Flexible Displays Made with Plastic Electronics

Plastic Logic has designed and constructed a full-scale manufacturing facility for flexible display modules fabricated using organic semiconductors ("plastic electronics"). These display modules are lightweight, flexible, and robust and are used in the QUEproReader, an e-reader device for mobile business professionals that was introduced in January 2010.

by Seamus Burns

FOR MANY YEARS, there has been healthy market interest in flexible displays because they will enable display products that are thin, lightweight, robust, and ultimately rollable or foldable. These products and related technologies are finally starting to enter the mainstream after a long period of development. Display media technologies such as those of E Ink Corp. and SiPix Imaging have come of age in the last 6 years and are a key enabler for flexible displays. Generally referred to as e-paper, these technologies indeed share many of the properties of paper in that they are reflective and flexible and their images remain in a non-power-consuming state between image updates (a property known as image stability). They are commercially available in high volume with consistent electro-optical performance and in a form that is easy to integrate by directly laminating the display media foil onto the display backplane. Although these display technologies have been mainly used on rigid glass displays, they have also opened the door to high-resolution flexible displays and to products that use them.

Another complex component, in addition to the e-paper display media, that is needed in order to make a high-resolution flexible display is a flexible active matrix. An active matrix is necessary for the majority of fast-update high-resolution displays: it is an array of electrodes and tiny electronic switches that is normally formed by multiple vacuum deposition and photolithography steps. These processes for deposition and patterning work well on glass – indeed, almost all displays in mobile phones, laptops, LCD TVs, and many other electronic devices are addressed by an active matrix built on a glass substrate. However, these deposition processes transfer poorly to flexible plastic substrates, and this has been a barrier to the implementation of flexible displays.

Flexible Prototypes

Flexible active-matrix display demonstrators have been fabricated by university laboratories and by corporate R&D departments many times since 1999 when Professor Ernst Lueder and his team at the University of Stuttgart made a flexible active-matrix liquid-crystal display (AMLCD) on a poly(ethersulfone) substrate. However, industrialization of these techniques has been less straightforward, despite the wealth of research and development in all aspects of flexible active-matrix manufacture and the intense market interest. There have been very few examples of products with flexible displays. However, a number of technologies are maturing that will enable flexible-display products to be available in the near future.

Recently, the Dutch company Polymer Vision announced the Readius, a beautiful and pioneering e-reader comprising a roll-out flexible e-paper display. This device is unique in that it is pocket size when its 5-in. display is rolled up. It was also to be the first consumer-electronics product to use organic thin-film transistors (TFTs). However, the Readius has yet to be made available to consumers.

Other approaches that are near commercialization are typically based on the adaptation of silicon display-making processes. Prime View International, a pioneering e-paper display-module manufacturer, has a flexible-display process based on laser release known as EPLaR. A very thin polymer film is deposited on a rigid substrate and conventional silicon processes are then used to form the active-matrix display. The polymer layer is then released from the rigid substrate by laser processing. Another company developing a flexible-display technology is LG Display, which has a process for flexible-display manufacture that uses thin, flexible, stainless-steel foil. This material can withstand the temperatures needed for silicon deposition and is sufficiently stable to allow mask alignment. For several years, LG Display has showcased flexible electro-phoretic display modules in a range of shapes and sizes, including 12-in.-diagonal displays with a resolution of 1600 ´ 1200 and 174 ppi. Also worthy of mention is AUO, which demonstrated flexible e-paper displays in October 2009 with a 6-in. flexible display module using SiPix electrophoretic display media. However, the company has said little about the underlying technology used.

Organic semiconductors

Spun out from the University of Cambridge in 2000, Plastic Logic was created to commercialize advances in organic semiconductors that allowed TFTs to be printed on flexible plastic, a route to flexible electronics. Organic semiconductors enable flexible electronics by two means. First, unlike silicon, organic semiconductors can be deposited from solution at room temperature. Flexible plastic can therefore be used as a substrate material, as opposed to silicon deposition in which the temperatures required would cause the plastic substrate to melt or distort. Second, organic semiconductor devices can be formed without using mask alignment. Conventional silicon electronics require the subsequent alignment of a series of masks. This cannot be done in a straightforward way on a plastic substrate. Shifts in temperature and solvent absorption cause the substrate to distort between mask steps. With organic materials, mask alignment can be avoided by using print-based processes, which overcome distortion in plastic substrates. This is achieved by locally aligning the print heads to compensate for distortion while performing a print step. A further advantage of organic materials when used for flexible electronics is their inherent flexibility.

