beams mold tiny holes
Technology Research News
Tiny holes can do amazing things. In living
things, nanopores play an important role in regulating the way substances
flow through cell membranes. Tiny holes are also a key component of the
junctions and switches that allow electronic devices to do the logical
operations that make up computing.
Researchers have historically looked to nature to make the tiniest and
most precise holes. A cell’s pores, for instance, can be as small as 0.3
nanometers in diameter, which is an order of magnitude smaller than current
hole-making techniques can produce with precision, and 225,000 times smaller
than the diameter of a human hair.
Scientists at Harvard University have discovered a way to make holes nearly
as small as nanopores in a way that allows the researchers to precisely
control their size.
The ion-beam sculpting technique uses beams of ions, which are negatively
or positively charged atoms, to manipulate matter atom by atom. The analogy
refers to the way a sculptor working with clay takes material from one
place and puts it in another. With ion-beam sculpting, the atoms are the
clay, and an ion beam is the sculpting tool.
The researchers discovered ion-beam sculpting when they were trying to
create a nanopore by exposing a thin layer of silicon nitride to an ion
beam. The membrane had a cavity on one side and a flat surface on the
other. The idea was to use ion-beam sputtering, which is a process like
sand blasting but on a much smaller scale, to create a tiny hole, said
Jene Golovchenko, a professor of applied physics at Harvard University.
The researchers set out to blast away the flat side of the membrane, atomic
layer by atomic layer, until the surface intercepted the bottom of the
The researchers found that they could remove layers of the membrane until
it became very thin between the flat surface and the bottom of the cavity,
but could not break through the membrane to create the pore.
To investigate the problem, they shot an ion beam through an existing
hole, and something curious happened. The hole began to shrink.
This was a shock, said Golovchenko. “My colleagues found this hard to
believe. Something else was going on that we hadn’t thought much about,”
That something else is the surprising behavior of atoms when exposed to
an ion beam. Though the ion beam removes many of them, some pesky atoms
stick around. Those atoms adhere to the edges of the pore, causing the
pore to shrink. If the process continues, the pore eventually closes.
In ion-beam sculpting, scientists begin with a larger hole, which they
then shrink using an ion gun and a device that counts the number of ions
passing through the hole. The smaller the number of ions, the smaller
the hole has become. The researchers set the apparatus to automatically
turn off when the hole reaches a certain size.
Whether the hole opens or shrinks depends on the intensity of the ion
beam and the temperature of the material. The researchers found that the
hole could be made larger or smaller by lowering or raising the temperature.
This is because at lower temperatures removal wins out and the hole widens,
and at higher temperatures more atoms stick around and the hole shrinks.
The researchers created a nanopore in silicon nitride to detect a single
molecule of DNA. To do this, they placed the silicon nitride between two
electrically separated areas of a salt solution. Using a small amount
of voltage, they coaxed a current of ions to flow through the pore, making
one side of the saline solution negatively charged, and the other positively
When the researchers put double-stranded DNA in the negatively-charged
side and applied a voltage designed to draw the strands through the nanopore,
they found that the DNA molecules partially blocked the current of ions
flowing through the hole. This measurable change in the ion current signaled
that a DNA molecule was passing through the hole.
Scientists could use the phenomenon to make DNA-sensing devices that measure
the number, length, and chemical make-up of DNA molecules more quickly
than current technologies, said Golovchenko. Previous sensing devices
have been made of less rigid organic materials.
The discovery of ion beam sculpting also has implications in the semiconductor
industry, where both ion beams and materials like silicon nitride are
used widely. “Ion beams can be controlled very nicely,” said Golovchenko.
The researchers’ discovery is promising because it could allow for an
unprecedented level of control over tiny holes, said Jie Han, a senior
research scientist at NASA. “It has been a great challenge to reproducibly
fabricate nanopores whose size is smaller than 5 nanometers,” he said.
“If ... 2-nanometer nanopore structures can be fabricated in a controlled
and economic way, it may be applied to single molecular DNA sequencing,
genotyping and clinical diagnostics first,” said Han, pointing out that
potential markets for such technologies are in the billions of dollars.
The researchers plan to continue experimenting with ion beam sculpting
using other materials, such as silicon dioxide, said Golovchenko.
Golovchenko’s research colleagues were Jiali Li, Derek Stein, Ciaran McMullan,
Daniel Branton and Michael J. Aziz. They published the research in the
July 12, 2001 issue of the journal Nature. The research was funded by
the U.S. Defense Advanced Research Projects Agency (DARPA), the National
Science Foundation (NSF) and the U.S. Department of Energy (DOE).
TRN Categories: Materials Science and Engineering; Nanotechnology
Story Type: News
Related Elements: Technical paper, "Ion Beam Sculpting at
Nanometer Length Scales” Nature, July 12, 2001.
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