Wubin Bai

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.

Universal pattern transfer methods for metal nanostructures by block copolymer lithography

A universal block copolymer pattern transfer method was demonstrated to produce Co nanostructures consisting of arrays of lines or dots from a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) diblock copolymer. Three processes were used: liftoff, a damascene process, and ion beam etching using a hard mask of tungsten, including a sacrificial poly(methyl methacrylate) layer under the PS-b-PDMS for the etch and liftoff processes. The ion beam etch process produced the most uniform magnetic arrays. A structural and magnetic comparison in terms of uniformity, edge roughness and switching field distribution has been reported.

Perpendicular Block Copolymer Microdomains in High Aspect Ratio Templates

Perpendicular orientation of lamellar microdomains in a high interaction parameter block copolymer was obtained within high aspect ratio gratings functionalized with a preferential sidewall brush. The experiments used polystyrene-block-polydimethylsiloxane (PS-b-PDMS) with molecular weight 43 kg/mol within trenches made using interference lithography. The perpendicular alignment was obtained for both thermal and solvent annealing, using three different solvent vapors, for a range of film thicknesses and trench widths. A platinum (Pt) layer at the base of the trenches avoided the formation of a wetting layer, giving perpendicular orientation at the substrate surface. The results are interpreted using self-consistent field theory simulation and a Ginzburg-Landau analytic model to map the energies of lamellae of different orientations as a function of the grating aspect ratio and the surface energies of the sidewalls and top and bottom surfaces. The model results agree with the experiment and provide a set of guidelines for obtaining perpendicular microdomains within topographic features.

Morphology, directed self-assembly and pattern transfer from a high molecular weight polystyrene-block-poly(dimethylsiloxane) block copolymer film

The self-assembly of block copolymers with large feature sizes is inherently challenging as the large kinetic barrier arising from chain entanglement of high molecular weight (MW) polymers limits the extent over which long-range ordered microdomains can be achieved. Here, we illustrate the evolution of thin film morphology from a diblock copolymer of polystyrene-block-poly(dimethylsiloxane) exhibiting total number average MW of 123 kg mol-1, and demonstrate the formation of layers of well-ordered cylindrical microdomains under appropriate conditions of binary solvent mix ratio, commensurate film thickness, and solvent vapor annealing time. Directed self-assembly of the block copolymer within lithographically patterned trenches occurs with alignment of cylinders parallel to the sidewalls. Fabrication of ordered cobalt nanowire arrays by pattern transfer was also implemented, and their magnetic properties and domain wall behavior were characterized.

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)