AMOLED Production: Entering a New Era?
Active-matrix organic light-emitting-diode (AMOLED) displays have many attractive features that have led companies to attempt to manufacture them, but one country – Korea – currently accounts for the vast majority of AMOLED-display manufacturing capacity. Ambitious investment plans in China, Japan, and Taiwan could change this balance over the next several years.
by Paul Semenza
AMONG the many compelling features of active-matrix organic lighting-emitting-diode (AMOLED) displays are image quality (wide color gamut, viewing angle, and high contrast), thinness, and weight (no backlight and the potential to be made on a single substrate, which could be plastic or metal), and manufacturing process (simple stack, small amounts of materials, and few optical films). During the decade since the first AMOLED displays were produced, dozens of companies have made at least some efforts toward mass production. Despite the promise of the technology, the reality has been that developing the equipment, materials, and manufacturing processes to make OLED displays at high yield rates has been very difficult. Even the dominant firms, Samsung Display and LG Display, have struggled to scale the technology to make TV panels. At the same time, TFT-LCD technology has continued to improve, most recently in the form of 4K × 2K panels.
Through 2012, the vast majority of AMOLED-display manufacturing capacity has been in Korea, primarily with Samsung Display but increasingly with LG Display as well through investments in Gen 8 fabs for TV panels (Fig. 1). There is a great deal of uncertainty with regard to future production plans, but it is likely that there will be several new fabs in China, as well as a resurgence from companies such as Japan Display, Inc. (JDI), Panasonic, AU Optronics Corp. (AUO), and Innolux, all of which have implemented restructuring and consolidation of existing AMOLED-display fabs. The key to building out new capacity will be to master manufacturing technologies that enable scaling to large substrate sizes.
Korea: Samsung Display and LG Display
Samsung Display, the result of the merger of Samsung Mobile Display and Samsung Electronics’s LCD business, is the dominant producer of AMOLED displays, starting mass production in 2007 and accounting for more than 95% of shipments in 2012. The company’s capacity thus far has been built around Gen 4 and, starting in 2011, Gen 5.5 lines, which focus on displays for smartphones and other mobile devices.
These lines use the low-temperature poly-silicon (LTPS) approach for the fabrication of the active-matrix backplanes, which are then cut in half (for the Gen 4) or quarters (for the Gen 5.5) for organic-material deposition. Vapor deposition of RGB subpixels is achieved primarily via fine-metal-mask patterning. Samsung has been using its Gen 5.5 A2 line, which consists of five phases, to experiment with new manufacturing technologies, including laser-induced thermal imaging (LITI) for materials deposition, flexible substrates, and thin-film encapsulation. (For more about current large-area OLED-display manufacturing processes, see the article “RGB Color Patterning for AMOLED TVs” in this issue.)
LG Display did not start mass production of AMOLED displays for mobile devices until 2011, as it focused on high-resolution and wide-viewing-angle LCDs for mobile devices. The company also started out with Gen 4 production, but within one year, pilot Gen 8 production began, focusing on AMOLED-TV panels. The company began shipping 55-in. TV panels at the end of 2012, but is not expected to manufacture in large volumes until 2014, when it starts deposition on full Gen 8 substrates (it is currently using half Gen 8 sheets for organic-material deposition) in its M2 fab.
LG Display has adopted two new approaches in its Gen 8 fab – oxide-TFT backplanes and white OLEDs with a color filter. Both of these approaches are believed to enable scaling to larger substrate sizes – the oxide-TFT approach because it can use modified a-Si TFT processes and the white-OLED approach because it does not require the deposition of individual red, green, and blue organic emitters on the subpixels. In order to fabricate RGB subpixels, color-filter material is then patterned on top of the white-OLED emitting area. The process to add color filters is much easier than the process to deposit RGB organic materials.
Samsung Display has also announced that it will begin Gen 8 production, although the exact timing is not clear. This new fab will cut the LTPS backplane into six sheets for organic-material deposition. Samsung has been developing a proprietary evaporation technique called small-mask scanning, in which a linear vapor-deposition source is scanned along the mask, which is believed to allow for scaling to larger substrate sizes than is the case when using a fine metal mask with a point source. Samsung is also believed to be developing oxide-TFT backplanes and white-OLED deposition for future Gen 8 production, as it may be easier to scale than LTPS backplanes.
