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General
Flat Panel Display Technologies
Flat panel displays are utilized in a multitude of products and applications. However, each one has different technical requirements and specifications for an electronic display. A cash terminal (ATM machine) for example requires a display with a very narrow viewing angle to protect privacy, while a television set should ideally have a wide viewing angle in order to allow multiple users to enjoy the program. A single display technology meeting all technical requirements does not exist – each one has its pros and cons. This chapter gives an overview. One can roughly divide displays into self-emissive displays, which act as a light source themselves, and non-emissive displays which need an external light source to function. Moreover, a display is characterized by a light modulation or -generation technique and a driving scheme (active or passive matrix driving; AM and PM). AM and PM can therefore be used in conjunction with different light modulation or -generation techniques. Currently available flat panel display (FPD) technologies are shown in the figure on the right.
More details, pictures and a glossary of terms can be found in the DFF Competence brochure "European Technologies: Flat Panel Displays".(PDF, 3.4 MB)
Liquid Crystal Displays (LCD)
Liquid crystals were discovered already in 1888, but it took about 80 years before the materials and electronics were advanced enough to practically use them. Back in 1971, the twisted nematic cell (TN-cell) was invented by Martin Schadt and Wolfgang Helfrich, two researchers from Switzerland. The TN-cell is currently the most widely used type for active matrix LCDs.
A TN-cell consists of two parallel glass substrates, typically only 0.7 to 0.35 mm thick, which are coated with optically transparent, electrically conductive films of Indium-Tin-Oxide (ITO) on their inner surfaces. These ITO films form electrodes which are coated with a transparent orientation layer made from an organic material (e. g. polyimide, only several nanometers thin). Between these films sits a mixture of liquid crystals. On their outer sides the glass substrates are coated with polarizer films, perpendicularly aligned.
LCDs are non-emissive and the liquid crystal cell acts as a “light valve”. In the transmissive mode light sources are backlights (e. g. cold cathode fluorescent lamps [CCFL] or LEDs), while the reflective mode uses ambient light reflected by a mirror foil behind the display. So-called transflective LCDs (often used in cell phones or car stereos) use both light sources.
In a TN-cell, the optically birefringent liquid crystals cause a rotation of polarization in the incident light by about 90 degrees when the cell is activated by an electric field applied through the ITO-electrodes. When the cell is inactive, the light passes through without modification (a so-called normally-white cell). Vice versa, by using parallel polarizing films on either side of the display, one obtains a normally black cell, which allows the light to pass only when an electrical field is applied.
Early displays for calculators or wrist watches mostly used the TN mode. In 1984 the supertwisted nematic (STN) mode was invented, which vastly improved the contrast ratio. In this mode the LC molecule twists the polarization plane of the light by 270 degrees. The response characteristic
of the material is steeper resulting in a better black and white appearance of the display,
compared to TN materials.
Different developments (cf. e. g. MLA, DSTN, FSTN, CSTN) significantly improved the display performance and even made the other LC phases, smectic (“soap-like”) and cholesteric (“cholesterol-like”) LC, usable for display applications. Common disadvantages of LCDs like narrow viewing angles, slow response times, temperature and shock sensitivity are steadily being improved.
The LCD technology is an established, mature technology for a broad range of applications. The new Generation 5, 6, and 7 production fabs enable the production of large screen sizes (up to 52 inches) in diagonal. For desktop PC monitor applications LCDs are already the dominant technology. The large LCDs have the potential to replace the cathode ray tubes (CRT) in the TV sector as well, if lower manufacturing costs can be achieved.
Passive matrix driving scheme
Early displays which had only a few elements were built with segments or pixels connected together electrically in a multiplexed manner. (Pixels are the crossing points of “data columns” and “scan rows” in dot matrix displays.) This allowed for a reduction in the number of driver chips and connecting wires, while giving reasonable performance for small displays.
A big advantage of these passive matrix (PM) displays is their low-cost producibility which facilitates a broad range of applications. Most segment 8, alphanumeric and small graphic displays are using this multiplexing scheme. However, with increasing numbers of row lines to be multiplexed, the contrast of the displays decreases because the driving voltage is present less time at a single pixel. Without a storage element the driving voltage drops very fast.
Active matrix driving scheme
With the larger number of lines multiplexed together, the liquid crystal material is not driven often enough, and starts to relax back into its natural state. Furthermore, passive driven displays have a rather long response time (>100 ms), resulting in a “smearing effect” when images change quickly. The solution to this is to change from passive to active matrix driving – one that addresses each pixel separately. The commonly used method today employs a thin film transistor (TFT) for each pixel. Silicon transistors directly integrated into each pixel amplify the driving signals, whereas the voltage is stabilized by capacitors between the drive pulses. The AM driving scheme allows the display material to be constantly driven, with the transistor circuit providing a short-term memory for the image-state of the display.
