Category Archives: Flexible Displays

When the new iPhone came out, customers complained that it could be bent — but what if you could roll up your too big 6 Plus to actually fit in your pocket? That technology might be available sooner than you think, based on the work of USC Viterbi engineers.

For many decades, silicon has been the heart of modern electronics — but as a material, it has its limits. As our devices get smaller and smaller, the basic unit of these devices, a transistor, must also get tinier and tinier. Bottom line: the size of the silicon transistor is reaching its physical limit. As silicon devices are based on what is called a top-down cutting method, it is increasingly difficult for silicon to be made even smaller. Consumers also demand phones to be lighter, faster, smaller, more flexible, wearable, bendable, etc. Yet silicon is also rigid — one can’t bend your smart phone or computer. These physical limitations have driven the race for new materials that can be used as semiconductors in lieu of silicon.

The demand for a silicon material aided the discovery of graphene, a single layer of graphite — which won the Nobel Prize in Physics in 2010. Since this time, scientists and engineers have developed many two-dimensional (2D) material innovations — layered materials with the thickness of only one atom or a few atoms. One such layered 2D material is black arsenic phosphorous. Now, a team of scientists at USC Viterbi, in collaboration with Technische Universität München, Germany, Universität Regensburg, Germany, and Yale University, have developed a new method to synthesize black arsenic-phosphorous without high pressure. This method demands less energy and is cheaper, and the synthesized materials have some incredible new properties.

The innovation, developed by USC Viterbi researchers, including Bilu Liu, the paper’s lead author and postdoctoral researcher; Ahamad Abbas, graduate student; Han Wang, assistant professor; Rohan Dhall, graduate student; Stephen B. Cronin, associate professor; Mingyuan Ge, research assistant; Xin Fang, graduate student; and Professor Chongwu Zhou of the Ming Hsieh Department of Electrical Engineering, in concert with their collaborators, is documented in a paper titled “Black Arsenic-Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties.” The paper appeared in Advanced Materialson June 25, 2015.

What the researchers are most excited about is the ability to adjust the electronic and optical properties of these materials to a range that cannot be achieved by any other 2D materials thus far. This includes manipulating the materials’ chemical compositions during materials synthesis and the materials’ ability to sense long wavelength infrared (LWIR) waves due to their small energy gaps. This particular electromagnetic spectral range of LWIR is important for a range of applications such as LIDAR (light radar) systems, basically because LWIR waves are highly transparent in earth atmosphere. This wave range also has great application for the soldiers in the military who rely on infrared thermal imaging technology and for flexible night vision glasses. Another intriguing aspect of these new layered semiconductors is their anisotropic electronic and optical properties, which means the materials have different properties along x and y direction in the same plane. The researchers believe these are marked improvement from existing materials and devices and would lead to unique applications.

In addition, the researchers anticipate that it could also lead to important improvement for devices that monitor the environment. “We believe these materials are important members in a large family of 2D materials, because they fit into the long-wavelength-infrared light range and deliver properties that any other currently existing 2D materials cannot,” said Zhou, the research team leader.

According to Liu, the paper’s lead author: “As these are rather new materials, we anticipate there is lots of exciting fundamental physics research as well as engineering work to be done. For example, what’s the electronic and optical properties of a truly single layer black arsenic phosphorus?”

Researchers from North Carolina State University have created stretchable, transparent conductors that work because of the structures’ “nano-accordion” design. The conductors could be used in a wide variety of applications, such as flexible electronics, stretchable displays or wearable sensors.

“There are no conductive, transparent and stretchable materials in nature, so we had to create one,” says Abhijeet Bagal, a Ph.D. student in mechanical and aerospace engineering at NC State and lead author of a paper describing the work.

“Our technique uses geometry to stretch brittle materials, which is inspired by springs that we see in everyday life,” Bagal says. “The only thing different is that we made it much smaller.”

The researchers begin by creating a three-dimensional polymer template on a silicon substrate. The template is shaped like a series of identical, evenly spaced rectangles. The template is coated with a layer of aluminum-doped zinc oxide, which is the conducting material, and an elastic polymer is applied to the zinc oxide. The researchers then flip the whole thing over and remove the silicon and the template.

What’s left behind is a series of symmetrical, zinc oxide ridges on an elastic substrate. Because both zinc oxide and the polymer are clear, the structure is transparent. And it is stretchable because the ridges of zinc oxide allow the structure to expand and contract, like the bellows of an accordion.

“We can also control the thickness of the zinc oxide layer, and have done extensive testing with layers ranging from 30 to 70 nanometers thick,” says Erinn Dandley, a Ph.D. student in chemical and biomolecular engineering at NC State and co-author of the paper. “This is important because the thickness of the zinc oxide affects the structure’s optical, electrical and mechanical properties.”

The 3-D templates used in the process are precisely engineered, using nanolithography, because the dimensions of each ridge directly affect the structure’s stretchability. The taller each ridge is, the more stretchable the structure. This is because the structure stretches by having the two sides of a ridge bend away from each other at the base – like a person doing a split.

The structure can be stretched repeatedly without breaking. And while there is some loss of conductivity the first time the nano-accordion is stretched, additional stretching does not affect conductivity.

“The most interesting thing for us is that this approach combines engineering with a touch of surface chemistry to precisely control the nano-accordion’s geometry, composition and, ultimately, its overall material properties,” says Chih-Hao Chang, an assistant professor of mechanical and aerospace engineering at NC State and corresponding author of the paper. “We’re now working on ways to improve the conductivity of the nano-accordion structures. And at some point we want to find a way to scale up the process.”

