UNIST researchers boost 2D semiconductor current tenfold by blocking photoresist residue

Researchers have developed a technology that blocks contaminants generated during semiconductor fabrication at the source, boosting the performance of atom-thin two-dimensional semiconductors tenfold. The advance addresses process contamination — long considered a major obstacle to ultrafine semiconductor development — and is expected to aid in the creation of smaller, faster next-generation chips.

Professor Kim Myung-soo of the Department of Electrical and Computer Engineering at the Ulsan National Institute of Science and Technology (UNIST) and Professor Kim Byung-jo of the Graduate School of Semiconductor Materials and Components announced Monday they had developed a semiconductor process technology that uses a thin metal film to prevent photoresist residue from adhering to the surface of molybdenum disulfide (MoS₂).

Photoresist is a light-sensitive polymer used to etch fine circuits onto semiconductor surfaces. When photoresist applied to molybdenum disulfide is selectively removed by plasma after patterning, some of it hardens and bonds stubbornly to the surface.

The resulting residue is difficult to remove with conventional solvent cleaning. Ultrasonic cleaning strong enough to dislodge it risks lifting the atomically thin molybdenum disulfide off its substrate, while high-temperature heat treatment can damage the device — making both approaches impractical.

The research team solved the problem by first coating the molybdenum disulfide with a 5-nanometer-thick "sacrificial metal film." The metal layer acts as a protective barrier, preventing the photoresist from coming into direct contact with the semiconductor surface.

Once circuit fabrication on top of the metal film is complete, the film is dissolved away — taking any adhered photoresist residue with it. This keeps the two-dimensional semiconductor surface clean without requiring aggressive cleaning or high-temperature treatment.

The performance gains were significant. The contact resistance of molybdenum disulfide transistors fabricated using the new process measured 2.58×10⁵ Ω·μm — roughly one-tenth that of devices made with conventional methods. The reduction in contact resistance translated to an approximately tenfold increase in on-state current flow through the transistors.

Notably, the crystalline structure of the molybdenum disulfide showed no significant damage after the metal film was applied and subsequently removed.

The team also used computational analysis to identify why photoresist residue bonds so strongly to the molybdenum disulfide surface. Oxygen in the plasma was found to alter the chemical structure of the photoresist, roughly doubling its binding affinity to the molybdenum disulfide surface.

"Molybdenum disulfide is a material that can enable semiconductor devices such as transistors to be made smaller and thinner than is currently possible," Professor Kim Myung-soo said. "By resolving the process contamination problem that had been a major obstacle to high-performance devices, we expect this work to contribute to the development of high-density logic and memory chips."

The findings were published in the international journal Small.