It is well known that to make integrated circuits faster and more powerful, the individual elements within them must be made smaller, leading Intel cofounder Gordon Moore to postulate that the number of transistors per square inch within integrated circuits (ICs) would double every 18 months. However, it is also well known that this trend cannot continue forever; at some point, those elements will reach atomic proportions, and where do we go from there?
FIG. 1: IBM''s nanotube-based ring oscillator is one-fifth the width of a human hair.
IC elements haven't achieved atomic sizes yet, but research at IBM's Thomas Watson Research Center (www.research.ibm.com) in New York could bring them close to the molecular level. IBM scientists recently announced that they have created the first complete circuit based on a single carbon-nanotube molecule using standard semiconductor processes.
Carbon nanotubes are molecules of carbon atoms that resemble microscopic rolls of chicken wire that are 50,000 times thinner than a human hair. Their unique electrical properties, such as ultralow resistance, could allow them to carry higher current densities than any interconnecting material used in today's chips, which are becoming limited by an effect called phonon scattering. As more transistors are packed into chips, an electron's movement is hindered when it becomes coupled to vibrations in the surrounding lattice, increasing resistance and limiting speed. That does not occur in a one-dimensional conductor such as a carbon nanotube.
The circuit built by the IBM team is called a ring oscillator, and it consists of 12 field-effect transistors laid along a carbon nanotube 18 microns long, which is about one-fifth the width of a human hair (see Fig. 1). Ring oscillators are used by chip makers to evaluate new manufacturing processes or materials, because their properties provide a good indication of how potential technologies will perform when used to construct complete ICs. As a result, IBM can now use a ring oscillator to test the performance of nanotube-based transistors and circuits in complete chip designs.
“Carbon-nanotube transistors have the potential to outperform state-of-the-art silicon devices,” says Dr. T. C. Chen, vice president of science and technology at IBM Research. “However, scientists have focused so far on fabricating and optimizing individual carbon-nanotube transistors. Now we can evaluate the potential of carbon-nanotube electronics in complete circuits — a critical step toward the integration of the technology with existing chip-making techniques.”
The nanotube ring oscillator was able to achieve a clock speed of 50 MHz, which is positively poky compared with the gigahertz speed of today's microprocessors. However, it's almost a million times faster than previously demonstrated circuits consisting of multiple nanotubes. IBM believes that new fabrication processes will eventually lead to superior performance from nanotube electronics.
According to IBM research staffer Joerg Appenzeller, “This isn't about making the circuits smaller; it's about making them faster. Nanotubes fit the characteristics we need to advance high-end processing. We are exploring moving beyond silicon-based chips to significantly boost the performance of PCs. Silicon chips will continue to improve, but we need to look ahead decades into the future. There are a lot of issues still to be resolved, but we believe that the nanotubes can complement silicon chip technology.” This is great news for all electronic musicians, but don't get too excited — it will be years before nanoprocessors become commercially available. Still, we can dream, can't we?