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Glass Technology

Quality in Glass Substrates Holds Key to Improvements in LCDs

Dr. Peter L. Bocko

Corning Incorporated

LCDs play a pivotal role in daily life, including home, office and public spaces. The evolution of glass substrates for active-matrix LCDs has occurred through rapid technological advances and the growing demand for ever-larger glass sizes on a continually decreasing timeline.

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lat-panel displays, mainly LCDs, retain their appeal to researchers and users alike, evolving at impressive speeds. Generation 2 (Gen 2) glass, available in 1993, four years after Gen 1, offered a 40 percent larger surface area than its predecessor. This represented a significant jump. Yet in another fouryear span, from 2000 to 2004, the display industry introduced Generations 4 through 7, achieving an increase of nearly 700 percent in substrate area (Fig. 1). Although recently it may have seemed that glass substrates have begun to approach their upper limits in size, the introduction of Generation 8 is just on the horizon. Already, the industry is starting to discuss Gen 9. Clearly, these size increases have resulted from great advances in the equipment and from the broad range of materials for the fabrication of LCD panels (Fig. 2). Central to these advances are innovations in glass composition and forming, which have had a profound impact on LCD panel manufacturing and on product performance. The close collaboration between glass suppliers and LCD panel manufacturers enables the substrates to offer an optimum level of value in the panel manufacturing process. As a result, the LCD platform has improved in throughput and yield, and finished display devices today exhibit vastly improved brightness, resolution, and viewing angles. Important substrate attributes derive from both the extrinsic and intrinsic glass properties

Fig. 3: Extrinsic and intrinsic properties of glass for substrates

(Fig. 3). The salient extrinsic properties, which arise from the manufacturing process, include surface quality, flatness, and dimensional stability. Because fine-resolution LCDs incorporate up to 2 million pixels (6 million red, green and blue sub-pixels) the panels are sensitive to any small particles measuring in the single micron range that may contaminate the glass substrate. Thus a pristine surface is crucial. Substrate flatness has become increasingly critical in LCD TVs, where new technologies for wide viewing angles make the image susceptible to thickness variations in the glass, even if those variations are only on the order of tens of nanometers. Finally, maintaining the dimensional stability of the substrate through the panel fabrication process is essential because high-resolution displays demand increasingly tight align-

Fig. 1: Dramatic gains in glass size during the past 15 years

Fig. 2: Large substrates enable economies of scale

Display Devices Fall '05

Copyright 2005 Dempa Publications, Inc. Not for public use without prior permission.

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Fig. 4: Advantages of Corning's fusion process

Fig. 6: Benefits of low-density glass for large panels

ment between the thin-film transistor (TFT) and the color filter arrays occupying the two glass substrates that comprise the LCD panel. Fusion Formation Corning's proprietary fusion-forming process has been exceptional for meeting the quality demands of large-generation glass substrates. The glass sheet does not contact a foreign surface during forming. Therefore, control of the surface properties depends strictly on the interplay of the visco-elastic properties of the glass and the thermal gradients, and on the mechanical forces in the draw. These conditions result in a pristine surface. Meticulous engineering of the thermal and mechanical variables in fusion technology can yield an extremely flat surface as well as a residual stress characteristic that is appropriate to achieving tight dimensional tolerance (Fig. 4). Basic Characteristics The substrate's intrinsic qualities, that is, the basic properties arising from glass composition,

also have played a significant role in bringing the industry to its current state (Fig. 5). Large-generation glass is continuing its dramatic evolution. The LCD platform has benefitted as trends in LCD substrate glass have shifted to compositions that exhibit lower density, lower coefficient of thermal expansion (CTE), and higher chemical durability than before. Low density decreases the overall weight of the glass in the panel. It also provides an indirect benefit by enabling the use of thinner substrates than in the past. This is a key contribution in reducing the weight of notebook computers and other portable equipment. Low-density glass brings a number of other benefits to the panel manufacturing process, including decreased gravitational sag during substrate handling and diminished distortion under the inevitable thermal gradients (Fig. 6). During the critical patterning stage, temperature variations induce a certain amount of error. High thermal expansion of the substrate will exacerbate the amount of error. Even a small variation of a few degrees can result in visible pixelalignment defects in the final display. Substrates with low coefficient of thermal expansion are

less sensitive to these effects than those with a high CTE. Moreover, fast processing through the thermal cycles of an LCD process induces glass stress that is proportional to the substrate's thermal expansion. Low-expansion substrates have less breakage in a sufficiently severe thermal gradient than high-expansion glass. Etching takes place after the photolithography process. Etching the thin films during LCD panel patterning requires the use of harsh chemicals, so it is essential that the glass substrate exhibit high chemical durability. Glass with the correct level of durability will experience only minimal surface damage during etching. This translates directly into improved yields and throughput during panel manufacturing. Steady Research Corning has conducted continuous research in the composition of glass for LCDs since 1985. A succession of innovative Corning products have paced the growth of LCDs. Most recently, Corning's Eagle2000 glass has supported significant progress in the manufacture of very large-generation substrates. The low density, low CTE, and high durability of Eagle2000 enable the production of increasingly affordable, larger, lighter, thinner, and finer-resolution displays. Silica Content In very broad terms, the trend in glass composition has been moving toward glass with a higher silica content than ever before (Fig. 7). Research initially moved in this direction because scientists were looking for a glass that closely matched the thermal expansion of silicon. While working on this problem, though, Corning came up with a solution that provided additional benefits: The higher silica of Gen 2 LCD glass improved the mechanical reliability of the glass under stress. Early in the 1990s, the advent of enlarged panels for notebook computers handling sophisticated information content required gate metals with extra electrical conductivity. This led to the adoption of increasingly complex alloys of re-