The Plastic Logic process is capable of depositing high-resolution electrical components on flexible plastic. The processing steps are at or near room temperature, so they are fully compatible with inexpensive plastic substrate materials. The standard substrate material used is poly(ethylene terephthalate) or PET, a very common plastic otherwise used in food and drink containers and synthetic fibers. The process can deposit features of down to a minimum size of 2 μm. The layer-to-layer alignment accuracy is typically ±5 μm. This is illustrated in Fig. 1.

Fig. 1: An active-matrix backplane array made on a flexible plastic substrate appears in a close-up view. Print-based processes are used to compensate for substrate distortion by locally aligning the print head during a patterning step. "A" is the feature size, which can be as low as 2 μm. "B" is the layer-to-layer registration accuracy, which is typically ±5 μm.

An active matrix allows displays with a very high resolution that have a comparatively low number of connections. It consists of an intersecting grid of row and column electrodes. At each intersection there is an electrical switch, typically a TFT. The transistor is used to move charge on or off a capacitor, which changes the voltage across the display medium and, in turn, changes the optical state of the medium. An active-matrix display is ideally capable of addressing the display media such that it achieves its optimum contrast ratio and refresh rate, as though the medium were directly driven.

In order to meet these requirements for an e-paper device, a number of electrical criteria need to be satisfied. The TFT "ON" current needs to be sufficiently high to move charge on to and off of the pixel during the line address time. This is governed by the transistor mobility. The TFT "OFF" current needs to be low so as to maintain the pixel voltage once the gates are turned off. Achieving a low OFF current requires a stable TFT threshold, which is determined by the purity of the organic semiconductor and dielectric materials. Control of process conditions is vital here. Gate leakage needs to be minimized, along with any other spurious inter-electrode electrical leakage path (e.g., source-to-gate leakage; source-to-common leakage). Pinhole-free dielectric layers with a high dielectric breakdown voltage are a critical requirement to avoid these leakage paths. Again, control of process conditions and the use of a clean manufacturing environment are prerequisites. The gate and source lines need to be sufficiently "fast," i.e., able to transmit changes in voltages quickly, requiring a high conductivity and a low capacitance. Minimizing capacitance requires fine-feature-size definition, as discussed earlier. Another desirable characteristic is to have a low kickback voltage that is consistent across all pixels. The kickback voltage is the voltage offset between the pixel electrode voltage and the data line voltage that is introduced when the gate line turns off. Because it is related to the overlapping area of electrodes, minimizing this parameter again requires a process with fine feature sizes.

The above-described process is capable of making flexible, active displays that are sufficiently large and of sufficiently high resolution to be used in e-paper display applications. The organic semiconductor used is a poly-fluorene-based material that is solution-processable and has a typical mobility of 0.03 cm²/V-sec. This allows for ON currents approaching 1 μA. The ON/OFF ratio of devices made with this process is 10⁵–10⁶, and the gate leakage is below 10–11 A. The first commercial display modules produced in the Plastic Logic factory have a resolution of 1280 ´ 960 and a diagonal dimension of 10.7 in. The pixel density is 150 ppi, sufficient for monochrome and gray-scale e-reader applications. The display medium used is Vizplex electrophoretic foil from E Ink Corp. This display is engineered to exploit the full contrast and speed of the medium for mono-chrome and gray-scale modes. An image of a display made with this process is shown in Fig. 2.

A New Flexible-Display Facility

Plastic Logic built a full-scale manufacturing facility in Dresden, Germany, in order to produce high volumes of display modules cost effectively. Dresden is one of the high-tech hubs in Germany and is host to a large number of electronics companies, among them silicon-wafer fabs. The presence of these companies has ensured the necessary support infrastructure to fabricate electronic devices such as display modules, as well as providing access to an educated and experienced workforce. The equipment for the fully automated line was sourced mainly from the Far East, typically, but not exclusively, from the flat-panel-display industry. In many cases, it was "off-the-shelf" or required only minor customization. The line is housed in a Class-100 clean room. Manufacturing is done by batch processing with the flexible PET substrate mounted on a rigid glass carrier during patterning. The substrate size is equivalent to Gen 3.5. A series of patterning steps are performed to produce nine active-matrix displays per mother substrate. The active matrices are then tested for pixel and line yield prior to lamination with the display media. The yielded displays are then encapsulated to maintain a constant level of humidity throughout product lifetime and ensure consistent display media operation. Flexible encapsulation is feasible because organic TFTs are not highly sensitive to oxygen and moisture, unlike OLED or PLED devices. This is followed by the bonding of high-voltage display drivers and outer lead bonding, in which the connections are made with aniso-tropic conductive film (ACF). Finally, the touch sensor is laminated onto the display module, which also comprises optical coatings and UV barriers.

