Manufacturing transistors that combine silicon with transition metal dichalcogenides (TMDs) requires an extremely precise process. Engineers need to remove atoms only from the top sulfur layer while leaving the underlying layers intact.
The most common technique for doing this relies on plasma, an ionized state of matter also found in the Sun and stars, and a field extensively studied for decades at the Princeton Plasma Physics Laboratory. Under carefully controlled conditions, plasma particles collide with the TMD surface and dislodge sulfur atoms.
However, the process demands a delicate balance – too little force fails to remove the sulfur layer, while too much can damage the molybdenum layer beneath, compromising the material’s performance.
Achieving clean removal of the top sulfur layer without harming the layers underneath remains a major manufacturing challenge due to the narrow process window. Using computer simulations, the research team found that pre-treating Molybdenum disulfide with oxygen or fluorine significantly improves control during the etching process.
Their findings, published in the Journal of Physical Chemistry Letters, showed that surface coatings substantially reduce the energy required to dislodge sulfur atoms. On an untreated surface, removing a sulfur atom requires around 30 electron volts, but that threshold drops to roughly 10 electron volts with fluorine treatment and about 14 electron volts with oxygen, lowering the risk of damaging deeper layers.
A lower energy threshold is important because plasma ions do not all strike the surface with identical energy levels; some variation is unavoidable during the process. On an untreated surface, the margin between removing sulfur atoms and damaging the underlying molybdenum layer is extremely small, meaning some ions can easily cause unwanted damage.
Reducing the sulfur-removal threshold to around 10 or 14 electron volts significantly expands that safety window, providing manufacturers with a more practical operating range in which the top layer can be removed cleanly while preserving the structural integrity of the material below.
Rather than depending purely on physical force to strip away atoms, the researchers introduced a chemically assisted approach. When an incoming ion hits an oxygen-coated Molybdenum disulfide surface, two oxygen atoms react with a nearby sulfur atom to form sulfur dioxide, a stable gas molecule that can detach and disperse naturally.
Fluorine produces a similar effect by creating sulfur-fluorine compounds that are easier to remove. According to lead author Yury Polyachenko, this method works by generating intermediate chemical products that weaken the bonds, making sulfur atoms far easier to separate from the surface.
The research is now moving from proof-of-concept toward a more detailed assessment of reliability and scalability. The immediate priority is to quantify the extent of any material degradation caused by the process, rather than simply determining whether damage occurs at all.
Beyond that, the team plans to test whether the same chemically assisted approach can be extended to a wider range of related materials. This includes substituting molybdenum with tungsten or replacing sulfur with selenium in similar layered structures, with a goal to evaluate how universal the technique might be and whether it can support a broader class of next-gen semiconductor materials.
Bojan Stojkovski is a freelance journalist based in Skopje, North Macedonia, covering foreign policy and technology for more than a decade. His work has appeared in Foreign Policy, ZDNet, and Nature.
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