Molecules are among the smallest building blocks available for making next-generation devices. Their unique, customizable properties enable promising applications in emerging computing, sensing, optical, and quantum technologies.
But integrating molecules into functional devices at scale remains a challenge. Traditional semiconductor manufacturing processes can damage small and fragile molecular materials. Now, MIT researchers have developed a scalable fabrication technique that incorporates delicate molecular materials into electronic devices on a chip without causing damage.
Their method extends the capabilities of standard semiconductor manufacturing processes to accommodate molecules. The researchers first prefabricate the device components using traditional processes. Then, they introduce the molecules and harness nanoscale surface forces to mechanically transform the fabricated device, which self-assembles without damaging the molecules.
The team demonstrated the robustness and scalability of their technique by fabricating more than 1,000 devices using sub-nanometer molecular layers.
"Our platform combines the scalability of conventional semiconductor manufacturing with the precision and control of self-assembly. This establishes a new fabrication framework for the scalable, high-throughput integration of emerging nanoscale and quantum materials, including molecules, into functional devices with architectures and capabilities that were previously infeasible," says Farnaz Niroui, an associate professor of electrical engineering and computer science (EECS), a member of the Research Laboratory of Electronics (RLE), and senior author of a new paper describing the work .
She is joined on the paper by co-lead authors Sarah Spector and Peter Satterthwaite, EECS graduate students; Jeremiah A. Johnson, the A. Thomas Guertin Professor of Chemistry at MIT; and others at MIT. The research appears today in Nature Nanotechnology.
Building with molecules
Molecules are small clusters of atoms with structures and chemistries that can be precisely designed. This allows their properties to be engineered across a wide design space.
Once integrated into device architectures, these molecules could enable next-generation electronics and computing platforms that are smaller, faster, and more adaptable, as well as higher-performance photonic devices and emerging quantum technologies.
To build a functional system, molecular building blocks need to be integrated with other device layers. In electronic systems, a critical step is making electrical contacts to the molecules by interfacing them with metallic surfaces. However, the harsh chemicals and processes needed for traditional chip manufacturing damages these fragile molecular materials, reducing reliability and performance.
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To leverage the scalability of standard fabrication techniques while achieving the precision needed for handling molecules, the MIT researchers developed a decoupled, two-step approach.
They first fabricate all the device components using standard semiconductor manufacturing, then incorporate the molecular material after-the-fact to finish building the device.
"By bringing the delicate materials into the process only after we have fabricated the main device elements, it allows us to use conventional processes that are normally not compatible with these nanomaterials," Satterthwaite says.
In their demonstration, the researchers fabricated a scaffold with two metal electrodes separated by a precisely sized gap. Then, they deposited the molecular layer on the electrode surfaces.
Finally, the researchers leverage nanoscale forces to gently pull the top electrode onto the molecules, forming the final device in a nondestructive way. This creates a self-aligned, damage-free electrical contact to the molecules.
Using the forces
While gravity is a dominant physical force that holds our world together, different forces dominate at the nanoscale. One, called the capillary force, causes liquid to get sucked into small spaces. (Plants rely on capillary forces to draw water into their stems.)
By carefully engineering the stiffness of the electrodes, when the solution containing the molecules evaporates, capillary forces gently pull the two metal surfaces together with the molecules sandwiched in between.
Once the two electrodes are in place, the researchers must hold them in a stable state. To do so, they rely on another nanoscale force known as the van der Waals force.
Van der Waals forces cause surfaces to attract one another. By controlling the device surface area and molecules properties, the researchers ensure these forces will be strong enough to hold the electrodes in a stable structure without damaging the molecules.
"Nanoscale forces play a critical role in our approach. Instead of fabricating exactly the structures we ultimately want, we make something mechanically mobile and use forces to transform it into an architecture that would otherwise be impossible to fabricate," Spector explains.
They used this technique to fabricate more than 1,000 devices with molecular layers less than 1 nanometer thick. Even at this tiny scale, the fabricated chips comprised a high yield of working devices, 96 percent on average. The robust devices also endured tens of thousands of electrical cycles without showing any sign of degradation.
"The stability really stands out. This is a critical feature for moving molecular devices toward practical applications, but it has been a persistent challenge in the field," Satterthwaite says.
Importantly, this versatile technique allows circuit- and system-level integration of molecular devices, pushing the field beyond the study of isolated devices, the researchers say. They demonstrated this by building an interconnected array of molecular memory devices which could have applications in next-generation computing platforms.
Their technique can also be extended to other materials and device architectures.
In the future, the researchers want to build on this platform to investigate and develop new classes of multifunctional computing and sensing devices and systems.
"By enabling the pristine integration of emerging molecular materials and other atomic-scale matter into functional devices at scale, our platform accelerates discovery and design of these materials with tailored functionalities and their deployment in emerging technologies," Niroui adds.
This research was funded, in part, by the U.S. Defense Advanced Research Projects Agency (DARPA), the Semiconductor Research Corporation, the U.S. National Science Foundation (NSF), the MathWorks Fellowship, and the Netherlands Organization for Scientific Research. Device fabrication was carried out, in part, using MIT.nano facilities.
