Take a bow, flexible chip. This week at the International Solid-State Circuits Conference, in San Francisco, European researchers will introduce the world’s first microprocessor made with organic semiconductors. The 4000-transistor, 8-bit logic circuit has the processing power of only a 1970s-era silicon model, but it has a key advantage—it can bend. The device’s designers say the chip could lead the way to cheaper flexible displays and sensors. Wrapped around pipes, for example, sheets of sensors with these processors could record average water pressure, and wrapped around food and pharmaceuticals, they might indicate that your tuna is rancid or that you forgot to take your pills.
The key to the chip’s design was taming the somewhat unruly organic transistor, says Jan Genoe, a polymer and molecular electronics researcher at Belgian nanotech research center Imec, in Leuven, who led the research with colleague Kris Myny. One advantage silicon has over organics is its monocrystalline structure, which allows for well-behaved switches. If you increase the transistor gate’s voltage above a known threshold, the current turns on. But today’s organic transistors—which swap silicon for a polymer—are unpredictable. Each one can have a slightly different switching threshold.
In applications where organic transistors are already taking hold, such as turning pixels on or off in some e-reader displays, a few transistors don’t affect the overall performance. Yet in logic circuits, a single transistor can stop the show. ”If only one is a little bit off, then nothing works,” Genoe says.
So Genoe’s team built an extra gate into the back of each organic transistor. He says this back gate allows the researchers to better control the electric field in the semiconductor, and thus avoid accidental switching.
Fabricating the 25-micrometer-thick chip starts with a substrate made from polyethylene naphthalate—a plastic. ”You could compare it to the material that you use to wrap your sandwiches,” says Genoe. ”It’s very flexible.” On top, the team placed a 25-nanometer-thick layer of gold, patterned to make the circuit. Above that sits an organic dielectric, followed by a second patterned gold layer, and finally the organic semiconductor, made of pentacene.
After fabricating the chip, Genoe’s team tested it by running a 16-line program to average changing input values with those stored in memory, the software for which they had hardwired into a second flexible chip. The processor, he says, could execute about six instructions per second.
Genoe hopes such chips can be made at a tenth of the cost of a similar silicon circuit But to realize that promise, manufacturers will need to translate the IMEC researchers’ carefully controlled, photolithography-based, laboratory-scale fabrication technique into a commercial one—such as those being used for large-area, printed electronics.
”It’s not as difficult as one might think,” says Dan Gamota, cofounder and president of the electronics printing company Printovate Technologies, in Palatine, Ill. Gamota, who was not involved in the research, taught commercial printing press operators how to modify their traditional ink-on-paper printing techniques to manufacture an early printed electronics display while a director at Motorola in the late 2000s.
Still, he says, printing logic circuits will have some special requirements. For today’s printed electronics, such as those proposed for lighting devices, he says, the thickness of the materials is crucial, but for logic circuits, manufacturers will also need to align the circuit’s layers more precisely. That will require both new measuring tools and new reliability training programs for printing press operators. ”A printed electronics operator is like a mechanic who knows how to work on a Ferrari,” Gamota says, ”while a traditional printer knows how to fix a Ford.”
Though manufacturing will improve, Gamota says, he doesn’t believe organic logic circuits will ever have the hundreds of millions of transistors found in today’s silicon chips. Instead, he says, many in the field look to use organics as a relatively dim-witted sidekick for silicon processors. As an example, he describes shopping for a new pair of pants by using your smartphone to communicate directly with plastic circuits inside the clothing. The circuits will tell you how the pants will look on you so you can try the trousers on virtually.
Like Gamota, Gerwin Gelinck, who worked on the IMEC chip, also believes that organics will make their start as a complement for silicon. Gelinck is a program manager at the Holst Centre, in Eindhoven, Netherlands, a research organization with commercial partners that include the display companies Polymer Vision and Panasonic. He believes that eventually more-complex organic logic may replace ”peripheral” silicon chips in devices like displays, to lower these gadgets’ cost and size.
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Organic and oxide semiconductors are new classes of semiconductors which allow to make thin film semiconductor devices on flexible plastic substrates. At imec we span the research range from fundamentals of such semiconductor materials and devices over integration technology to design.
The ultimate use of these semiconductors is in backplanes for new flexible displays, electronic paper, as well as for active components made directly on flexible plastic substrates (e.g. flexible chips, smart tags, electronic skin, etc...).
- Charge carriers in organic semiconductors are localized on individual molecules, and charge transport occurs hopping, which is fundamentally different from band transport. The localized nature of charge carriers and the low dielectric constant of the materials mean that electrostatic binding forces of carriers to fixed charges, dipoles or multipoles are strong. This raises interesting questions as to the mechanism of electrical doping, the threshold voltage control of transistors and so on.
- Amorphous oxide semiconductors have been introduced in 2004, and enjoy a considerable and growing interest from display industry. At imec, Ph.D. research is possible on the understanding and characterization of the band structure, and the relation between defect generation in the bands with electrical properties of transistors. Furthermore, most of the oxide semiconductors are n-type, i.e., conduct electrons, and only very few systems are known to allow measurable hole transport in the valence band. It will be very important to acquire a better understanding of hole transport in these semiconductors, because that would ultimately result in complementary oxide semiconductor systems, with great technological impact.
- Using organic and oxide semiconductors, we develop new active devices for integration on flexible pastic substrates. Two classes of devices are non-volatile memory transistors, and high-frequency rectifying diodes. Thin-film non-volatile memories can be made directly on plastic substrates, and can be integrated 3-dimensionally. The research challenges are to arrive at integration of non-volatile plastic memories with plastic (or oxide) write- and read-out circuits on flexible plastic foil. Such banks of non-volatile memory on foil will be the memory of smart cards, smart tags, etc...
- Using thin-film transistors made with organic and oxide semiconductors, we develop integration technologies for display backplanes, sensor backplanes, and circuits on flexible substrates. How can we integrate p-type and n-type transistors on the same substrate, in technologies where a p-type semiconductor and an n-type semiconductor are different materials? Can we make use of electrical doping to improve contact injection? How can we improve device operation speed, is there a scaling law that can be applied? Bias stress and degradation are important for the application of the technology, but many effects still have to be properly characterized and solved by technology. And finally: we hope to be able to ultimately define circuits by printing technologies instead of by lithography, but many challenges remain to apply these technologies to circuit manufacturing!
- Specific Ph.D. topics exist also on design of circuits with thin-film organic and oxide transistors. They are in co-promotorship with the MICAS group of ESAT (Prof. Steyaert, Prof. Dehaene, Prof. Gielen). Examples are: challenges of analog design for sensor signal amplification and treatment for future electronic skin; programming and read-out of organic or oxide thin-film non-volatile memory arrays using thin-film transistor technologies on foil; design and operation of active-matrix display or sensor backplanes.
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