A recent paper in Nano Letters illustrates both why researchers are so interested in carbon nanotubes as an interconnect medium and why making the nanomaterials a useful reality is such a hurdle. Researchers at Stanford University, working with support from Toshiba and foundry services from TSMC (Taiwan Semiconductor Manufacturing Co), have successfully connected ring oscillators using nanotubes from a commercial third party. The hit-or-miss results displayed oscillator frequencies as high as 1 GHz—a first for this new material. TSMC fabricated a die with 256 ring oscillators; one interconnect link was missing from each ring. The Stanford/Toshiba team then attempted to make this one missing connection using commercially manufactured metallic-mode nanotubes. They succeeded for 19 of the 256 circuits. Of those 19, 16 of the connections showed impedance low enough to allow the oscillators to operate at frequencies greater than 800 MHz, and one achieved 1.02 GHz. The team used nanotubes measuring 50 to 100 nm in diameter and approximately 5 microns long. HS Phillip Wong, professor of electrical engineering at Stanford and co-author of the paper, observes that significant improvements in both the quality of the nanotube stock and the technology of connecting the tubes to transistor contacts would be necessary before the technique approached production viability. But the experiment shows that the nanotubes can at least achieve reasonable frequencies.
