Stanford Researchers Develop New Manufacturing Technique for Flexible Electronics
June 30, 2021 | Andrew Myers, Stanford UniversityEstimated reading time: 4 minutes
Ultrathin, flexible computer circuits have been an engineering goal for years, but technical hurdles have prevented the degree of miniaturization necessary to achieve high performance. Now, researchers at Stanford University have invented a manufacturing technique that yields flexible, atomically thin transistors less than 100 nanometers in length – several times smaller than previously possible. The technique is detailed in a paper published June 17 in Nature Electronics.
With the 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.
Technical difficulties
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.
The engineering challenge to date has been that forming these almost impossibly thin devices requires a process that is far too heat-intensive for the flexible plastic substrates. These flexible materials would simply melt and decompose in the production process.
The solution, 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, is to do it in steps, starting with a base substrate that is anything but flexible.
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 the 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 layering technique, known as chemical vapor deposition (CVD), grows a film of MoS2 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 work. By comparison, the flexible substrate – made of polyimide, a thin plastic – would long ago 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 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.”
Co-authors include postdoctoral scholars Sam Vaziri and Kevin Brenner, doctoral candidates Victoria Chen, Ça??l Köro?lu, Ryan Grady, Connor Bailey and Kirstin Schauble, and research scientist Hye Ryoung Lee.
Funding for this research was provided by the Swiss National Science Foundation’s Early Postdoc Mobility Fellowship, the Beijing Institute of Collaborative Innovation, the U.S. National Science Foundation and the Stanford SystemX Alliance.
Suggested Items
IDTechEx Examines the Opportunities for Wearables in Digital Health
04/19/2024 | IDTechExIDTechEx’s report, “Digital Health and Artificial Intelligence 2024-2034: Trends, Opportunities, and Outlook”, covers this ongoing trend in the consumer health wearables market and includes analysis of the opportunities and roadmap for biometric monitoring.
ZESTRON Welcomes Whitlock Associates as New Addition to their Existing Rep Team in Florida
04/19/2024 | ZESTRONZESTRON, the leading global provider of high-precision cleaning products, services, and training solutions in the electronics manufacturing and semiconductor industries, is thrilled to announce the addition of Whitlock Associates to its esteemed network of sales representatives.
IPC Bestows Posthumous Hall of Fame Award to Industry Icon Michael Ford
04/18/2024 | IPCIPC honored the late Michael Ford, Aegis Software, for his extraordinary contributions to the global electronics manufacturing industry with the IPC Raymond E. Pritchard Hall of Fame Award at IPC APEX EXPO 2024. IPC’s most prestigious honor, the Hall of Fame Award is given to individuals who have provided exceptional service and advancement to IPC and the electronics industry. Ford, an industry leader and valued IPC volunteer, died suddenly in January 2024.
SEMI Applauds U.S. Chips Act Award for Samsung Electronics Facilities to Strengthen Domestic Semiconductor Supply Chain
04/17/2024 | SEMISEMI, the industry association serving the global electronics design and manufacturing supply chain, applauded the United States Department of Commerce’s announcement of a Preliminary Memorandum of Terms for an award under the CHIPS and Science Act to support the expansion of Samsung Electronics’ presence in Texas and the company’s development and production of leading-edge chips.
Northrop Grumman honors Calumet Electronics with Supplier Excellence Award
04/17/2024 | Calumet ElectronicsNorthrop Grumman Corporation has recognized Calumet Electronics during the company’s 2024 Supplier Excellence Awards for “exceptional performance and unwavering commitment to delivering with excellence.” Calumet is one of 70 suppliers recognized from across the globe. In its award category of “Supplier Strategic Excellence,” Calumet was honored alongside global corporations such as Amazon Web Services, Dell Technologies, and Eaton Corporation.