Displays go organic
08/01/2003
Manufacturers of organic light emitting diode (OLED) displays eat up contamination control lessons learned from flat panel pioneers
By Sheila Galatowitsch
The outlook for the flat panel display industry is anything but flat. Spared the economic malaise that's afflicting the rest of the global electronics community, the worldwide display market will likely reach $60 billion this year, up from $50 billion last year and $43 billion in 2001, according to the market research firm iSuppli/Stanford Resources (El Segundo, Calif.).
Credit the bulk of this growth to thin-film transistor liquid crystal displays (TFT-LCDs), ubiquitous in laptop computers and quickly supplanting cathode ray tubes in desktop computer monitors. Plasma displays in TVs are also fueling growth. These two dominant flat panel display technologies are setting the stage for further innovations, such as active matrix organic light emitting diode (OLED) displays. OLED displays, with their high brightness and low power-consumption advantages over TFT-LCDs, are ripe for commercialization.
In fact, the world is just waking up to the technological and economic impact of advanced displays, says Michael Ciesinski, president and CEO of the United States Display Consortium (USDC; San Jose, Calif.). "How we view information in our home, office and car—and even on our person—will change significantly over the next three to four years from both hardware and content points of view."
Production in Asia
While the U.S. is home to much of the materials science research in advanced displays, several countries in Asia—Korea, Taiwan, Japan, Singapore and China—monopolize large-scale TFT-LCD and plasma display manufacturing. LG.Philips LCD and Samsung Electronics in Korea, AU Optronics in Taiwan, and Sharp and Hitachi in Japan are leading manufacturers in a field of dozens of companies making everything from TV to cell phone displays.
Most equipment suppliers are also located in Asia, with Photon Dynamics Inc. (San Jose, Calif.) and Applied Materials' AKT (Santa Clara, Calif.) two notable exceptions. These companies supply test and repair, and chemical vapor deposition equipment, respectively.
Asian companies cornered the manufacturing business through "patient capital and the long-term vision of the top leadership of these large companies," says David Mentley, iSuppli/Stanford Resources' senior vice president. "There are no small companies involved in the TFT side."
Mentley adds, "Everybody on the planet is going to continue to need displays for TVs, computers and cell phones. The demand is not going to go away, but it takes so much money—$1 billion to $1.5 billion—to build a factory, and such a long time to make a profit. Even making a profit is uncertain in a field with this number of competitors. These factors didn't scare off the Asian giants, but they scared off all Western corporate and venture capital."
Companies that previously operated commercial manufacturing facilities in the U.S. either went out of business or moved production to Asia. One of the last holdouts was Planar Systems Inc. (Beaverton, Ore.), which closed its fabs in Wisconsin and Oregon last year and sent production overseas.
Another former LCD manufacturer, Three-Five Systems Inc., has converted its LCD commercial manufacturing line in Tempe, Ariz. over to high-volume production of microdisplays. These extremely small, high-resolution displays depend on processes more akin to semiconductors than flat glass panels (see sidebar, "Microdisplays made in the U.S.A.," page 16).
Meanwhile, flat panel innovator companies in the U.S., Europe and elsewhere are engaged in pilot manufacturing of prototype OLED technologies, which they plan to license to display and device manufacturers. In addition, one major U.S.-based innovator, Eastman Kodak Company (Rochester, N.Y.), has created a joint venture manufacturing partnership in Asia to mass-produce its active matrix OLED displays.
"The only reason OLEDs have been brought to manufacture so rapidly is that there is such a lot to be learned from the rest of the flat panel industry," says Johan van deVen, chief technology officer for Philips Mobile Display Systems (Hong Kong), which is pursuing commercialization of its Polymer LED (PolyLED) technology—a type of OLED display.
Employing clean principles
Indeed, the flat panel display industry's chief contamination concerns—particles and electrostatic discharge (ESD)—are now under control, thanks to an increasing appreciation of clean manufacturing principles.
"The early factories were not designed to pay attention to contamination," says Michael O'Halloran, director of technology at IDC (Portland, Ore.), a cleanroom design and construction firm that has worked on several flat panel display fabs—including the latest fifth- and sixth-generation facilities.
