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Display Glass

Corning Profiles Its AwardWinning EAGLE XGTM Glass

Dr. Peter L. Bocko

Corning Incorporated

Glass is a fundamental material in almost all electronic displays. Besides adapting to different display technologies, it also must conform to stringent demands beyond the scope of its display performance.

T

he story of Corning Inc.'s environmentally friendly glass composition began in the mid-1990s, on the banks of a river in central Japan. During that time, the company was supplying a glass known as Corning 7059, and had recently introduced Corning's 1737 (Fig. 1), which was designed to meet the needs of the lighter weight notebook computers then emerging in the marketplace. From the vantage point of some customers, both of these compositions had a flaw: to ensure that the glass met customers' evolving quality requirements, it was necessary to add heavy metals as fining agents to eliminate microscopic bubbles in the substrate. Why did some regard heavy metals as a problem? To construct an LCD screen, Corning's customers must create micron-scale electronic features on glass, like thin-film transistors (TFTs). This is accomplished through iterations of the photolithography process, in which a thin film is deposited and patterned via photoresist application and exposure through a photomask. The portion of the film not necessary for the device structure is then etched away. The problem was that during the etching step, part of the glass was dissolved as well. The resulting effluent from the etch process was becoming an increasing concern, as the heavy metal content approached the range where it could be considered a hazardous material.

On that memorable day nearly a decade ago, the executives from the Japanese customer took several Corning representatives to view a river that flowed near their newest plant. The executives made it clear that maintaining the purity of this river was something they took very seriously. As their valued strategic supplier, Corning realized the need to accept that responsibility. Technical Considerations Glass used in windows and automotive windshields is a sodalime composition that dates back to antiquity. Today's LCD glass is as different from this sodalime glass as a Formula One racecar is from an oxcart. There is simply no comparison. The modern LCD glass is a highly optimized composition of alkaline earth boroaluminosilicate. Its key features are low thermal expansion and high thermal durability. Customers have driven the amorphous silicon process to temperatures as low as 300°, which in conventional glass technology does not seem to be a particularly challenging thermal requirement. However, in examinations at a micron scale, the glass shows expansions, contractions and distortions as it undergoes the heating and cooling rigors of TFT fabrication. Early TFT LCD researchers came from the semiconductor industry, and their expe-

Fig. 1: Three generations of Corning display glass

Display Devices Fall '07

Publication, layout and design copyright ©2007 Dempa Publications, Inc. Text and graphics copyright ©2007 Corning Incorporated