Fig. 2: This flexible display module made with plastic electronics in the Dresden facility in late 2009 has a resolution of 1280 ´ 960 and 150 ppi. The display media is Vizplex provided by E Ink Corp.

The QUEproReader

The current generation of e-reader devices can be traced back to the SonyLIBRIé, which was launched in 2004. This was the first e-reader with an electrophoretic display and was considered sufficiently groundbreaking to win the SID/Information Display Display Product of the Year Award in 2005. Since then, a plethora of e-reader devices with different sizes, weights, user interfaces, and intended purposes have been introduced. The majority of these devices were designed primarily for reading books, usually with additional functionality for storing, purchasing, downloading, and otherwise facilitating book reading.

Figure 3 shows an image of the QUE-proReader from Plastic Logic. QUE is specifically designed as a reader for the business professional. It supports the document formats PDF, GIF, JPEG, PNG, BMP, ePub, text, and printable formats such as Microsoft Office (2003/2007), e-mail, calendar, HTML (e.g., maps), and RTF and can handle a file cabinet's worth of customer-generated documents. It also features powerful tools for interacting with and managing content. In addition, the QUE store allows users to buy and download business and professional newspapers, periodicals, and eBooks, with access to over 1 million eBooks through Barnes & Noble. Books and newspaper content can be downloaded via WiFi or (in the 3G version) via a cellular network.

QUE has been designed to differ from other readers in that it is aimed at the business market, but it's also unique for its shatterproof plastic display, which also makes it ultra-thin and lightweight. It is the size of an 8.5 x 11-in. pad of paper, measuring about 1/3 in. thick. It weighs about 1 pound. Reliability tests have confirmed that the display is more resilient to being dropped and to objects being dropped onto the display than are equivalent glass-based products. QUE also has the largest capacitive touch screen in the industry.

Technology Outlook

Future products that will require more sophisticated displays that fully exploit the rapidly evolving properties of e-paper display media are being developed by Plastic Logic as well as other companies. Most of the companies developing these media are engaged in a color program, and it is expected that full-color e-readers will become available in the next year. Plastic Logic is currently working on a flexible-display platform for a full-color e-reader device.

Color displays have a higher ppi than monochrome displays, requiring a higher performance and higher-resolution display. This, in turn, leads to requirements such as higher-conductivity gate lines, higher-performance TFTs, and finer levels of patterning. Plastic Logic's process development for the next generation of displays is undertaken in the development line in Cambridge, UK. Here, there are programs to improve backplane performance and meet these higher requirements. Next-generation TFT devices use a new organic semiconductor materials set with a mobility approaching that of amorphous-silicon. Displays with a higher number of pixels per inch, required for color and the necessary TFT performance, have been fabricated with these improved processes. Once fully proven on the development line, these materials and processes will be transferred to the manufacturing facility, where they will be scaled up, verified, and ultimately incorporated in the display-making process.

Flexible displays are comparatively an early-stage technology, and their huge potential remains to be exploited from the possibilities of organic TFT devices to the adaptation of more conventional electronics. The plastic substrate can be used to house progressively more of the system electronics, either by directly bonding discrete electronic components or by replacing silicon high-voltage display drivers with printed electronics. The latter was demonstrated in concept in 2004 by Polymer Vision (then Philips Research Laboratories), which integrated part of the gate-driver display circuitry onto a flexible display. Ultimately, it is anticipated that organic TFTs will be powerful and stable enough to drive current-driven media such as OLEDs and PLEDs in consumer products. This, in turn, would make possible a flexible emissive display with integrated high-voltage drivers fully manufactured using printing processes. There are huge challenges that need to be overcome in order to allow this possibility; however, given the pace of development in this field, it is easy to imagine this happening by the middle of this decade.

This is to say nothing of the range of non-display applications that may use printed electronics. RFID is one of the expected applications, in circumstances where a small number of electrical gates are needed, and it is less economically viable to use silicon. Flexible plastic sensors are another possible application, one that resembles an active-matrix display in architecture. There are further opportunities associated with disposable and "on-the-fly" configurable electronics.

Ten years ago, plastic electronics was a nascent field focused on materials research in a handful of universities and corporate research laboratories. This technology has since been shown to be viable for use in cutting-edge display products. In the coming years, we can realistically expect significant further advances in the commercialization of this expanding field. •

Fig. 3: The QUEproReader from Plastic Logic was introduced at CES in Las Vegas in January of 2010. The company says it will ship in April 2010.