The first is a thin-film transistor (TFT) array backplane, a substrate for vapor-deposited silicon that, in turn, forms the basis for the transistor array.
The second is the color filter. The glass surface is the substrate for the color filter materials that enable the full-color displays we see on our screens.
Between these sheets is a very thin layer of liquid crystal. Precise registry between the two substrates is essential to ensure the picture elements, or pixels, operate seamlessly, resulting in brilliant, fast-moving images.
Both the glass composition and the manufacturing process must contribute to the surface quality and other attributes that display customers need – like low density or extreme stability. Even minuscule changes in elements or melting conditions can cause major changes in the atomic state and structure of the glass.
That’s because glass is not a single material. Rather, it’s a diverse platform that Corning scientists and engineers can exploit in nearly limitless ways, always tuning glass properties for different applications.
As displays have evolved, customer requirements have become increasingly demanding. Still, some basic requirements for AMLCD substrates remain consistent – and they point to the complexity of the manufacturing challenge and the value Corning continues to deliver.
1) The substrate must behave as a rigid solid – with extremely precise, stable geometry – through the entire range of processing conditions to which it will be exposed in TFT and color filter manufacturing.
Most glasses would lose rigidity under temperatures exceeding 600 degrees C, but Corning’s LCD substrates are specially formulated to remain solid and stable, even at the atomic scale.
2) The substrate must be extremely flat.
Warp, curvature, other distortions would destroy the usefulness of the LCD substrate. Corning has developed metrology methods to detect any deviations and ensure the perfection of the glass plane.
3) The substrate surface must be, as nearly as possible, free of variations and defects.
The tiniest bubble can bend light away from the display surface. A large part of Corning’s LCD composition and process research activities are focused on eliminating such defects.
4) The substrate must be durable enough to withstand the chemical treatments it will experience during both manufacturing and long-term normal conditions during use.
Both TFT and color filter substrates undergo intense cleaning and etching processes. The chemical resistance of the substrates must be compatible with the processing technology as it evolves.
5) The substrate must not interact chemically or electronically with the TFTs.
Sodium, in particular, could interfere with TFT performance if it leaches out of the glass.
Corning scientists have used these guidelines – and learned more about the demands of each one – over the course of nearly four decades, helping to drive ongoing innovation and leadership in the LCD industry. More stringent requirements are sure to arise in the future. Corning scientists are embracing this challenge as an exciting avenue for future composition research.