While Samsung will have the largest AMOLED-display manufacturing capacity through 2014, it is possible that LG Display could catch up in 2015 if it continues on its current Gen 8 investment path. Samsung would likely maintain its market share for some time, however, as it continues to increase smartphone-display production and is moving toward the production of panels for tablet PCs and other larger-screen sizes. The outcome may depend on whether Samsung pursues OLED TV as rapidly as LG Display or focuses instead on smaller, mobile displays.
Japan: Fewer, Larger Players
Japan Display, Inc., (JDI) was formed via the merger of the mobile-display businesses of Hitachi, Toshiba, and Sony, and produces AMOLED displays using Gen 2 (formerly Hitachi) and Gen 4 (formerly Toshiba) fabs; Sony did not transfer its AMOLED-display assets to the new company. JDI is planning to add new capacity to the Gen 4 line (organic deposition on half-substrates), with the capability to process WOLED or RGB. JDI has also acquired Panasonic’s Gen 6 a-Si LCD line and will install equipment to convert the fab to LTPS production in 2013, with organic-material deposition on the substrate cut into sixths.
Sony, historically a leader in AMOLED-display production, previously produced AMOLED displays for its Clie PDAs, as well as the first AMOLED TVs. Sony has maintained its facilities and integrated into Sony Semiconductor and is currently producing very-high-end AMOLED monitors for master video and medical applications.
Panasonic has been developing AMOLED-display production for years at its R&D center. In 2012, it installed ink-jet tools at a line in Himeji for AMOLED-TV pilot production. The company has been pursuing AMOLED-display production with oxide backplanes, ink-jet printing, and Sumitomo Chemical’s (much of the original patent portfolio was acquired from Cambridge Display) soluble polymer AMOLED materials (as opposed to more common small-molecule materials that are vapor deposited through a fine metal mask). In July 2012, Panasonic announced a joint development program with Sony based on printing technologies.
Other companies in Japan with ongoing AMOLED-display efforts include Epson, Ortus, and Sharp; however, none are anticipated to begin mass production in the near future.
Taiwan: Consolidation, Limited Production
AUO has two OLED lines for the production of small-to-medium OLED products, based on LTPS backplane and evaporation color patterning, a Gen 3.5 line, and a half Gen 4 line (AFPD in Singapore). In 2012, the company started a Gen 6 line using oxide-TFT backplanes and the full deposition of white OLEDs; it is believed that Sony’s 56-in. RGB 4K × 2K AMOLED TV was produced by AUO. Innolux owns a former TPO Gen 3.5 fab that uses LTPS backplanes with white OLEDs and a Chimei EL Gen 3.5 fab that previously produced bottom-emission AMOLEDs. Wintek has also been developing AMOLED-display production.
China: Many Announcements, Little Capacity Thus Far
There are many panel makers in China that have built R&D lines for AMOLED production, including BOE, CCO, Shenzhen China Star Optoelctronics, IRICO, Tianma, UDT, and Visionox. Most of these fabs are Gen 2, with the exception of Shenzhen China Star Optoelectronic Technology’s (half Gen 4) fab, and all use LTPS backplanes and RGB patterning. BOE, China Star, Hehui Opto, IRICO, Lai Bao, and Truly have announced their intention to go into production with Gen 4 fabs over the next few years; BOE, China Star, Tianma, and Visionox are planning Gen 5.5 fabs, and Foxconn has announced its intention to build a Gen 6 line. Finally, BOE is planning a Gen 8 AMOLED-display fab for making TV panels in Beijing, using oxide TFTs and white OLEDs.
For a number of these companies, the schedule and probability of success are uncertain. Many are driven by provincial initiatives to develop high-technology manufacturing, but the supply chain and human-resource pool are limited in some cases. However, a steady stream of new ventures is targeting AMOLED-display production. Among the newest players are IMEC, which announced that it intends to build a Gen 3.5 AMOLED fab in Nanjing, with help from JGroup. Blue Excited Tech-nology (BEX) plans to produce small AMOLED displays based on blue-coating technology developed at Xinyang Normal University; the company plans to build a Gen 4 fab using oxide-TFT blue OLED material and color conversion for color patterning.
