As e-paper products begin to emerge into the marketplace, it is interesting to examine the roadmap of how these technologies work their way from the development stage into mass manufacturing. Here is a case study that examines how BiNem e-paper displays make use of the existing LCD ecosystem and production lines.

by Jacques Angelé

ONE of the more intriguing developments in display technology in the past decade, electronic-paper displays (EPDs) continue to attract widespread public awareness and business interest, beginning with the pioneering work of Nick Sheridon at Xerox's Palo Alto Research Center in the 1970s and the first steps of the electrophoretic technology invented by Joseph Jacobson (who later co-founded E Ink Corp.) in the 1990s. Electronic-paper displays are competing to achieve the same comfortable reading experience as that of printed paper in ambient lighting conditions, and devices equipped with EPDs are gaining extended battery life due to their "zero power" feature (between image updates). Since 2006, more than ten different models of e-books embedding EPDs were announced, including Amazon's Kindle.

Electronic-paper display technologies are based on a number of surprisingly different physical principles at various stages of technological or industrial process development¹: electrophoretic (displacement of charged colored particles), bistable liquid crystal (using nematic or cholesteric liquid crystals), microelectromechanical systems (MEMS), electro-chromics, ferroelectric, etc. Because of the low cost of printed paper and the diversity of its applications, many feel that we are still a long way from significant diffusion of e-paper devices. However, the technologies are rapidly maturing. Growth and market-size predictions consistently send green lights to move forward. Big players such as Sony and Amazon have entered the game. It may be the right time for content providers, e-paper terminal manufacturers, as well as others to grab opportunities and invent creative economic models to leverage value from this remarkable innovation.

This article will provide a roadmap for how one e-paper technology, BiNem displays, went from concept to prototype to product (Fig. 1). BiNem, which stands for "Bistable Nematics," is a liquid-crystal-based e-paper display technology invented by researchers from the French National Center for Scientific Research (CNRS). The start-up company Nemoptic was founded in 1999 to develop and prepare the commercial exploitation of BiNem displays. BiNem technology has been designed to be compatible with existing LCD production lines, ecosystems, and supply chains to achieve competitive cost compared to other e-paper display technologies. BiNem displays have made substantial progress in improving the economies of scale in this area.

Structure of BiNem Displays

BiNem displays have a simple structure that is quite similar to that of twisted-nematic (TN) or supertwisted-nematic (STN) LCDs. They are sandwich-type cells filled with a nematic liquid-crystal mixture; the main differences are the thinner cell gap (~ 1.5 μm) and the use of a BiNem alignment layer and a BiNem liquid-crystal mixture with specific anchoring properties. Two polarizers are attached to the cell: the top polarizer is transmissive while the bottom polarizer is reflective. Two stable crystal textures exist in BiNem displays, which appear black and white, respectively, in the standard reflective configuration.

The top and bottom substrates are coated with different alignment layers and are rubbed in the same direction. One substrate is coated with the BiNem alignment layer, the other with a conventional polyimide. The BiNem layer gives a nearly planar anchoring with moderately strong zenithal energy, while the polyimide layer gives a tilted anchoring with strong zenithal energy. Both azimuthal anchoring energies are kept strong enough to maintain the azimuthal orientation of the liquid crystal on the alignment layer.

Operation Principles

Passive-matrix BiNem displays can achieve very high resolution (more than 1200 lines for an A4 active area) because Alt and Pleshko's iron law of multiplexing² does not hold for them. Alt and Pleshko's law sets a limit to the resolution of any multiplexed display that respond to the root mean square (RMS) of the applied voltage; for example, it limits STN-LCDs to a maximum practical resolution – video graphics array (VGA). BiNem displays escape the limits defined by the Alt and Pleshko law of multiplexing because this law is only valid for RMS-responding display technologies. A simple example illustrates the non-RMS response of BiNem displays: A rectangular driving pulse immediately preceded by a gradual on-ramp switches pixels in the white state (with appropriate voltage and timing conditions), while the same pulse followed by a similar off-ramp switches pixels in the black state. The two driving waveforms have exactly the same RMS voltage, but drive the display in opposite optical states.

