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Alex Alexander Ruyack, CNF Fellow - Jan. 2017 - Parts re-purposed from 2017 issue of the Nanometer
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Introduction
Nanolithography is a fundamental requirement for the future of electronics patterning. Current trends indicate the end of Moore’s Law for traditional lithography processes. Directed self-assembly (DSA) of block copolymers (BCPs) can generate ordered, periodic arrays of various structures down to single nanometer (nm) size scale. The heterogeneous nature of these structures act intrinsically as their own mask, enabling nanometer (nm) scale resolution with a flood exposure and no traditional photo mask. BCP lithography offers low-cost processing of nm scale periodic structures typically only available by e-beam lithography, and can act as a complementary technology to conventional photolithography.
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Below, we detail our specific work on the PS-b-PMMA BCP while also providing insights necessary for determining if BCP lithography is right for your needs, and if so, how to chose and polymer and develop a process.
If you are interested in developing a process, please contact myself or Vince Genova.
Block Copolymer Lithography Overview
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Polymer Selection & General Information
Resolution & The Flory–Huggins Parameter
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The ultimate feature size of the BCP you select will be a strong function of the Flory–Huggins Parameter (χ), which is dependent on the two polymers that comprise the BCP. This is a measure of the energy of interaction between the two polymer blocks. The higher χ is, the more the two blocks want to separate. Thus, higher χ enables smaller features, but it also complicates fabrication. This is because with more disparate surface energies, the more one phase will prefer the substrate surface or air/vacuum to minimize interfacial energy, making perpendicular morphology alignment (which is desired for lithography) more difficult.
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Beyond selecting the two components of the BCP, there may also be a choice of functional end group. As long as the polymer is above ~2kg/mol, this should have little to no effect (e.g., the end group chemistry is insignificant when it is a small constituent of the polymer overall) [2].
Brushes
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To make the substrate non-selective to a particular polymer phase and promote perpendicular ordering, a neutral brush layer can be used. This is typically a surface coating/monolayer whose surface energy is nearly equal for both phases of the BCP. Random co-polymers of the same blocks as the BCP to be applied work well, if they are available. Some literature reports success with homopolymer brushes. If using a homopolymer brush, it is suggested to followed established literature for your particular system [3]. It is also possible that a brush will not be necessary, such as in some applications of PS-b-PDMS BCPs.
Keep in mind that surface chemistry will dictate phase preference. Additionally, if you are performing grapho or chemoepitaxy, you may not need a brush (see below for more information).
Morphology
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Typical BCP morphologies follow the trend above. The left plot shows the theoretical phase diagram (χN vs. block/molecular weight ratio, where N is the number of monomers). On the right is an actual experimental plot for poly(isoprene-styrene) diblock copolymers. For lithography, the C/C’ (cylindrical) and L (lamellar) phases are most useful. These are relative easy phases to select using near 1/4 (or 3/4) or 1/2 block ratio, respectively [4].
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Some other considerations: Typically, higher molecular weight improves phase segregation. Additionally, it has been shown that increasing molecular weight and/or the polydispersity increases the lattice parameter/domain spacing of the resulting film [5, 6].
Etch selectivity
: You will want to select a BCP whose phases have good etch selectivity. For example, PS-b-PMMA has excellent etch selectivity. PS is crosslinked and PMMA is cleaved with 220 nm UV light, making development easy. Another is example is PS-b-PDMS, where PDMS is relatively unaffected by O2 plasma due to it's silicon content.
Grapho and Chemoepitaxy
: For lithography purposes, it is sometimes desired to not only have an ordered array produced by the BCP, but also have some additional control over placement or orientation. For example, to have aligned lamellar structures. This can be accomplished using chemical or topological pre-patterned substrates (chemo- and grapho-epitaxy, respectively) to achieve directed self-assembly. If interested, I suggest reading the below paper for general information [7].
General reading
: For general reading about BCPs, I suggest [81].
References
[1] C. Bates, et al., “Block Copolymer Lithography,” Macromolecules, 2014, 47 (1), pp 2-12.
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[5] N. Lynda, A. J. Meuler, M. A. Hillmyer, “Polydispersity and block copolymer self-assembly,” Prog. Polym. Sci., 33 (9), 2008, pp. 875-893.
[6] Yushu Y. Matsushita, et al., “Molecular Weight Dependence of Lamellar Domain Spacing of Diblock Copolymers in Bulk,” Macromolecules, 1990, 23 (19), pp 4313-4316.
[7] J. Kim, et al., “Directed self-assembly of block copolymers for next generation nanolithography,” Mater. Today, 16 (12), 2013, pp 468-476.[8] C. Bates, et al., “Block Copolymer Lithography,” Macromolecules, 2014, 47 (1), pp 2-12.
Upcoming: PS-b-PDMS for <10nm lines and pillars!