The researchers are also experimenting with the technique using other conductive materials to determine their usefulness in creating non-transparent, elastic conductors.

They are thin, light-weight, flexible and can be produced cost- and energy-efficiently: printed microelectronic components made of synthetics. Flexible displays and touch screens, glowing films, RFID tags and solar cells represent a future market. In the context of an international cooperation project, physicists at the Technische Universität München (TUM) have now observed the creation of razor thin polymer electrodes during the printing process and successfully improved the electrical properties of the printed films.

Solar cells out of a printer? This seemed unthinkable only a few years ago. There were hardly any alternatives to classical silicon technology available. In the mean time touch screens, sensors and solar cells can be made of conducting polymers. Flexible monitors and glowing wall paper made of organic light emitting diodes, so-called OLEDs, are in rapid development. The “organic electronics” are hailed as a promising future market.

However, the technology also has its pitfalls: To manufacture the components on an industrial scale, semiconducting or insulating layers – each a thousand times thinner than a human hair – must be printed onto a carrier film in a predefined order. “This is a highly complex process, whose details need to be fully understood to allow custom-tailored applications,” explains Professor Peter Müller-Buschbaum of the Chair of Functional Materials at TU München.

A further challenge is the contacting between flexible, conducting layers. Hitherto electronic contacts made of crystalline indium tin oxide were frequently used. However, this construction has numerous drawbacks: The oxide is more brittle than the polymer layers over them, which limits the flexibility of the cells. Furthermore, the manufacturing process also consumes much energy. Finally, indium is a rare element that exists only in very limited quantities.

Polymers in X-ray light 

A few months ago, researchers from the Lawrence Berkeley National Laboratory in California for the first time succeeded in observing the cross-linking of polymer molecules in the active layer of an organic solar cell during the printing process. In collaboration with their colleagues in California, Müller-Buschbaum’s team took advantage of this technology to improve the characteristics of the polymer electronic elements.

The researchers used X-ray radiation generated in the Berkley synchrotron for their investigations. The X-rays are directed to the freshly printed synthetic layer and scattered. The arrangement and orientation of the molecules during the curing process of the printed films can be determined from changes in the scattering pattern.

“Thanks to the very intensive X-ray radiation we can achieve a very high time resolution,” says Claudia M. Palumbiny. In Berkeley the physicist from the TUM investigated the “blocking layer” that sorts and selectively transports the charge carriers in the organic electronic components. The TUM research team is now, together with its US colleagues, publishing the results in the trade journal Advanced Materials.

Custom properties

“In our work, we showed for the first time ever that even small changes in the physico-chemical process conditions have a significant influence on the build-up and properties of the layer,” says Claudia M. Palumbiny. “Adding solvents with a high boiling point, for example, improves segregation in synthetics components. This improves the crystallization in conducting molecules. The distance between the molecules shrinks and the conductivity increases.

In this manner stability and conductivity can be improved to such an extent that the material can be deployed not only as a blocking layer, but even as a transparent, electrical contact. This can be used to replace the brittle indium tin oxide layers. “At the end of the day, this means that all layers could be produced using the same process,” explains Palumbiny. “That would be a great advantage for manufacturers.”

To make all of this possible one day, TUM researchers want to continue investigating and optimizing the electrode material further and make their know-how available to industry. “We have now formed the basis for pushing ahead materials development with future investigations so that these can be taken over by industrial enterprises,” explains Prof. Müller-Buschbaum.

The research was supported by the GreenTech Initiative “Interface Science for Photovoltaics” (ISPV) of the EuroTech Universities together with the International Graduate School of Science and Engineering (IGSSE) at TUM and by the Cluster of Excellence “Nanosystems Initiative Munich” (NIM). Further support came from the Elite Network of Bavaria’s International Doctorate Program “NanoBioTechnology” (IDK-NBT) and the Center for NanoScience (CeNS) and from “Polymer-Based Materials for Harvesting Solar Energy” (PHaSE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences. Portions of the research were carried out at the Advanced Light Source which receives support by the Office of Basic Energy Sciences of the U.S. Department of Energy.

4K LCD TV panel shipments continue to rise, driven by forces on both the supply side and the demand side. Shipments of 4K TV panels in April 2015 exceeded 3 million units for the first time, comprising 14 percent of all TV panels shipped globally during the month, according to IHS Inc. (NYSE: IHS), a global source of critical information and insight. Of all TV panels forecast to ship in 2016, one in five is forecast to be a 4K TV panels, sometimes marketed as ultra-high definition (UHD), due to the trend toward higher resolution panels in the high-end TV segment and improved production efficiency of panel makers. 

“Prices for 4K TV panels continued to decline in 2014 and early this year, causing a rise in their adoption,” said Linda Lin, senior analyst, IHS Technology. “Most global TV brands have now launched 4K UHD products and are introducing more 4K models to their television offerings.”

When AU Optronics (AUO) introduced the first 55-inch 4K TV panels in Taiwan in 2012, fewer than 100 units were shipped each month. That same year, Innolux introduced the first 50-inch 4K TV panels; however, due to higher manufacturing costs, shipments still totaled fewer than 10,000 units per month. In 2013, panel makers managed to improve their 4K TV panel yield rates, but shipments still made up less than 2 percent of all TV panels shipped, according to recent data from the IHS Monthly TFT LCD Shipment Database.

After panel makers instituted aggressive promotions in the Chinese TV market, using 4K resolution of 3840 pixels by 2160 pixels as a point of differentiation in the high-end TV market, 4K TV panel market share reached 8 percent. Red-green-blue-white (RGBW) 4K sub-pixel technology is now widely accepted in the Chinese TV market and has even begun to penetrate the global market.

