This technique, recently published in the prestigious journal Nature Communications, was developed by researchers from the Graphene Flagship at RWTH Aachen University, the Bundeswehr in Munich, AMO GmbH in Germany, and the KTH Royal Institute of Technology in Sweden, in collaboration with Protemics GmbH.
Integrating 2D materials, such as graphene, with silicon (or any other substrate used in integrated circuits) presents a number of challenges. “There is always a critical step when you transfer the 2D material from the substrate where it is synthesized to the final substrate that you will use to manufacture electronic components such as sensors,” says Arne Quellmalz, co-author of the paper and a researcher at KTH, a center associated with the Graphene Flagship. “You might want to combine a graphene photodetector for on-chip optical communications with a readout on silicon electronic circuits, but the temperature for manufacturing these materials is too high; you can’t do this process directly on the substrate of the electronic devices.”

Until now, methods developed for transferring 2D materials from the substrate where they are synthesized to electronic components have either been incompatible with mass production or produced defects in the 2D material that compromised its electronic properties. But the technique developed by Quellmalz and his team at the Graphene Flagship leverages technologies and tools already used in semiconductor fabs: it utilizes a standard dielectric material called bisbenzocyclobutane (BCB) and equipment for assembling silicon wafers.
“Basically, we glue the two wafers together using a BDB resin,” Quellmalz explains. “We heat the resin until it becomes viscous, like honey, and then we glue the 2D material on.” When it cools to room temperature, the resin solidifies and forms a bond between the 2D material and the silicon wafer, he adds. “To stack materials, we simply repeat the heating and pressing steps. As the resin becomes viscous again, it acts as a cushion, like a waterbed, supporting the 2D materials and conforming to their surface.”
Researchers at the Graphene Flagship demonstrated their technology for transferring graphene and molybdenum disulfide (a material in the metal dichalcogenide family). They also fabricated heterostructures of 2D materials using graphene, molybdenum disulfide, and boron nitride. Once transferred to the wafer, they analyzed the materials and observed exceptionally high quality. The silicon coating was uniform on wafers up to 100 millimeters in diameter, and the 2D materials showed no signs of stress or defects.

“Our transfer method can be applied to any 2D material, regardless of its size or the substrate on which it was synthesized,” says Max Lemme, co-author of the paper and a researcher at AMO GmbH and RTWH Aachen, two centers in Germany that are members of the Graphene Flagship. “Furthermore, because it uses tools and methods that are very common in the semiconductor industry, it could accelerate the market penetration of a new generation of devices where 2D materials are integrated with conventional circuits and microchips. This work represents a major step forward toward this goal, and although there are still challenges to overcome, the possibilities it opens up are enormous: applications in photonics, sensors, and even neuromorphic computing. The integration of 2D materials could be a game-changer for the European technology industry.”
The European Commission recently announced a €20 million project to bridge the gap between 2D material fabrication methods in the lab and large-scale electronic device production. The project, launched under the Graphene Flagship, is known as the “2D Experimental Pilot Line (2D-EPL).” “This paper is a great example of the work being done by the 2D-EPL project,” says Cedric Huyghebaert, program director for new materials and integration at imec, Belgium, and technical director of 2D-EPL. “This is one of the most pressing pending tasks: developing tools, methods, and manuals for fabricating devices that harness the potential of 2D materials and are compatible with semiconductor industry standards,” says Huyghebaert. “The next step will be to demonstrate the potential of these processes by fabricating sensors and optoelectronic devices in a pilot plant.”

Reference
“Large-area integration of two-dimensional materials and their heterostructures by wafer bonding”, A. Quellmalz, (2021), DOI: 10.1038/s41467-021-21136-0.