Silicon may underpin the computers that surround us, but the rigid inflexibility of the semiconductor means it cannot reach everywhere. The first computer processor and memory chips made out of plastic semiconductors suggest that, someday, nowhere will be out of bounds for computer power.
Plastic power: This microprocessor is made from organic materials. It is puny compared to most silicon processors, but is flexible and cheap.
Researchers in Europe used 4,000 plastic, or organic, transistors to create the plastic microprocessor, which measures roughly two centimeters square and is built on top of flexible plastic foil. "Compared to using silicon, this has the advantage of lower price and that it can be flexible," says Jan Genoe at the IMEC nanotechnology center in Leuven, Belgium. Genoe and IMEC colleagues worked with researchers at the TNO research organization and display company Polymer Vision, both in the Netherlands.
The processor can so far run only one simple program of 16 instructions. The commands are hardcoded into a second foil etched with plastic circuits that can be connected to the processor to "load" the program. This allows the processor to calculate a running average of an incoming signal, something that a chip involved in processing the signal from a sensor might do, says Genoe. The chip runs at a speed of six hertz-on the order of a million times slower than a modern desktop machine-and can only process information in eight-bit chunks at most, compared to 128 bits for modern computer processors.
Organic transistors have already been used in certain LED displays and RFID tags, but have not been used to make a processor of any kind. The microprocessor was presented at the ISSCC conference in San Jose, California, last month.
Making the processor begins with a 25-micrometer thick sheet of flexible plastic, "like what you might wrap your lunch with," says Genoe. A layer of gold electrodes are deposited on top, followed by an insulating layer of plastic, another layer of gold electrodes and the plastic semiconductors that make up the processor's 4,000 transistors. Those transistors were made by spinning the plastic foil to spread a drop of organic liquid into a thin, even layer. When the foil is heated gently the liquid converts into solid pentacene, a commonly used organic semiconductor. The different layers were then etched using photolithography to make the final pattern for transistors.
In the future, such processors could be made more cheaply by printing the organic components like ink, says Genoe. "There are research groups working on roll-to-roll or sheet-to-sheet printing," he says, "but there is still some progress needed to make organic transistors at small sizes that aren't wobbly," meaning physically irregular. The best lab-scale printing methods so far can only deliver reliable transistors in the tens of micrometers, he says.
Creating a processor made from plastic transistors was a challenge, because unlike those made from ordered silicon crystals, not every one can be trusted to behave like any other. Plastic transistors each behave slightly differently because they are made up of jumbled, amorphous collections of pentacene crystals. "You won't have two that are equal," says Geneo. "We had to study and simulate that variability to work out a design with the highest chance of behaving correctly."
The team succeeded, but that doesn't mean the stage is set for plastic processors to displace silicon ones in consumer computers. "Organic materials fundamentally limit the speed of operation," Genoe explains. He expects plastic processors to appear in places where silicon is barred by its cost or physical inflexibility. The lower cost of the organic materials used compared to conventional silicon should make the plastic approach around 10 times cheaper.
"You can imagine an organic gas sensor wrapped around a gas pipe to report on any leaks with a flexible microprocessor to clean up the noisy signal," he says. Plastic electronics could also allow disposable interactive displays to be built into packaging, for example for food, says Genoe. "You might press a button to have it add up the calories in the cookies you ate," he says.
But such applications will require more than just plastic processors, says Wei Zhang, who works on organic electronics at the University of Minnesota. At the same conference where the organic processor was unveiled, Zhang and colleagues presented the first printed organic memory of a type known as DRAM, which works alongside the processor in most computers for short-term data storage. The 24-millimeter-square memory array was made by building up several layers of organic "ink" squirted from a nozzle like an aerosol. It can store 64 bits of information.
Previous printed memory has been nonvolatile, meaning it holds data even when the power is off and isn't suitable for short-term storage involving frequent writing, reading, and rewriting, says Zhang. The Minnesota group was able to print DRAM because it devised a form of printed, organic transistor that uses an ion-rich gel for the insulating material that separates its electrodes.
The ions inside enable the gel layer to store more charge than a conventional, ion-free insulator. That addresses two problems that have limited organic memory development. The gel's charge-storing ability reduces the power needed to operate the transistor and memory built from it; it also enables the levels of charge used to represent 1 and 0 in the memory to be very distinct and to persist for as long as a minute without the need for the memory to be refreshed.
Organic, printed DRAM could be used for short-term storage of image frames in displays that are today made with printed organic LEDs, says Zhang. That would enable more devices to be made using printing methods and eliminate some silicon components, reducing costs.
Finding a way to combine organic microprocessors and memory could cut prices further, although Zhang says the two are not yet ready to connect. "These efforts are new techniques, so we cannot guarantee that they will be built and work together," says Zhang. "But in the future, it would make sense."
By Tom Simonite
From Technology Review