Researchers from South Korea’s Pusan National University have reportedly developed corrosion-free copper thin films.

A major component used in the electronics, semiconductor and electro-optics industries thanks to its electrical conductivity, copper is vulnerable to oxidation, a type of surface corrosion, which often leads to increased electrical resistance and, eventually, limited lifespan. As such, the Pusan researchers sought to develop a way to fabricate oxidation-resistant thin films of copper that could potentially replace gold in semiconductor devices, thereby cutting their costs.

Source: Pusan National University

Copper oxidation is a type of corrosion that occurs in a three-step process wherein copper oxidizes to copper oxide, then to cuprous or cupric sulfide, and, ultimately to copper carbonate, resulting in a green copper layer, or patina, which forms over time. These steps serve as a source of adatoms (adsorbed atoms) to interact with oxygen and provide a place for oxides to grow.

To enable single-crystalline copper to demonstrate resistance to oxidation, the researchers used a process called atomic sputtering epitaxy to devise tightly coordinated flat single-crystal copper films. The team reportedly used noise reduction systems to limit electrical and mechanical noises, thereby keeping copper surfaces almost entirely defect-free and fabricating atomically flat films.

Using high-resolution transmission electron microscopy (HR-TEM) to study the copper films, the researchers discovered that the film grew in the [111] direction and demonstrated a nearly flat surface with only occasional mono-atomic steps. The team then went on to compare the single-crystal copper (111) films (SCCFs) with other copper films featuring a higher surface roughness. Their findings revealed that the SCCFs were oxidation-resistant due to the difficultly for oxygen to penetrate the mono-atomic step edge, according to the researchers.

Further, the team then used a microscopic model of copper oxidation based on "density functional theory" to assess how SCCFs interact with oxygen, discovering that the SCCF’s surface was protected by oxygen, once 50% of its surface was inundated with oxygen atoms.

Likewise, the high energy barrier created by the researchers suppressed absorption of oxygen atoms on the SCCF.

The article, Flat-surface-assisted and self-regulated oxidation resistance of Cu(111), appears in the journal Nature.