“While panel makers in Taiwan initially developed and stimulated 4K TV panel production, South Korean panel makers are now leading the 4K TV panel market,” Lin said. “In fact, LG Display and Samsung Display have risen to become the largest global manufacturers of 4K displays.”

The IHS Monthly TFT LCD Shipment Database provides the latest panel shipment numbers from global large-area panel makers. The database includes monthly shipments of all major TFT LCD suppliers, detailing revenues and average selling prices, as well as shipments by unit, display area, application, size and aspect ratio for each supplier.

Phase change random access memory (PRAM) is one of the strongest candidates for next-generation nonvolatile memory for flexible and wearable electronics. In order to be used as a core memory for flexible devices, the most important issue is reducing high operating current. The effective solution is to decrease cell size in sub-micron region as in commercialized conventional PRAM. However, the scaling to nano-dimension on flexible substrates is extremely difficult due to soft nature and photolithographic limits on plastics, thus practical flexible PRAM has not been realized yet.

Low-power nonvolatile PRAM for flexible and wearable memories enabled by (a) self-assembled BCP silica nanostructures and (b) self-structured conductive filament nanoheater. CREDIT: KAIST

Low-power nonvolatile PRAM for flexible and wearable memories enabled by (a) self-assembled BCP silica nanostructures and (b) self-structured conductive filament nanoheater.
CREDIT: KAIST

Recently, a team led by Professors Keon Jae Lee and Yeon Sik Jung of the Department of Materials Science and Engineering at KAIST has developed the first flexible PRAM enabled by self-assembled block copolymer (BCP) silica nanostructures with an ultralow current operation (below one quarter of conventional PRAM without BCP) on plastic substrates. BCP is the mixture of two different polymer materials, which can easily create self-ordered arrays of sub-20nm features through simple spin-coating and plasma treatments. BCP silica nanostructures successfully lowered the contact area by localizing the volume change of phase-change materials and thus resulted in significant power reduction. Furthermore, the ultrathin silicon-based diodes were integrated with phase-change memories (PCM) to suppress the inter-cell interference, which demonstrated random access capability for flexible and wearable electronics. Their work was published in the March issue of ACS Nano: “Flexible One Diode-One Phase Change Memory Array Enabled by Block Copolymer Self-Assembly.”

Another way to achieve ultralow-powered PRAM is to utilize self-structured conductive filaments (CF) instead of the resistor-type conventional heater. The self-structured CF nanoheater originated from unipolar memristor can generate strong heat toward phase-change materials due to high current density through the nanofilament. This ground-breaking methodology shows that sub-10nm filament heater, without using expensive and non-compatible nanolithography, achieved nanoscale switching volume of phase change materials, resulted in the PCM writing current of below 20 uA, the lowest value among top-down PCM devices. This achievement was published in the June online issue of ACS Nano “Self-Structured Conductive Filament Nanoheater for Chalcogenide Phase Transition.” In addition, due to self-structured low-power technology compatible to plastics, the research team has recently succeeded in fabricating a flexible PRAM on wearable substrates.

Professor Lee said, “The demonstration of low power PRAM on plastics is one of the most important issues for next-generation wearable and flexible non-volatile memory. Our innovative and simple methodology represents the strong potential for commercializing flexible PRAM.”

In addition, he wrote a review paper regarding the nanotechnology-based electronic devices in the June online issue of Advanced Materials entitled “Performance Enhancement of Electronic and Energy Devices via Block Copolymer Self-Assembly.”

Organic semiconductors now offer the performance, cost and route to adoption, for foldable displays; from ultra-thin, conformal, wearables to truly foldable smartphones and tablets.

BY DR. MICHAEL COWIN, SmartKem Ltd, St Asaph, Wales

Buoyed by consumer demand for fresh innovation and fierce industry competition, the display industry exists in a cycle of continuous improvement.

Today a new breed of semiconductors – a key enabling component in the evolution of active matrix displays – are competing to offer manufacturers a route to the production of high performance curved, foldable and even roll-able displays.

There are two key factors that define the impact and adoption of any new enabling technology like this; namely how will it perform and what will be the cost.

This article demonstrates that the performance of organic thin-film transistors (OTFT) for display backplane application has reached a tipping point into market adoption. OTFTs are now equal and arguably greater than competitive technology solutions while also offering ultra-flexibility and a significant cost advantage in production and ownership over the more traditional inorganic equivalents. OTFTs are now a serious contender to fill a critical gap in the market for high performance, ultra-flexible TFT backplanes to drive the next generation of conformal displays.

At first, low-temperature polysilicon (LTPS) was considered the most likely solution to replace hydro-genated amorphous silicon (a-Si:H) as the TFT channel layer for rigid flat panel display backplanes, until the advent of indium gallium zinc oxide (IGZO). While the vastly superior mobility of LTPS gave uplift in mobility over traditional a-Si TFT, it came at a price of significantly higher manufacturing costs through high CAPEX, complicated processing and much lower yields, some of which were as low as 20% in early 2014.[1]

However, the recent aggressive drive to manufacture OLED, EPD and LCD display products with new form factors so they are lightweight, conformal or flexible has placed new challenging demands on the TFT material characteristics. This has allowed new technology platforms such as OTFTs to enter into the supply chain to compete head on with LTPS and IGZO as a TFT channel material based on the same metrics of performance and cost.

