Ion beams mold tiny holes

By Susanna Space, 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 cavity.

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,” he said.

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 charged.

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).

Timeline:  Now
Funding:   Government
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.


August 22/29, 2001

Page One

Nets mimic quantum physics

Teamed filters catch more spam

Software eases remote robot control

Ion beams mold tiny holes

Unusual calms tell of coming storms


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