 Among several platforms under development at Universal Display, this OLED prototype uses a plastic substrate instead of glass for a highly flexible display. |
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"Flat panel display cleanrooms are now being designed specifically to keep the substrates—the glass itself—clean. It's standard contamination control protocol—implementing the appropriate cleanliness class requirements and airflow management, and improving tools and material handling," says O'Halloran. "That's especially true for the newer facilities, which have been designed to some extent using the knowledge of the semiconductor industry to mitigate particles and ESD."
The advent of automated handling systems has also helped protect the glass substrate, which today measures one meter and is getting larger with each new generation.
The glass substrates for OLED displays are not nearly that large yet, but their contamination control requirements are similar. OLEDs are "truly thin film devices," says Janice Mahon, vice president of technology commercialization for Universal Display Corp. (Ewing, N.J.), a developer of OLED technologies with more than 400 patents issued and pending. "Particulate or surface imperfections can lead to a short in the plate, which leads to a pixel defect and an undesirable display."
 Philips Mobile Display Systems produces LCDs for mobile phones, as well as passive PolyLED displays that can be used in aircraft cockpit instrumentation systems. |
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Universal Display is working on several OLED platforms, including one prototype that uses a plastic substrate instead of glass for a highly flexible display. Another concept, which is targeting applications such as automotive windshields, makes the display transparent when turned off. In addition, the company's phosphorescent OLED (PHOLED) employs a class of new organic materials "that will enable OLEDS to be more power efficient than previously thought possible," says Mahon.
The company plans to license its patents to display manufacturers. Initial products will incorporate PHOLEDs into small displays suitable for cellular phones and other portable electronics.
Philips Mobile Display Systems already produces LCDs for mobile phones with manufacturing partners in Japan and China, and it also has a mass production line in The Netherlands for passive matrix PolyLED displays. The company recently secured an order from L-3 Communications' Display Systems Division for PolyLED displays that will be used in commercial and military aircraft cockpit instrumentation systems.
Other companies pursuing OLED development include Cambridge Display Technology (Cambridge, U.K.), Osram Opto Semiconductors (Regensburg, Germany) and eMagin Corp. (Hopewell Junction, N.Y.).
Manufacturing OLEDs
Among U.S.-based innovator companies, DuPont Displays (Wilmington, Del.) and Eastman Kodak are closest to full-scale manufacturing of active matrix OLEDs.
In addition to its pilot line in Santa Barbara, Calif., DuPont has a full production line with RiTdisplay in Hsinchu, Taiwan, which will begin ramping up to commercial production of active matrix OLEDs later this year. In April, the company launched the Olight brand name for its OLED display products.
Kodak's and Sanyo's joint manufacturing venture in Japan—SK Display Corp.—has a pilot production line in operation, which is testing manufacturing processes on Generation 1 glass (300 mm ¥ 400 mm). A separate full-scale facility, working with Generation 3 glass (550 mm ¥ 670 mm), will begin operation later this year.
In addition to its manufacturing strategy, Kodak has licensed its NuVue-branded passive matrix OLED technology to more than a dozen display and device manufacturers .
SK Display's full-scale facility was originally built to make TFT-LCDs and was converted to produce active matrix OLED displays. The two technologies are comparable in that both use thin film transistor arrays, where every pixel is addressed by a separate active switch on the glass. (OLEDs typically use a minimum of two transistors per pixel.) That makes the substrates' manufacturing processes similar, with both technologies using some of the same equipment and cleanroom classifications.
 LG.Philips LCD in South Korea is one of the world's leading manufacturers of large-scale TFT-LCDs and plasma displays. |
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The difference lies in the active materials. A TFT-LCD uses liquid crystal suspended between the TFT array and a second transparent glass sheet. OLED displays, however, require coating a stack of several thin, uniform layers of organic materials directly onto the array. The layers are deposited in a number of steps, where the active layer is patterned and deposited through an evaporative mask process or inkjet-printing technique.
These organic layers are extremely sensitive to moisture and oxygen. Critical steps in the deposition process are performed in either vacuum or dry nitrogen to minimize contamination. Then, the newly constructed stack of organic layers is encapsulated in glass or metal along with a desiccant to absorb any residual moisture.