Two stable quasi-planar liquid-crystal textures exist in BiNem displays: the zero-twisted texture U (for uniform) and the half-turn (180°) twisted texture T. The U texture appears black and the T texture white in the standard reflective configuration (Fig. 2). The T texture acts as a medium having a small optical rotatory power: it induces a rotation of only a few degrees of the plan of polarization of the light, while the U texture acts as a simple half-wave plate. The liquid-crystal mixture is doped with a chiral guest to equalize the U and T energies by making the LC spontaneous pitch 4x the cell gap. This compensated cell achieves infinite bistability because it is impossible to transform U to T by continuous bulk deformation in the absence of external fields and defects. The U and T textures are "topologically distinct" – they cannot change each other into the opposite texture by continuous reorientation of the liquid-crystal director inside the cell (with paper and scissors, you would need to "cut" the texture and reconnect it differently to transform it in its opposite); the displayed information will remain stable for an infinite time because the textures cannot transform spontaneously.

The switching between the two textures is obtained by breaking the anchoring of the liquid crystal on the BiNem alignment layer. If the pixel driving voltage is a two-step waveform, the dark state U is induced. If the pixel driving voltage is a single pulse, the cell relaxes rapidly to the bright T texture, after a transient bent state. The texture selection mechanism relies on the magnitude of the liquid-crystal backflow, which is voltage dependant. The texture switching mechanism is illustrated in Fig. 3.

Performance

The typical reflectance of BiNem displays is about 33% at normal incidence in the white-balanced mode and up to 40% in the recently developed high-efficiency mode. The contrast ratio is typically greater than 10 at normal incidence and greater than 4 in a zone extending out to polar angles of 50° with a white paper-like appearance with no discernable color shift over these angles. Typical performance data for BiNem displays are listed in Table 1. A nearly achromatic white state is achieved by BiNem displays, as shown in Fig. 4.

Gray Scale and Color in BiNem Displays

BiNem displays have gray-scale capability. Nemoptic has developed a method to display bistable gray scales called the "curtain effect." This method provides up to 32 different stable grays at the pixel level. The gray levels appear to be spatially uniform to the eye when viewed normally, but magnification shows they are composed of microscopic white and black domains. This method is similar to the spatial gray scales widely used in ink-jet or offset printing technology to obtain gray levels on physical paper. It involves the control of the liquid-crystal flow inside the pixels. When the driving voltage is switched off, a liquid-crystal flow is created. The part of the flow that has a speed higher than a threshold value produces the bright T texture, the other one produces the dark U texture. With suitable driving voltages, it is possible to continuously adjust the spatial extension of the two domains that fill the pixel; one in the T texture, the other in the U texture. A constant texture filling ratio is directly perceived by the eye as a spatially uniform gray level.

Color BiNem displays are built with color-filter substrates. Color BiNem displays driven with the "curtain effect" gray levels can display large numbers of different colors (up to 32,768 different colors have been demonstrated). The RGBW configuration (each pixel is composed of red, green, blue, and white dots) has been recently introduced in a number of conventional and e-paper display technologies to improve color display performance.³ The quad subpixel structure has been implemented in a BiNem display module to improve the brightness while keeping a good level of contrast and reasonable color purity in reflective mode. A 5.1-in.-diagonal 400 x 300-pixel color BiNem display with 100-ppi resolution has been recently developed (Fig. 5). It achieves ~20% reflectance (white state) with a maximum contrast ratio exceeding 10:1 under diffuse illumination. Optimization of the brightness/color-saturation trade off is in progress.

