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 have invented a manufacturing technique that yields flexible, atomically thin transistors less than 100 nanometers in length — several times smaller than previously possible.
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. 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 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, researchers formed 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 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 a few additional fabrication steps, the results are flexible transistors capable of several times higher performance than any produced before with atomically thin semiconductors. 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.
The devices can handle high electrical currents while operating at low voltage, as required for low 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.
The team is looking into integrating radio circuitry with the devices, which will allow future variations to communicate wirelessly with the outside world — another step toward viability for flextronics, particularly those implanted in the human body or integrated deep within other devices.