A Better Way to Make Nano Stuff

Nano mold: This scanning-electron micrograph shows platinum metallic glass patterned with pillars 55 nanometers in diameter. This structure can be used as a mold for nanoimprint lithography. Credit: Nature

Without better techniques for manufacturing nanoscale devices, many will remain in the lab. The chip industry also needs better nanoscale manufacturing techniques to create ever denser data storage and ever faster microprocessors. A technique called nanoimprint lithography that uses finely patterned molds is promising, but the molds are either limited in resolution or fragile. Now researchers at Yale University have demonstrated that these molds can be created from more durable materials--an advance that could make the technique commercially viable.

"Conventional lithography is at the limit right now," says Yale mechanical-engineering professor Jan Schroers, who led the work using molds made out of metallic glasses for nanoimprint lithography. Such stamps patterned at the nanoscale promise to be simpler and cheaper than conventional photolithography.

In nanoimprint lithography, a mold made of a hard material such as metal or silicon is pressed into a softer material, often molten silicon itself or a polymer. The molds can then be reused. But both metals and silicon have limitations as mold materials. Silicon is brittle, and molds made of the material fail after about a hundred uses, says Schroers. Metal molds are more resilient but are grainy, and their features can't be any smaller than the grains in the metal itself--about 10 micrometers.

"In many ways, metallic glasses are an ideal material for nanoimprint lithography," says John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign, who was not involved in the work. "They're extremely strong, and they can be molded at extremely high resolution." Ordinary metals are crystalline. But bulk metallic glasses, which are created by cooling liquid metals very rapidly to prevent crystallization, lack such structure. Like silicon molds, they can be patterned very finely.

Schroers says that metallic-glass molds can be used millions of times to pattern materials, including polymers like those used to make DVDs. The Yale group has used the molds to create three-dimensional microparts such as gears and tweezers, as well as much finer structures. This week in the journal Nature, Schroers's group describes making molds with features as small as 13 nanometers.

"Theoretically, the size limit is the size of a single atom," says Schroers of the metallic-glass molds. Indeed, the Yale researchers hope to make molds that can form even finer structures by controlling the surface chemistry of the metallic glasses. But the main limitation on the molds is the structure of the metal and silicon templates used to make them. In the hope of further increasing the molds' resolution, Schroers is now developing templates made of nanostructures such as carbon nanotubes only one to two nanometers in diameter.