Fig. 4: (Left) Measured iso-reflectance contour of a BiNem display in diffuse light. Reflectance in diffuse light at normal incidence is ~ 35%. (Right) Experimental determination of the hue of the bright-state BiNem display in the CIE 1976 Chromaticity diagram It lies inside the iso-perception circle centered at the ideal white point.

Compatibility with Flexible Substrates

Flexible e-paper displays answer the need for thinner devices with higher information content and larger display size. Flexible displays bend but do not break under moderate mechanical stress, an important feature for nomadic applications. Flexible BiNem display samples based on polyethersulfone (PES) substrates with barrier layers have already been demonstrated with satisfactory results in the lab.⁴ Because of their simple passive-matrix structure, flexible BiNem displays do not do suffer from the technical or economical drawbacks caused by the implementation of organic or silicon-based TFTs on flexible substrates in other e-paper technologies.

Applications for BiNem Displays

Electronic shelf labels (ESLs), point of purchase, and promotional displays are major applications for BiNem displays. Graphical ESLs are a large market segment enjoying rapid growth because dot-matrix designs are needed to implement larger amounts of product-related information, such as product names, origin, and barcode. Such barcodes can be read by optical barcode readers as well as by mobile phones with camera capability. BiNem ESLs display barcodes in the labels in order to increase automation in price- and product-logistics management at the supermarket or store level. The contrast of a BiNem display remains large regardless of the resolution, and the brightness is relatively constant, depending on the viewing angle.

Display sizes range from less than 3–10 in. on the diagonal or more, with HVGA or higher resolution for large-sized displays. These displays are used in an environment where cost is challenged on an everyday basis, and the price premium for additional functionalities has to remain modest. BiNem electronic-paper technology provides these features by being able to use existing LCD ecosystem and manufacturing plants.

Electronic-newspapers and e-books are high-growth high-potential markets. Large-sized high-resolution BiNem displays can provide the required high information content with excellent legibility. In addition, the simple passive-matrix structure significantly reduces the cost of customization of the BiNem module to the specific sizes and resolutions required by the application.

The potential applications of A4-sized 200-dpi electronic-paper displays are huge, especially if the display has some degree of flexibility. A4-sized BiNem displays can offer multimillion-pixel resolution without active-matrix backplanes, bringing significant economic and technical advantages compared to technologies requiring organic or silicon-based TFTs on plastic substrates. These features make BiNem flexible displays an attractive solution for large-sized high-resolution cost-effective e-document applications.

From Lab to Fab

The original BiNem displays invented by Durand, Martinot-Lagarde, and Dozov⁵ included a special alignment layer made by vacuum evaporation of inorganic material under oblique orientation. This method required the use of specific R&D manufacturing tools and was only applicable to small-sized substrates because a uniform and precisely controlled grazing evaporation direction (15±0.5°) was required over the substrate area; this condition is incompatible with the angle deviation induced by a substrate size exceeding a few inches for a reasonable distance between the source and the substrates.

Nemoptic decided to develop a new BiNem manufacturing process that was completely compatible with that of STN-LCDs (Fig. 6). This approach reduces significantly the level of investment required to address volume markets because there is no need to finance dedicated production infrastructure from zero. Volume production is performed by LCD-manufacturer partners after adjusting their manufacturing processes to meet the BiNem process requirements. These adjustments are light and reversible, and the line configuration is unchanged – runs of BiNem and STN displays can be launched the same day. Competitive costs result from the use of vested and mature infrastructures.

The cost structure for BiNem displays is similar to that of STN-LCDs, and the LCD supply chain is directly applicable. BiNem displays benefit from the low-price/high-volume availability of LCD base materials (substrates, liquid-crystal mixtures, orientation layers, optical films, and spacers) and electronic components (drivers, controllers, and power supply). They are compatible with any standard driver packaging (TAB, TCP, and COF). Moreover, BiNem displays do not require an active-matrix backplane.