Electrical performance: It’s all about power

While a semiconductor technology’s cost of ownership outlines the market entry opportunities, no TFT platform will even be considered a viable alternative to incumbent semiconductors unless it meets, and surpasses key criteria. When defining these criteria it is vital that context to the end application and how this might improve the user experience is considered. Power consumption is one such aspect becoming critical in defining the battery life of mobile and wearable displays and any new TFT channel material, such as OTFT needs to demonstrate either equal or better performance to add value to the user experience in end product form.

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The progression from a-Si semiconductors to alternative materials for rigid displays was originally driven by the charge carrier mobility bottleneck, as manufacturers tried to move to higher resolution active matrix LCD displays. The same requirement exists for AMOLED displays, and as such a parallel can be drawn to the arguments for and against the competing materials systems, but with the increasingly important necessity for physical flexibility.

Each semiconductor platform has its own advantages and disadvantages. For instance while LTPS has a very high carrier mobility it could be debated whether it’s necessary in the average pixel driver circuit for a high quality LCD or OLED display where a mobility of 5-10 cm2/V.s is more than adequate. Indeed IGZO and the latest generation of OTFTs meet this requirement with ease. In contrast TFT electrical (bias stress) stability is an issue with IGZO, usually resulting in more complexity in the TFT drive circuitry for each pixel to compensate for this short coming. From a general perspective each of the above mentioned three contenders are more than suitable as a channel semiconductor. However, these options also need to be considered in context; which of these offers the potential to add real uplift in the user experience at a price point the market will accept? Most displays today are mobile-enabled and are soon to become wearable with the advent of the smartwatch. The power consumption of these displays and its impact on battery life may well be a defining factor in the choice of TFT channel semiconductor for many manufacturers.

An important contribution to this argument was made by Sharp with the introduction to the market of IGZO. Sharp highlighted the importance of TFT leakage current which led to a clearer understanding of the mechanisms responsible for these leakage currents. The causes are found to be predominantly dependent on the smoothness of the interface between the insulator and the channel semiconductor.

So while LTPS has a rough polycrystalline surface its leakage current is higher; IGZO in contrast has smooth amorphous surfaces at this key interface and as such much lower leakage currents.

The context of lower leakage currents is that it will become a very desirable quality since less current is dissipated when the TFT is off and as such the TFT switch capacitor/s can retain an internal charge for a longer period of time. Thus the display refresh rate can be reduced which leads to a potentially dramatic reduction in power consumption – especially for displays that will have static images – ideal for wearable and mobile based displays. As such IGZO has a clear advantage over LTPS for this display based application.

However, recent advances in OTFT technology reported here for the first time show the potential for low leakage currents equivalent to IGZO; but achieved using OTFTs. By designing into solution based organic semiconductor ‘inks’ the preferred features of the single- crystal organic semiconductor combined with semiconducting polymers or ‘binders’ an amorphous semiconductor layer can be achieved. This material combination offers the high mobility of single crystals but with highly uniform processing charac- teristics required for device uniformity. Furthermore, the amorphous nature of these materials offers very smooth interfaces between the solution processed insulator and solution processed semiconductor.

The results in FIGURE 1 demonstrate that the low leakage current levels achieved by a single gate OTFT. This could be lowered further by use of a dual gate OTFT stack as with commercial IGZO TFTs.

FIGURE 1. TEM of copper hillocks

FIGURE 1. TEM of copper hillocks

Therefore OTFTs represent serious competition to IGZO as a channel material in the context for application to wearable and mobile displays for extended battery life. Coupled with the further advantages of excellent bias stress stability and low temperature processing, the case for OTFT adoption rather than IGZO becomes more attractive from a performance perspective.

Physical performance: The foldable frontier

Recently there have been a number of commercial products launched based on curved AMOLED displays such as the Galaxy Round, LG G Flex and Galaxy Note Edge with curved features (and slight flex in the case of the G Flex), all based on LTPS TFT backplanes on plastic. When the user context is taken into account it could be suggested that these products have not offered much value differentiation from glass based equivalent devices.

As such the real ‘wow’ factor in the consumer experience or user value-add has yet to be achieved.

Next generation smart and wearable technology will come with the introduction of flexible and foldable devices such as wearables, smartphones and tablets; but this demands a semicon- ductor platform with entirely new physical properties and a form factor capability which in turn raises a unique set of challenges for traditional and new TFT technologies to overcome.

The current limiting factor is the inability of LTPS and IGZO technologies to offer robust and acute bend capability in TFT form. Even with the use of exotic and expensive strain management layering techniques the maximum bend radius of these technologies have hit a roadblock at around 5 mm.

To genuinely offer a differentiated product with a compelling value-add proposition to the consumer experience, manufacturers must turn to the use of material technologies that enable truly foldable mobile devices or fully bendable, robust and light- weight smartwatches (FIGURE 2). The solution to the limitations presented by LTPS and IGZO in bend capability is the use of OTFTs. It has long been understood that the polymeric nature of OTFTs is ideally suited for bendable applications, and it has widely been reported that products such as Smart- Kem’s tru-FLEX® can withstand 10,000 bends below 1mm with minimal effect on device performance. As such OTFT technology is now considered a key enabler for a wide range of highly robust bendable and foldable display based products; and the market timing could not be better with the recent upturn in demand for smartwatch based products.

FIGURE 2: Display form factor dependency on bend radius.

FIGURE 2: Display form factor dependency on bend radius.

In the context of performance it may be suggested that while the initial market entrants in curved display products have been manufactured with LTPS, and that there is further development potential in the IGZO platform, a complete technology solution already exists – OTFT.

