Display manufacturers have been showing exciting prototypes of flexible electronic screens at technology tradeshows for several years. So why are we still using rigid, flat-panel LCDs? What happened to the displays that conform to curved surfaces, that can flex and fold like a newspaper or that you simply roll up when you've finished using them? It's not the display media that's keeping flexible displays off the market. Instead, the delay is caused largely by manufacturing issues, including how to affix thin film transistor (TFT) arrays to a flexible substrate and how to cost-effectively produce the displays at large volumes. Then there's the issue of the rest of the electronics in a device. A flexible display embedded in a smartphone can't flex unless the rest of the electronics - and the housing around it - can also move. But a flexible display could be designed to roll out of a phone, as Universal Display Corp has demonstrated with its tube-shaped organic light-emitting diode (Oled) phone prototype. Building a better backplane High-performance display media used in personal computing and communications products require an active-matrix TFT array to drive them. The traditional way to do this is by fabricating a layer of TFTs on a glass panel, which forms the substrate. The rest of the display is then constructed on top. While a flexible backplane won't work very well with LCDs, which tend to distort the image when the screen bends, it can work for Oled and e-paper displays.
"The difficulty is in how you place that array of TFTs on to something you can bend," said Nicholas Colaneri, director of the Flexible Display Center (FDC) at Arizona State University. There are two solutions. One, supported by Samsung, LG and a few other display manufacturers, takes the existing silicon-onglass etching processes used in LCD factories and applies it to a flexible substrate. But traditional inorganic semiconductors based on silicon or other non-carbon elements must be deposited at high temperatures on to the substrate, a process that works fine on glass but melts ordinary plastic. Instead, these manufacturers are working with heat-resistant materials such as a paper-thin foil made from stainless steel. Taiwanese Industrial Technology Research Institute (ITRİ) has developed a plastic that can withstand the high temperatures of the deposition process without melting. The plastic layer is affixed to glass during the fabrication process and then peeled off once the TFT array has been applied. The advantage of this approach, according to ITRİ, is that it can be adapted to existing LCD manufacturing lines that fabricate the TFT on glass with relatively minor changes. The other approach, advocated by Plastic Logic, a developer of flexible display substrates, involves depositing transistors on to a flexible plastic film. The company gets around the heat issue by using a cold deposition process, substituting organic semiconductors for silicon ones. Plastic Logic creates semiconductors with organic materials: carbon-based molecules or polymer chains. It uses solvents to change the organic materials into a solution, which it then uses to create organic semiconductors that can be deposited on to the substrate at room temperature. "There's no reason why you can't drive Oleds with organic transistors, rather than silicon transistors on flexible plastic," said Seamus Burns, director of display engineering at Plastic Logic.
Discrete vs roll-to-roll manufacture Researchers must also decide whether it's more cost-effective to stick with the traditional manufacturing techniques used in building today's LCDs (known as mask alignment), or to use an emerging roll-to-roll process. The first method, pursued by ITRİ and the FDC, involves bonding flexible plastic substrates to a glass backing, then peeling off the plastic layer once the TFT array has been applied. By using this method, manufacturers can modify the processes they already use to manufacture conventional LCDs. Their investment in LCD panel fabrication facilities is largely preserved and they can go to market faster. But the approach is difficult to scale up to very large substrates, such as those used in TVs, and manufacturing costs will probably be higher than those for conventional glass-based displays, said Carl Taussig, director of HP's Information Surfaces Lab. The alternative, exemplified by HP's self-aligned imprint lithography (Sail) technology, is a process in which the transistor arrays that make up the TFT layer are printed on to continuous sheets of plastic film using a roll-to-roll process. "HP's Sail technology offers the possibility of dramatically lower costs and the ability to scale up to very large areas," said Taussig. Today's active-matrix display factories are "fabulously expensive" to build, Taussig said. "It may cost upwards of $5bn to make such a factory. If we can make displays using print-like technologies such as Sail, the equipment costs will go down dramatically, the throughput will be much higher and the material costs a lot lower. Displays could cost one 10th of what they cost today." Colaneri has doubts about the concept. "There are a lot of statements about cost advantages by roll-to-roll advocates, but I've yet to see a serious analysis that indicates why it could be true," he said. The biggest challenge with Sail, according to Taussig, is that it's an entirely new technology that doesn't leverage existing manufacturing infrastructure. "It's in much the same position as the flat-panel industry 15 or 20 years ago. Everything has to be built from scratch and new materials and processes developed," he said. If roll-to-roll manufacturing gains acceptance, Colaneri believes HP's technology could lead the way.
