Self-Stacking Nanogrids Could Lead to Tinier Chips

MIT researchers detail a technique for using block copolymers and mesh structures to find new ways to build memory, optical and computer chips.

Copolymers MIT

Researchers at MIT have been studying whether molecules called block copolymers can be used as a new way to manufacture processors for memory and optical chips, and possible processors for computers.

Now, the researchers are outlining a technique for creating what they call "mesh structures" by using block copolymers that can spontaneously self-assemble into particular shapes, a process that could be used to build future chips. The team from the Massachusetts Institute for Technology published its findings in a new paper in the journal Nature Communications.

"There is previous work on fabricating a mesh structure—for example, our work," Amir Tavakkoli, a post-doctoral researcher in MIT's Research Laboratory of Electronics and one of three first authors on the paper, said in a statement. "We used posts that we had fabricated by electron-beam lithography, which is time-consuming. But here, we don't use the electron-beam lithography. We use the first layer of block copolymer as a template to self-assemble another layer of block copolymer on top of it."

The research was funded by the National Science Foundation and chip foundry Taiwan Semiconductor Manufacturing Corp. (TSMC).

Computer chips for five decades have been built via a process called photolithography, but it has almost run its course, according to the MIT researchers. As the features on processors have shrunk—some have gotten smaller than the wavelength of light—it's forced manufacturers to continually modify the photolithography process

To keep up with Moore's Law, engineers have been looking for new techniques for manufacturing chips. For MIT researchers, that has meant block copolymer, molecules that spontaneously self-assemble into shapes. By stacking layers of block-polymer wires in particular ways, mesh structures can be produced, which could lead to more practical ways for manufacturing chips.

Polymers are long molecules that comprise basic molecular units that are strung into chains, the researchers said, pointing to plastics and biological molecules—like DNA and protein—as examples. Copolymers are created by joining two different molecules together. To create block copolymers, the polymers that are chosen are chemically incompatible, and as they try to push away from each other, they self-organize.

The MIT researchers used two polymers, one carbon-based and the other silicon-based. The silicon-based polymers, as they tried to escape from their carbon-based counterparts, folded in on themselves, creating cylinders with loops of silicon-based polymer on the inside and the carbon-based polymer on the outside. Exposing the cylinder to oxygen plasma causes the carbon-based polymer to burn away and the silicon to oxidize, leaving glass-like cylinders attached to a base.

Researchers created a second layer of cylinders by repeating the process using copolymers with slightly different chain lengths, and the cylinders in the new layer naturally orient themselves perpendicularly to those in the first, creating a mesh structure, they said. If the surface of the first group of cylinders is chemically treated, they will line up in parallel rows, and the second layer of cylinders will then also form parallel rows, again perpendicular to those in the first.

However, if the cylinders in the bottom layer are allowed to form haphazardly—"snaking out into elaborate, looping patterns," the researchers said—the second layer will keep their relative orientation, creating their own elaborate, perpendicular patterns. While the orderly mesh structure has applications for which it can be used, the haphazard, disorderly one is "the one the materials scientists are excited about," Sam Nicaise, a graduate student in electrical engineering at MIT and another co-first author of the paper, said in a statement.

The glass-like wires by themselves can't be used directly for electronic applications, but they could be seeded with other types of molecules that can make them electronically active. They also could be used as templates for depositing other materials, the researchers said.