The Boron Behind Your Little Screen
A Japanese view of liquid crystal displays
by Mitch Funahashi
January 1997

With today's continuous technological revolution, there is a risk that this article will become obsolescent before it is published, yet liquid crystal displays (LCDs), particularly active matrix LCDs based on thin film transistors, now seem almost certain to signal offices and homes into the 21st century. Boric oxide will be there with, or rather behind them.

Very light, incredibly thin, and with minimal power consumption, active matrix LCDs are already behind the screens of most laptop computers, hand-held TVs, camcorder viewfinders, video phones, and work stations. Soon they should come to desktop computers, large TVs, and projection TV. The little screens are getting bigger year by year, if not month by month.

LCDs are essentially glass sandwiches: two sheets of glass about one millimeter thick with a high-tech filling of liquid crystals, semi-conductors, insulators, and electrodes. Part of this filling - five to ten microns thick (human hair is normally about 200 microns in diameter) - is the thin film transistor or TFT. The liquid crystal itself is an organic compound in an intermediate state between solid and liquid. By interaction with, for example, an electrical or thermal potential, it will change from being an unstructured to a structured material, as in the change from a liquid to a crystal. Its function is analogous to that of a camera shutter.

Borosilicate glass is a brilliant material for display panels on account of its transparency, thermal characteristics, durability, scratch resistance, and chemical resistance. The rear glass panel is all-important since the critically sensitive TFT must be deposited on it in layers without any risk of contamination. Fabrication of the TFT comprises several heating cycles which deposit thin films and chemical etching. To host these layers, the glass substrate must have very high heat and chemical resistance and a coefficient of thermal expansion which matches that of silicon.

Above all, the glass must be free of alkali metals. The diffusion of alkali ions to the film or the liquid crystal leads to instability in, and breakdown of, the TFT. Sodium - an important ingredient of so many glasses - is particularly harmful, in fact, the worst contaminant of all. Until recently 'low alkali' glass usually denoted materials with 1000 to 2000 parts per million of sodium oxide. Today, borosilicate with less than five ppm can be made - a perfectly tolerable level.

Boric oxide is a key constituent of alkali-free LCD glass. It acts as a network former in the glass structure, it improves optical properties, and it reduces the melting temperature. Generally, the melting temperature of alkali-free glass is higher than that of alkaline glasses; the presence of boric oxide overcomes this drawback.

In Japan,essentially two glass forming processes are used for active matrix LCDs - the fusion process as used by Corning Japan (originally developed by Corning in the U.S.) and the float process used by Asahi Glass. Both processes have their own advantages and benefits: the float process appears to be lower in glass-melting and glass forming costs but requires polishing to meet LCD customer requirements, while the fusion process which might be higher in glass-melting and forming costs does not require expensive polishing.

AN635 is the code name of Asahi's no-alkali borosilicate substrate for active matrix LCDs. Excellent chemical resistance is derived from the use of boric oxide. The glass surface retains its smoothness when washed with alkali or hydrochloric acid during the manufacturing process. When examined under high illumination, this glass exhibits not even the faintest haze due to chemical corrosion. AN635 has a high strain point (635°C) and therefore high heat resistance. This counteracts any tendency to deform or shrink during application of the TFT.

Corning, which has been making electronics glasses since 1956, is the acknowledged leader of the industry. In the late 1980s, its fusion-made LCD glass, code 7059F, containing about 15 percent boric oxide, became the first mass-produced TFT glass available, and set the early standards for the industry.

In 1994, Corning developed and introduced a new glass, code 1737F, designed to lower the cost of producing the current amorphous silicon LCDs, as well as to enable commercialization of the new poly-silicon TFT processes. This glass, about 8.5 percent boric oxide, has a much higher strain point (666°C), an expansion match closer to silicon, improved chemical durability, and lower density. It has quickly become the new standard for Corning.

Summary

Liquid crystal displays were originally used in electronic calculators and digital wrist watches. Remarkably increased display capacity and dramatically improved color and picture quality are opening up new uses as LCD imaging catches up with that of the cathode ray tube while offering the benefits of low voltage operation, low power consumption, and ultra-thin structure. Flat screen displays seem certain to play a key role in the coming age of interactive multimedia communication.

By the turn of the century, active matrix LCDs will almost certainly be providing large, flat-screen, high definition TV, as the cathode ray tube, having served the modern world so well for so long gradually becomes yesterday's technology.

Mitch Funahashi, general manager of Borax Japan, thanks Asahi Glass and Corning Japan for their help in the preparation of this article.