Kevin W. Gotrik

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.

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.

Templated self-assembly of Si-containing block copolymers for nanoscale device fabrication

Block copolymers have been proposed for self-assembled nanolithography because they can spontaneously form well-ordered nanoscale periodic patterns of lines or dots in a rapid, low-cost process. By templating the selfassembly, patterns of increasing complexity can be generated, for example arrays of lines with bends or junctions. This offers the possibility of using a sparse template, written by electron-beam lithography or other means, to organize a dense array of nanoscale features. Pattern transfer is simplified if one block is etch resistant and one easily removable, and in this work we use a diblock copolymer or a triblock terpolymer with one Sicontaining block such as polydimethylsiloxane or polyferrocenylsilane, and one or two organic blocks such as polystyrene or polyisoprene. Removal of the organic block(s) with an oxygen plasma leaves a pattern of Sicontaining material which can be used as an etch mask for subsequent pattern transfer to make metallization lines or magnetic nanostructures with feature sizes below 10 nm and periodicity below 20 nm.

Assembling nanoparticle catalysts with nanospheres for periodic carbon nanotube structure growth.

We have developed a novel method to grow carbon nanotubes in a periodic structure using a simple one-step self-assembly process. In this approach, monodispersed nanospheres are utilized to assemble smaller nanoparticle catalysts into an ordered periodic pattern. Using this process, we have grown carbon nanotube bundles into a honeycomb structure. The proposed method eliminates the need for lithography and material deposition, greatly reducing the fabrication complexity and cost.

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)