Polymer mould makes perfect silicon nanostructures

Polymer mould makes perfect silicon nanostructures

10:30 AM, 12th July 2015
Polymer mould makes perfect silicon nanostructures
Scanning electron microscopy micrographs show a periodically ordered mesoporous gyroidal resin template (A and B) and the resulting laser-induced crystalline silicon nanostructure after template removal (C and D).

ITHACA, US: Using moulds to shape things is as old as humanity. In the Bronze Age, the copper-tin alloy was melted and cast into weapons in ceramic moulds. Today, injection and extrusion moulding shape hot liquids into everything from car parts to toys.

For this to work, the mould needs to be stable while the hot liquid material hardens into shape. In a breakthrough for nanoscience, Cornell polymer engineers have made such a mould for nanostructures that can shape liquid silicon out of an organic polymer material. This paves the way for perfect, 3D, single crystal nanostructures.

The advance is from the lab of Uli Wiesner, the Spencer T Olin professor of engineering in the department of materials science and engineering, whose lab previously has led the creation of novel materials made of organic polymers. With the right chemistry, organic polymers self-assemble, and the researchers used this special ability of polymers to make a mould dotted with precisely shaped and sized nano-pores.

Normally, melting amorphous silicon, which has a melting temperature of about 2,350 degrees, would destroy the delicate polymer mould, which degrades at about 600 degrees. But the scientists, in collaboration with Michael Thompson, associate professor of materials science and engineering, got around this issue by using extremely short melt periods induced by a laser.

The researchers found the polymer mould holds up if the silicon is heated by laser pulses just nanoseconds long. They essentially tricked the polymer mould into retaining its shape at temperatures above its decomposition point.

When the mould was etched away, the researchers showed that the silicon had been perfectly shaped by the mould. This could lead to making perfect, single-crystal silicon nanostructures. They haven’t done it yet, but their Science paper shows it’s possible. In work published in 2010, Wiesner and colleagues showed the pathway for this process, using an oxide mould.

Wiesner called the breakthrough “beautiful.” In materials science, the goal is always to get well-defined structures that can be studied without interference from material defects.

Most self-assembled nanostructures are either amorphous or polycrystalline – made up of more than one piece of a material with perfect order. Discovery of single-crystal silicon - the semiconductor in every integrated circuit - made the electronics revolution possible. Today, nanotechnology allows incredibly detailed nanoscale etching, down to 10 nanometers on a silicon wafer.

Semiconductors like silicon don’t self-assemble into perfectly ordered structures like polymers do. It’s almost unheard of to get a 3D structured single crystal of a semiconductor. Wiesner’s group has made the mould. The way they made the mould was itself a breakthrough. They had previously learned to self-assemble highly ordered, porous nanomaterials using specially structured molecules called block copolymers.

They first used a carbon dioxide laser in Thompson’s lab to “write” the nanoporous materials onto a silicon wafer. A film, spin-coated on the wafer, contained a block copolymer, which directed the assembly of a polymer resin. Writing lines in the film with the laser, the block copolymer decomposed, acting like a positive-tone resist, while the negative-tone resin was left behind to form the porous nanostructure. That became the mould.

The paper’s first author is Kwan Wee Tan, a former graduate student in the Wiesner Lab. 

© Cornell University News

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