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Micro/Nano Lithography

Self-assembling polymer patterns could shrink lithographic limits

New strategies to direct the self-assembly of block copolymers could provide an alternative patterning approach to fabricate very small features below the resolution limits of traditional optical lithography.
30 January 2013, SPIE Newsroom. DOI: 10.1117/2.1201301.004650

Block copolymers (BCPs) are polymers made of two or more distinct monomer or block units covalently bonded together in a variety of different architectures. Due to their differing chemistries, the blocks tend to phase separate like oil and water; but because of their covalent linkage, this microphase separation occurs over length scales determined by the length of the BCP molecules, typically ranging from a few nanometers to a hundred times that. A thin film of a BCP can be used like photoresist, by etching one block away and using the resulting self-assembled structure as a hard mask for patterning the underlying substrate.

A challenge with BCP self-assembly is that it is limited to forming periodic patterns without long range order or registration on a substrate. We overcome this by patterning the substrate with nanoscale template features that guide the self-assembly, producing device-like geometries such as parallel lines, line segments, bends, junctions, meshes, and gridded arrays at specific locations on the substrate. A further challenge is that in order to obtain the smallest feature sizes, a high degree of chemical repulsion between the blocks is required. BCPs with this characteristic are called high-chi BCPs. A high chi does, however, hinder the microphase separation of the BCP, making it difficult to obtain self-assembled patterns in a sufficiently fast process for integration into semiconductor device manufacturing. We are investigating a range of different polymer systems and developing a suite of methods for controlling the self-assembly through a combination of annealing techniques and top-down patterning.

Recently we have investigated how best to control the self-assembly of a high-chi BCP, polystyrene-block-polydimethylsiloxane (PS-PDMS).1 We employ a strategy known as solvent vapor annealing, where the thin film of PS-PDMS is introduced to a vapor of organic solvents that penetrate into the polymer film, causing it to swell to over twice its initial thickness. The BCP is allowed to anneal in this swelled state to form the desired patterns, a process that can take minutes or seconds, compared to hours for standard thermal annealing. Additionally, by precisely controlling the type and amount of different solvent vapors that interact with the BCP thin film, both the periodicity and the pattern morphology (lines, dots, holes, etc.) can be controlled as in Figure 1.

Figure 1. Precision control of solvent vapor pressures allows for a wide range of morphologies to be formed by the self-assembly of a block copolymer (BCP). MFC: Mass flow controller. (Reproduced with permission.1)

Obtaining well-ordered patterns from thin films of BCPs requires templated or directed self-assembly. For example, some of our recent work has used small posts (diameter 15nm and height 40nm), made by electron-beam lithography, as guiding features for the self-assembly of one or two layers of cylinder-forming PS-PDMS (see Figure 2). Depending on the periodicity and placement of these post patterns, complex structures with sub-10nm half pitch can be made that would be very difficult to achieve using traditional optical lithography methods.2, 3

Figure 2. Left and middle: Electron-beam lithography is used to produce post features, shown as bright dots, which direct the self-assembly of a double layer of BCP cylinders to form a mesh-like structure. The untemplated region (green outline) shows a random pattern. Right: A single layer of BCP cylinders is templated to produce a four-way junction. (Reproduced with permission.2, 3)

In addition to AB diblock copolymers such as PS-PDMS, which include two different blocks, ABC linear or star triblock terpolymers with three different blocks are also being explored.4,5 We have shown that a polyisoprene-b-polystyrene-b-polyferro-cenylsilane linear triblock terpolymer can form a square symmetry array, one of the key geometries for device architecture. This polymer can produce a square array of either posts or holes with tunable diameter. Guiding features such as small posts or sidewalls were used for long range ordering of the square pattern, as shown in Figure 3. Furthermore, we have recently demonstrated the self-assembly of miktoarm star terpolymer (3μ-ABC) thin films. This polymer architecture is one of the rare material systems that gives access to Archimedean tilings, resembling bathroom tile patterns at the nanometer scale. Using different 3μ-ABC systems, we have generated ‘three-colored’ surfaces with interesting symmetry groups, including octagonal, square, triangular, or hexagonal shapes that cannot be obtained from other block copolymer architectures. These patterns could be useful in defining vertical contacts in integrated circuits, or for making devices such as patterned recording media or photonic crystals.

Figure 3. Square symmetry arrays of dots and holes produced from a triblock terpolymer. The positions of the features can be controlled using posts or sidewalls (bright structures). (Reproduced with permission.4, 5)

The directed self-assembly of BCPs is one strategy that may enable us to reach smaller and smaller feature sizes in a high throughput, economical manufacturing process. Understanding and controlling BCP self-assembly by combining top-down templating techniques with precision annealing is required in order to reach this goal. The next step will be to explore annealing strategies that can produce well-ordered patterns with low defect levels in timescales around a minute, and combine these approaches with increasingly complex multi-block copolymer architectures, which can vastly expand the repertoire of possible geometries.

We would like to acknowledge the generous financial support of the National Science Foundation, Semiconductor Research Corporation, Tokyo Electron, and Taiwan Semiconductor Manufacturing Company.

Caroline Ross, Kevin Gotrik, Hong Kyoon Choi, Karim Aissou, Adam Hannon, Wubin Bai
Massachusetts Institute of Technology (MIT)
Cambridge, Massachusetts

Caroline Ross has been a faculty member in the Department of Materials Science and Engineering at MIT since 1997. Prior to that she spent six years at Komag, a hard disk manufacturer, and carried out a postdoctoral fellowship at Harvard University.

Kevin Gotrik, Wubin Bai, and Adam Hannon are graduate students and Hong Kyoon Choi and Karim Aissou are postdoctoral researchers at MIT.

1. K. W. Gotrik, A. F. Hannon, J. G. Son, B. Keller, A. Alexander-Katz, C. A. Ross, Morphology control in block copolymer films using mixed solvent vapors, ACS Nano 6(9), p. 8052-8059, 2012. doi:10.1021/nn302641z
2. A. Tavakkoli K. G., K. W. Gotrik, A. F. Hannon, A. Alexander-Katz, C. A. Ross, K. K. Berggren, Templating three-dimensional self-assembled structures in bilayer block copolymer films, Science 336(6086), p. 1294-1298, 2012. doi:10.1126/science.1218437
3. J. K. Yang, Y. S. Jung, J. B. Chang, R. A. Mickiewicz, A. Alexander-Katz, C. A. Ross, K. K. Berggren, Complex self-assembled patterns using sparse commensurate templates with locally varying motifs, Nat. Nanotech. 5(4), p. 256-260, 2010. doi:10.1038/nnano.2010.30
4. J. G. Son, J. Gwyther, J. B. Chang, K. K. Berggren, I. Manners, C. A. Ross, Highly ordered square arrays from a templated ABC triblock terpolymer, Nano Lett. 11(7), p. 2849-2855, 2011. doi:10.1021/nl201262f
5. H. Choi, J. Gwyther, I. Manners, C. Ross, Square arrays of holes and dots patterned from a linear ABC triblock terpolymer, ACS Nano 6(9), p. 8342-8348, 2012. doi:10.1021/nn303085k