Silicon types used for active matrix displays
The most common type of active matrix display in use today is one fabricated from hydrogenated amorphous silicon (a-Si:H) deposited on a glass substrate. It is used in most high-performance notebook computers, monitors, and even in handheld video games for children. However, the size of crystalline grains in amorphous Si is rather small, which results in rather low electron mobility and therefore big transistors and large pixels. The demand for higher resolutions and therefore smaller pixel sizes especially for small high-density camcorder and projection applications has driven efforts to manufacture TFTs with polycrystalline silicon (p-Si). Low Temperature Poly Silicon (LTPS) is commonly prepared by recrystallizing a pre-deposited amorphous silicon layer by an Excimer Laser Annealing (ELA) process. There has been a considerable development in the field of microdisplays which have to utilize even smaller transistors with a single-crystal silicon substrate. They integrate display material directly over the top of the silicon. The technology was enabled by a combination of events in the integrated circuit industry:
- significant reduction of cost for processed silicon wafers,
- greatly increased number of circuits that can be put on an individual die,
- improvement in yield of these large die, and
- inclusion of chemical-mechanical-polishing (CMP) as a standard IC process.
Plasma Display Panels (PDP)
One technology that has been very successful for large-format displays is the Plasma Display Panel (PDP). This technology has the benefits of a Cathode Ray Tube (CRT), but can be built in a much thinner structure. Plasma Displays are typically filled with a gas such as neon, and driven in a rowcolumn passive-matrix manner. They require high voltages to ignite the plasma, and careful current limiting to prevent display heating. Since the actuation mechanism ionizes gas at each pixel, PDPs create radio frequency emissions, which must be carefully controlled.
PDP technology is important for large area viewing, but due to the size limitation of the plasma channels, small high-resolution displays cannot be realized, so PDPs will not be significant for portable and handheld devices in the future. Manufacturing costs will decide which technology, LCD or PDP, will be the winner for the large format display market (e. g. TV sets).
Vacuum Fluorescence Displays (VFD)
VFDs are an established technology still widely used as low information content displays in audio-/video devices or household appliances. The VFD technology uses the fluorescence of phosphors under electron bombardment similar as in cathode ray tubes (CRT). However, the device structure is quite different from CRTs and resembles the classical triode: Electrons evaporate from the metal cathode, a filament with around 10 µm thickness. They are accelerated by a grid voltage around 50 V. VFDs can be easily identified by the honeycomb structure of that grid which is fabricated by etching a very thin steel foil. As soon as the electrons penetrate the anode at around 100 V, light is being emitted. VFDs are robust, reliable, with a high contrast ratio and long life span. One disadvantage is their large spatial dimensions compared to the active display area.
Electroluminescence Displays (ELD)
ELDs have a very simple device structure and can entirely be built employing solid state thin film technologies. Between two electrically conducting slabs (e. g. glass with structured ITO stripes in matrix configuration) with applied insulating layers a thin electroluminescent layer is deposited. Doped zinc sulfate ZnS, or strontium sulfate SrS with a rather broad emission spectrum (“white”) are used as EL compounds. Conventional color filters generate RGB colors. With the EL layer being only about 100 µm thick, fully transparent displays, like for OLEDs, can be achieved. Typical driving voltages are chosen around 200 V AC at up to 10 kHz which necessitates rather expensive driver ICs. With an AM driving scheme (AMEL) employing a transistor matrix on a silicon substrate, high-resolution microdisplays have been demonstrated.
Light Emitting Diodes (LED)
(Inorganic) Light Emitting Diodes (LED) are widely used as large-area video walls or displays for tickers. These LED displays are commonly monochrome or multicolor and are composed of commercially obtainable LEDs. Meanwhile high-efficiency blue LEDs are available, making full-color large-area LED displays possible. LEDs exhibit high luminescence, high efficiency and long life time, which makes them particularly attractive for outdoor use. However, LEDs are rather spacious. Therefore, medium-sized displays for monitors or PDAs are not feasible with this technique. Monolithic integration of LEDs on a single chip, however, can be used for virtual (monochrome) displays.
MEMS (DMD)
Another microdisplay-oriented technology is based on Micro-Electro-Mechanical Systems (MEMS). In these types of displays, silicon and other materials are machined using standard semiconductor processes to make miniature mechanical structures. In the case of a Digital Micromirror Device (DMD), the structure is a mirror supported by a hinge, which can be actuated by placing a charge on plates connected to an underlying memory cell. The size of each mirror is about the width of a human hair. This device has gained acceptance widely in portable business projectors and home theater projectors.