Fig. 5: Innovations in melting, forming and glass composition

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Copyright 2005 Dempa Publications, Inc. Not for public use without prior permission.

Display Devices Fall '05

Glass Technology

Glass with higher silica content exhibits fewer lateral cracks along the scribed edge.

Fig. 7: Trend toward glass with higher silica content

fractory metals, for deposition as films with a high level of intrinsic stress. As a result, the level of stress at the glass-to-gate metal interface could exceed the design stress of the glass surface. Failure of this interface became a key issue in the Gen 2 LCD manufacturing process. The movement to a higher silica content than before in the glass improved the robustness of the glass-togate-line interface. Raising the silica content in the glass, and judicious optimization of glass modifiers (alkaline earth oxides and alumina) form a path to enhanced chemical durability. In the early development of the LCD process, the adoption of etchants from the semiconductor industry exposed new phenomena in glass substrates. Fluoride etchants (buffered hydrofluoric, buffered HF) created faceted bumps on the glass surface, known as hillocks (Fig. 8). These features resembled crystals in morphology, but microscopic analysis indicated a composition identical to the bulk glass. An investigation of the mechanism indicated that hillock formation started with the nucleation of alkaline earth fluoride crystals on the surface. The hillock then masked the glass underneath while the aggressive fluoride etchant dissolved the surrounding glass. As the surface crystal grows, it progressively masks more of the glass surface. Rinsing removes the crystal, but a replica of the crystal remains on the glass surface

afterwards. The hillock mechanism diminishes in glass with higher silica content. During the score-and-break process that takes place late in panel manufacture, the force of the scribe head literally crushes the glass, causing lateral cracking. This can lead to an unpredictable separation (break) along the scribe. In the final display, the existence of lateral cracks can reduce the strength of the panel under mechanical shock. Glass with extra silica content responds more elastically to the scribe force than highly modified glasses. Therefore, lateral cracks along the scribed edge (Fig. 9) are less likely to form in glass with higher silica content. Advances in Glass Composition? As the growth of substrate sizes has accelerated, panel manufacturers have modified their processes to improve their handling of large glass. These improvements mitigate concerns about gravitational sag, and lessen the impact of low densities in substrate handling. The industry may have reached a point of diminishing returns in the benefits possible from further reductions in density and CTE in substrate glass. At the same time, the rapid growth in largedisplay applications like LCD TV and public signs diminishes the impact of low CTE. When panels use large pixels, thermal expansion is not a key concern. Corning Eagle2000 constitutes the industry standard in glass substrates. As development continues, this product may represent the near-terminal composition for active-matrix LCD glass substrates, because its composition leaves little room for dramatic improvements like those occurring in recent years (Fig. 10). Yet customer demand may drive improvements to glass composition in two areas. The first involves the use of polysilicon technology that makes it possible to fabricate the TFTs during panel manufacture. However, this approach to TFTs involves substantially higher temperatures than when using amorphous silicon, the workhorse of the LCD

Fig. 9: Scribed edge where lateral cracks may occur

Fig. 10: Comparison of Corning's Eagle2000 brand of glass and other glass substrates

platform. Therefore, the substrate glass must offer incremental dimensional stability in the thermal regime. Obviously, this represents a challenge for glass manufacturers. Opportunities for compositional improvement also may occur with the elimination of environmentally questionable materials from consumer electronics. All glass substrates today contain heavy metal oxides that will come under increasing scrutiny as environmental initiatives evolve. Manufacturers will find it challenging to comply with regulatory and customer requirements to eliminate all heavy metals while maintaining key substrate attributes. Conclusion LCD substrates that offered state-of-the-art quality several years ago would be wholly unacceptable today. Panel manufacturers will continue to push for maximum yields and productivity in their fabs, aiming to create display devices with sophisticated performance. New enhancements to the manufacturing processes for glass substrates will raise the quality of the glass, offering cleaner, flatter, and more stable substrates than ever before. Ongoing collaboration between glass suppliers and panel manufacturers will enable both to identify opportunities for delivering value through innovations in glass composition.

About This Article The author, Dr. Peter L. Bocko, is Division Vice President and Director of Commercial Technology for Displays at Corning, Inc. (www.corning.com).

Fig. 8: Mechanism of hillock formation

Display Devices Fall '05

Copyright 2005 Dempa Publications, Inc. Not for public use without prior permission.

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