In an effort to keep Moore’s Law going, a team of engineers from MIT, the University of Chicago, and the Argonne National Laboratory has developed a technique to make microchip wire patterns tinier. They’ve accomplished this by making those patterns assemble themselves from a particular type of polymer. With this method, called directed self-assembly, the resulting features are one-quarter the size of features made using today’s chip patterning techniques. Because the technique relies on several tools already commonly used in semiconductor manufacturing, the engineers believe it could easily integrate into the fabrication process.
This research, detailed last week in Nature Nanotechnology, resulted in chip features with a pitch dimension—the distance between the midpoint of any two features—of 18.5 nanometers. Chips in production today are capable of smaller, but this was a proof of concept demonstration.
“We’re not saying they’re the smallest features, by any means, that have been demonstrated,” says Paul Nealey, a University of Chicago professor of molecular engineering who worked on this project.
While the focus is to ultimately create even smaller nanostructures, this experiment focused on refining fabrication methods. “It was really more about perceived integration using semiconductor manufacturing-friendly tools.”
Chris Mack, a lithographer who did not work on the project, thinks this research is important. “I would characterize this as a useful, incremental step,” says Mack. He sees self-assembly as an intriguing solution to the problem of creating smaller chips, because it’s an inexpensive and accessible technique.
This research team’s technique relies on the creation of multiple layers. First, a pattern of 74 nanometer-wide trenches is made using traditional lithographic methods. Lithography is the process of shining patterns of light onto a photosensitive surface. The areas touched by light harden, while the negative space remains soft and gets washed away. Ordinarily, the negative spaces might be filled with copper to form interconnects, but here the hardened pattern then serves as a template for the next layer, a film of a chemical called a block copolymer.
Block copolymers are made of two molecules that want to do different things but are bound together. Mack described the concept using political parties.
“You’ve got one Democrat handcuffed to a Republican. And so you’ve got a whole room full of people like that and they line up so that every Democrat has a Democrat to talk to, because they hate talking to the Republican.”
In this case, the block copolymer forms horizontal layers within the trench, because one part of the copolymer preferred the surface energy (the “political leanings”) of the air interface. But such an arrangement doesn’t make the overall circuit pattern any finer, so Nealey and his team had to find a way to turn the layers vertically. The solution was to add a neutral layer on top of the block copolymer, so neither side is drawn upward more than the other and the trench is filled with vertical layers of polymer. That meant the trench was now filled with 4 narrower trenches.
Karen Gleason, professor of chemical engineering at MIT, came up with a way to deposit this crucial top coat. This method, called initiated chemical vapor deposition (iCVD), deposits the neutral layer from a vapor phase and in the process creates a layer with the same interfacial properties as the block copolymer layer.
The research promises to make directed self-assembly more viable for manufacturing sub-10-nanometer chips, but it’s not quite there yet. Meanwhile, researchers are making headway using other methods as well. To keep pace, Mack says, researchers have to set their goals very high. “In the last 10 years, as researchers have been trying to develop directed self-assembly as a real-life solution to patterning really small features, the needs of the industry keep progressing,” he says.