The OTFT technology platform offers the transistor performance for exciting new applica- tions while also holding two ‘aces’ when it comes to product-specific performance for this new generation of wearable and mobile displays; low leakage for significant battery life extension and ultra-flexibility for foldable mobile devices and bendable smart- watches.

How much will it cost?

Beyond the performance benefits of OTFTs, a commercially viable TFT channel semicon-
ductor must provide favourable characteristics for integration into a robust and cost-effective semiconductor manufacturing process. The savings in manufacturing costs compared with inorganic materials as well as the low risk approach of re-purposing existing a-Si production lines to pilot OTFT backplanes on plastic is an appealing prospect.

One of the major advantages of organic semiconductors comes from their ease of application. Solution based semiconductor inks can be applied to substrates through a range of additive processes and print production systems such as slot dye coating as well as low temperature process (FIGURE 3). Although modern organic semiconductors are stable up to 300°C the ease by which these solution-based materials can be processed at low temperatures offers manufacturers a wide range of cost effective stack materials and substrates, and easier bond/de-bond and inter-layer alignment due to less expansion and contraction. This all adds up to significantly improving production yield (over high temperature processing) and thereby reducing production costs over any area of substrate.

FIGURE 3. Commercial organic semiconductors, such as SmartKem’s tru-FLEX® material, offer a total technology solution, combining high performance mobility, low temperature processing and true flexibility.

FIGURE 3. Commercial organic semiconductors, such as SmartKem’s tru-FLEX® material, offer a total technology solution, combining high performance mobility, low temperature processing and true flexibility.

An independent study has been commissioned by SmartKem comparing the cost of key features within the TFT stack that would show the maximum variance between technology platforms; the semiconductor and gate dielectric layer. This will ensure a complete understanding of the difference in the cost of ownership and cost of production for the alternate TFT channel materials for backplane manufacture for flexible displays.

The four technology platforms chosen for the TFT array devices were: a-Si, LTPS, IGZO and SmartKem’s OTFT semiconductor tru-FLEX®. The overall cost of TFT device manufacture included manufacturing overheads to produce the two layers, depreciation of equipment (amortized over five years of production of 1.8 million substrates) and the direct materials costs.

The CAPEX for each fabrication process is determined from the type and quantity of equipment needed for producing the semiconductor and gate insulator layers with an assumed input capacity of 30,000 substrates per month. In this study, the assumed equipment and materials are shown in Table 2. The summary findings of the on-going study have shown the cost of manufacturing TFT arrays with organic semiconductors is almost half that of LTPS and a third lower than a-Si and IGZO. The most significant findings (to be published in a white paper) were that the manufacturing overheads and depreciation costs for OTFT were ten times less than LTPS and four times less than a-Si and IGZO.

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It was found that the depreciation cost of production for a ‘greenfield’ OTFT line is vastly smaller than competing technologies and could be further reduced by the re-purposing of an a-Si production line; OTFTs thus offer an easy route to adoption for the cost-down manufacture of superior performance flexible TFT backplanes.

The future is organic

The value proposition of organic semiconductors now makes sense to an industry eager for differentiated products that can be adopted and scaled with low risk. From a performance and cost perspective the immediate value-add to the consumer is longer battery life and fully foldable mobile displays. While the cost of production is reduced with OTFT, the extremely low cost of ownership offers a low risk industrialization strategy through the building of a ‘greenfield’ line or by the re-purposing of an existing a-Si line.

One of the most exciting and eagerly awaited outputs of this rapid evolution in material perfor- mance and cost is the advancement and commercialization of bendable and foldable displays. From ultra-thin, conformal, wearables to truly foldable smartphones and tablets, organic semiconductors now offers the performance, cost and route to adoption for the manufacture of a new generation of OLED, EPD and LCD displays with entirely new physical properties and form factors.

References

1. http://www.displaysearchblog.com/2014/08/waiting-for-the-apple-iwatch/

DR MICHAEL COWIN is Head of Strategic Marketing, SmartKem Ltd., St Asaph, Wales

How gases are used in the manufacture of displays is being impacted by new technologies, consumer demand, and the burgeoning China market.

BY EDDIE LEE, Linde Electronics, Hsin Chu, Taiwan

While the display market is no longer enjoying double-digit annual growth rates, it is experiencing resurgence due to increasing customer demands for larger flat-panel displays, OLED and 4K technology, ultra-slim form factor, curved and wearable displays, automotive displays, and more. This growth is particularly conspicuous in China, a late comer to the market, which is now the fastest growing region in display manufacturing.

These new technologies and markets require very large quantities of ultra-high purity bulk and electronic specialty gases and a dependable supply chain for these gases. This article will explore the impact of these technologies, consumer demand, and the burgeoning China market on the gases used in the manufacture of display.

Display market

According to IHS DisplaySearch, in 2014 the global display market saw revenue of $134 billion and is expected to grow 6% in 2015. The demand is being driven in large part due to new technologies and new uses for existing display technologies such as 4K, OLED, curved, and flexible displays.

Gases used in display

This love affair that consumers have of interacting with devices large and small not only increases the volume of displays to be manufactured, it also increases the volume of gases needed to make the displays. In the 20 years since the initial development and commercialization of the first Thin Film Transistor (TFT) LCD display panel, the gases market for the display sector has grown to around $450 million.

As shown in FIGURE 1, display manufacturing today uses a wide variety of gases, which can be categorized into two types: Electronic specialty gases (ESGs) and Electronic bulk gases (EBGs).

Displays 1 Displays 1-2

 

FIGURE 1. Market breakdown for the two types of gases used in display manufacturing. 

