Soul of a new machine

A roll-up computer requires four distinct elements, says Michael Sauvante, Rolltronics’ boss. It needs flexible logic circuitry (to do the actual computing), flexible memory or storage, a flexible power source, and a flexible display. This last element is the focus of much attention from researchers trying to make “digital paper” that can be reconfigured to display arbitrary text and images. Rolltronics will leave others to develop this technology. The power source is also taken care of: several firms make thin-film batteries, which could be recharged by flexible solar cells already being manufactured in a roll-to-roll process by Iowa Thin Film Technologies (ITFT), a company based in Boone, Iowa. So Rolltronics has formed a partnership with ITFT to concentrate on the two remaining pieces of the puzzle: logic circuitry and storage.

Making flexible circuitry is difficult because logic circuits are traditionally made of transistors etched into a crystalline silicon wafer, which is rigid. But it is possible to lay down a thin layer of silicon on a flexible plastic sheet, through a process called vacuum deposition. This silicon can then have transistors etched into it in the usual way. The problem, however, is that the silicon layer is amorphous, rather than crystalline. This affects its electrical properties, and means the transistors must be much larger, and switch much more slowly, than the transistors in a conventional chip—bad news if you are trying to build a computer. And while amorphous silicon can be turned into crystalline silicon by heating it, the high temperature required causes plastic to melt.

One way around this is to devise novel transistors that are themselves made of plastic, and therefore flexible. Rolltronics is taking a different approach. The company has licensed several patents from America’s Lawrence Livermore National Laboratory, where a team led by Paul Carey devised a clever way to turn amorphous silicon into the crystalline kind without high temperatures. By zapping the silicon with a laser it is possible to heat it, and cause it to form crystals, without melting the plastic. The resulting silicon can be treated with the usual chip-making processes to make transistors that are small, flexible and reasonably fast.

Rolltronics is working towards this flexible circuitry in two steps. To start with, it is developing a roll-to-roll process to make amorphous-silicon transistors on plastic film. Although these will be big and slow, they will have their uses. They could be used to make cheap radio-frequency identity tags—in essence, printable bar-codes that transmit an identifying code in response to a pulsed radio signal. Such tags, the size of a grain of rice, are currently used to tag everything from pets to televisions, and cost a few dollars apiece. Making cheaper tags using roll-to-roll would dramatically extend the range of uses. Amorphous-silicon transistors could also be used to make the “backplane” circuitry inside flat-panel computer displays. At the moment, this circuitry is made on glass. Using flexible plastic instead would be lighter, cheaper and less likely to break in a laptop computer that was dropped.

After that, says James Sheats, a researcher at Hewlett-Packard who is also Rolltronics’ chief technical adviser, the next step will be to apply Dr Carey’s work and make smaller, faster transistors on plastic film. Dr Sheats is optimistic that it will be possible to achieve performance equivalent to an Intel 80286 microprocessor (with a clock speed of around 10MHz) without too much difficulty. This would be enough for many applications, such as pocket calculators or handheld organisers, and there is room for further improvement.

That just leaves the memory, for which Rolltronics has licensed patents from a group of researchers at the University of Texas led by Allen Bard. Dr Bard and his colleagues have developed a novel form of memory that consists of a thin layer of organic liquid crystal sandwiched between two sheets of glass. Each glass sheet has rows of conducting wires running across it, and the two sets of wires are at right angles, so that a voltage can be applied across the liquid crystal at any chosen intersection. Although the exact mechanism is still not entirely understood, it turns out that illuminating the liquid crystal, applying a voltage across a small region of it, and then turning off the light source causes a small amount of charge to be stored in that region. This means it can be used as a memory.

By replacing the glass sheets with plastic, Rolltronics plans to manufacture a flexible version of this memory using a roll-to-roll process. Its storage potential is enormous: Dr Sheats says that initially, a single layer of memory the size of a sheet of writing paper will be able to store one billion bytes (a gigabyte) of data. Admittedly, this unusual memory requires an internal light source, but the amount of illumination required is tiny and consumes less power than the logic circuits that are used to read data in and out. And as with the flexible circuitry, this technology has commercial applications on its own, in devices such as portable computers or digital cameras.

The long-term plan is to make a complete flexible computer in a laminated sandwich just a couple of millimetres thick. Of course, it is all still vapourware. But it is worth noting that one of Rolltronics’ early supporters was Clayton Christensen of Harvard Business School, the champion of the notion of “disruptive technologies”—that is, innovations that take the incumbents in a particular industry by surprise. If Rolltronics’ plan pans out, it could prove to be a canonical example.