Field Emission Displays (FED)
In an effort to create a thin CRT display, several companies have been developing Field Emission Displays (FEDs). FEDs resemble thin CRTs, but without the heating element in the cathode; in addition, they are organized in a one cathode per pixel passive matrix organization. Like the Plasma Displays Panels, FEDs typically require a high voltage to operate, anywhere between 200 V and 6 kV. These displays can be very thin, but thus far the production costs of manufacturing facilities have kept them out of mainstream commercial products.
Organic Light Emitting Diodes(OLED)
One of the most promising display technologies to come along in the past 25 years are Organic Light Emitting Diodes (OLEDs). Light emission from thin films of small molecule organo-metallic compounds was first discovered by Kodak in 1987. Three years later, a research group at Cambridge University, UK, observed similar properties in conjugated polymers, consisting of long carbon chains with alternating single and double or triple bonds. Meanwhile, oligomers and dendrimers are also utilized as OLED materials.
An OLED is made from a stack of organic layers, forming a p-n junction, similar to an inorganic LED. When a voltage is applied in forward direction, light is emitted from the region where injected holes and electrons recombine. As the organic material is very susceptible to water vapor and oxygen, thorough encapsulation is indispensable.
OLEDs are self-emissive, highly efficient, and show excellent optical properties. They have high potential to be mass-produced on flexible substrates which would enable processing in a roll-to-roll manner. Moreover, the possibility to simply print the organic material makes fabrication very inexpensive.
A wide range of applications from simple monochrome large-area lighting to full color, videocapable graphic displays can be covered by OLED technology. Commercialization started in 1999 with the introduction of a multicolor OLED display in a car stereo.
Emerging Display Technologies
Mark Fihn, Veritas et Visus
E-paper
Several technology candidates have laid claim to the e-paper nomenclature, including electrophoretic, electrochromic, electrowetting, cholesteric liquid crystal, as well as some other novel display approaches. A quick summary of these various approaches:
The most successful e-paper technology to date is used in electrophoretic displays. This means that visible images are formed by rearranging charged pigment particles using an applied electric field. Perhaps the most well-known manufacturer of electrophoretic displays is E Ink, whose frontplane technology utilizes tiny titanium dioxide particles that are dispersed in a hydrocarbon oil. Pigments and charging agents are added to the oil, giving the particles an electric charge. When a voltage is applied, the particles migrate to the surface bearing the opposite charge from the particles. Images can be created by way of an active or passive matrix backplane by applying appropriate voltages to the display, creating patterns of reflecting and absorbing light.
Another approach to electrophoretic displays has been developed by Sipix. SiPix e-paper is created by inserting electrically-charged white particles and dielectric fluid within SiPix’s Microcup design. Once the e-paper is laminated to a patterned conductor with adhesive, the e-paper display may be driven. The Microcups serve the same function as the microcapsules employed by E Ink.
A third approach to create color electrophoretic displays was recently demonstrated by Bridgestone using their Quick-Response Liquid Powder Display (QR-LPD).
Still in startup mode, Zikon has developed the proprietary REED technology (Reverse Emulsionbased Electrophoretic Display). The REED technology takes advantage of properties associated with self-assembled nano-droplets in electrical fields. The technology takes advantage of two phases: a continuous phase which is non-polar and insulated; and a dispersed phase, which utilizes self-assembled nano-droplets, polar dyes, polar solvents, and surfactants. REED can also take advantage of chiral-electric properties, whereby the droplets are polarizable at certain frequencies.
Electrophoretic solutions suffer from limitations associated with motion video and full color rendering, although all of the manufacturers are developing solutions to address both of these limitations.
At least three different e-paper technologies have been developed based on the concept of electrochromism. Electrochromism is a chemical reaction whereby colors are changed reversibly when a charge is applied. An example of an electrochromic material is polyaniline which can be formed either by the electrochemical or chemical oxidation of aniline. If an electrode is immersed in hydrochloric acid which contains a small concentration of aniline, then a film of polyaniline can be grown on the electrode. Other electrochromic materials include tungsten oxide (WO3), which is the main chemical used in the production of electrochromic windows or smart windows. Several companies, including Ntera, Acreo, Siemens, and Aveso have developed electrochromic displays, but so far they have not ventured into color e-paper displays, focusing instead on small monochrome devices for smart cards and similar devices.
