Micrograph of recession and clumping in gold electrodes after NIST researchers applied 1.7 V of electricity to the carbon nanotube wiring for an hour. The NIST reliability tests may help determine whether nanotubes can replace copper wiring in next-generation electronics. Image: M. Strus/NIST |
Carbon nanotubes offer big promise in a small package. For
instance, these tiny cylinders of carbon molecules theoretically can carry
1,000 times more electric current than a metal conductor of the same size. It’s
easy to imagine carbon nanotubes replacing copper wiring in future nanoscale
electronics.
But—not so fast. Recent tests at NIST suggest device
reliability is a major issue.
Copper wires transport power and other signals among all the
parts of integrated circuits; even one failed conductor can cause chip failure.
As a rough comparison, NIST researchers fabricated and tested numerous nanotube
interconnects between metal electrodes. NIST test results show that nanotubes
can sustain extremely high current densities (tens to hundreds of times larger
than that in a typical semiconductor circuit) for several hours, but slowly
degrade under constant current. Of greater concern, the metal electrodes
fail—the edges recede and clump—when currents rise above a certain threshold.
The circuits failed in about 40 hours.
While many researchers around the world are studying
nanotube fabrication and properties, the NIST work offers an early look at how
these materials may behave in real electronic devices over the long term. To
support industrial applications of these novel materials, NIST is developing
measurement and test techniques and studying a variety of nanotube structures,
zeroing in on what happens at the intersections of nanotubes and metals and
between different nanotubes. “The common link is that we really need to
study the interfaces,” says Mark Strus, a NIST postdoctoral researcher.
In another, related study, NIST researchers identified
failures in carbon nanotube networks—materials in which electrons physically
hop from tube to tube. Failures in this case seemed to occur between nanotubes,
the point of highest resistance, Strus says. By monitoring the starting
resistance and initial stages of material degradation, researchers could
predict whether resistance would degrade gradually—allowing operational limits
to be set—or in a sporadic, unpredictable way that would undermine device
performance. NIST developed electrical stress tests that link initial
resistance to degradation rate, predictability of failure, and total device
lifetime. The test can be used to screen for proper fabrication and reliability
of nanotube networks.
Despite the reliability concerns, Strus imagines that carbon
nanotube networks may ultimately be very useful for some electronic
applications. “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,” Strus
says.
Overall, the NIST research will help qualify nanotube
materials for next-generation electronics, and help process developers
determine how well a structure may tolerate high electric current and adjust
processing accordingly to optimize both performance and reliability.