Scaling Up to Larger Substrate Sizes
Most existing AMOLED-display fabs are Gen 4 or smaller. This is due to limitations in scaling both the TFT backplane array and organic materials deposition to larger substrate sizes. As with TFT-LCDs, there is an imperative in the AMOLED-display industry to move to larger fab sizes in order to increase revenues and reap economies of scale; so much of the industry focus is in these two areas of manufacturing technology.
In the past, AMOLED backplanes have been limited to Gen 4 substrates. This was because LTPS was the only TFT technology that was able to drive the OLED devices, which are diodes, with sufficient levels of current density. LTPS fabs were limited to Gen 4 because of the need for an annealing step that typically uses excimer lasers. Due to limitations in laser power, the Gen 4 substrate was the largest area that could be annealed by the laser in a reasonable amount of processing time. However, developments over the past recent years have reduced some of these barriers. First, more powerful excimer lasers, as well as multiple scanning, have enabled Gen 6 and potentially larger LTPS backplanes. Second, oxide-TFT technology, which is similar to standard a-Si TFTs but provides higher levels of current density, has been brought into mass manufacturing, enabling Gen 8 AMOLED backplanes to be produced.
It is common in AMOLED-display manufacturing to cut the TFT-array substrate down to a smaller size for the back-end process of organic-material deposition. This is due to limitations in the standard process for evaporating organic material from a point source through a fine metal mask to define the subpixels. The use of this technique at Gen 4 and larger sizes has been difficult, due to issues such as sagging of the mask, shadowing and angular-distortion induced by the large angles, and the necessity of cleaning the masks. Samsung has recently been working on scanning a linear source across a mask, which could address some of these challenges; another approach is vertical scanning, in which both the mask and substrate are oriented vertically.
To scale up to very large substrate sizes, however, it is likely that new approaches are needed (Fig. 2). Two that have been in development for some time are LITI and ink-jet printing. In LITI, the organic material is embedded in a sacrificial film that is put on top of the substrate and then exposed to a laser, which causes a transfer of the organic materials to the substrate. In ink-jet printing, the organic materials are suspended in solution, which is then forced through ink-jet print heads onto the substrate. While these techniques show great promise, neither has been put into mass production. (The January/February Industry News section of ID magazine discusses new ink technology developed by Merck and Epson for the manufacture of large OLED displays.)
The newest approach to be tried is the white-OLED approach combined with the use of color filters, which LG Display is using (as described earlier); in this approach, a single layer of white-emitting material is deposited on the substrate and a color-filter layer is used to define the RGB (sometimes RGBW) subpixels. This approach eliminates the need for separate deposition steps for each of the primary colors, a source of many of the challenges with fine-metal-mask deposition. A downside is the added complexity of a color-filter layer and the potential for lower efficiency due to absorption losses.
Next Steps
Several upcoming fabs are planning to adopt both oxide-TFT backplanes and white OLEDs, as these technologies appear to be most easily scaled to larger substrate sizes. It should be noted that both of these techniques are new to mass production, and it is likely that there are unforeseen barriers to mass production that could impact schedules and output. Finally, it cannot be forgotten that AMOLED displays currently face competition from TFT-LCDs in virtually every screen size and application. Thus, it is imperative that AMOLED-display manufacturing costs, currently a multiple of equivalent LCDs in large sizes, come down dramatically. In some cases, notably in China, AMOLED displays are viewed as an entry into the FPD industry that has greater potential than LCD manufacturing, which has become mature and extremely competitive.
A different approach to the production of AMOLED displays could be in flexible displays. It is extremely difficult for LCDs to be made in fully flexible, or even curved, formats, due to challenges with cell-gap maintenance and backlighting optics. Technologies such as electrophoretic technology are very amenable to flexible formats, but do not have the visual performance required for video and other key applications. AMOLED-display technology is one of the few display technologies that has demonstrated the ability to be made in thin, flexible formats with little diminution of display performance.
Both Samsung Display and LG Display are working on the production of AMOLED displays on flexible substrates and face a different set of manufacturing challenges; notably, manufacturing yield and performance of the TFT backplane on flexible substrates and the requirement for some sort of thin-film encapsulation to protect the organic materials. Neither of these challenges has been surmounted in mass production, and it is likely that we will see a series of steps, first using flexible substrates to make fixed, possibly curved, displays, and then moving to full flexibility in future product generations. •