The photolithography process used to create the features on computer chips has remained largely unchanged in the last 50 years. But as chip manufacturers continue to cram more and more circuits onto a chip the limitation of this process is rapidly being reached. Potential solutions to keep apace with Moore’s Law include using DNA molecules as scaffolding, replacing copper interconnects with graphene and using plasma beams. Now researchers at MIT are developing a process that could see tiny circuits being created using molecules that automatically arrange themselves into useful patterns.
Photolithography involves chips being built up layer by layer. A layer of silicon, metal, or some other material is deposited on a chip and coated with a light-sensitive material, called a photoresist. Light shining through a kind of stencil - a “mask” - projects a detailed pattern onto the photoresist, which hardens where it’s exposed. The unhardened photoresist is washed away, and chemicals etch away the bare material underneath.
This process has served chip makers well for the last 50 years but the problem is that chip features are now significantly smaller than the wavelength of the light used to make them. Manufacturers have developed various tricks to get light to produce patterns smaller than its own wavelength, but they won’t work at smaller scales.
The obvious way to continue shrinking chip features would be to use beams of electrons to transfer mask patterns to layers of photoresist. But unlike light, which can shine through a mask and expose an entire chip at once, an electron beam has to move back and forth across the surface of a chip in parallel lines, like a harvester working along rows of wheat. This makes electron-beam lithography significantly more expensive than conventional optical lithography.
MIT researchers led by Caroline Ross, the Toyota Professor of Materials Science and Engineering, and Karl Berggren, the Emanuel E. Landsman Associate Professor of Electrical Engineering, have developed a new approach that uses electron-beam lithography much more sparingly, to create patterns of tiny posts on a silicon chip. They then deposit specially designed polymers — molecules in which smaller, repeating molecular units are linked into long chains — on the chip. The polymers spontaneously hitch up to the posts and arrange themselves into useful patterns.
In previous research, getting molecules to self-assemble has required some kind of template on the chip surface - either a trench etched into the chip, or a pattern created through chemical modification. By eliminating that template, Berggren and Ross’ approach could make chip manufacture through self-assembly much more efficient.
The new technique uses “copolymers” made of two different types of polymer. Berggren compares a copolymer molecule to the characters played by Robert De Niro and Charles Grodin in the movie Midnight Run, a bounty hunter and a white-collar criminal who are handcuffed together but can’t stand each other. Ross prefers a homelier analogy: “You can think of it like a piece of spaghetti joined to a piece of tagliatelle,” she says. “These two chains don’t like to mix. So given the choice, all the spaghetti ends would go here, and all the tagliatelle ends would go there, but they can’t, because they’re joined together.”
Whichever analogy you prefer, in their attempts to segregate themselves, the different types of polymer chain arrange themselves into predictable patterns. By varying the length of the chains, the proportions of the two polymers, and the shape and location of the silicon hitching posts, Ross, Berggren, and their colleagues were able to produce a wide range of patterns useful in circuit design.
One of the two polymers that the MIT researchers used burns away when exposed to a plasma (an electrically charged gas), while the other, which contains silicon, turns to glass. The glass layer could serve the same purpose that a photoresist does in ordinary lithography, protecting the material beneath it while that around it is etched away.
Ross and Berggren are working to find arrangements of their nanoscale posts that will produce functioning circuits in prototype chips, and they’re trying to refine their technique to produce even smaller chip features.
Their research appears in the paper, Complex self-assembled patterns using sparse commensurate templates with locally varying motifs, which appears in the journal Nature Nanotechnology.
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