The thinnest films of copper look flat, but they aren’t

Like the surface of an alien planet, thin sheets of copper display a complex topography of ridges and valleys. These never-before-seen undulations may spell trouble for electronic gadgets: The zigzagging surface could contribute to the electrical resistance of miniature copper wires that snake throughout computer chips.

Using a scanning tunneling microscope, scientists observed nanoscale peaks and dips on a sheet of copper, with angles of a few degrees, researchers report in the July 28 Science. “When we saw that, we were absolutely shocked,” says materials scientist John Boland of Trinity College Dublin. Conventional wisdom was that the copper would lay mostly flat.

Copper and other metals are a conglomeration of smaller bits, known as grains. Within each grain, the atoms are neatly arranged, but at grain boundaries, the pattern is disrupted. In the type of copper the researchers studied, nanocrystalline copper, the grains are particularly small; each has around 1 million atoms. Boland and colleagues showed for the first time that, in films of nanocrystalline copper just tens of nanometers thick, peaks and dips appear where misaligned grains meet.

“This is a completely new observation,” says materials scientist Peter Nellist of the University of Oxford, who was not involved in the study. Metals that have the same crystalline structure as copper might show similar behavior, he says.  

Copper’s rippled surface could add to the electrical resistance of nanowires made from the metal. Electrons traveling through the material would have to change direction to navigate the landscape, impeding their progress. In an electronic device, extra resistance can generate heat or drain battery power faster.

“Now that we know it’s happening, we can think about how we can control it,” Boland says. For example, scientists might be able to add another material to copper wires, such as aluminum, which could change how the grain edges meet and keep the metal from angling upward. Or, scientists could affix the copper to a stiff material that the metal bonds to very strongly. That might help keep copper flat, clearing the way for electrons to flow more easily.