HP's plastic low power screen that's on a roll

Etech 09: Budget bendable displays to usurp newspapers?

The electronics behind plastic screens  the etched SAIL backplane has layers of silicon and chrome on a plastic substrate  The first ever active matrix display printed entirely on a roll

Glass screens are fragile, expensive and difficult to make. True epaper displays won't just look like paper; you might print out what you want to read on the reusable screens like paper too. For printer experts HP, that's both a threat and an opportunity, and Carl Taussig, Director of the Information Surfaces Lab at HP Labs (and the man behind DVD+RW) has been leading a project to print paper-like displays for eight years.

The printed plastic sheets he showed off at the Emerging Technology conference this week aren't actually screens yet - just the backplanes for them - but HP has just set up a company, called Phicot, to make plastic displays using E-Ink. Phicot is a collaboration with PowerFilm Solar, a spin-off of 3M that has spent 18 years developing the technology to print solar cells on to plastic.

Lessening the cost

Plastic screens could easily cost a tenth of the cost of glass displays, says Taussig; cheap enough to replace a lot of paper, though not a replacement for today's screens. "What we're working on is something that doesn't have all the capabilities of laptops, but for reading email and looking at text we're starting to get there."

The price goes down partly because plastic is cheaper than glass, and you need less of it: the plastic sheet is just 40 microns thick and a roll with a kilometre length of substrate ready to print would be only a few feet around.

Compare that to the glass for an LCD which is 0.7mm thick. It can't be much thinner because it wouldn't be strong enough to stand up to the manufacturing process, which has to be done in clean rooms with the glass moved around by industrial robots from baths of liquid to vacuum chambers. The printers that handle rolls of plastic are much smaller, greener and a hundred times cheaper – and because the printer is a sealed unit, it doesn't need to be in a clean room.

Building precision components on something as flexible as plastic isn't easy though, says Taussig. "For a laptop display, each pixel is about 200 microns on each side and within that square is a tiny transistor with a channel length on the order of 5 or 6 microns.

If your surface isn't very flat you can't make small features. Plastic is not very flat - that's why it's cheap to make. Plastic is very unstable; sneeze on it, look at it cross-eyed, change the temperature and you can get dimensional instability." So aligning different layers of electronics on the sheet of plastic is almost impossible.

How it works

Instead, self-aligning imprint lithography deposits all the materials at once. Then it's covered by a polymer that cures in ultra-violet light. The system etches away the polymer mask and areas of the substrate alternatively in 3D, to create four levels of components that make up the electronics.

If the substrate gets distorted, the match stretches with it. That lets Phicot create transistors with gates just a micron long, which makes for much faster performance; in fact the process can imprint lines just 40 nanometers wide, which is the finest line ever created on plastic.

There's a long way to go before Phicot can create 40-inch screens with every pixel perfect; cracks, pinholes, bubbles and particles can all cause defects that reduce the yield. Today the displays use monochrome E-Ink (also printed on a roll); colour screens will follow in a couple of years.

And large screens might not be that easy to manage because rolling them up too tightly could cause damage. Taussig suggests screens that fold out in several pieces; "the seams aren't ideal but they would be much more portable".

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