Recently, 10000 working transistors, made purely of carbon nanotubes, have been placed and tested on a single chip through existing semiconducting fabrication processes. In this new approach to carbon nanotube technology, scientists from IBM have opened the path to commercial fabrication of computer chips that are faster, smaller and more powerful. Such a device has high hopes of replacing current silicon technology. There has been a continuing trend over the years of miniaturization of computing parts, leading the way for future microelectronics.
Silicon transistors are not new to us. They are tiny switches that carry information on a chip and they have been part of the basis of computing technology for many years now. Year after year, they have been made smaller and smaller, to the point where the ability to shrink chips will be physically limited. When silicon starts to measure only a few nanometers across, it starts to lose its effectiveness as a semiconductor. After a few more generations, classical methods of shrinking semiconductors will no longer be viable in creating low cost and faster processors.
The solution to this lies in carbon nanotube technology. They are more attractive than silicon for a couple of reasons, especially at the nanoscale where measurements are atomically sized. Electrons tend to travel easier in carbon transistors and because of this, data can be transported much faster. Moreover, nanotubes form the ideal physical shape for atomic sized transistors.
IBM is paving the way for circuit fabrication with a high density of carbon nanotubes, at discrete positions on a substrate. The ability to isolate semiconducting nanotubes and placing so many on a single chip is crucial, as over one billion transistors will need to be integrated for future chips. Right now, we are hovering at around 100. They have also shown that carbon nanotubes operate as excellent switches at molecular dimensions of less than 10 nanometers (this is less than half of the current silicon transistors).
Carbon nanotubes come naturally as a mix of metallic and semiconducting species. For operation, only the semiconducting parts are useful and not the metallic parts. Thus, there is a need to differentiate the two. To make working electronic circuits, these tubes need to be aligned and placed perfectly on a wafer through large scale integration.
IBM researchers have turned to a novel method based on ion-exchange chemistry allowing the precise and controlled placement of carbon nanotubes in high densities, at one billion nanotubes per square centimeter. First, they mixed the carbon nanotubes with a surfactant, a soap that makes it soluble in water. Then, they made a substrate comprised of two oxides with trenches made of modified hafnium oxides and silicon dioxide everywhere else. Afterwards, the substrate was submersed in the carbon nanotube solution where the tubes would chemically bond to the regions of hafnium oxide while the rest of the surface would remain clean. By doing this, they have managed to fit more than 10000 transistors onto a single chip. In addition, it is possible to do rapid testing of thousands of devices by using high volume characterization tools because of its compatibility with standard commercial processes. This process can be readily implemented as it involves the use of common chemicals and existing semiconductor fabrication techniques. It will allow the industry to work with carbon nanotubes at a larger scale and of course, to further the development of carbon electronic technology.
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