Nealey Group

The Nealey group is a pioneer of the emerging field of block copolymer (BCP) directed self-assembly (DSA). A block copolymer is a molecule consisting of two or more blocks of dissimilar chemical composition. By careful selection of these chemistries, BCPs can be fabricated such that the blocks have some degree of thermodynamically unfavorable interaction; this interaction is encapsulated by the product of χ (chi), the Flory-Huggins interaction parameter, and N, the molecular weight of the polymer. Above a critical χN where the dissimilar blocks have sufficient strength to repel one another, a BCP system will spontaneously phase-separate into chemically pure ordered domains of regular geometry. Specifically, a diblock copolymer system self-assembles into an ordered array of spherical, cylindrical, or lamellar domains, depending on the relative size of the two blocks. While self-assembled BCPs can take on a variety of nanoscale structures, the Nealey group works actively to engineer, control, and direct the size, spacing, orientation, and shape of these structures for many applications.

 

Nanolithography

One of the most promising areas for implementation of BCP DSA is in next-generation nanolithography. Traditional lithographic techniques are rapidly approaching their lower size limit, but are still the most reliable top-down fabrication tools for high-throughput, large-area patterning. Meanwhile, the bottom-up BCP self-assembly process allows precise control over high density features, albeit with low long-range order in the absence of direction. By combining these two techniques, the Nealey group has been able to exploit the best features of both top-down and bottom-up to develop a state-of-the-art lithographic process which is now being implemented in industry.

To further improve and expand the potential of BCP DSA, members of the Nealey group are working on a wide variety of projects. These individual study topics investigate polymer physics, interface science, etch selectivity, and other specific areas of focus germane to the group’s overarching research themes. One particular area of emphasis is synthesis and characterization of high-χ BCPs for smaller feature size and the corresponding challenges of directing the assembly of these materials. Another focus area is improved control over interfacial interaction between the BCP thin film and both the supporting and free surfaces.

The Nealey group has developed strong collaborations worldwide and maintains close ties to world-leading research facilities and industrial partners. PI Paul Nealey holds a joint appointment at Argonne National Laboratories and all the group members conduct experiments at the ANL facilities, using X-ray scattering techniques to characterize polymer assembly behavior and quasi-equilibrium structures. Additionally, the group has several members working in the industry research cleanrooms of imec corporation in Leuven, Belgium.

While much of the Nealey group’s focus is on projects with nanolithography implications, the developed DSA process can be used for fabrication of many other nanoscale structures. Ordered arrays of immobilized nanoparticles can be realized through this process and have enormous potential for photonic and plasmonic devices and are another major focus of the Nealey group. Similarly, the patterning techniques used for BCP DSA also provide an excellent experimental platform for studying liquid crystal (LC) anchoring and assembly behavior.

 

Sub-10nm Features

• Taking BCP DSA to the Next Level

Block copolymers' incredible promise for many applications comes from their ability to self-assemble into nanoscale features. Achieving precise control over the size, shape, and orientation of these features at increasingly smaller dimensions is a major focus of the Nealey Research Lab.

• High-χ Materials

Block copolymer self-assembly behavior is a direct result of chemical dissimilarity of (and therefore thermodynamically unfavorable interactions between) the several polymer blocks. The degree of the effective repulsion between the blocks is described by the product χN, where χ is the Flory-Huggins interaction parameter and N is the molecular weight of the overall polymer; microphase separation and ordering of the BCP structures only occurs when χΝ is above a critical value.

The size of the BCP domains scales monotonically with the molecular weight (N) of the individual polymer chains, so it is intuitive that one way to achieve shorter length scales is to decrease N. However, to achieve assembly, χ must be proportionally increased to prevent crossing the order/disorder transition.  Unlike N, which can be controlled simply through the synthesis mechanism, χ is an parameter intrinsic to the particular chemistry of the individual system.

The Nealey group is actively involved in several projects to identify high-χ chemistries, and, by extension, to synthesize BCPs meeting these requirements. One avenue to achieving this type of control is through selection of dissimilar chemistries in the constituent blocks of the polymers themselves. In a different approach, recent work with the research group of Dr. Frank Bates at the University of Minnesota has explored the incorporation of statistical copolymers for tunable χ control.

• Top-Coat Neutral Layers

Block copolymers self-assemble into a variety of morphologies depending on the relative size of the respective blocks. In the bulk, these morphologies exhibit no particular long-range order or overall orientation. In thin films, however, certain types of BCP morphology—specifically cylinders and lamellae—adopt an orientation which can be described by how the domains are aligned with respect to the supporting substrate. In these thin films, we distinguish between parallel and perpendicular morphologies. Parallel assembly occurs when one of the blocks has a preferential affinity for either the supporting substrate or the free surface. Conversely, if both blocks have similar surface energy and balanced interaction with the substrate, perpendicular assembly occurs.

For many commercial applications, assembling perpendicular BCP structures on rigid substrates is of considerable importance. To accomplish this goal it is necessary to generate non-preferential substrates—i.e. those with equal affinity for the dissimilar blocks–which can be done with grafted copolymer brushes.  More difficult, however, is ensuring that the top surface of the BCP thin film is also non-preferential. For an arbitrary block copolymer, it is statistically improbable that the two blocks will have equal surface energies. At the higher χ necessary for smaller features, it becomes even less likely, since highly dissimilar (and therefore repulsive) chemistries are almost certainly dissimilar in their surface energy. When assembling these polymers on a thin film, a horizontal “wetting layer” of the lower-surface-energy block invariably forms at the free surface of the film.

The Nealey group is working to overcome this tendency and ensure through-film perpendicular morphology though the use of neutral top-coats: layers of controllable chemistry and interfacial energy placed on the block copolymer thin film. The BCP film is thus effectively sandwiched between two tunable surfaces, allowing precise control of the energetic boundary conditions and therefore the resulting assembly behavior.

• Solvent Annealing

Block copolymer thin films self-assemble into nanometer-scale features with controllable shape and orientation, but without any long-range order. The thermodynamic interactions between the blocks of a block copolymer are the driving force behind self-assembly. As the polymer chains explore real space though thermal fluctuations, they adopt those configurations with a free energy minimum. However, this assembly process is very slow for long-range ordering, so BCP films are often annealed, or given additional mobility to accelerate or direct the assembly. One common example is thermal annealing, where a sample is heated to give the individual molecules additional thermal energy.

Some BCP materials, which may otherwise be promising candidates for sub-10nm architectures, cannot be assembled through this standard annealing process, as one or more of the blocks undergoes thermal degradation at the elevated temperatures required for adequately enhanced mobility. To overcome this challenge, these materials can instead be solvent annealed, a process that takes place at room temperature. In solvent annealing, a small-molecule vapor diffuses into the film, plasticizing the polymer chains and increasing their mobility. The shielding effect of the solvent molecules allows the polymers to move around more freely and adopt an ordered configuration, at which point the solvent is removed from the film though simple evaporation. In collaboration with our industrial partners at HGST Storage, the Nealey group is involved in the ongoing investigation of the use of solvent annealing in BCP DSA.

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