Electrowetting technology also allows much faster switching speeds than most other electronic paper technologies, enabled by use of selectively charging hydrophobic and hydrophilic materials. The technology is particularly compelling for flexible substrates because there are no moisture/gas permeation issues, peak fabrication temperatures are only 90°C, only four main process steps are required, images are bistable and Lambertian, even on a curved surface, and they can show ultrahigh brightness. Several entities are known to be working on electrowetting displays: LiquaVista is the most advanced, but others, like PVI, adt, the University of Cincinnati and Linkoping University in Sweden have also demonstrated electrowetting solutions.
Cholesteric Liquid Crystal displays (ChLCDs) have been around for several years, but until recently have been relegated to markets needing low-cost, bistable, and monochrome display solutions. Several companies have recently demonstrated flexible color ChLCDs. Kent Displays leads the industry in the development and commercialization of ChLCD technology and have demonstrated fully printed solutions. Other companies involved in ChLCD development work are Nemoptic, Fujitsu Frontech, Hitachi, Fuji Xerox, ZBD Display, and Image Lab.
Qualcomm is commercializing a MEMS approach based on Interferometric Modulation (IMOD) technology, which requires no backlighting and can be viewed in bright sunlight and in a wide range of environments. The IMOD displays work by reflecting light so that specific wavelengths interfere with each other to create pure, vivid colors. The phenomenon that makes a butterfly’s wings shimmer is the same process that gives the Qualcomm displays their color.
Polymer-Dispersed Liquid Crystals (PDLC)
More than a decade ago, Taliq tried to commercialize PDLC displays. PDLCs consist of liquid crystal droplets that are dispersed in a solid polymer matrix. By changing the orientation of the liquid crystal molecules with an electric field, it is possible to vary the intensity of transmitted light. Although numerous other companies have attempted to do so, the technology is mostly relegated to such things as privacy windows and automobile sun-roof solutions, introduced by companies such as Kent Optronics and Saint Gobain. The Kent Optronics solutions employ a variation called polymer-stabilized cholesteric-texture (PSCT) coating.
Blue Phase LCs
Samsung Electronics announced that it has developed the world’s first “Blue Phase” LCD panel – which will offer more natural moving images with an unprecedented image-driving speed of 240 Hz. The Samsung Blue Phase mode does not require liquid crystal alignment layers, unlike today’s most widely used LCD modes such as twisted nematic, in-plane switching or vertical alignment. The new Blue Phase mode can make its own alignment layers, eliminating the need for any mechanical alignment and rubbing processes. The Blue Phase mode features a superior response rate, allowing images to be reproduced at 240 Hz or higher without the need for any overdrive circuit. Samsung intends to commercialize the technology by 2011.
OCB LCDs
Optically Compensated Bend (OCB) is a technology that realizes performance capabilities comparable to those of cathode ray tube (CRT) displays, offering fast response time and wide viewing angles which have been the challenge of conventional liquid crystal displays (LCDs). Toshiba Matsushita Display Technology has led the industry in the development of OCB LCDs and has been applying the technology in both amorphous silicon and low-temperature poly-silicon TFT LCDs.
Laser Projection
Several companies, led by Novalux, are working to develop technology that exploits a new type of laser architecture that matches the micro-mirrors of a DLP light engine with an array of lasers to replace the more conventional lamp solutions used in projection TVs. These mirrors switch on and ff thousands of times per second, and the lasers shine on the mirrors in varying intensity, mixing the fundamental red, green, and blue. The result is a high color gamut with high contrast ratios. Mitsubishi is the first company to commercialize the technology, although reduced market penetration by rear-projection TVs makes the future of the technology uncertain.
3D Technologies
Volumetric displays (from companies such as Actuality systems) are those that emit, redirect, diffuse, or re-image light from a localized true volume as integrated over the system’s refresh rate. Examples of volumetric displays include swept-screen multi-planar displays, projection onto a stack of LC panels, two-step up-conversion in doped solids, and even projection into fog.
Holographic displays (which some consider to be a form of volumetric display) utilize lasers to reconstruct objects whose scattered light is received by a photographic plate during recording. The front, sides and back of the object can be recorded on three, four or more photographic plates. Such holograms can give 360 degree views of the object. The main difficulty of such displays has been due to the enormous information content of the holograms as well as the difficulty of representing a full color spectrum.
Stereoscopic displays create a right eye-view and a left-eye view that are reconstructed with the use of special glasses, which can be based on differences in color, time, or sequence. Stereoscopic displays are currently the most common and are commercially available in LCD, PDP, and as both rear and front projection systems.
Autostereoscopic displays create multiple viewpoints so the viewer does not have to wear special glasses. This can be accomplished by headtracking solutions or by reducing resolution to enablemultiple viewpoints.
More details, pictures and a glossary of terms can be found in the DFF Competence brochure "European Technologies: Flat Panel Displays".(PDF, 3.4 MB)
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