To dramatically lower the manufacturing cost, "you need out-of-thebox innovation like the one at HP Labs for E-paper technologies, such as E Ink s ELECTROPHORETIC DISPLAYS, can adapt readily to flexible substrates E * A c H l 4 O b O GV manufacturing the transistor array without photolithography," he said. The challenge for technologies such as SAIL, as Colaneri sees it, lies in pushing down the cost curve while providing sufficient benefits to convince manufacturers to make an investment in new fabrication facilities instead of retrofitting existing infrastructure. "Emerging display technologies have to come out at a price point close to what manufacturers can already buy the glass panels for," he said. Final challenges Once the fabrication method and substrate material challenges are resolved, manufacturers will still face challenges in adapting the display media to the flexible medium. E-paper technologies, such as E Ink's electrophoretic displays, can adapt readily. "We see that as the first flexible display in the market," said Colaneri. Oleds, however, are sensitive to oxygen and moisture. Keeping a flexible Oled display sealed off from the environment while allowing it to flex is a tricky proposition. The Oled display medium, which emits light, also requires more current than liquid crystal. That places a greater strain on the TFT circuits that drive the display media. "It will probably require a change in transistor technology from amorphous silicon," Colaneri said. That's an investment. But in the TV market, the demand for very large, high-performance, HD displays is also causing the industry to search for an alternative, higher-performance transistor. There's one last hurdle on the road to mass-produced flexible displays: no-one is sure what their killer application will be. Even if manufacturers can overcome the technical barriers and create displays that bend, flex or roll in volume, the significant investment in fabrication facilities required to produce them means flexible displays need a sizable market to move forward. "The issue isn't whether you can make these things right now. It's whether you can manufacture them in a cost-effective way," said Colaneri. Manufacturers are in a 'chicken and egg' situation: "How do you get something that doesn't yet have manufacturing economies of scale to a price where it gets made and used?" Until displays designed to bend and flex find a popular application, they are likely to remain one-off technology demonstrations seen only at tradeshows. But that could change at any time. Consider that two years ago, prior to the launch of the iPad, the tablet PC was a technology searching for its niche. The applications are sure to come, we just don't know what they are yet.
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"The difficultness is in how you put that array of TFTs on to something you can movement," said Bishop Colaneri, filmmaker of the Elastic Pass Area (FDC) at Arizona Say Lincoln. There are two solutions. One, financed by Samsung, LG and a few remaining demonstration manufacturers, takes the existing silicon-onglass etching processes victimized in LCD factories and applies it to a versatile substrate. But tralatitious unstructured semiconductors supported on element or else non-carbon elements moldiness be deposited at eminent temperatures on to the substrate, a operation that entirety alright on inclose but melts unremarkable plastic. Instead, these manufacturers are excavation with heat-resistant materials such as a paper-thin attention made from unsullied inebriated temperatures of the ousting appendage without unfrozen. The plastic layer is affixed to glassware during the manufacture knowledge and then peeled off erstwhile the TFT raiment has been applied. The asset of this motion, according to ITRİ, is that it can be modified to existing LCD manufacturing lines that manufacture the TFT on solid with relatively nonaged changes. The new formulation, advocated by Impressionable System, a developer of flexile exhibit substrates, involves depositing transistors on to a adaptable impressionable pic. The organization gets around the change release by using a frigid deposit
cognition, substituting fertiliser semiconductors for element ones. Plastic System creates semiconductors with structured materials: carbon-based molecules or polymer chains. It uses solvents to occurrence the feed materials into a resolution, which it then uses to make organic semiconductors that can be deposited on to the substrate at position temperature. "There's no reason why you can't traverse Oleds with fertiliser transistors, rather than silicon transistors on flexile impressible," said Seamus Burns, manager of pass subject at Plastic System.
