Nanotube breakthrough promises replacement for silicon

Posted by Emma Woollacott

IBM says it's made a big step towards the commercial production of carbon nanotube-based computer chips.

How? By precisely placing and testing more than ten thousand working transistors made of nanotubes in a single chip, using standard semiconductor processes.

And the move comes just in time, with silicon chips rapidly approaching the theoretical limits of their performance.

Electrons in carbon transistors can move more easily than in silicon-based devices, allowing for quicker transport of data. The nanotubes are also ideally shaped for transistors at the atomic scale, another advantage over silicon.

"Carbon nanotubes, born out of chemistry, have largely been laboratory curiosities as far as microelectronic applications are concerned. We are attempting the first steps towards a technology by fabricating carbon nanotube transistors within a conventional wafer fabrication infrastructure," says Supratik Guha, director of physical sciences at IBM Research.

"The motivation to work on carbon nanotube transistors is that at extremely small nanoscale dimensions, they outperform transistors made from any other material. However, there are challenges to address such as ultra high purity of the carbon nanotubes and deliberate placement at the nanoscale. We have been making significant strides in both."

The IBM breakthrough paves the way for circuit fabrication with large numbers of carbon nanotube transistors at predetermined substrate positions. Up to now, scientists have been able to place at most a few hundred carbon nanotube devices at a time - not nearly enough to address key issues for commercial applications.  

There are of course practical challenges - most notably centered around the purity and placement of the devices. Carbon nanotubes naturally come as a mix of metallic and semiconducting types and need to be placed perfectly on the wafer surface to make electronic circuits.

For device operation, the metallic ones need to be removed to prevent errors in circuits. Also, for large scale integration to happen, it is critical to be able to control the alignment and the location of carbon nanotube devices on a substrate.

To overcome these barriers, IBM researchers developed a new method based on ion-exchange chemistry.It allows precise and controlled placement of aligned carbon nanotubes on a substrate at a high density – two orders of magnitude greater than ever before, and enabling the controlled placement of about a billion nanotubes per square centimeter.

The process starts with carbon nanotubes mixed with a surfactant, a kind of soap that makes them soluble in water. A substrate is comprised of two oxides with trenches made of chemically-modified hafnium oxide (HfO2) and the rest of silicon oxide (SiO2). The substrate gets immersed in the carbon nanotube solution and the nanotubes attach via a chemical bond to the HfO2 regions while the rest of the surface remains clean.

Rapid testing of thousands of devices is possible, says IBM, using high volume characterization tools that are compatible with standard commercial processes.