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Regulatory Issues With the current concern for the environment, everyone in the distribution chain from manufacturers to consumers is becoming much more aware of the environmental impact of their product and lifestyle choices. At present, there is no legislation that prohibits the use of arsenic in LCD glass, yet the overall Fig. 3: Benefits of EAGLE XG throughout the product life cycle trend is for governments to create regulations prohibiting materials that cause problems cern, or by using halides (like chlorine and fluothroughout the product life cycle (Fig. 2). The rine) to address the bubble problem. However, number of prohibited substances has increased Corning felt that these methods merely shifted continually over time and is somewhat prethe environmental impact of the glass from one dictable. point of manufacturing to another, rather than adOccasionally, however, eliminating a subdressing the fundamental problem. stance presents an insurmountable technical challenge. In these cases, legislators make acThe Danger of Bubbles commodations, allowing time to develop a way What happens if a bubble gets through the to remove the material. This is why the European manufacturer's quality assurance process and Union (EU) has granted a variance on its Restricreaches the end product? The consequences are a tion on Hazardous Substances (RoHS) regulafunction of the size of the bubble, its geometry, tion, allowing manufacturers of plasma panels to and its location, including whether it is buried continue using lead. deep in the center of the glass or near the surface. For glass manufacturers who need to employ Conceivably one could develop a specification materials of environmental concern, one reasonthat took these variables into account and then able approach might be to wait for the regulasimply "inspect out" any bubbles that would retions to catch up with them and then introduce a sult in a customer defect. However, it is not that technical solution that meets the minimum resimple. Experience teaches that there is no miniquirements of the regulation. However, Cornmum size for an imperfection: Under the right ing's value-driven approach has been to proconditions, any unwanted feature on the glass actively address emerging requirements, thereby could cause a defect on the customer's device. achieving technology leadership in its chosen Nothing is too small to have defect-forming poapplications. The company has sustained more tential. This has been a difficult lesson for glass than 150 years of technological innovation by manufacturers to learn. delivering technology solutions ahead of the Circulating within Corning is a near-legcustomers' timelines. Despite the technical diffiendary microphotograph, illustrating a microculties of finding a scientific breakthrough and scopic bubble well beyond the ability of the huthe significant investment necessary to minimize man eye to detect in the glass. This tiny bubble the number of materials of environmental concaused a disruption of a thin-film transistor and a cern in its glass, Corning elected to develop a dead pixel. The photograph has been used in insolution in advance, because it believes in taking numerable technical presentations at Corning, decisive actions to future-proof its products and and it has created a culture that supports deliverprotect its customers against impending regulaing customer quality by eliminating the source of tions (Fig. 3). the defect altogether, rather than by developing Recognizing that creating this new glass better or more sensitive quality inspection methwould be a multi-year effort, Corning established ods or specifications that are more stringent. a goal in the late 1990s to eliminate environmentally objectionable materials in its Moving to Larger Generations glass. At that point, the comAbout the year 2000, the industry saw a very pany did not know if the sorapid acceleration in LCD substrate sizes. In the lution would reside in the early days of the display industry, there seemed composition, the process, the to be a tacit assumption that the ultimate subequipment, or a combination strate size would be about one meter square of all three. (1sq.m). In the late 1990s, as Gen 4 platforms Until this point, environwere being introduced for larger-format desktop mental requirements could monitors just below that benchmark, it seemed be circumvented by substithat manufacturers were approaching a maxituting a more obscure heavy mum size for glass substrates. metal for the material of conFig. 2: Increasingly strict environmental standards worldwide riences with silicon did not prepare them for the complex viscoelastic behavior of glass. This drove the industry to adopt LCD glasses that had both low thermal expansion and a maximum temperature capability hundreds of degrees above the customer's process temperature. This added to the challenge for LCD glass manufacturers to provide substrates of exceptional inclusional quality. High thermal requirement means higher melting temperatures, and no glass manufacturer had succeeded in melting refractory glasses at this quality level. Aluminosilicates belong to an ancient and versatile glass family, but they can also be a challenge in glass melting, particularly when a high thermal capability is necessary. Compounding this is the fact that TFTs and high-performance liquid crystals cannot tolerate the presence of sodium. As one constituent in the glass designer's toolbox, sodium oxide acts to reduce melting temperatures, allowing manufacturers to achieve large-scale production of reasonablequality, commodity glass. Yet for LCD glass, this option is off the table. This is where heavy metals come in. Heavy metals are powerful tools that allow the glass chemist to achieve the conflicting attributes of low expansion and high thermal capability. Glass chemists' favorite options are barium, arsenic and antimony. Barium is particularly useful as a melting aid for high-temperature glasses: It acts as a helping hand to stabilize the glass network in an aluminosilicate composition. Arsenic and antimony are powerful agents in reducing bubble inclusions in glass because they have the ability to coexist in glass in multiple oxidation states. How does it work? During the glass melting process, arsenic and antimony oxides can chemically reduce, thereby releasing oxygen in the form of bubbles that sweep through the molten glass, picking up other dissolved gases. However efficient this process may be, there is always some amount of residual dissolved gas. During cooling, more bubbles can be generated, just as dissolved air in water releases small bubbles that make an ice cube translucent rather than clear. When arsenic and antimony are added to a glass, their oxides can chemically oxidize during glass cooling, absorbing the dissolved oxygen before it has a chance to precipitate into a bubble defect in the final substrate. This is why glassmakers previously found heavy metals essential in the manufacture of high-performance glass.

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Publication, layout and design copyright ©2007 Dempa Publications, Inc. Text and graphics copyright ©2007 Corning Incorporated

Display Devices Fall '07

Display Glass

However, much to the surprise and amusement of industry watchers, a 1sq.m substrate never materialized. Instead, with the leap from Gen 4 to Gen 5, panel manufacturers broke through that barrier. It soon became clear that LCD could make a serious play in TV, and there seemed to be no end in sight. As substrates grew rapidly, another development in the industry made the glass technologists' work even more difficult. In the late 1990s, a glass design team had begun to create what would later be deemed the new industry standard, the EAGLE2000® composition. Among many benefits, EAGLE2000 substrates offered improvements in both density and thermal expansion coefficient, mitigating the issues that prevented engineers from scaling TFT processes to progressively larger sizes. When added to the existing requirements for thermal durability, the quest for a version of EAGLE2000 glass that was free of heavy metals was even more challenging than the glasses it made obsolete. Increasing substrate size, stringent performance specifications and impending heavy metal regulations combined to form the environment in which a breakthrough product would be engineered. Creating EAGLE XG Corning's innovation process is highly collaborative, and the number of glass scientists who participated in this decade-long effort was extensive. Among the heroes of the research that paved the way for EAGLE XG are people like Dr. Rand Murnane, who squinted through a hotstage microscope for hundreds of hours, watching how bubbles grow and shrink in molten glass. There are many others: at least two generations of glass composition experts scoured the periodic table for the magic bullet in the quest for an environmentally friendly, bubble-free glass, only to find that no such magic bullet exists. However, Dr. Adam Ellison, inventor of EAGLE XG, realized that if certain glass constituents were formulated in exactly the right proportion, the glass could be melted much more easily and therefore be less likely to form bubbles. This was the breakthrough that hatched EAGLE XG. The late Dr. J.C. Lapp then formed the critical bridge between the work in the laboratory and the way to ensure great performance in the customer's process. After all, LCD glass substrates are not designed for the convenience of the glass technologist but for the value they bring to the customer's process and product. From Lab to Production Aside from its extraordinary cleanliness and high level of automation, the modern LCD glassmaking facility may look very similar to more conventional glass melting units. The difference is in the process. Armed with compositions that were intrinsically more amenable to forming bubble-free glass, as well as deep insights into bubble nucleation, bubble formation and dissolution kinetics, the development team was ready to