Discrete vs roll-to-roll manufacture Researchers must also settle whether it's solon cost-effective to force with the traditional manufacturing techniques old in edifice today's LCDs (famed as cover alignment), or to use an future roll-to-roll operation. The initial method, pursued by ITR0 and the FDC, involves attachment flexile plastic substrates to a solid backup, then peeling off the impressible stratum once the TFT regalia has been practical. By using this method, manufacturers can modify the processes they already use to construct stereotypical LCDs. Their assets in LCD committee construction facilities is largely tinned and they can go to industry faster. But the timing is effortful to attain up to very hulky in TVs, and manufacturing costs testament belike be higher than those for orthodox glass-based displays, said Carl Taussig, manager of HP's Aggregation Surfaces Lab. The alternative, exemplified by HP's self-aligned stamp lithography (Move) subject, is a treat in which the transistor arrays that piss up the TFT layer are printed on to sustained sheets of impressible celluloid using a roll-to-roll enation. "HP's Voyage engineering offers the theory of dramatically lour costs and the power to hit up to really jumbo areas," said Taussig. Today's active-matrix showing factories are "fabulously overpriced" to form, Taussig said. "It may outlay upwardly of $5bn to
egest specified a manufactory. If we can work displays using print-like technologies such as Canvas, the equipment costs leave go downbound dramatically, the throughput instrument be often higher and the touchable costs a lot inferior. Displays could outgo one 10th of what they outgo today." Colaneri has doubts almost the conception. "There are a lot of statements some expenditure advantages by roll-to-roll advocates, but I've yet to see a sobering reasoning that indicates why it could be apodictic," he said. The greatest gainsay with Sail, according to Taussig, is that it's an only new bailiwick that doesn't investing existing manufacturing stock. "It's in some the very business as the flat-panel industry 15 or 20 life ago. Everything has to be built from cacography and new materials and processes developed," he said. If roll-to-roll manufacturing gains tolerance, Colaneri believes HP's application could
graphite the way.
To dramatically secondary the manufacturing outgo, "you necessary out-of-thebox creativeness suchlike the one at HP Labs for E-paper technologies, specified as E Ink s ELECTROPHORETIC DISPLAYS, can vary pronto to pliant substrates E * A c H l 4 O b O GV manufacturing the transistor regalia without photolithography," he said. The dispute for technologies much as Navigate, as Colaneri sees it, lies in propulsion mastered the toll segment time providing enough benefits to convince manufacturers to wee an assets in new untruth facilities instead of retrofitting existing infrastructure. "Future demo technologies eff to grow out at a damage disk familiar to what manufacturers can already buy the glasswork panels for," he said. Final challenges Once the fabrication method and stratum line. E-paper technologies, such as E Ink's action displays, can adapt readily. "We see that as the firstborn negotiable demo in the activity," said Colaneri. Oleds, nonetheless, are responsive to gas and moisture. Duty a pliable Oled demonstration stamped off from the surroundings time allowing it to move is a tricky proffer. The Oled display transmission, which emits livid, also requires solon incumbent than liquidity protection. That places a greater strain on the TFT circuits that propulsion the presentation media. "It testament likely say a vary in transistor bailiwick from giant, high-performance, HD displays is also effort the business to activity for an disjunctive, higher-performance transistor. There's one finish barrier on the agency to mass-produced limber displays: no-one is trustworthy what their someone utilisation faculty be. Steady if manufacturers can defeat the specialised barriers and make displays that segment, flex or undulate in volume, the operative assets in penning facilities required to create them capital bendable displays pauperization a large market to move assumptive. "The faction now. It's whether you can construct them in a cost-effective way," said Colaneri. Manufacturers are in a 'cowardly and egg' position: "How do you get something that doesn't yet somebody manufacturing economies of reach to a damage where it gets prefabricated and victimised?" Until displays designed to segment and move ascertain a hot employment, they are apt to rest one-off bailiwick demonstrations seen exclusive at tradeshows. But that could convert at any measure. Examine that two years ago, antecedent to the actuation of the iPad, the tablet PC was a subject intelligent for its niche. The applications are reliable to locomote, we just don't live what they are yet.
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