 

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Electronic specialty gases (ESGs)

Silane, nitrogen trifluoride, fluorine (on-site generation), sulfur hexafluoride, ammonia, and phosphine mixtures make up 52% of the gases used in the manufacture of displays and are available in both cylinder and bulk supply.

Of the major countries that manufacture displays, Taiwan and China import most of their ESGs while Korea and Japan have robust domestic production of ESGs.

Silane: SiH4 is one of the most critical molecules in flat panel manufacturing. Silane is used for deposition of amorphous Si (silicon), the most critical layer in the TFT transistor.

Nitrogen trifluoride: NF3 is the single largest Electronic Material from spend and volume stand- point for flat panel display (FPD) production. NF3 is used for cleaning the PECVD (plasma-enhanced chemical vapor deposition). This gas requires scalability to get the cost advantage necessary for the highly competitive market. Over 70% of the global capacity of NF3 comes from Korea and Japan.

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Electronic bulk gases (EBGs)

Nitrogen, hydrogen, helium, oxygen, carbon dioxide, and argon make up 48% of the gases used in the manufacture of displays.

Nitrogen: For a typical large TFT-LCD fab, nitrogen demand can be as high as 30,000 Nm3/ hour so an on-site generator, such as the Linde SPECTRA®-N 30,000, is a cost-effective solution that has the added benefit of an 8% reduction in CO2 footprint over conventional nitrogen plants.

Helium is used for cooling the glass during and after processing. Manufacturers are looking at ways to decrease the usage of helium because of cost and availability issues due it being a non-renewable gas.

New technologies and implications for gases

Currently about 20% of smartphones – the ones with lower resolution displays – use a-Si display process. Higher resolution devices and new effects such as curved displays require higher performance transistors and improvements in electron mobility. This can be achieved by switching from amorphous silicon (a-Si) transistors to low temperature polysilicon (LTPS) or metal oxide (MO), also known as transparent amorphous oxide semiconductor (TAOS).

LTPS is used in about 44% of high-end LCD smart- phone displays as it has the highest performance. Due to its higher costs and scalability limitations, LTPS is less suited for large screen displays

Small displays with very high pixel resolution are produced with LTPS. High-definition large displays can be made using MO. Metal oxide semiconductors can remain in an active state longer than traditional LCD and can cut power consumption by up to 90%, which is a huge benefit.

New process requirements

Metal Oxide TFT and LTPS: To meet the changes in technology, N2O, C2HF5, C4F8, BF3, and laser gases are replacing or at least reducing the requirement of NH3, BCl3, and SiH4.

The use of N2O is expected to double from 5,000 TPA (tons per annum) in 2013 to 10,000 TPA in 2017. Why nitrous oxide? The move from a-Si to MO requires a change in the TFT device structure where the a-Si layers (g-SiNx, a-Si, n+) are being replaced by the MO layers (g-SiOx + indium gallium zinc oxide). This requires a change from NH3 to high- volume, high-purity N2O.

LTPS process also uses N2O for its oxide layer deposition. In addition, LTPS uses XeCl (xenon monochloride) excimer lasers for annealing after the silicon deposition to change the silicon structure to polysilicon. High-performance laser gases, such as Ne, Xe, and Kr from Linde, are well-suited for this process.

Transparent Conductive Films (TCF) and ITO Replacements: TCFs are used in most high-tech displays and touchscreens, and particularly in displays that are bent or curved. Currently the electronics industry relies primarily on Indium Tin Oxide (ITO) to make electro-conductive films for display. ITO presents challenges: it is brittle and cracks so new TCFs are needed for structural flexibility.

New materials to potentially replace ITO are metal mesh, Ag nanowire (agNW), and carbon nanotube (CNT), which are all highly flexible with comparable transparency and resistance to ITO. Metal mesh is good for large displays, but is restricted on small and medium displays due to its wire width (typically 6 μm). AgNW demonstrates excellent transmittance and flexibility with small wire diameter (20 – 100 nm), but haze is an issue. CNT has excellent conductivity, transmit- tance, and flexibility, but the supply chain needs to be developed. Single walled carbon nanotubes (SWNT) technology from Linde uses liquid ammonia to produce solubilized carbon nanotubes in the form of inks, which can then be deposited as films and has the added benefit of zero carbon footprint.

F2 as replacement for NF3 and SF6: For a typical large TFT-LCD fab, chamber cleaning gas demand can exceed 300 tons per year. Traditionally NF3 has been used. The GWP100 (100-year Global Warming Potential) for NF3 is 17,200; for the replacement F2, the GWP100 is 0.

Switching to fluorine not only significantly reduces environmental footprint, but also leads to material cost savings and up to 50% reduction in cleaning time, increasing productivity (FIGURE 2). Fluorine can also be used to replace Sulfur hexafluoride (SF6), which is used in dielectric etching. The GWP100 for SF6 is 22,800, which surpasses that of NF3. Significant improvements in etch rate and etch uniformity have been measured with the shift to F2.

FIGURE 2. Switching to fluorine reduces environmental footprint, material costs cleaning time.

FIGURE 2. Switching to fluorine reduces environmental footprint, material costs cleaning time.

On-site fluorine generation, like that available from Linde, eliminates large-volume, high-pressure storage, and modular generators meet all flow and volume requirements for the largest scale fabs.

The China factor

Currently Korea is the leader in display manufacturing, with Taiwan and China on
its heels and Japan a distant fourth (FIGURE 3). This is changing, though, as China rapidly gains market share. China, which started in most traditional manufacturing industries as “factory to the world,” is a relative late comer in the display sector due to technology barriers.

