Stacked Circuits
Multilayered structures bring carbon-nanotube processors closer to reality

Source: “Monolithic three-dimensional integrated circuits using carbon nanotube FETs and interconnects” and “VMR: VLSI-­Compatible Metallic Carbon Nanotube Removal for Imperfection-Immune Cascaded Multi-Stage Digital Logic Circuits using Carbon Nanotube FETs”
Hai Wei et al. and Nishant Patil et al.
International Electron Devices Meeting, December 6-9, 2009, Baltimore, MD

Results: Researchers at Stanford demonstrated the most advanced computing and storage elements yet made from carbon-nanotube transistors. They also stacked these transistors in three layers, yielding the first multilayered integrated circuits made from carbon nanotubes.

Why it matters: Carbon-­nanotube transistors are potentially faster and more energy efficient than their silicon equivalents, but integrating them into complex circuits has been challenging–in part because when researchers grow arrays of nanotubes in the lab, only some of them are the well-formed semiconducting nanotubes that can be used in circuits. Some are defective, and others are metallic, a property that causes circuit malfunctions. The researchers found a way to work around these problems, making the circuits immune to such imperfections. And the techniques allow for multilayered circuits, which can be faster and more efficient than single-layered circuits.

Methods: The researchers grow carbon nanotubes on a quartz substrate at 800 °C and use an adhesive tape to transfer them to a silicon wafer. Then they lay down a special pattern of metal electrodes on top. To eliminate metallic nanotubes, the researchers use the silicon substrate as the back gate to turn off the semiconducting nanotubes; then they burn the rest out with a blast from the electrodes. Next, they use a chemical etching process to remove electrodes not needed for the final chip design. A variant of this technique is repeated to create multiple layers. Layering the circuits is possible because the transfer process can take place at relatively low temperatures (under 130 °C), so the underlying electrodes don’t melt.

Next steps: The researchers will work to increase the complexity of the circuits by refining the designs and by developing ways to grow the nanotubes more densely. So far, they have grown five to 10 nanotubes per micrometer; high-performance circuits will require 10 times this density.