The most complex flexible microprocessor to date

This month marks the 50th anniversary of the microprocessor. But the developments of these semiconductor chips, which are at the heart of every electronic device, continue with enthusiasm. Researchers from UK semiconductor chip companies have now made the world’s largest flexible microprocessor, which they say should help efforts to add smart circuitry to everyday objects like clothing and packaging.
The low-cost chip carries over 12 times more transistors than previous flexible microprocessors, and is ultra-thin and flexible with good performance. The team fabricated the chip from metal oxide transistors using conventional manufacturing equipment. And although the performance of the chip may not match that of silicon circuits, “It’s more about putting small amounts of computing power into everyday consumer goods”, says Catherine Ramsdale, senior vice president of technology at PragmatIC Semiconductor. Such chips could lead to sensors on milk jugs and foods to detect spoilage, on dressings to monitor wound healing, or on retail products to track their passage from the factory to the shelves.
Today’s high performance computer chips are based on silicon microprocessors. But the high processing temperatures, stiffness, and cost of silicon are unsuitable for flexible electronic components for clothing, paper, and plastic. For these applications, most researchers have turned to organic or metal oxide semiconductor materials. Some researchers have also used transistors made of 2D materials such as carbon nanotubes or molybdenum disulfide. For more information, see the IDTechEx report on Flexible Hybrid Electronics 2020-2030: Applications, Challenges, Innovations and Forecasts.

Ramsdale, Emre Ozer of Arm Ltd., and their colleagues used indium gallium zinc oxide (IGZO) to fabricate their thin film transistors. They use traditional techniques and machines to create the devices by depositing different materials layer by layer on a polyimide film.

The team coated the film on a glass backing during the manufacturing process and then peeled it off after the chip was completed. Compared to making silicon chips, this process is simpler and uses lower temperatures. “We can manufacture the chip in a few days instead of weeks for silicon”, Ramsdale said.

With more than 39,000 transistors, the microprocessor is the most complex and flexible manufactured to date, she says. The key to achieving this feat was careful design and testing to downsize transistors and other devices in order to make quality, consistent devices. The chip speed isn’t impressive at 29kHz – silicon transistors run 1000 times faster – but these flexible chips wouldn’t be used in the same applications as silicon.

Building a flexible circuit with such a large number of transistors is remarkable, says Ravinder Dahiya, an engineer at the University of Glasgow. A downside is that IGZO only produces n-type transistors, which conduct electrons. Complex computer circuits also rely on p-type transistors that conduct positive charges called holes. A chip that uses only n-type transistors consumes significantly more power than a chip that uses both, says Dahiya. This increased power demand limits the number of devices you can put on a microprocessor. “Going beyond 100,000 transistors will be a challenge with IGZO”, he says.
Yet this complex microprocessor is a game-changer for flexible electronics, explains Kris Myny, researcher at the IMEC Electronics Research Center. “This is an impressive advance in the state of the art and extremely valuable to the field of thin film integrated circuits on foil.”

Top image: Nature

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