The world has long waited for electronic skin. But the wait can soon be over. Stanford researchers have invented a manufacturing technique that produces atomically-thin transistors under 100 nanometers in length, which is several times smaller than previously possible. The technique is detailed in a paper published on June 17 in Nature Electronics.
The researchers said that with the advancement, ‘flextronics’ move closer to reality and can offer bendable, shapeable, yet energy-efficient computer circuits that can even be worn on or implanted in the human body to perform myriad health-related tasks. It will even be a boon for the coming ‘internet of things’ era.
However, accomplishing this was no easy feat. While 2D semiconductors are ideal, they require too much heat to make, which melts the flexible plastic and decompose in the process.
According to Eric Pop, a professor of electrical engineering at Stanford, and Alwin Daus, a postdoctoral scholar in Pop’s lab, who developed the technique, the solution is to do it in steps, starting with a base substrate that is anything but flexible.
The new approach covers the glass-coated silicon with a super-thin semiconductor film (molybdenum disulfide) overlaid with nano-patterened gold electrodes. As the step is performed on the conventional silicon substrate, the nanoscale transistor dimensions can be patterned with existing advanced pattering techniques, achieving a resolution otherwise impossible on flexible plastic substrates.
Known as chemical vapor deposition (CVD), the layering technique grows a film of molybdenum disulfide on layer of atoms at a time. This produces a film which is just three atoms thick, but requires temperatures reaching 850 C (over 1500 F) to work. But a thin plastic would have long ago lost its shape at round 360 C (680 F), and completely decomposed at higher temperatures.
The Stanford university researchers first patterned and formed these critical parts on rigid silicon and then allowed them to cool, which enables them to apply the flexible material without damage. With a simple bath in deionized water, the entire device stack peels back, now fully transferred to the flexible polyimide.
After few additional fabrication steps, the results are flexible transistors capable of several times higher performance than any produced before with atomically thin semiconductors. The researchers said that while entire circuits could be built and then transferred to the flexible material, certain complications with subsequent layers make these additional steps easier after transfer.
“In the end, the entire structure is just 5 microns thick, including the flexible polyimide,” said Pop, who is senior author of the paper. “That’s about ten times thinner than a human hair.”
While the technical achievement in producing nanoscale transistors on a flexible material is notable in its own right, the researchers also described their devices as “high performance,” which in this context means that they are able to handle high electrical currents while operating at low voltage, as required for low power consumption.
“This downscaling has several benefits,” said Daus, who is first author of the paper. “You can fit more transistors in a given footprint, of course, but you can also have higher currents at lower voltage – high speed with less power consumption.”
Meanwhile, the gold metal contacts dissipate and spread the heat generated by the transistors while in use – heat which might otherwise jeopardize the flexible polyimide.
Promising Future
With a prototype and patent application complete, Daus and Pop have moved on to their next challenges of refining the devices. They have built similar transistors using two other atomically thin semiconductors (MoSe2 and WSe2) to demonstrate the broad applicability of the technique.
Meanwhile, Daus said that he is looking into integrating radio circuitry with the devices, which will allow future variations to communicate wirelessly with the outside world – another large leap toward viability for flextronics, particularly those implanted in the human body or integrated deep within other devices connected to the internet of things.
“This is more than a promising production technique. We’ve achieved flexibility, density, high performance and low power – all at the same time,” Pop said. “This work will hopefully move the technology forward on several levels.”
