HP flexible, unbreakable TFT display demonstrated

Friday, Dec 12th 2008 by Chris Davies

HP and Arizona State University have demonstrated what they’re calling the first prototype of “affordable, flexible electronic displays”. Constructed using the self-aligned imprint lithography (SAIL) technology invented by HP Labs, the display also uses E Ink’s Vizplex bi-stable electrophoretic imaging film that requires no power to maintain an image; the end result is a low-power, low-manufacturing-cost display suitable for color text and graphics.

Rather than being produced in sheets, in a batch process, SAIL allows the different layers to be combined in a roll-to-roll process. That continuous build reduces manufacturing complexity and material usage (up to 90-percent less materials by volume, in fact) and thus cost. It’s also being called “unbreakable”, with the patterning information imprinted in such a way that distortion does not impact perfect alignment.

HP envisage the new display technology to be used in laptops, smartphones and other electronic devices. Not only will form factors be able to change, thanks to the flexible screen, but the could be cheaper, too.

Press Release:

HP and Arizona State University Demo Flexible, Unbreakable Displays

Summary - HP and the Flexible Display Center (FDC) at Arizona State University (ASU) today announced the first prototype of affordable, flexible electronic displays.

Flexible displays are paper-like computer displays made almost entirely of plastic. This technology enables displays to become easily portable and consumes less power than today’s computer displays. Popular applications for the technology could include electronic paper and signage.

The production feat is a milestone in the industry’s efforts to create a mass market for high-resolution flexible displays. Plus, from an environmental standpoint, the displays leapfrog conventional display processes by using up to 90 percent less materials by volume.

Mass production of such displays can enable production of notebook computers, smart phones and other electronic devices at much lower costs since the display is one of the more costly components.

The unbreakable displays were created by the FDC and HP using self-aligned imprint lithography (SAIL) technology invented in HP Labs, HP’s central research arm. SAIL is considered “self aligned” because the patterning information is imprinted on the substrate in such a way that perfect alignment is maintained regardless of process-induced distortion.

SAIL technology enables the fabrication of thin film transistor arrays on a flexible plastic material in a low-cost, roll-to-roll manufacturing process. This allows for more cost-effective continuous production, rather than batch sheet-to-sheet production.

“The display HP has created with the FDC proves the technology and demonstrates the remarkable innovation we’re bringing to the rapidly growing display market,” said Carl Taussig, director, Information Surfaces, HP Labs. “In addition to providing a lower-cost process, SAIL technology represents a more sustainable, environmentally sensitive approach to producing electronic displays.”

Production of flexible displays

The first practical demonstration of the flexible displays was achieved through collaborative efforts between the FDC and HP as well as other FDC partners including DuPont Teijin Films and E Ink. To create this display, the FDC produces stacks of semiconductor materials and metals on flexible Teonex® Polyethylene Naphthalate (PEN) substrates from DuPont Teijin Films.

HP then patterns the substrates using the SAIL process and subsequently integrates E Ink’s Vizplex™ imaging film to produce an actively addressed flexible display on plastic. E Ink’s Vizplex bi-stable electrophoretic imaging film enables images to persist without applied voltage, thereby greatly reducing power consumption for viewing text.

“Producing a photolithography-free, flexible active-matrix display is an excellent example of the Flexible Display Center’s world-class development and manufacturing infrastructure,” said Shawn O’Rourke, director, Engineering, Flexible Display Center at Arizona State University. “It demonstrates how multiple industry partners can collaborate on innovative solutions, including roll-to-roll compatible technology that addresses the rapidly growing market for flexible electronics.”

“Flexible electronic displays are playing an increasingly important role in the global high-tech industry, serving as the crucial enabling technology for a new generation of portable devices, including e-readers and similar products designed to combine mobility with compelling user interfaces,” said Vinita Jakhanwal, principal analyst, Small and Medium Displays, iSuppli. “We expect the flexible display market to grow from $80 million in 2007 to $2.8 billion by 2013. The Flexible Display Center at Arizona State University is a key participant in helping to develop the technology and manufacturing ecosystem to support this market.”

