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The Cutting Edge | Science Watch

Shades of Gray Could Be the Color of Success for Flat-Panel Design

April 05, 1999|LEE DYE

It's a scene repeated over and over at any computer store: dreamers standing in front of the latest laptop computer, marveling at the clarity of the screen and sobbing over the price.

There's a reason laptops are still priced high above comparable desktop computers. Those liquid crystal screens are costly to manufacture, and a large flat-panel display can cost several times as much as a typical monitor with a cathode ray tube.

Researchers around the world have been trying to come up with ways of reducing that cost, and now one team claims some success, at least in a laboratory setting.

Physicists at Kent State University say they have developed a new technology that should result in flat-panel screens that are cheaper, more rugged, sharper and faster than anything available today.

Experts in industry call the technology interesting but say it is not clear whether it will have a wide range of commercial uses.

Physicist Satyendra Kumar of Kent State sees applications for the technology for everything from flat-panel television sets to "smart cards" rugged enough to be carried in a wallet and containing all sorts of vital information.

Liquid crystal displays have become ubiquitous, showing up on everything from control panels on home appliances to sophisticated "heads up" information displays for pilots of high-performance aircraft.

Several types are now in production, but they all operate on similar principles. Basically, a thin layer of liquid crystal is sandwiched between two pieces of glass or plastic, forming the cell. When an electric field is applied, it alters the molecular alignment of the liquid crystal, thus changing the light that is passed through it. That turns small windows of light known as pixels "on" and "off."

To work, the liquid crystal must be in a particular orientation, or pointed in the right direction, and this is achieved in the manufacturing process by "rubbing" the two plates with a polymer, creating a grid like rows of corn.

The most common kind of liquid crystal display on the market today is a "passive" form in which all the pixels in each row are tied together, thus reducing the need to control each pixel independently. But that means the pixels remain somewhere between on and off, resulting in a loss of contrast. It also produces annoying ghost images, especially of moving objects.

Researchers have been struggling for years to develop cost-efficient "active" displays in which each pixel is controlled by its own transistor. "Active matrix" displays are now available on laptops, but they are expensive to manufacture, partly because the transistors can be so easily damaged during the fabrication of the display screen.

Some technologies that are not currently on the market could potentially reduce the cost, including ferroelectric liquid crystal cells in which a thin film of transistors would be used to control the pixels individually.

In a ferroelectric screen each pixel would be either on or off, thus producing an image that is light and dark, like the numbers on a digital watch. Before the Kent State work, it was thought that ferroelectric screens could not produce "gray" scales such as those needed to produce a photograph, thus severely limiting its use.

Furthermore, fabrication has proven extremely difficult. When the polymer is rubbed across the film of transistors to provide alignment for the liquid crystal, it causes mechanical damage and electrostatic charges that can play havoc with the transistors.

"The yield of the transistors then goes down drastically," Kumar said. The best way to bring the price of such screens down, he added, is to keep the yield of the transistors up.

It is precisely those two problems--producing a gray scale in a ferroelectric display and aligning the liquid crystal without damaging the transistors--that Kumar said he has solved. Kumar and his associate, Valery Vorflusev, announced the results of their work in the March 19 issue of the journal Science.

The key to their success seems disarmingly simple. They poured a mixture of liquid crystal and a polymer into a narrow space between two sheets of glass. Then they turned on a 200-watt ultraviolet light for about five minutes.

That resulted in something called phase separation. The polymer formed a thin film on the glass panel closest to the light, and the liquid crystal formed a perfectly uniform film on the plate farthest from the light.

The scientists found that the liquid crystal aligned correctly even if the alignment layer was rubbed onto only one plate. So they could rub just the plate without the transistors, thus avoiding the damage that had doomed so many previous efforts.

And perhaps most important of all, the researchers found that their devices exhibited a natural gray scale, yielding the various tones needed for so many applications.

That surprised even the scientists, who admit they do not fully understand their ability to produce gray scales.

"It just happened as a bonus in this method," Kumar said.

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