Carbon nanotubes not so great for interconnect

The advantages of carbon nanotubes over aluminium and copper for fine geometry interconnect have been questioned by the US National Institute of Standards and Technology (NIST).

"Carbon nanotubes theoretically can carry 1,000 times more electric current than a metal conductor of the same size, but recent tests at NIST suggest device reliability is a major issue," said the organisation.

Researchers fabricated and tested numerous nanotube interconnects between metal electrodes.

The tests showed that nanotubes can indeed sustain extremely high current densities - tens to hundreds of times more than typically found in interconnect - for several hours but slowly degrade under constant current.

They also showed that interfaces between metal and nanotubes are vulnerable (see photo below).

"The metal electrodes fail, the edges recede and clump, when currents rise above a certain threshold. The circuits failed in about 40 hours," said NIST, which is developing measurement and test techniques for nanotube-containing electrical structures.

"The common link is that we really need to study the interfaces," said Dr Mark Strus of NIST.

In a related study, NIST researchers identified failures in carbon nanotube networks where electrons hop from tube to tube.

Failures in this case seemed to occur between nanotubes, the point of highest resistance, said Strus.

In this case NIST is trying to established if degradation is steady and therefore predictable, or unpredictably sporadic.

Despite the concerns, Strus sees a future for nanotube conductors.

"For instance, carbon nanotube networks may not be the replacement for copper in logic or memory devices, but they may turn out to be interconnects for flexible electronic displays or photovoltaics," he said