. By careful control of the parameters discussedin this paper – thermal stress (offline stabilization and reducingprocessing temperatures) (discussed in Section 1.3.4), envi-ronment to minimize the effect of moisture (discussed inSection 1.3.6), and mechanical stress (exploiting rigiditythrough film thickness and control of tension through process-ing) (discussed in Section 1.3.7) – a dimensional reproducibil-ity of less than 100 ppm at 150°C has been reported.14 Thisexceptional performance demonstrates that the film can bepushed beyond its data sheet specification if careful controlof the key factors above is achieved and by matching andoptimizing device fabrication against the property set of agiven film.

5 Concluding remarksThis paper has only addressed the base substrate on whichsubsequent processing including conductive and barriercoatings in addition to electronic-device fabrication will becarried out. The demanding property set required for flex-ible electronics is pushing the existing commercially avail-able films to the limits of their performance, but as has beenillustrated in this paper the approach of the substrate devel-oper working closely with the device manufacturer and eachunderstanding the limitations of their respective technolo-gies is enabling significant progress in fabricating flexibleelectronics devices to be made.

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16 W A MacDonald, Organic Electronics: Materials, Manufacturing andApplications, ed. H Klauk (Wiley-VCH, 2006), Chap. 7.

17 W A MacDonald, K Rollins, D MacKerron, R Eveson, R Rustin, RAdam, M K Looney, and K Hashimoto, Proc 25th International DisplayResearch Conference, 36–41 (2005).

18 W S Wong, R Lujan, J H Daniel, and S Limb, “Digital lithography forlarge-area electronics on flexible substrates,” J Non-Crystalline Solids352, 1981–1985 (2006).


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