Carving beams shrink circuits

By Chhavi Sachdev, Technology Research News

Molecular electronics is a brave new vision of computing, poised to make smaller and faster circuits when silicon chips have reached their size threshold and can shrink no further. Scientists at IBM's T.J. Watson Research Center have taken a step towards using molecules to compute by diminishing the size of electrode gaps by an order of magnitude.

Electrical devices use the presence or absence of electrical current to represent the ones and zeros of computing. Current flows from an annode, or positive electrode, to a cathode, or negative electrode. In order to make electronic devices that can take advantage of the small size of molecules, the electrodes that carry signals between them must also be small.

"The distance between... electrodes is very important for nanoapplication in nanoscale science and technologies," Kun Liu, who was a member of the IBM research team and is now a research staff scientist at FEI Company. Electrodes are necessary to connect molecules and nanoparticles to the larger world of computers and machines, he said. Nanoparticles can be as small as a couple of nanometers in diameter. A DNA molecule has a width of two nanometers. A nanometer is a millionth of a millimeter.

The IBM researchers used a modified type of electron beam lithography, a method of chiseling a substance by bombarding it with electrons, to make anode-cathode gaps as small as 3 nanometers, which is about the size of 30 hydrogen atoms lined up in a row.

This electrode gap is at least 10 times smaller than current methods can produce, according to Liu. In addition, these methods, which include electrochemical plating, shadow evaporation and electron beam lithography, involve complicated processes that are not easy to control and produce only one or a few gaps at a time, said Liu. Existing electron beam methods, for instance, require a very small size beam controlled by a precise alignment system, he said.

The researchers' modifications took away these requirements, and at the same time increased yields, according to Liu. "We made 972 gaps... and found almost all of them [were] below 10 nanometers," said Liu. In theory, "you can fabricate millions of narrow gaps in a 4- by 4-square millimeter area."

Yields are nearly 100 percent for 8- to 9-nanometer gaps, but drop to 15 percent for 3- to 4-nanometer gaps, according to Liu. The gaps are electronically clean, have high insulating resistance, and do not leak, meaning little energy is wasted, he said.

The method can be implemented in any lab equipped with a standard electron beam system, said Liu.

To make the gaps, the researchers used a silicon wafer layered with the plastic polymethylmethacrylate (PMMA). They formed tiny gaps by bombarding the wafer with a pair of overlapping electron beams, said Liu.

The researchers were able to control the beams so that an area of the polymethylmethacrylate between 3 and 10 nanometers wide received a lower dose than the amount needed to break down the plastic, leaving a thin wall. Then they added titanium-silver to the wafer to form electrodes. The researchers then removed the plastic walls, leaving tiny gaps between the electrodes, according to Liu. The electrodes were 100 to 200 nanometers wide and 25 to 30 nanometers thick.

For the 3- to 4-nanometer gaps, the yield is smaller because the walls are often are too thin to withstand the process, said Liu.

The researchers have pulled off a couple of clever ideas, said Mark Ratner, a professor of chemistry at Northwestern University. "This is a technical achievement... along the road to really doing molecular junctions."

"It's interesting that their gap leakage resistance is so big, which means that they do not have spurs across [the gap,] or flaky metal or anything of the kind," he said. "You can make very, very good isolated junctions but by the time you get down to 4 nanometers, [such low leakage] is really good," said Ratner.

While it's not clear how stable the system is when heated and cooled repeatedly, the data seems sound, Ratner said.

The researchers are planning next to use smaller electrodes and test the technology in nanodevices, said Liu. They have shown that this technology could be used to make electrodes that allow only one electron through at a time, he said. "Experiments show single-electron coulomb blockage behavior, which means one electron could tunnel at a time."

The researchers also plan to make electrodes with different metals, said Liu. "We would expect this technology to be in practical use in labs immediately and in industries within next few years," he said.

Liu's research colleagues were Phaedon Avouris, Jim Bucchignano, Richard Martel, and Shouheng Sun at IBM and Josef Michl at the University of Colorado. They published the research in the February 4, 2002 issue of Applied Physics Letters. The research was funded by IBM's Research Division and the University of Colorado.

Timeline:  now; 3 years
Funding:  Corporate; University
TRN Categories:  Integrated Circuits
Story Type:   News
Related Elements:  Technical paper, "Simple Fabrication Scheme for Sub-10 Nanometer Electrode Gaps Using Electron Beam Lithography," Applied Physics Letters, February 4, 2002.


March 20/27, 2002

Page One

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