HP SAIL technology is one example of the technologies available for licensing from the HP Intellectual Property Licensing Group.

Further information about DuPont Teijin Films is available at www.dupontteijinfilms.com.

About ASU Flexible Display Center

The FDC is a government–industry–academia partnership that’s advancing full-color flexible display technology and fostering development of a manufacturing ecosystem to support the rapidly growing market for flexible electronic displays. FDC partners include many of the world’s leading providers of advanced display technology, materials and process equipment. The FDC is unique among the U.S. Army’s University centers, having been formed through a 10-year cooperative agreement with Arizona State University in 2004. This adaptable agreement has enabled the FDC to create and implement a proven collaborative partnership model with more than 20 engaged industry members, and to successfully deploy world-class wafer-scale R&D and GEN-II display-scale pilot production lines for rapid flexible display technology development and manufacturing supply chain commercialization. More information about FDC is available at http://flexdisplay.asu.edu/.

About HP

HP, the world’s largest technology company, simplifies the technology experience for consumers and businesses with a portfolio that spans printing, personal computing, software, services and IT infrastructure. More information about HP (NYSE: HPQ) is available at http://www.hp.com/.

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CNT-TFTs, flexible displays, ANI, University of Stuttgart

Alternative Displays

August 26, 2008

Single-wall carbon nanotube (CNT) thin-film transistors (TFTs) are now possible for flexible displays and electronics, thanks to breakthroughs from the collaboration between the University of Stuttgart, Germany, and Applied Nanotech, Inc (ANI). Dr Paul Beatty an expert in the displays industry now follows up with some additional details and insights.

The team announced June 26 it had obtained improved yield from its proprietary printing method, which avoids expensive photolithography. Furthermore, high mobility (100 cm2/Vs) and high on/off ratio (105) were achieved, which is far better than printed TFTs using organic semiconductors.

Such high mobility means these TFTs can be made small enough to avoid obscuring too much light, and therefore do not need to be transparent or hidden on the other side of substrates or display layers. The on/off ratio compares with a value of under 10 for previous attempts at the University of Maryland in 2005, in which printing of CNTs also was used.

No details were given of the precise yield, pending more data, or the particular printing method used, but ANI said ink-jet and microcontact printing methods may work. Dr. Zvi Yaniv, president and CEO of ANI, said yield is likely to be more a function of CNT purity, particularly semiconducting versus metallic types. Improvements in preparing purer CNTs has enabled monolayer CNTs to form the TFT semiconducting channels, which avoids the tremendous variations in mobility and threshold voltages found earlier. ANI considers the additional costs of higher purity to be inconsequential because so little of the material is needed in a display.

In the past, the significant proportion of metallic rather than semiconducting CNTs led to lower on/off ratios, and this can short-circuit the transistor. In fact, a previously reported method of removing the metallic type was by attacking with nitronium ions (NO2+) in a mixture of nitric and sulfuric acids (e.g. Cheol-Min Yang at Sungkyunkwan University, Republic of Korea, J. Phys. Chem. B, 2005, 109). ANI has its own methods, but also buys CNTs from other suppliers, and some of the latest separation methods are said to be more commercially viable and also allow selection of CNTs having the same "chirality."

Dr.Yaniv said, "Chirality relates to the skew of the rolled-up graphitic sheet of carbon atoms. This determines the semiconducting energy band gap affecting the mobility and threshold voltage." So, having CNTs with all the same chirality allows a smaller variation in the mobility and threshold voltage.

Of particular interest for flexible displays, electronic circuits and sensors is the ability to deposit at low temperature compatible with flexible plastic substrates. For more information about ANI's thin-film transistor approach see Solution-deposited carbon nanotube layers for flexible display applications, published in Physica E 37, Issue 1-2 (March 2007). There, researchers obtained a mobility of 1cm2/Vs, but not yet the homogeneity and reproducibility that has been addressed in this latest work.

Interestingly, Dr.Yaniv agreed that any adsorbent from the atmosphere on the CNTs can change the TFT characteristics, and that encapsulation by passivation will be necessary. But he said effects of gas and water vapor on the gate part of the TFT is less severe than for TFTs made with a-Si. (See also plastic vs. metal foil substrates as mentioned in the FlexTech Alliance contract searching for other metal foils besides stainless steel.) Overall, Dr. Yaniv did not see a problem with lifetime for these CNTs.

