Slimming down

A revolutionary technology could make ultralight -- and utrathin -- laptops and TVs a reality

Published: Sunday 31 January 1999

Thin is in: new laptops will s (Credit: Amit Shanker /CSE)this is an age when a good figure almost always helps: the thinner it is, the better it is supposed to be. We are talking, of course, about television ( tv ) screens and miscellaneous visual display systems. These have been getting thinner and thinner over the past decade. Recently, a multinational consumer electronics giant unveiled its new 'flatscreen' tv . Of course, it is a different matter that you have to have a fat wallet to take these thin beauties home.

To make things thinner still, scientists working for the Japanese electronics giant Sharp are thinking of using something called continuous grain silicon ( cgs ) screens to make computers and tv s out of a single sheet of glass. Unbelievable? Not really. cgs screens act like huge microchips and can carry processing circuitry as well as that required for display.

Sharp recently demonstrated a prototype that uses three small liquid crystal displays ( lcd s) based on the new technology to create a back-projected tv image.

Flat panel displays, such as the screens usually seen in laptop computers, work on a thin film transistor ( tft ) principle: a layer of amorphous silicon is deposited on the glass and then etched to create an array of transistors that switch current through tiny cells of liquid-crystal materials to vary their optical properties.

However, silicon does not have a crystalline structure, the transistors cannot switch current quickly. And when such screens try to show a fast-moving object, the image 'smears' the screen ( Electronics , Vol 178, No 350).

The company's scientists found that this problem could be solved if the screen is made from a thin slice of silicon formed from a single crystal. This solution, however, is not very feasible economically, as it is not only very expensive but is impossible unless the screens are not more than a few centimetres wide.

cgs displays overcome the problem, but differently. To make them, a glass substrate is cleaned and polished to remove all blemishes larger than 0.1 micrometres (m). It is then coated with a layer of silicon 0.4 m in thickness. This is done by exposing the glass sheet to hot silicon vapour. The flow of the material is monitored carefully ensuring that the silicon grains take up a regular pattern, with their borders neatly aligned. This encourages a smooth flow of electrons through the surface, just like it happens in a single crystal.

The silicon layer is then processed to create a mosaic of transistors that acts like a microchip made out of a single crystal of silicon. Displays with cgs technology are about 600 times faster than the conventional amorphous silicon tft s.

After working secretly for three years with Japan-based Semiconductor Energy Laboratory, Sharp recently showed the astonishing results. A high-definition rear-projection tv that uses three 6.5-cm cgs panels , each one lit by a different coloured beam to create extremely clear, bright colour images, with no trace of smear on moving images. Displaying 1,280 by 1,024 pixels, the resolution of Sharp's tv is almost twice that of regular tv s.

"Theoretically, the size of the cgs panels can be unlimited as we are using coated glass, not a silicon wafer," says Masaya Hijikigawa of Sharp's lcd Development Group. "Other circuit components, even a computer processor, can be formed on the same silicon layer. Our three-year target is to make a single-card tv or a personal computer ( pc )."

Hijikigawa says the next step is to make the panel reflective by exploiting a high reflectance aluminium system now being used in the screen which Sharp produces for the latest range of digital cameras and hand-held computers.

This, says Hijikigawa, would bring down the cost of cgs systems by as much as 80 per cent by allowing natural light to create a colour image, instead of back light.

Eventually Hijikigawa and his team plan to integrate a solar panel with the display, so that the single-card tv or pc could operate without a battery.

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