Materials science

On the straight and narrow

Nanoscale chemical patterns written on a substrate can direct the self-assembly of polymer overlayers with remarkable precision. These polymer films, in turn, can be used as templates for nanofabrication.

Nanostructures of precisely defined size and position are essential for any addressable nanoscale device — such as an ultrahigh-density hard drive, in which each bit of material stores a bit of information. Amphiphilic organic molecules are attractive templates for making such structures, as they spontaneously form supramolecular aggregates (such as micelles) of near-uniform size, generating a multitude of identical nanoscopic objects in parallel. But this same feature causes a problem: the aggregates form simultaneously in many uncorrelated locations and so the nanostructures are never precisely positioned. On page 411 of this issue, Kim and co-workers1 overcome this obstacle by directing the self-assembly of polystyrene–poly(methyl methacrylate) — or PS–PMMA, a 'diblock copolymer' — on a lithographically patterned surface.

Diblock copolymers comprise two chemically distinct but individually homogeneous polymer chains joined end to end (Fig. 1). In bulk, repulsions between the unlike blocks induce the blocks to form 'microdomains', whose dimensions are set by the polymer's chain length. When the two blocks have roughly equal volumes, lamellar microdomains form, resembling a stack of playing cards, with each layer only a few tens of nanometres thick. In the absence of any guiding template or field, these lamellar stacks are randomly oriented.

Figure 1: Self-assembly of block copolymers on substrates.
figure1

Polystyrene–poly(methyl methacrylate), or PS–PMMA, is a diblock copolymer — it comprises two polymer chains, of PS (blue) and PMMA (red). The polar PMMA block adsorbs to a uniformly polar substrate surface (a), driving the PS and PMMA lamellae, with a period of about 50 nm, to lie parallel to the substrate (b). If, instead, the substrate is patterned with alternating polar and nonpolar stripes (c), with a period that is similar to that of the PS–PMMA, the block copolymer self-assembles epitaxially, with lamellae forming perpendicular to the surface and in precise register with the underlying pattern (d). Further chemical modification of the block-copolymer film, such as depolymerization of the PMMA block (e), can translate the original chemical pattern on the substrate into a template for patterned functional materials.

But when diblock copolymers are deposited as thin films on substrates, chemical differences between the blocks can generate a preferential orientation; for example, polar blocks will wet polar substrates, but nonpolar blocks will wet the film's air surface. So, when polar substrates are coated with a film of PS–PMMA several lamellar spacings thick, adsorption of the polar PMMA block to the substrate and the nonpolar PS block to the air surface causes the lamellae to stack up with remarkable regularity2 (Fig. 1b). Kim et al.1 exploit differences in block polarity to generate 'standing' lamellae (Fig. 1d), by first creating a pattern of polar and nonpolar stripes on the substrate (Fig. 1c) with a repeat distance, or period, that closely matches the natural period of the block copolymer (which is 48 nm for the PS–PMMA they used). This reproduction of the substrate structure in the deposited overlayer is known as 'epitaxy'. The pattern, 50 × 400 µm overall, is created by extreme-ultraviolet interference lithography (EUV-IL) on substrates coated with a nanometre-thick, organic self-assembled monolayer (SAM); regions of the normally nonpolar SAM exposed by EUV-IL are then oxidized to generate the chemical pattern. When the periods of the pattern and copolymer are the same, the substrate pattern directs the block copolymer to assemble its lamellae vertically, in register with the pattern.

If the PMMA block is then depolymerized, the standing polystyrene lamellae that remain can be used as a template (Fig. 1e), for example for the deposition of magnetic metals3, or for patterned etching of the substrate4. Even more interesting would be applications in which the diblock itself — looking beyond PS–PMMA — has useful optical, electronic or magnetic properties. Alternatively, an active device could be created through selective chemical transformation: for example, cleavage of a polymethacrylate block to poly(methacrylic acid) would produce nanofluidic channels whose permeability could be controlled through pH or ionic strength5. The substrate patterns need not be limited to stripes; a second exposure to EUV-IL of the same substrate, rotated by 60°, would generate a pseudohexagonal lattice of lozenges, onto which arrays of cylinders could assemble vertically.

The epitaxial self-assembly described by Kim et al.1 complements other work in which oriented block-copolymer films are used to generate, rather than replicate, the substrate pattern. For example, block-copolymer microdomains can be aligned with in-plane electric fields6, or through patterning the substrate on a length scale coarser than the microdomain spacing7,8. Epitaxial self-assembly has the distinct advantage that precise registration of the microdomains with substrate features can be achieved: undulations of the lamellae9 are completely suppressed, yielding straight and narrow stripes. Kim et al. have fabricated stripes that are all 24 nm wide, running the full 400-µm length of the substrate stripe in the best cases (Fig. 2). Moreover, they have produced regions up to 5 × 8 µm that are completely free of defects, even dislocations9 (which are lamellae that end prematurely, like a torn card inserted in the middle of the deck).

Figure 2: Precise patterning.
figure2

Kim et al.1 have used a striped polar–nonpolar substrate to control the self-assembly of the diblock copolymer PS–PMMA. In this scanning-electron-microscope image (corresponding to a top view of Fig. 1d), the light and dark regions are 'standing' lamellae of PS and depolymerized PMMA, respectively. On the right are straight, narrow stripes, each 24 nm wide, following the stripe pattern of the substrate; the stripes extend to lengths of up to 400 µm across the width of the substrate. On the left, where the diblock copolymer rests upon an area of unpatterned substrate, the PS and PMMA lamellae still stand edge-on, but their orientation in the plane of the substrate is random.

Dislocations destroy the translational order of the lamellar structure. They form easily (their energetic cost is small) and are difficult to eliminate. Consequently, many approaches6,7,8 to guided self-assembly can produce long-range orientational order, but they achieve only finite translational order. But by patterning the substrate, Kim et al. raise the energetic cost for dislocations and undulations — both of which move the lamellae out of register with the substrate pattern — so these detrimental features are less likely to form. The efficacy of epitaxy rests critically upon the match between the period of the substrate pattern and the period of the diblock copolymer; when the former exceeds the latter by 5% or more, the lamellae tilt and twist to compensate, creating a defective pattern1,10. But this commensurability requirement can be turned to an advantage, as decoration of the substrate with diblock immediately reveals defective regions in the underlying chemical pattern (such as the unpatterned region of substrate on the left in Fig. 2).

Kim and colleagues' method for substrate prepatterning requires specialized EUV tools and masks, but alternative methods for generating SAM-patterned substrates may be emerging. Microcontact printing of SAMs has typically been used to define features as small as 1 µm (ref. 11), but recent advances in contact printing12 have demonstrated edge resolutions of 10 nm. If pushed to even finer resolution, contact printing could be used to pattern substrates for block-copolymer epitaxial self-assembly, using stamps that are replicas of complex masters written by electron-beam lithography. Such direct stamping would permit the fabrication of nonrepetitive patterns that could guide lamellae to curve, join, split or end, perhaps forming templates for electronic or nanofluidic circuitry.

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Register, R. On the straight and narrow. Nature 424, 378–379 (2003). https://doi.org/10.1038/424378a

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