The active layers in OLED displays are less than 1-µm thick, which makes them vulnerable to particles of that size and smaller. Panel production begins in an ISO Class 5 environment, with some local conditions of ISO Class 4, and finishes in an ISO Class 7 cleanroom after encapsulation.
Through a contract funded in part by the USDC, Dow Corning Corp. (Midland, Mich.) is investigating protective barrier layer materials that would resist permeation of water and oxygen. Vitex Systems Inc. (San Jose, Calif.) is seeking to improve OLED encapsulation technology.
Protecting the inkjet printing process from particle contamination is another area of continuing research. "The droplets that create a pixel are on the order of tens of picoliters in size and require a landing precision of a few microns," says Philips Mobile Display Systems' van deVen. "Every single pixel has to work. If even one fails, you will see the effect on the screen."
Philips has worked with printing companies to develop next-generation inkjet equipment that can handle full-scale manufacturing demands and is now installing the equipment at its PolyLED plant in The Netherlands.
Future in high resolution
Active matrix OLED displays will initially penetrate small-screen formats, says Dan Gisser, Kodak's director of strategic marketing. "Portable consumer applications, such as cameras, mobile phones, personal digital assistants and portable DVD players, are a target market for these screens because of their great image quality. Plus, the thinness of these screens means you can make devices smaller and reduce battery consumption."
In another five years, OLED displays will start showing up in computer monitors, 10- to 30-inch TV screens and other larger formats. "We don't need great new inventions, just small improvements," says Gisser.
Industry observers predict OLED displays may someday compete directly with TFT-LCDs. Yet, even if TFT-LCDs remain dominant and OLEDs occupy niche markets, both will face demand for higher resolutions. And as the number of pixels-per-inch increase, so will the contamination control requirements.
Sheila Galatowitsch, a special correspondent to CleanRooms, is based in Denver, Colo. She can be reached at: [email protected]
The world of flat panels defined
The world of flat panel displays encompasses those that need backlights (LCDs) to those that produce their own light (LEDs). Here are a few technologies as defined by the United States Display Consortium. For the complete lexicon, visit USDC's newsroom at www.usdc.org.
Liquid crystal displays (LCD): A display composed of liquid crystal suspended between two transparent sheets. The display is composed of pixels or other shapes, which can be turned on or off with electrical stimulation. Typically, a light is passed through the LCD to illuminate the pixels. Custom passive LCDs address most display needs and requirements for high performance, increased information content, low power and low cost.
Active matrix liquid crystal displays (AMLCD): AMLCDs are composed of a rear glass substrate patterned with thin-film transistors (TFTs), a front glass substrate with color filters and a liquid crystal material filling the middle between the glass "sandwich." The array of TFTs on the rear substrate is attached to electronic drivers that receive impulses from a computer chip attached to the host system. Each TFT acts as an on/off switch to activate a pixel, force the liquid crystal to twist, and allow light to pass through and form images on the display. Most AMLCDs use either an amorphous silicon or poly-silicon layer of semiconductor material within the TFT array.
Liquid crystal on silicon (LCoS):An alternative way to create high-resolution images with liquid crystals involves the use of LCoS devices. LCoS devices use only one glass substrate and employ a silicon surface for the back of the display. Silicon processing technology is advanced to the point that patterning several million pixels and their related drivers on a one-inch square section of crystal is easily done. The pixels are then generally coated with a reflective aluminum layer, and then a polyimide alignment layer. Thus, the liquid crystal industry can piggyback off of existing silicon technology to allow for a high-resolution microdisplay that is easy and inexpensive to manufacture.
Light-emitting diode (LED): An LED is an electronic component, which glows when electricity flows through it. LEDs light like a bulb, but they only use a fraction of the power. This means that batteries will last longer if LEDs are used instead of bulbs.
Microdisplay: A small, high-resolution display (size of a thumbnail) that when combined with projection optics has the unique advantage of being able to magnify and project an image that is much larger than the display itself. Microdisplays are typically less than 1.0-inch diagonal, but can offer resolutions from 1/4 VGA (78,000 pixels) to UXGA+ (over 2 million pixels). Microdisplays are ideal for wireless, portable, lightweight products.
