(Left) Illustration of the novel transfer process for a 2D semiconductor with nanopatterned contacts. (Right) Photograph of the flexible transparent substrate with transferred structures. Image: Victoria Chen/Alwin Daus/Pop Lab.
(Left) Illustration of the novel transfer process for a 2D semiconductor with nanopatterned contacts. (Right) Photograph of the flexible transparent substrate with transferred structures. Image: Victoria Chen/Alwin Daus/Pop Lab.

Ultrathin, flexible computer circuits have been an engineering goal for years, but technical hurdles have prevented the degree of miniaturization necessary for achieving high performance. Now, researchers at Stanford University have invented a manufacturing technique that yields flexible, atomically thin transistors less than 100nm in length – several times smaller than previously possible. The researchers report their novel technique in a paper in Nature Electronics.

With this advance, said the researchers, so-called 'flextronics' move closer to reality. Flexible electronics promise bendable, shapeable, yet energy-efficient computer circuits that can be worn on or implanted in the human body to perform myriad health-related tasks. What's more, the coming 'internet of things', in which almost every device in our lives is integrated and interconnected with flexible electronics, should similarly benefit from flextronics.

Among suitable materials for flexible electronics, two-dimensional (2D) semiconductors have shown promise because of their excellent mechanical and electrical properties, even at the nanoscale, making them better candidates than conventional silicon or organic materials.

Up to now, however, forming these almost impossibly thin devices has required a process that is far too heat-intensive for flexible plastic substrates. Such plastic materials would simply melt and decompose in the production process.

The solution, according to Eric Pop, a professor of electrical engineering at Stanford University, and Alwin Daus, a postdoctoral scholar in Pop's lab, who developed the technique, is to do it in steps. In this way, they can utilize a base substrate that is anything but flexible for the initial heat-intensive process.

Atop a solid slab of silicon coated with glass, Pop and Daus form an atomically thin film of the 2D semiconductor molybdenum disulfide (MoS2), overlaid with small nano-patterned gold electrodes. Because this step is performed on a conventional silicon substrate, the nanoscale transistor dimensions can be patterned with existing advanced patterning techniques, achieving a resolution otherwise impossible on flexible plastic substrates.

The researchers use the layering technique known as chemical vapor deposition (CVD) to grow the film of MoS2 on the silicon substrate one layer of atoms at a time. The resulting film is just three atoms thick, but requires temperatures reaching 850°C (over 1500°F) to form. By comparison, the flexible substrate – made of polyimide, a thin plastic – would have lost its shape somewhere around 360°C (680°F) and completely decomposed at higher temperatures.

By first patterning and forming these critical parts on rigid silicon and allowing them to cool, the Stanford researchers can then apply the flexible material without damage. With a simple bath in deionized water, the entire device stack peels back from the silicon substrate, allowing it to be transferred to the flexible polyimide.

After a few additional fabrication steps, the end result is a flexible transistor 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 5µm thick, including the flexible polyimide," said Pop, who is senior author of the paper. "That's about 10 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'. In this context, that means 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 that might otherwise jeopardize the flexible polyimide.

With a prototype and patent application complete, Daus and Pop have now moved on to the next challenge 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 also looking into integrating radio circuitry with the devices, which will allow future variations to communicate wirelessly with the outside world. This represents 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."

This story is adapted from material from Stanford University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.