The next-generation semiconductor devices use beyond-silicon technology, which relies on a high-performance field-effect transistor (FET). A research team has unveiled a technology revolution that can significantly change the devices we use.
Challenges in Beyond-Silicon Technology
The current three-dimensional silicon technology experiences degradation of FET performances when the device is scaled down past sub-3 nm. One of the major problems is the presence of residues, which occur during fabrication.
Beyond-silicon technology requires the use of ultrahigh-performance field-effect transistors. Transition metal dichalcogenides serve as an ideal material platform. Still, interfacial residues produced from transfer procedures usually limit the device's performance, like mobility, on/off ratio, and contact resistance.
Polymethyl methacrylate (PMMA) is traditionally used as a supporting holder for device transfer. This material is known for leaving insulating residues on transition metal dichalcogenides (TMD) surfaces. This often leads to mechanical damage to the fragile TMD sheet during transfer.
Several other polymers have been proposed as supporting holders in replacement for PMMA. These include polyvinylpyrrolidone (PVP), ethylene vinyl acetate (EVA), polycarbonate (PC), polystyrene (PS), polyvinyl alcohol (PVA), polydimethylsiloxane (PDMS), and organic molecules such as naphthalene, cellulose acetate, and paraffin. Despite having these alternatives, mechanical damages and residues are still inevitably introduced during transfer, which can lead to the degradation of FET performances.
Enhanced FET Fabrication
Addressing this problem has been the main goal of a group of researchers from the Center for Integrated Nanostructure Physics within the Institute for Basic Science (IBS) in South Korea. Led by Professor Lee Young Hee, they successfully harnessed polypropylene carbonate (PPC) for residue-free wet transfer.
A bismuth semimetal contact with an atomically clean transistor was incorporated on a hexagonal boron nitride substrate, allowing them to approach the quantum limit. PPC not only eliminated residue but also allowed the researchers to produce wafer-scale TMD through chemical vapor deposition. Previous efforts to manufacture large-scale TMDs resulted in wrinkles during the transfer process. In the method developed by the IBS team, the weak binding affinity between the PPC and the TMD eliminated both the residues and the wrinkles.
According to study first author Ashok Mondal, their chosen PPC transfer method allowed them to fabricate centimeter-scale TMDs. In previous studies, the production of TMDs was limited to using a stamping method that generates flakes that only measure 30-40 µm.
The team developed the FET device using a semimetal Bi contact electrode with MoS2 monolayer transferred by the PPC method. Meanwhile, less than 0.08% of PPC residue remained in the MoS2 layer.
The result of the study is the first in the world to demonstrate the possibility of producing wafer-scale and transferring CVD-grown TMD. The pioneer FET device developed this way has electrical properties that far exceed the values from previous studies. The researchers are confident that this technology can be implemented easily using integrated circuit manufacturing technology.
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