FIGURE 3. Currently Korea is the leader in display manufacturing, with Taiwan and China on its heels and Japan a distant fourth. This is changing, though, as China rapidly gains market share. Source: IHS Displaysearch and Linde Internal.

FIGURE 3. Currently Korea is the leader in display manufacturing, with Taiwan and China on its heels and Japan a distant fourth. This is changing, though, as China rapidly gains market share. Source: IHS Displaysearch and Linde Internal.

Currently there are about five major domestic display manufacturers in China; they cater primarily to domestic mobile display and large screen markets. China has been aggressively investing in display fabs over the last five years and has gained market share from other regions.

It is expected that China will account for more than 50% of display capacity investment in the next four years. China capacity is expected to double with aggressive investments especially in the leading technology Low Temperature Polysilicon (LTPS) and Metal Oxide (MO).

Gas supply issues in China

Bulk gases are produced in China, mostly by large international gas companies. There are domestic producers of some ESGs (NH3, N2O, and SF6); other gases currently are mostly imported.

Silane (SiH4): Silane, primarily extracted as an interim process gas during poly silicon production, is one of the most critical molecules in FPD manufacturing. Chinese producers have a very small capacity of silane as they entered the market late. Considering the need for extensive qualification, technical support to achieve that, and the lack of scalable production base, local Chinese poly silicon producers are not able to offer a complete package and thus China still imports more than 80% of its silane and produces locally only 2% of the global capacity of silane.

The current consumption of silane in China display manufacturing is about 300 TPA, which is 7.5% of the global demand, and is expected to double in the next four years. Considering the complexity of the supply chain, import regulations, and storage requirements, companies are actively moving towards local transfilling and analytical capability.

Nitrogen trifluoride (NF3): Similar to silane, the China display manufacturing consumption of NF3 is expected to double to greater than 2000 TPA in the next four years. Considering the volume used and spend on NF3 and the rapid expansion of FPD manufacturing in China, more production will be done locally to minimize customs duties and to support domestic sourcing requirements. NF3 is relatively easy to qualify for chamber cleaning, but ISO supply to large customers is the biggest challenge since most producers do not have large-scale production and equipped facilities to make NF3 cost-effective to make. This is a major area of investment for local producers.

LTPS, Metal Oxide, and the Increase in Demand for N2O: N2O is a regional and localized product due to its low cost, making long supply chains with high logistic costs unfeasible. Currently, in the region, Korea manufactures about 63% of high-purity N2O, Taiwan about 30%, and China only about 7%. As China leap frogs its display industry into the cutting- edge metal oxide, or LTPS nodes, the demand for N2O will triple from its current requirement to 3,000 TPA in 2017 with the adoption of LTPS and MO.

Enablers of the growth of the China display industry

The key priorities for materials manufacturers to enable the growth of the China display industry are:

  • Commitment to invest in local infrastructure such as as on-site bulk gas plants
  • Localization of production facilities for high-purity gas and chemical manufacturing
  • Collaboration with global materials suppliers for development of new materials

Conclusion

To accommodate the boundless appetite that consumers have for the latest, most innovative, and highest definition displays – both large and small – display manufacturers must partner with gas suppliers to:

  • Identify the most appropriate gas and display technology match-up
  • Globally source electronic materials to provide customers with stable and cost-effective gas solutions
  • Develop local sources of electronic materials
  • Improve productivity
  • Reduce carbon footprint and increase energy efficiency through on-site gas plants

EDDIE LEE is Head of Global Market Development and OEMs Display, Linde Electronics, Hsin Chu, Taiwan

Revenues for flat panel display (FPD) manufacturing equipment are expected to grow for the third consecutive year to reach $9.1 billion, according to IHS Inc. (NYSE: IHS), a global source of critical information and insight. This level of FPD equipment spending, the highest level since 2011, is being driven by new liquid crystal display (LCD) and active-matrix organic light-emitting diode (AMOLED) panel factories targeting both large-area television and smartphone applications.

In terms of technology, spending will be split nearly evenly between amorphous silicon (a-Si) TV and low-temperature polycrystalline silicon (LTPS) smartphone plants, according to the latest IHS Quarterly FPD Supply/Demand and Capital Spending Report, LTPS investments in both 2015 and 2016 are expected to exceed all-time highs. 

“Over the past five years, spending on new LTPS LCD and AMOLED factories has been even more volatile than the overall FPD equipment market,” said Charles Annis, senior director at IHS. “LTPS-related equipment expenditures are now expected to peak in 2015 and 2016, before dropping off again in 2017, Recently announced projects are generating unprecedented levels of LTPS equipment expenditures, including new fab plans for JDI in Japan and Foxconn in Taiwan; expansions of current lines at both Samsung and LG Display in Korea; and new LTPS plants in China being built by AUO, BOE, Tianma and China Star.”

In addition to all the current LTPS fab activity, in 2015 makers continue to invest in a-Si Gen 8 factories targeted at TV applications, mainly in China. Much of this investment is the result of growing demand for large-area panels, which increased 14 percent last year – significantly outstripping capacity growth of 6 percent. This increased demand caused tight supply and firm prices last year, encouraging panel makers to extend capacity expansions. This year large-area demand and supply are forecast to grow at similar rates of 6 percent. Although factory utilization remains at relatively high levels, and there are concerns that growing set inventories will continue to push prices down in the third quarter (Q3) of this year, large-area supply and demand will be balanced for the year.