"The collaboration with the University of Stuttgart is very productive," he said. "Their expertise and facilities for microelectronic processes are well known and are very suitable for our need to transition from an idea to a proof of concept." The university's emphasis was on the deposition of CNTs in flexible displays, while ANI concentrated on the CNT material.

Dr. Yaniv maintains that there will be no problem going up in substrate size for larger displays or lower-cost volume production as the equivalent to large mother glass. Compared to organic TFTs, the numbers of addressed pixels should be greater, although any need for very short channel lengths may limit conductivity as the "percolation" mechanism for the fishnet monolayer of CNTs may not work. Ultimately, this might limit the pixel density, but the specific number has yet to be determined, and depends also on the final levels of the metallic CNT impurities. Furthermore, it appears the CNT-TFTs are compatible with the electrical requirements of all the applicable flexible display technologies, although the initial development work is with LCDs.

An attribute for use in displays is the transparency of electrodes. In related work on use of transparent CNTs as replacement for the usual thin-film transparent indium tin oxide (ITO) pixel electrodes, Prof. Dr. Ing Norbert Fruehauf at the University of Stuttgart presented a paper in May at SID '08 revealing a working demonstration of a 4-inch diagonal 320 x RGB x 240 a-Si TFT-LCD made in this way. Prepared entirely at the university's facilities, CNTs were deposited by a low-cost spray method. Sheet resistance for electrodes does not need to be so low, but high transmittance is more important. The researchers found purified CNTs prepared by the HiPCO process gave a transmittance up to about 94% for a sheet resistance of 2,000 to 3,000 Ohms/square. Using conventional a-Si TFTs with such electrodes resulted in an on/off ratio of 106 and mobility of 0.4 -0.6 cm2/Vs.

APNT is a holding company with wholly owned operating subsidiaries Applied Nanotech and Electronic Billboard Technology Inc. (EBT). ANI's business model is to license its technology to partners that will manufacture and distribute products using the technology. Dr. Yaniv said, "Ideally for us would be to find a strategic partner that would want to take this to a pilot line."nTogether, the companies have more than 250 patents or patents pending, with at least one on this development, and one held by the University of Stuttgart.

by Dr Paul Beatty

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Flexible Display Center redefines ultrathin display process

The Flexible Display Center at Arizona State University has developed a new process for manufacturing high-performance flexible displays on transparent plastic.

FDC researchers, working with industrial partners DuPont Teijin Films and E Ink Corp., have developed a method for making high-performance amorphous silicon thin film transistors on planarized Teonex® PEN films. The FDC team integrated 3.8-in. QVGA arrays of these transistors with Vizplex-100™ imaging layer film from E Ink to fabricate glass-free high-performance flexible electrophoretic displays that are only 15 mils (375 micrometers) thick.

The displays are quite rugged and readily withstand severe vibration and impact tests performed at industry partner General Dynamics’ labs. To download video highlights of these tests go to http://flexdisplay.asu.edu/Flex-display-test_revB.wmv.

The FDC process uses a proprietary technique for temporarily bonding the planarized Teonex PEN film (from DuPont Teijin) to a rigid carrier using a specially developed adhesive. Amorphous silicon circuits then are fabricated with conventional flat panel display manufacturing equipment. Despite exposure of the bonded film to temperatures as high as 200 C (392 F) during the fabrication process, essentially no plastic substrate distortion is observed. The film bearing the completed transistor arrays is removed from the carrier using a mechanical force that is gentle enough to permit automation of the process.

“Most of the technology development in our pilot line environment is realized through steady improvements over several cycles of learning,” said Greg Raupp, director of FDC. “In this case, integrated learning came together as we viewed the entire flexible substrate system of carrier, adhesive, substrate, planarization and associated process protocols to point to a directed solution that yielded a dramatic technical advance.”

