Canon is the leader in LCD lithography systems
A common process used in the production of large TFT displays is LCD lithography. Ultrafine transistor structures are applied to coated display thin glass by exposure and etching. With a world market share of some 80 percent, Canon is the leading supplier of LCD lithography systems for TFT displays. The Japanese company hopes to fabricate even more sophisticated lithographic tools with the corresponding precise optical components made with special glasses and other optical materials. “ZERODUR®” has long proved its worth in such applications. This glass ceramic, which has made a name for itself among astronomers worldwide as a mirror substrate material for large-scale telescopes, also has decisive advantages for application in LCD lithography. Among these advantages are its zero thermal expansion, its good processability and its very low surface roughness.
SCHOTT supplies Canon with, among other things, pressings of optical glass for use in cameras, projectors, webcams, mobile phone cameras and digital cameras for surveillance systems. Canon has also received “ZERODUR®” components for its LCD lithography tools for more than 20 years. And in the spirit of a true partnership, the German technology company has taken a major step in response to the Asian company’s increase in output because of the market situation: SCHOTT has set up and expanded its production capacities. At an investment in the two-digit million-euro range, a new melting tank including installations for further processing was constructed. “We can now react quickly to the fast tempo in the market for flat displays. This will help Canon strengthen its market position, and we can build a long-term partnership,” explains Dr. Thomas Kessler, Director, Marketing & Sales, Europe, Japan & Korea in the Advanced Optics Business Segment.
Highly precise components
LCD lithography: a trend toward larger formats
LCD lithography technology was developed at the end of the 1980s for the production of the first TFT displays in the United States. The leading display manufacturers in Japan, Korea and Taiwan now mainly use lithography tools for the optical application of transistors with whose help the individual pixels of active LCDs or TFT displays are triggered to illuminate. These exposure tools function according to projection technology. Light is conveyed through an optical system and directed through a mask. The mask exposes ultrafine microstructures onto the display glass plate to be processed, which has been coated with a photosensitive resist and moves simultaneously with the mask. After this photolithographic exposure, an etching process is used to fix the transistor circuits.
The optical components for directing the light are composed of special glasses and other optical material. In Canon’s system (see illustration), which makes use of “ZERODUR®” components, the light first strikes a prism with a triangular cross section after passing through the mask. From here it is deviated onto a large concave mirror, then onto a small convex round disk, before it reaches the display via the concave mirror and the prism.
This procedure follows the general rule that the larger the surface of the concave mirror, the better the dimensions of the display glass to be exposed can be defined. This is the way to expose the targeted large-format TFT displays in a single operating cycle and thus to realize next-generation processes for the fabrication of still larger TFT formats. In the meantime, the generation 5 for TFT formats, which measures approximately 1,100 by 1,250 millimeters, is increasingly becoming the production standard. Leading manufacturers are already tackling the challenge of generation 6 (1,500 x 1,800 millimeters), while generation 7 (1,800 x 2,100 millimeters) will be the objective of the next two to three years.
This trend towards large-format TFT displays has led to high-volume exposure facilities weighing some 18 metric tons and more. They are fully automatic and integrated into production lines. The fabrication process requires the highest levels of precision and cleanliness. This is why LCD lithography tools are encapsulated in special casings and fabrication takes place in clean rooms.