Adam F. Hannon

Templating three-dimensional self-assembled structures in bilayer block copolymer films

To the Next Level Block copolymers will spontaneously separate into a range of microstructures that depend on the polymer block lengths and chemical compositions, and have been used as a templating material because one can selectively etch or functionalize one of the blocks. However, creating a template that is more than one layer thick is challenging. Tavakkoli K. G. et al. (p. 1294) used an array of posts to provide independent and simultaneous control of the morphology and orientation of two layers of block copolymers and were able to create local variations in the curvature and spacing of the domains. An array of posts guides the bilayer assembly of block copolymers with independent control of morphology and orientation. The registration and alignment of a monolayer of microdomains in a self-assembled block copolymer thin film can be controlled by chemical or physical templating methods. Although planar patterns are useful for nanoscale device fabrication, three-dimensional multilevel structures are required for some applications. We found that a bilayer film of a cylindrical-morphology block copolymer, templated by an array of posts functionalized with a brush attractive to the majority block, can form a rich variety of three-dimensional structures consisting of cylinder arrays with controllable angles, bends, and junctions whose geometry is controlled by the template periodicity and arrangement. This technique allows control of microdomain patterns and the ability to route and connect microdomains in specific directions.

Hierarchical Nanostructures by Sequential Self-Assembly of Styrene-Dimethylsiloxane Block Copolymers of Different Periods

Poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) block copolymers with a period as low as 13 nm have been self-assembled on a template formed from PS-b-PDMS of a 34–40 nm period, which is itself templated by micron-scale substrate features prepared using conventional lithography. This hierarchical process provides a simple method for directing the self-assembly of sub-10 nm features and registering them on the substrate.

Aligned Sub-10-nm Block Copolymer Patterns Templated by Post Arrays

Self-assembly of block copolymer films can generate useful periodic nanopatterns, but the self-assembly needs to be templated to impose long-range order and to control pattern registration with other substrate features. We demonstrate here the fabrication of aligned sub-10-nm line width patterns with a controlled orientation by using lithographically formed post arrays as templates for a 16 kg/mol poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) diblock copolymer. The in-plane orientation of the block copolymer cylinders was controlled by varying the spacing and geometry of the posts, and the results were modeled using 3D self-consistent field theory. This work illustrates how arrays of narrow lines with specific in-plane orientation can be produced, and how the post height and diameter affect the self-assembly.

Design rules for self-assembled block copolymer patterns using tiled templates

Directed self-assembly of block copolymers has been used for fabricating various nanoscale patterns, ranging from periodic lines to simple bends. However, assemblies of dense bends, junctions and line segments in a single pattern have not been achieved by using sparse templates, because no systematic template design methods for achieving such complex patterns existed. To direct a complex pattern by using a sparse template, the template needs to encode the key information contained in the final pattern, without being a simple copy of the pattern. Here we develop a set of topographic template tiles consisting of square lattices of posts with a restricted range of geometric features. The block copolymer patterns resulting from all tile arrangements are determined. By combining tiles in different ways, it is possible to predict a relatively simple template that will direct the formation of non-trivial block copolymer patterns, providing a new template design method for a complex block copolymer pattern.

Optimizing Topographical Templates for Directed Self-Assembly of Block Copolymers via Inverse Design Simulations

An inverse design algorithm has been developed that predicts the necessary topographical template needed to direct the self-assembly of a diblock copolymer to produce a given complex target structure. The approach is optimized by varying the number of topographical posts, post size, and block copolymer volume fraction to yield a template solution that generates the target structure in a reproducible manner. The inverse algorithm is implemented computationally to predict post arrangements that will template two different target structures and the predicted templates are tested experimentally with a polydimethylsiloxane-b-polystyrene block copolymer. Simulated and experimental results show overall very good agreement despite the complexity of the patterns. The templates determined from the model can be used in developing simpler design rules for block copolymer directed self-assembly.

Rectangular symmetry morphologies in a topographically templated block copolymer.

Using an array of majority-block-functionalized posts makes it possible to locally control the self-assembly of a block copolymer and achieve several morphologies on a single substrate. A template consisting of a square symmetry array of posts produces a square-symmetry lattice of microdomains, which doubles the areal density of features.

Sacrificial-post templating method for block copolymer self-assembly.

A sacrificial-post templating method is presented for directing block copolymer self-assembly to form nanostructures consisting of monolayers and bilayers of microdomains. In this approach, the topographical post template is removed after self-assembly and therefore is not incorporated into the final microdomain pattern. Arrays of nanoscale holes of different shapes and symmetries, including mesh structures and perforated lamellae with a bimodal pore size distribution, are produced. The ratio of the pore sizes in the bimodal distributions can be varied via the template pitch, and agrees with predictions of self consistent field theory.

Inverting the design path for self-assembled block copolymers

Recent success of inverse design methodologies in the realm of self-assembled materials has allowed us to envision an inverse path of discovery where we go from a desired target function to building blocks. In this review we examine recent advances of such inverse design methods in soft materials containing block copolymers, colloids, or DNA. By combining well-developed theoretical models with advanced inverse search algorithms, the design of such systems has been dramatically enhanced over the past decade. Advantages and disadvantages of the most prominent inverse search algorithms are discussed in the context of block copolymer directed self-assembly inverse design. The success of these methodologies in such systems shows great promise for the future of self-assembling materials, particularly for applications where the desired structure and properties of the system needed for a functional device are known.

Self-assembling polymer patterns could shrink lithographic limits

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 selfassembly of a high-chi BCP, polystyrene-block-polydimethylsiloxane (PS-PDMS).1 We employ a strategy known as solvent 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)