“Despite the maturing TV market, along with various concerns about the ability of all the new LTPS plants in China to ramp-up smoothly, FPD investment activity remains dynamic,” Annis said. “FPD equipment spending in 2016 is currently forecast to be flat or slightly down. BOE’s recent announcement to build a future Gen 8 factory in Fuzhou, and the world’s first Gen 10.5 fab in Hefei China, suggests that FPD makers still believe that building new factories will continue to lower costs and expand the range of applications.”

Discussion of these topics and more can be found in the IHS Quarterly FPD Supply/Demand and Capital Spending Report. The report covers the most important metrics used to evaluate supply, demand, and capital spending for all major FPD technologies and applications.

Smartwatch display unit shipments are expected to grow 250 percent year-over-year, reaching a record 34 million units in 2015, led by demand for the new Apple Watch, according to IHS Inc. (NYSE: IHS), a global source of critical information and insight. The display market is still assessing the staying power of smartwatch demand, so as not to overshoot display supply needs in the coming year, particularly for the year-end shopping season. Smartwatch display shipments are therefore forecast to decline to about 6.5 million units in the fourth quarter (Q4) of 2015, after reaching a high point of 10.5 million units in the third quarter.

Because both Apple Watch and Samsung Gear rely on active-matrix organic light-emitting diode (AMOLED) panels, that technology will comprise the majority (58 percent) of total smartwatch panels shipped. Based on the latest information from the IHS Quarterly Small/Medium Shipment and Forecast Report, Apple Watch is expected to make up 84 percent of AMOLED smartwatch panels and 49 percent of total displays for smartwatch shipped in 2015.

“Apple Watch has attracted a lot of attention from consumers, which has led to increased demand,” said Hiroshi Hayase, director of analysis and research for IHS Technology. “The display market is carefully watching consumer response to products in the smartwatch category, which should help to improve future display technologies.”

The IHS Quarterly Small/Medium Shipment and Forecast Report covers the entire range of small and medium (9 inches or smaller) displays shipped worldwide and regionally.

Researchers at Lehigh University have identified for the first time that a performance gain in the electrical conductivity of random metal nanowire networks can be achieved by slightly restricting nanowire orientation. The most surprising result of the study is that heavily ordered configurations do not outperform configurations with some degree of randomness; randomness in the case of metal nanowire orientations acts to increase conductivity.

The study, Conductivity of Nanowire Arrays under Random and Ordered Orientation Configurations, is published in the current issue of Nature‘s journal Scientific Reports. The research was carried out by Nelson Tansu, Daniel E. ’39 and Patricia M. Smith Endowed Chair Professor in Lehigh’s Center for Photonics and Nanoelectronics and Department of Electrical and Computer Engineering, and lead author Milind Jagota, a Bethlehem-area high school student.

Transparent conductors are needed widely for flat screen displays, touch screens, solar cells, and light-emitting diodes, among many other technologies. Currently, Indium Tin Oxide (ITO) is the most widely used material for transparent conductors due to its high conductivity and high transparency. However, ITO-based technology has several issues. The material is scarce, expensive to manufacture and brittle, a particularly undesirable characteristic for anything being used in this modern age of flexible electronics.

Researchers searching for a replacement for ITO are increasingly employing random networks of metal nanowires to match ITO in both transparency and conductivity. Metal nanowire-based technologies display better flexibility and are more compatible with manufacturing processes than ITO films. The technology, however, is still in an early phase of development and performance must be improved. Current research is focused on the effect of rod orientation on conductivity of networks to improve performance.

In this work, Lehigh researchers developed a computational model for simulation of metal nanowire networks, which should speed the process towards idealizing the configuration of nanowires. The model predicts existing experimental results and previously published computational results.

The researchers then used this model to extract results for the first time on how conductivity of random metal nanowire networks is affected by different orientation restrictions of varying randomness. Two different orientation configurations are reported.

In the first, a uniform distribution of orientations over the range (?θ, θ) with respect to a horizontal line is used. In the second, a distribution of orientations over the range [?θ] _ [θ] is used, also with respect to a horizontal line. In each case θ is gradually decreased from 90° to 0°. Conductivity is measured both in directions parallel and perpendicular to alignment.

Researchers found that a significant improvement in conductivity parallel to direction of alignment can be obtained by slightly restricting orientation of the uniform distribution. This improvement, however, comes at the expense of a larger drop in perpendicular conductivity. The general form of these results matches that demonstrated by researchers experimenting with carbon nanotube films. Surprisingly, it was found that the highly ordered second case is unable to outperform isotropic networks for any value of θ; thus demonstrating that continuous orientation configurations with some degree of randomness are preferable to highly ordered configurations.

Prior research in this field has studied the effects of orientation on conductivity of 3D carbon nanotube composites, finding that a slight degree of alignment improves conductivity. Computational models have been used to study how percolation probability of 2D random rod dispersions is affected by rod orientation. Others have developed a more sophisticated computational model capable of calculating conductivity of 3D rod dispersions, again finding that a slight degree of axial alignment improves conductivity.

“Metal nanowire networks show great potential for application in various forms of technology,” said Jagota. “This computational model, which has proven itself accurate through its good fit with previously published data, has demonstrated quantitatively how different orientation configurations can impact conductivity of metal nanowire networks.”

“Restriction of orientation can improve conductivity in a single direction by significant amounts, which can be relevant in a variety of technologies where current flow is only required in one direction,” said Tansu. “Surprisingly, heavily controlled orientation configurations do not exhibit superior conductivity; some degree of randomness in orientation in fact acts to improve conductivity of the networks. This approach may have tremendous impacts on improving current spreading in optoelectronics devices, specifically on deep ultraviolet emitter with poor p-type contact layer.”