Princeton builds tiniest tunnel,
reveals nanostamping process

PRINCETON, N.J. — Princeton’s Nanostructures Laboratory (NSL) is on a roll, with two significant small tech advances announced in one week.

In a paper published today in Applied Physics Letters, the group reported building the world’s smallest enclosed channel, less than 10 nanometers wide. In the June 20 issue of Nature, NSL announced a new method for creating nanoscale structures on the molten surface of a silicon wafer. They are calling the process Laser-Assisted Direct Imprint (LADI).

The work on the nanofluidic channel, led by NSL Director Stephen Chou and Princeton physics Professor Robert Austin, is based on Chou’s nanoimprint lithography (NIL) technique. NIL creates nanoscale patterns on a surface by direct contact, a highly miniaturized form of printing.

As Chou’s postdoctoral researcher Han Cao explained, millions of such tiny tunnels could function as a nanofluidic array for screening strands of DNA or proteins from a single cell. Companies such as U.S. Genomics and Cornell Professor Harold Craighead’s research group are already pursuing similar strategies to get tangled balls of DNA to unravel so they could be “read” by a laser as they were pulled through a very narrow channel.

The problem: No one had developed a small enough channel to control and confine the tiny filament of DNA. Without an extremely narrow conduit to thread the genetic string through, one might as well be trying to push a length of yarn through New York’s Lincoln Tunnel.

With NIL, the Princeton team could already make millions of nanoscale channels in a 100-millimeter surface. The trick, Cao said, was figuring out how to cover those open trenches to form an enclosed conduit. One way would be to bond a solid roof over the channels with another wafer. But at the nanoscale, such a roof would have to be almost perfectly flat to create a solid seal.

The solution Cao and his postdoctoral colleague Jonas Tegenfeldt devised entails depositing atoms of silicon dioxide by bouncing them at carefully controlled, changing angles. The idea is roughly analogous to snow falling at an angle and piling up along the side of a house. The result produced a cone-shaped channel inside the open trench. The smaller the trench they started with, the smaller the enclosed channel would be.

What excites Cao about the group’s success in creating tiny tunnels is the potential to build a device that could analyze the DNA from a single, specific cell, such as a tumor cell — as opposed to current techniques, which extract DNA from thousands of cells.

Printable Pentiums?

Chou’s new technique for nanoimprinting on silicon could change how computer chips are made. Today, circuitry is carved into silicon wafers through elaborate and time-consuming photolithographic steps of masking, depositing layers material and etching away unwanted structures.

Chou reported that, with his LADI system, complex structures and patterns as small as 10 nanometers could be embossed into silicon in as little as 250 nanoseconds. The process would be simple, fast, cheap and require less use of strong chemicals.

LADI begins with a 1-millimeter-thick quartz mold fabricated with conventional etching techniques. An excimer laser pulse shines through the quartz, briefly melting a thin upper layer of the silicon, enabling the mold to reshape the surface of the silicon. The silicon resolidifies very rapidly.

“You just imprint the pattern directly into the silicon,” said Chou. “You not only reduce the number of steps, you can do it in nanoseconds.”

Chou reported that the LADI process, which Princeton has filed to patent, also work with other materials such as polysilicon, making the method potentially a good tool for stamping out gates for field-effect transistors.

Graduate students Chris Keimel and Jian Gu co-authored the paper in Nature.


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