Organic LED (OLED): This type of display is made possible by the development of polysilicon technology (PolySi), which, because of its high carrier mobility, provides TFTs with high-current carrying capability and high switching speed. The passive matrix OLED display has a simple structure and is well suited for low-cost and low-information-content applications such as alphanumeric displays. In contrast to the passive matrix OLED display, an active matrix OLED has an integrated electronic backplane as its substrate and lends itself to high-resolution, high-information content applications, including videos and graphics.
Plasma display panels (PDP): Electrons are removed from atoms to produce ions, later recombining with the ions and releasing energy in the form of light. A certain trigger (or priming) voltage is required to start the ionization process, after which the process will continue at a lower voltage and the brightness of the emission will depend directly upon the current passing through the ionized gas, known as a plasma. Two sheets of glass with a conductive film and a mixture of gases that glow when excited by a current are being used for large-format color screens.
Microdisplays made in the U.S.A.
TFT-LCDs and OLEDs offer direct-display viewing, but another up-and-coming display technology requires magnification before it can be viewed. Microdisplays are tiny, high-resolution displays with military, entertainment and occupational applications.
The near-to-eye (NTE) version of the technology offers a head-mounted or mobile virtual viewing experience. Microdisplays can hold up to 2.6 million pixels in less than one diagonal inch for high-definition or ultra-high resolution displays.
Several Asian manufacturers are developing these displays for use in projection TVs, but there's also manufacturing here in the U.S. at Three-Five Systems' Tempe, Ariz., cleanroom. The company, which has 300 microdisplay patents either awarded or pending, employs one shift of personnel with a manufacturing capacity of 100,000 microdisplays a month.
Just over a year ago, Three-Five Systems converted its former LCD manufacturing plant into a line for microdisplays, which required a complete facility upgrade and new equipment. While OLEDs borrow from TFT-LCD manufacturing processes, microdisplays are more akin to semiconductor manufacturing. That's because microdisplays employ both LCD and liquid-crystal-on-silicon (LCoS) specific technologies, which call for round glass substrates and eight-inch silicon wafers.
The 20,000 square-foot manufacturing cleanroom includes ISO Class 6 and Class 5 areas, and is a "typical, semiconductor-type cleanroom with raised floors and laminar flow," says Henning Stauss, senior director of microdisplay manufacturing at Three-Five Systems. "For example, all handling of our product is done by robot from cassette to cassette. No operator is handling the wafers." As many as 200 microdisplays are produced on a single wafer.
Since microdisplays depend upon magnification for viewing, any particle defects inside the display are also magnified. In a 200-times magnification, a 1-µm particle will be a 200-µm spot on the screen, says Stauss. To control particle contamination in the cleanroom, Three-Five Systems uses online particle counters and monitors test wafers. It also strictly controls room humidity and uses a nitrogen blanket for those processes sensitive to oxygen. Most critical processes occur in ISO Class 5.
The microdisplays using projection end up in military cockpits and high-end audio-visual consumer electronics, such as TVs and home theater systems, where they compete with plasma displays. The NTE displays are integrated into wireless information appliances and head-mounted devices for the sight-impaired and medical markets.
In addition to the microdisplay plant, the company has four other factories—one in Redmond, Wash., and three in Asia—housing electronic assembly operations, including display module assembly. Recently the company decided to spin off its successful microdisplay business into a wholly owned subsidiary called Brillian.
The challenge facing microdisplay manufacturers in the U.S. and Asia is to boost yields, says Bob O'Donnell, director of personal technology for the research firm IDC (Framingham, Mass.). Low yields and high costs have stymied the market over the past five years.
The difficulties of making magnified displays were originally underestimated by the companies pursuing this technology, says Stauss. Many new techniques, such as wafer scale LCoS processing, spacerless cell construction, reflective modes and high operating temperature performance, had to be developed.
"Many microdisplay manufacturers in the past had a fabless concept and thought they could manufacture offshore in a foundry-type facility," says Stauss. "Although that approach would probably work once the technology is developed, it is difficult when working on a brand new technology. In an immature product, very quick feedback is needed from manufacturing to engineering and R&D. Manufacturing becomes part of engineering in that phase, and in-house manufacturing is superior to having a factory across the Pacific."