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The Frontline of Nanomanufacturing: Top Down Meets Bottom Up for Patterned Media Approaches to Magnetic Data Storage

The ability to create features using directed self-assembly from pre-patterned templates, as reported in two recent Science papers, may further address the needs of the integrated circuit industry for specific design approaches.

A review by Jeff Morse , PhD. National Nanomanufacturing Network

As present approaches to reduce the length scales associated with electronic and memory devices encounter intrinsic economical and technological limits via optical lithography, the so-called “engine of the semiconductor industry,” new approaches to forming nanoscale patterns exhibiting long-range order are receiving significant attention. While self-assembly techniques exploiting the properties of block copolymers result in pattern formations on the order of the length scale of the constituent blocks (˜3-50 nm), achieving long-range nanoscopic order over macroscopic dimensions has been elusive, along with high defect densities present in the patterns that further limit the applications for which these approaches are useful. In the case of magnetic storage devices, the demands by the industry on feature size and reproducibility outpace those of the integrated circuit industry in their goal to achieve >1Tbit/in2 by 2010. This translates to bit patterns of roughly 27 nm with similar spacing, which must be produced over a 95-mm diameter master template from which the storage media discs will be produced.
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Ruiz Figure
Process to create lithographically defined chemically prepatterned surfaces and subsequent directed assembly. From Ruiz, et al.
While these objectives seem daunting, two recent papers (Ruiz,, and Bita, et. al.) reported key results incorporating directed self-assembly methodologies that provide enhanced long-range order, low defect density, and pattern multiplication. The directed self-assembly techniques incorporate either chemical or physical pre-patterning steps, which effectively provide an anchor to which the block copolymer self assembly aligns. The pre-pattern template can be made using electron beam lithography to form a low density pattern in a resist layer from which either a chemical or topological pattern is formed on the substrate below. Subsequently the resist layer is removed, after which the block copolymer layers are cast. With appropriate design of the pre-pattern feature size and spacing along with control of the block copolymer composition and attributes, specific multiplication of patterned features by four times has been demonstrated for patterns in the 27-50 nm range. Furthermore, the patterns have shown the ability to repair defects present in the pre-patterned template, as well as maintain log-range order over length scales of 3 mm.

These methods, which impart improved ordering in block copolymer films, are referred to as chemical pre-patterning, or graphoepitaxy in the case of physical pre-patterning of the substrate surface. In both examples, a top-down lithographic process is combined with a bottom-up self-assembly process to achieve the desired result. The significant result is that critical nanomanufacturing issues are addressed in terms of defects, process scaleup, and process design. Future research directions will likely explore means for extending the pattern multiplication factors, as well as possibly forming the pre-pattern template through more conventional optical lithography capabilities. Ultimately, the ability to create features using directed self-assembly from pre-patterned templates may further address the needs of the integrated circuit industry for specific design approaches.

Image from Ruiz , R., et al., Density Multiplication and Improved Lithography by Directed Block Copolymer Assembly, Science, 321 (2008) 936-939. Reprinted with permission of the AAAS.