IBM’s directed self-assembly one facet in broad nanotech program

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YORKTOWN HEIGHTS, N.Y., Dec. 9, 2003 — Chuck Black and Kathryn Guarini demonstrated their nanotech breakthrough with strands of purple and red pipe-cleaner wire.

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In a preview of today’s announcement at the IEEE’s International Electron Devices Meeting in Washington D.C., the two IBM researchers explained how they coaxed two different polymers — stringy molecules they represented with different-colored pipe cleaners — to assemble themselves into a honeycombed template of 20-nanometer holes.

The honeycomb pattern, known as a diblock copolymer, was applied as a layer in the conventional process used to make transistors in a flash memory device, the thin cards used in digital cameras and handheld computers. The self-assembled pattern enabled silicon to form an ordered array of 20-nanometers crystals in the transistor’s gate, the area that stores the charge determining if the switch represents a 1 or 0 in binary code.

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The array of crystal dots could enable transistors to be more tightly packed into flash memory devices, increasing their storage capacity. The crystals also offered more redundancy so that flash memory cards could preserve data longer.

The scientists were quick to point out that while their process was patented, IBM wasn’t looking to jump into the flash memory business. Rather, they said it was a working example of how simple, self-assembled nanostructures were becoming practical in microelectronics. Indeed, they noted that the patterning process could be used in other applications, such as making more dense and orderly arrays of magnetic nanoparticles in data storage devices.

The self-assembly work came at the end of a recent show-and-tell day at IBM’s Thomas J. Watson Research Center. The center’s director of physical sciences, Tom Theis, set the stage earlier with an overview of how IBM expects to turn nanoscience into technology.

He emphasized that many scientists have for decades been working on what is being called nanotechnology today. And while the timetable for commercializing complex nanotech devices is longer than some companies and media reports have suggested, “now is the time to make basic investments,” he said.

Some of those investments at IBM include work on carbon nanotubes, nanowires, molecular transistors, quantum corrals, and self-assembly techniques like the one announced today.

Theis, a physicist, said that IBM scientists have also built what he characterized as the most complex structure every assembled atom-by-atom, a logic circuit that works via a cascade of carbon dioxide molecules that topple over, a little bit like rows of falling dominos. Of course, the single device took more than a day to make and only works once, but it is an indication, said Theis, of how microelectronics is gradually evolving into true nanoelectronics.

One of the under-appreciated challenges in shrinking electronics to the nanoscale, noted several IBM scientists, is that all elements have to be reduced, from transistors and circuits to the wiring that interconnects them all.

Phillip Wong, IBM’s senior manager for nanoscale materials, processes and devices, described some of the ways, for example, that researchers are trying to build nanowires to work with transistors made of molecules or carbon nanotubes circuits. One effort entails getting square nanoparticles — also known as ‘quantum cubes’ — of lead selenium to line up into neat rows that look something like narrow brick walkways.

As for the quintessential transistor, Wong noted that while IBM is working with different molecules only about 2.5 nanometers in size that could function as switches, the company has also produced an experimental silicon transistor with a gate length of a mere six nanometers. By comparison, today’s smallest commercial transistors are 65 nanometers. In Wong’s view, the tremendous expertise and inertia behind silicon technology will likely keep it the bedrock of information technology for years, if not decades, to come.

While Wong said that 2-3 nanometers may be the absolute limit at which any structure could function as a switch, he added that at such a physical boundary, the deciding factor will be price/performance, not absolute size. For example, he said that carbon nanotubes in IBM’s labs have already equaled the performance of the most optimized silicon transistors. Whether they can be fabricated into computing structures of a billion switches, and made more cheaply than with conventional semiconductor processes, is unknown.

In the meantime, Phaedon Avouris, manager of IBM’s carbon nanotube research program, said that his group remains focused on building and studying electronic devices and circuits made with single-walled carbon nanotubes. In addition to their surprising performance as transistors, Avouris noted that because carbon nanotubes can be either metallic or semiconducting depending on their structure, they could serve as both switching elements and nanowires for interconnecting atomic-scale circuits.

Avouris’s group has also found this spring that when attached to electrodes in special ways, a carbon nanotube can also function as an ultrasmall light emitter or photon detector. As he explained, when electrons and the absence of electrons known as “holes” collide in the middle of a carbon nanotube, they emit a tiny amount of infrared light. Similarly, a photon striking such specialized nanotube would create a small electrical signal.


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