beams shrink circuits
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
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
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.
Monkey think, cursor do
DNA solves big problem
Carving beams shrink
Lasers snatch free-floating
Etching could boost
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link