Manufacturing Process

From a manufacturing point of view, two main differences exist between BiNem and conventional LCDs: (1) the alignment layer and liquid-crystal mixture have special anchoring-energy properties and (2) the cell gap of BiNem displays (1.5 μm) is lower than that of LCDs (about 5 μm).

LCD production lines usually use flex-printing to transfer thin layers of polyimide alignment material on the substrates. Nemoptic has focused its efforts to develop weak-anchoring-energy alignment materials that are as simple to process as conventional polyimide alignment materials.^6,7 These efforts have led to the development of a polymeric BiNem alignment material optimized for flex-printer deposition and achieving the desired azimuthal/zenithal anchoring energies. This result was a major achievement that opened the way for volume production.

Anchoring properties depend on both the alignment material and the liquid-crystal mixture. Recent research at Nemoptic has focused on improving the LC mixtures to achieve better properties for this application, including addressing rate, response time, operating temperature range, driving voltage, optical constants, and reliability. Because of this, our new BiNem liquid-crystal mixtures usually contain more than 10 pure nematic compounds. The specific details of these compounds are beyond the scope of this article.

The cell gap of a BiNem display is fixed by the diameter of the spacers dispensed on the substrates. The manufacturing process uses the same standard spray-dispensing machines and assembly equipment as LCD manufacturing. The only difference is the use of spacers with lower diameter (about 1.6 μm). To prevent particle contamination during panel production, front-end process steps are carried out in the local environment, and cleaning steps such as wet cleaning and dry ultrasonic decontamination are used. Fine cell-gap adjustment and low panel-to-panel dispersion is achieved by adequate calibration of spacer density, pressure, and time parameters during end-seal.

The ready-for-use BiNem alignment solutions are printed on patterned ITO glass plates. The rubbing step is performed by using industrial rubbing machines that utilize a commercial velvet rubbing cloth. The assembly process is identical to the standard LCD process and uses thermal-curing sealant deposited using screen printers.

For front-end industrial processes, the key equipments are high-throughput printing machines, rubbing machines, assembly machines, and cleaning machines. The major processing factors are excellent surface cleanliness, low particle contamination, and excellent cell-gap and thickness-layer control. The back-end production process is unchanged compared to that used for conventional LCDs.

The e-paper market is developing fast. To play a central and active role in this market, Nemoptic has established a high-volume source of BiNem display modules with its manufacturing partner Seiko Instruments, Inc.

Nemoptic has targeted the electronic-shelf-label (ESL) market as one of its priorities because it combines simple b/w design and high-volume needs. Several display modules for small- and medium-sized ESLs are already in production ramp-up. Building up manufacturing experience and stimulating market demand is important in preparing new high-potential e-paper applications that require color, flexibility, higher resolution, and faster response time. Nemoptic is confident that its BiNem display technology will sustain the strong growth and enjoy the multiple business opportunities that are going to open up in e-paper applications.

References

¹P. S. Drzaic, "Reflective Displays: The Quest for Electronic Paper," SID Seminar Lecture Notes (2006).
²P. M. Alt and P. Pleshko, "Scanning limitations of liquid crystal displays," IEEE Electron Devices ED-21 (1974).
³B-w. Lee et al., "TFT-LCD with RGBW Color System," SID Symposium Digest Tech Papers 34, 1212-1215 (2003).
⁴C. Barron, J. Angelé, L. Bajic, I. Dozov, F. Leblanc, and S. Perny, Proc. Asia Display/IMID '04, 16.2 (2004).
⁵U.S. Patent No. 6,327,017 B2 (2001).
⁶I. Dozov et. al., "Recent improvement of the bistable nematic displays switched by anchoring breaking," SID Symposium Digest Tech Papers 32 (2001).
⁷P. Martinot-Lagarde and I. Dozov, SPIE Proc. 5003, 25-34 (2003).
⁸J. Angelé et al., "Development of 5.1-in. High-Speed SVGA Bistable BiNem Display for Electronic Document Applications," SID Symposium Digest Tech Papers 37, 1634-1637 (2006). •