Directed self-assembly (DSA) is arguably the most promising strategy for high-volume cost-effective manufacturing at the nanoscale. Over the past decades, manufacturing techniques have been developed with such remarkable efficiency that it is now possible to engineer complex systems of heterogeneous materials at the scale of a few tens of nanometers to support the ever-growing market for semiconductors, which exceeded $300 billion in 2010. Further evolution of these techniques, however, is faced with difficult challenges not only in feasibility of implementation at scales of 10nm and below, but also in prohibitively high capital equipment costs. Materials that self-assemble, on the other hand, spontaneously form nanostructures down to length scales at the molecular scale, but the micrometer areas or volumes over which the materials self-assemble with adequate perfection in structure is incommensurate with the macroscopic dimensions of working devices and systems of devices of industrial relevance.
Directed self-assembly refers to the integration of self-assembling materials with traditional manufacturing processes. The key concept of DSA is to take advantage of the self-assembling properties of materials to reach nanoscale dimensions and, at the same time, meet the constraints of manufacturing. In other words, DSA enables current manufacturing process capabilities to be enhanced and augmented, providing pathways for true nanomanufacturing at a drastically reduced cost.
DSA of block copolymer films on lithographically defined chemically nanopatterned surfaces is an emerging technology originally developed by Pritzker School of Molecular Engineering professors Paul Nealey and Juan de Pablo that is well-positioned to revolutionize sub-10nm lithography and the manufacture of integrated circuits and magnetic storage media. Block copolymer materials self-assemble to form densely packed features with highly uniform dimensions and shapes in ordered arrays at the scale of 3 to 50nm. Chemical pre-patterns are defined using traditional lithographic materials and processes such as 193 immersion or electron beam lithography at the scale of 20 to 40nm. By directing the assembly of block copolymer films on the chemical pre-patterns, the overall resolution of the lithographic process may be increased by three to four-fold or more.
Implementation across industry
Almost every major semiconductor conductor company (e.g. Intel, IBM, Global Foundries), materials and equipment supplier (e.g. Dow, DuPont, AZ Chemical, Applied Materials, Tokyo Electron), and hard drive manufacturer (e.g. Toshiba, Seagate, HGST, a Western Digital Company) supports programs in DSA of block copolymers based on our technology. The interest and exponential growth in research activity and expenditure is driven in the semiconductor industry by the prospect of manufacturing future generations of computer chips according to Moore’s law without having to invest billions in new fabrication facilities (i.e. based on extreme ultra violet lithography) that may or may not be able to meet the resolution requirements already being demonstrated by DSA. For hard drives, block copolymer lithography is the only known technology that is feasible to fabricate nanoimprint masters to manufacture bit patterned media at the required storage densities (at least greater than 2 terabit/inch squared). Currently DSA of polystyrene-block-methymethacrylate (PS-b-PMMA) films on lithographically defined chemically nanopatterned surfaces is the primary focus of activity. The main research objectives revolve around demonstrations that DSA can meet manufacturing requirements related to degrees of perfection, processing latitude, and integration of the technology with existing infrastructure, and device design for use with DSA patterns.
Patterns for all uses
The Semiconductor Industry Association defined through the Nanoelectronics Research Initiative a number of essential pattern geometries that are required for functional nanofabrication. These include 90-degree bends, periodic arrays, and T-junctions among other patterns. Using simulation and experiments, Nealey and de Pablo showed that these geometries are all possible using designed DSA. By directing the assembly, it is possible to define orientation, structural dimensions, and pattern density with consistency and precision.
In addition, the block copolymers show a tendency to repair defects in patterns made using traditional photolithographic techniques. As the polymers self-assemble, they maintain a tendency to fill in gaps missing within patterns already on the surface. By repairing defects, DSA has the potential to play a major role in the future of semiconductor fabrication.
Academic research to commercial success
Nealey and de Pablo, who both joined PME in 2012 from the University of Wisconsin-Madison, have been developing the methods behind DSA for almost 15 years. Currently, the first full-scale commercial semiconductor production line using DSA technology is being developed by the industry.