The FDC thin film transistors are produced using the highest semiconductor and gate-dielectric deposition temperatures reported for a process on Teonex PEN. The higher temperatures permit the fabrication of transistors with higher on-off ratio, better sub-threshold slope, and – most importantly – greater bias-stress stability. These performance characteristics translate directly into higher pixel densities for enhanced display resolution and an enlarged number of grey levels for improved image quality.

The ability to produce high quality arrays of thin film transistors with low defects is aided by the use of DTF’s planarized Teonex PEN, which has been developed to meet the needs of demanding display applications. The temporarily bonded Teonex PEN with its newly developed planarization coating provides a surface smooth enough and sufficiently defect-free to enable the fabrication of micrometer-scale electronics.

Development of methods for the handling of mechanically flexible substrates such as Teonex PEN in automated manufacturing equipment has been a significant challenge to creating practical and economical processes for flexible displays and electronics. The FDC advance in temporarily bonding plastic films to a carrier is a significant move forward for advancing engineering prototypes of flexible displays to commercial manufacturing.

Skip Derra, skip.derra@asu.edu

480-965-4823

Media Relations

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Nanotube Circuits - Carbon nanotubes combine high performance and flexibility for electronics.

Wednesday, July 23, 2008

By Lauren Rugani

Fast and flexible: An integrated circuit on a thin plastic sheet incorporates transistors made from single-walled carbon-nanotube networks. The carbon-based networks rival the performance of single-crystal silicon, but they can be easily printed onto the plastic from solution and have good mechanical properties that are useful for flexible electronics. Credit: Beckman Institute, University of Illinois

New research suggests that networks of single-walled carbon nanotubes printed onto bendable plastic perform well as semiconductors in integrated circuits. Researchers from the University of Illinois at Urbana-Champaign (UIUC) and Purdue University, whose work appears this week in Nature, say that these nanotube networks could replace organic semiconductors in applications such as flexible displays.

Development of flexible electronics has recently focused on organic molecules because, unlike silicon, they are compatible with bendable plastic substrates. Flexible electronics have potential in such applications as low-power electronic newspapers or PDAs that roll up into the size and shape of a pen. The problem with existing organic-electronic devices, however, is that "they aren't well developed for long-term reliability, and they perform far worse than silicon," says John A. Rogers, an engineering professor at UIUC and co-author of the Nature paper.

Carbon-nanotube networks, on the other hand, combine the performance of silicon with the flexibility of organic films on plastic. Rogers says that the speed of the nanotube device compares favorably with the speed of commercially used single-crystal silicon circuits. The transistors can also switch between on and off states in the range of several kilohertz, which is similar to the range of those used for liquid crystal displays and radio frequency identification (RFID) sensors. However, the on-off current ratio for carbon nanotubes is still a few orders of magnitude lower than that for silicon transistors.

The researchers made the networks by depositing nanotubes onto plastic by standard printing methods, which could lead to low-cost, large-scale fabrication. And the printed circuits can bend to a radius of about five millimeters without compromising the electrical performance of the device. "This method is good for flexible electronics that need to be printed over a large area," says Ali Javey, an assistant professor of electrical engineering at the University of California, Berkeley.

Using a technique called transfer printing, the researchers deposited randomly aligned carbon nanotubes onto a 50-micrometer-thick sheet of plastic, and then patterned gold electrodes and other circuit components onto the substrate. Because about one-third of the nanotubes in any network are metallic, which can short out the transistors, the researchers then etched narrow parallel lines through the network with soft lithography. By cutting the nanotubes, they can effectively eliminate the possibility of a purely metallic pathway connecting two electrodes while preserving the performance of the device.

Several challenges still remain before the nanotubes networks are ready for actual products. Devices need to be made in which the performance from device to device doesn't vary; billions of individual nanotubes have to be made with high purity and the right dimensions for optimal performance. The printing process also needs development, says George Gruner, a professor of physics at the University of California, Los Angeles. Gruner suggests that nanotubes could be dissolved into ink and then printed onto plastic. "These devices have to be cheap and disposable," especially for devices like RFID tags in food packaging, he adds.

Rogers's group's immediate goals are to work toward lower power and higher speed in the devices. "We want to push the limits to see how far we can go," he says.

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