ULSAN — Korean researchers have developed a technique that resolves a chronic process-contamination problem in two-dimensional semiconductors, a material drawing attention as a candidate for next-generation ultra-fine chip devices, while sharply improving device performance.
UNIST (Ulsan National Institute of Science and Technology) said on the 24th that a team led by Kim Myung-soo, a professor in the Department of Electrical and Electronic Engineering, and Kim Byung-jo, a professor at the Graduate School of Semiconductor Materials and Devices, developed a process that uses a thin sacrificial metal film to block process residue from adhering to the surface of two-dimensional semiconductors.
Molybdenum disulfide (MoS₂), just one atom thick, retains its semiconductor properties despite its thinness, making it a key material for highly integrated next-generation logic and memory devices. But because it is extremely thin, residue from photoresist — the light-sensitive coating used to fabricate circuits — clings to the surface, obstructing charge transport and sharply degrading device performance. Photoresist exposed to plasma processing hardens in particular, so it cannot be removed by ordinary solvent cleaning, while strong ultrasound or high-temperature heat treatment brings the added problem of damaging the semiconductor film.
The team first used computer simulations — molecular dynamics and density functional theory — to establish that oxygen in the plasma alters the chemical structure of the photoresist, more than doubling its adsorption energy and thereby creating defect levels that block the flow of charge.
The researchers then devised a "sacrificial metal mask lithography" method that blocks contamination at its source. Before applying photoresist, they first coat the molybdenum disulfide with a 5-nanometer (nm) sacrificial gold (Au) film to prevent direct contact with chemicals and plasma. Once the circuit is formed, dissolving the gold film with a chemical solution also removes the residue hardened on top, without damaging the film.
A molybdenum disulfide transistor made with this process showed contact resistance of about 2.58×10⁵ Ω·μm, one-tenth the previous level. As resistance fell, the on-current flowing when the transistor is switched on increased about tenfold. Microscopy and spectroscopy analysis showed no damage whatsoever to the semiconductor's crystal structure after processing.
Kim Da-hyun, the study's first author, said the process can be used not only in general semiconductor circuit fabrication but also when drawing finer circuits with an electron beam, and offers versatility in that it works regardless of how the molybdenum disulfide was synthesized.
Professor Kim Myung-soo said molybdenum disulfide is a material that can make semiconductor devices such as transistors smaller and thinner than they are now. By resolving the process-contamination problem that had been an obstacle to turning it into actual high-performance devices, the work can contribute to the future development of highly integrated logic and memory devices, he said.
The findings were published online on the 11th of this month in Small, an international journal in the nanoscience field.
