Flat nanoscale films of copper prove impossible

Materials World magazine
1 Sep 2017

A new understanding of the structure of nanocrystalline films reveals that perfectly flat copper nanoscale films cannot be formed. Kathryn Allen reports.

Until now, grain boundaries in nanocrystalline copper films have been thought to be perpendicular to the material’s surface. But new research shows that these grains are often rotated, forming ridges that cause surface roughness. 

This discovery is detailed in Nanocrystalline copper films are never flat by a team of researchers from AMBER, the Science Foundation Ireland-funded materials science centre based in Trinity College Dublin, Ireland, the University of Pennsylvania, USA, Imperial College London, UK, and the Intel Corporation Components Research Group, USA. 

The researchers used scanning tunnelling microscopy to study the grain boundaries of copper, finding that the presence of these boundaries creates a misaligned surface, with ridges and valleys formed by grain rotation. The grains are made up of millions of atoms and the rotation is created by reduced grain boundary energy. 

Professor John Boland, Investigator at AMBER and corresponding author of the paper, told Materials World, ‘Scanning tunnelling microscopy uses a sharp tip to follow the relative orientation of the grains as the tip is scanned over the surface. The vertical resolution is in the order of a billionth of a metre, so it can detect very slight misalignments.’ Using this technique, the team could measure the angles between adjacent grains. 

While the team focused on copper films, Boland claims the findings will also apply to silver, gold and potentially nickel, stating that ‘the principles [of the research] are relevant to any material where the elastic properties are different in different directions.’ 

This discovery at the nanoscale has implications for material use. The structure of a material’s grains affects the thermal, electrical and mechanical properties of the material. Boland said, ‘These properties depend on the connection between grains and the degree to which the atomic planes in one grain are aligned with those in neighbouring grains. Grain rotation will impede the flow of heat or electrons.’ 

As nanocrystalline copper and other nanocrystalline metals are used in integrated circuits as electrical contacts and interconnects, this improved understanding of grain structure can aid the development of more efficient and durable circuits and devices.  

Regarding the next step for this research, Boland said, ‘We hope to modify the interaction between the film and the substrate to control the degree of grain rotation and we hope this will affect the electrical, thermal and mechanical properties.’ Boland believes this research will not only impact consumer electronics, but also medical implants and diagnostics.