environmentally friendly glass substrate," says Jim Clappin, President of Corning Display Technologies. "EAGLE XG is a revolutionary product, and an extension of Corning's corporate commitment to continuously improve our environmental performance." Implications for the Environment For those outside the industry who have never visited the massive LCD fabs in Japan, Korea, Taiwan and China, the amount of glass being consumed there is staggering. It is a stretch of the imagination to picture a ribbon of LCD glass two football fields wide, extending all the way from Los Angeles to the rural community of Corning, New York, where EAGLE XG was invented. Nevertheless, that is the amount of glass the LCD industry probably will consume for all applications in 2008. Just how much has this glass innovation reduced the burden on the environment? If all LCDs between now and 2010 were made with an environmentally friendly glass like EAGLE XG, more than 15,000 metric tons of heavy metals would go unused. That is enough potentially hazardous material to fill more than 2,000 standard dump trucks. Clearly, it is better not to introduce this material into the supply chain. When consumers go to a store and put a new 32-inch TV into a cart, it's doubtful that they think about what's going to happen when the TV reaches the end of its useful life. Yet this is a consideration because, unlike other glass industries, closed-loop recycling for LCD is not an option. The extraordinary quality requirements of LCD and the difficulty in retrieving pure glass from a discarded panel bar device manufacturers from taking the glass back and remelting it into new LCD screens. What eliminating the heavy metal content does do is greatly increase the ability to recycle the material for other purposes, like glassphalt, in which ground-up glass is mixed with asphalt and used in road construction. Conclusion Even though there is no regulatory requirement to remove heavy metals like arsenic and antimony, Corning embarked on the 10-year odyssey to invent an environmentally friendly glass substrate because the company felt it was the right thing to do. Management had the patience and foresight to understand that basic science would someday deliver powerful dividends, including competitive advantage for the company, value for the customer and environmental benefits for society as a whole. Those tri-fold results are why EAGLE XG is an award-winning glass.

Corning has won two awards for EAGLE XG glass, one from SID and one from Finetech.

devise a way to transfer this understanding from the lab benchtop to the modern fusion plant. Keith Congleton was the program manager for EAGLE XG. Looking back on the glass development, he notes, "Instrumental to our success in developing EAGLE XG was applying progressively larger glass melting development tools to drive fundamental understanding into process insight." With plants experiencing a high demand for glass substrates, it can be challenging for a successful business to devote productive assets to helping develop advanced technology. Yet Corning felt that dedicating production resources to this goal was a worthwhile investment in the future of LCD. If the LCD TV revolution were to achieve its ultimate potential, it would be essential for the platform to be as environmentally green as possible. Reaction to a Revolutionary Product On March 21, 2006, Corning announced the commercialization of EAGLE XG, the world's first LCD glass to contain no added arsenic, antimony, or barium. The product was greeted with rapid acceptance worldwide, exceeding even Corning's own expectations. The company expects to be almost completely converted to EAGLE XG at the end of 2007, which makes this the company's fastest glass conversion in its 20-year history of glass innovation for LCD. Reaction from the industry has also been gratifying; in fact, in 2007 the industry honored Corning's composition with two major awards. In April, Corning received an Advanced Display of the Year award at Finetech, the largest trade show in the flat panel display industry. The following month, Corning received the Display Component of the Year Award from the Society for Information Display (SID). This award is granted for a novel component that significantly enhances the performance of a display. "With these awards, the TFT LCD industry is recognizing our ability to meet customer needs for superior attributes and product quality in an

About This Article The author, Dr. Peter L. Bocko, is Division Vice President of Display Futures at Corning, Inc. (www.corning.com).

Display Devices Fall '07

Publication, layout and design copyright ©2007 Dempa Publications, Inc. Text and graphics copyright ©2007 Corning Incorporated

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