This small prototype display demonstrates a breakthrough technology being developed at Hewlett-Packard Laboratories in Bristol, U.K., a bistable liquid crystal display that can be printed onto plastic.

A recurring dream in the display business is that it will one day be possible to produce a low-cost display that has the same clarity and resolution as a page in a magazine. The other part of the dream is that such a display can be cost-effectively “printed,” much in the same way one would print a magazine. Recent research is bringing that dream closer to realization.

Researchers at Hewlett-Packard Labora-tories in Bristol, U.K., have developed a prototype color, bistable, plastic display made by imprinting and lamination processes that eliminate the expensive vacuum deposition and photolithography used to make today’s generation of flat panel displays.

The 3 cm x 4 cm, 128 x 96 x RGB prototype liquid crystal display needs no active matrix and can display 125 colors. The technologies used to create the prototype are at an early stage, but are designed to scale to paper-like resolutions over large areas so that future products can affordably deliver full-color, print quality from a low-cost printed display. The development is targeted at applications such as electronic books and magazines and digital posters and photographs, rather than video displays such as television and computer monitors.

Current display technologies have price and performance points that suit them to high-value products such as notebook computers, televisions and mobile products including PDAs, mobile phones and digital cameras. The primary focus for these products has been to create displays that are video-enabled, and which usually have fewer than 2 megapixels. By contrast, current printers support fine detail and crisp edges using 20 to 40 times the number of pixels per unit area compared to the average computer monitor; and printers can deliver this resolution over large areas.

The prototype HP Labs display is a bistable passive matrix, which means that displays with as many pixels as desired can be built. Most of today’s LCDs have an active matrix — a TFT (Thin Film Transistor) embedded in each pixel to keep it turned on between periodic updates. In the HP Labs prototype, the pixel stays on or off without this help, remembering its state for as long as required.

Paper-like display technologies are beginning to emerge, but so far many developments still need active matrices and have focused on reflective, black-and-white displays.

“More and more content is produced in color. New display technologies cannot afford to ignore that if they are ever to compete with books, art, magazines and posters,” says Adrian Geisow, manager of Displays Research, HP Laboratories, Bristol.

Low cost, large area

Current flat screen displays are made on glass by processes very similar to those used for making silicon chips. As displays have gotten larger, the factories to make them have become extremely expensive. The new HP Labs prototype has been created out of plastic, which is easier to handle than large sheets of glass, and which reduces the weight and thickness of the display. However, it is more difficult to make good TFTs on plastic, so eliminating them avoids this problem and the costs associated with the complex manufacturing processes involved.

In the HP Labs prototype, the ability of the pixel to remember its state is produced by tiny posts less than a thousandth of a millimeter wide, which are imprinted on to the plastic. These posts hold the liquid crystal in one of two orientations, corresponding to on and off.

The display also has electrodes that are integrated with the printed color filters, further simplifying the device. The electrodes and color filters are made by imprinting shapes on to the plastic, and then using the shapes as templates for the color filter and electrode materials. This gives very precise control of features — such as metal lines 5 microns wide.

“All of the patterning in the prototype has been carried out by printing-like processes,” says Geisow. “The details of the processes are still being developed, and we expect it will take a few more years of further applied research to properly develop and assess their commercial potential. However, given HP’s position in the printing and imaging business, and the volume of display products that the company sells, any development that can deliver thin, light, and attractive displays at low cost is of great interest. This is particularly so if it can display images that would otherwise need to be printed. We believe that this is an important advance.”

Bistability

In the quest for a paper-like display, bistability has been a key area of focus. Many flat-panel displays are based on a thin film of nematic liquid crystals, liquids composed of rod-like molecules that tend to align themselves in the same orientation. The application of an electric field is used to control the direction of orientation, which controls the passage of light through a the cell, turning the pixel on or off.

One part of the HP researchers’ work investigated the interaction of the anisotropic elasticity of nematics with shaped surfaces, in particular the alignment effect of arrays of very small posts, measuring about 1 micron.

For short posts, the nematic tends to lie parallel to the ground surface that supports the posts, flowing around the post from one side to the other. As the posts get taller, the nematic tends to tilt upward and away from the ground surface, to minimize deformation. At around a micron high, the nematic develops two different, but stable, rates of tilt away from the ground.

The application of an electric field can switch the position to either tilted or horizontal, turning the pixel on or off. Most importantly, the liquid crystals maintain their orientation after the electric field is removed, allowing an image to be displayed without power. The display only consumes power when the image is changed.

(For a more in-depth explanation, see: “Controllable Alignment of Nematic Liquid Crystals around Microscopic Posts,” by S. Kitson and A. Geisow in Applied Physics Letters, Vol. 80, Number 19, May 13, 2002.)

Benefits

The microscopic posts on the HP bistable display are similar in size to the dimensional features on a common CD. They could, therefore, be cost-effectively manufactured using similar techniques used to imprint plastic.

Having a plastic form factor would significantly reduce both the thickness and weight of a display, while at the same time increasing its robustness. A plastic display also provides advantages for the industrial designer, since it provides greater freedom in terms of shapes, sizes, and aspect ratios.

The form factor and manufacturing method would also make this display technology scalable, permitting the development of high resolution displays in large formats, perhaps even as large as a billboard.