Directed self-assembly

One reward of exploring "Photonic Frontiers" every month for Laser Focus World is discovering new and emerging technologies that could have important impact. In investigating extreme-ultraviolet (EUV) lithography for my May Frontiers article, I discovered an intriguing concept called "directed self-assembly" which has surfaced since I last covered EUV development four years ago. Practical applications of directed self-assembly remain a ways off, but it could be crucial to sustaining the Moore's Law trend of shrinking electronic components on semiconductor chips.

Simple self-assembly builds structures from the bottom up. On a nano-scale, it starts with molecular building blocks that assemble themselves into larger structures. An example is atoms or molecules adding themselves to bonding sites on the edge of a growing crystal. DARPA has studied ways to self-assemble small building-block modules into robots that could reassemble themselves in different configurations for other purposes, like how children reassemble Lego blocks into new structures.

Robotic modules can be programmed to build desired structures, but external controls are needed to make atoms and molecules grow specific nanostructures. Directed self-assembly does that by applying forces from the top down to control assembly. For making semiconductor chips, the top-down control would come from patterns written by the photolithographic light source onto the material.

Dan Herr became intrigued by the idea of the functional self-assembly of materials while working on lithographic photoresists at Research Triangle Park, NC-based Semiconductor Research Corp. Resists are central to photolithography, and their chemistry can limit the minimum feature size, edge roughness, and writing speed. Conventional resists were designed to match visible and near-ultraviolet light sources, but EUV lithography poses additional challenges because the photons carry an order of magnitude more energy, enough to blast cascades of electrons from the resist.

"Directed self-assembly is a replacement for conventional resists," says Herr, who is now developing the materials at the University of North Carolina, Greensboro (Greensboro, NC). It's based on combining two polymers--one water-soluble, the other oil-soluble--which arrange themselves in regular patterns so the oil- and water-soluble parts can keep apart from each other. Light sources then direct their assembly by writing lines on the substrate for the polymers to start building upon.

Directed self-assembly involves three steps: writing a pattern, depositing the two block copolymers, and removing one to form a pattern. (Courtesy of Wikipedia)

The short-term focus is finding an alternative to conventional photoresists. But Herr says "the holy grail would be materials that self-assemble into shapes and structures that define active components of the circuit." That could change the rules for light sources as well as lithography.