Surfactant‐Assisted Orientation of Thin Diblock Copolymer Films

Abstract

The precise control of morphology and orientation of block copolymers (BCPs) in thin films is crucial to fully exploit the potential of these materials for applications in nanotechnology such as nanolithographic templates for nanodevices, [1][2][3][4][5][6][7][8][9] nanoparticle patterning [10][11][12][13] and biological applications.[14]The control of block copolymer morphology is easily achieved by varying the molecular weight and the volume fraction of each block. The control of the orientation of block copolymer thin films is far more challenging as the BCP nanodomains spontaneously self-assemble into the configuration that minimizes the total free energy of the system. A block with a lower interfacial tension with a substrate preferentially wets the substrate whereas a block with a lower surface energy tends to locate at the surface exposed to air. These energetic fields exerted by the substrate and the free surface are strong enough to induce the parallel orientation of nanodomains. In order to establish the perpendicular orientation of a BCP film, which would lead to higher potentials for practical applications, the first approach would be to apply a perpendicular external or internal field that overcomes the surface fields. Electrical field [15][16][17] and solvent evaporation field [18][19][20][21] have been successfully introduced and critically tested. Another method consists of minimizing the difference in interfacial tensions between each block and the substrate by adjusting the surface energy of the substrate. [22][23][24][25][26][27][28] This surface state is referred to as the substrate neutrality, which has been achieved by the deposition of self-assembled monolayers [24][25][26] or copolymer brushes on the substrate. [20][21][22][23] In this case, the BCP film adopts the perpendicular orientation from the bottom interface or substrate toward the surface exposed to air. For thicker BCP films, the perpendicular orientation can not propagate all the way to the upper part of the film, and the final morphology is mixed, consisting of perpendicularly oriented domains at the bottom and parallel oriented domains in the upper part of the film. [23,[28][29][30] In this communication, we report on a new approach to induce the perpendicular orientation from the top of BCP films toward the bottom substrate. Our concept is based on the basic properties of surfactants that naturally locate at any interfaces to tailor the surface properties of a material. We demonstrate that the addition of a low molecular weight surfactant, oleic acid (hereafter denoted as OA), in present case can easily create the energetically neutral conditions at the top of the polystyrene-block-poly(methylmethacylate) (PS-b-PMMA) diblock copolymer thin films, resulting in the desired perpendicular orientation. Figure 1 demonstrates the remarkable efficiency of OA to induce the perpendicular orientation of BCP films. Figure 1a corresponds to the reference atomic force microscopy (AFM) height image of a 52k-52k lamellae structured PS-b-PMMA...

A Top Coat with Solvent Annealing Enables Perpendicular Orientation of Sub‐10 nm Microdomains in Si‐Containing Block Copolymer Thin Films

“…In thin films of BCPs, a difference in surface energies between the blocks promotes microdomain orientation parallel to the surface, and there has been considerable work done on instead producing a perpendicular microdomain orientation in a thin film . Equalization of the surface energies of the blocks during thermal annealing promotes a perpendicular microdomain orientation in a thin film at the air surface, and equalization of the interface energies promotes a perpendicular orientation at the substrate interface.…”

“…In the case of polystyrene‐ block ‐poly(methyl methacrylate) (PS‐ b ‐PMMA), the blocks have similar surface energies, and substrate neutrality can be achieved by coating the surface with a random copolymer brush or a self‐assembled monolayer (SAM) . Neutrality of the top surface of PS‐ b ‐PMMA films has also been obtained at specific temperature (≈250 °C) for the identical surface energy or by the addition of surfactants . However, this neutralizing approach is limited to a narrow range of surface energy differences and the perpendicular domain orientation is dependent on the film thickness .…”

A Facile Synthesis of Dynamic, Shape‐Changing Polymer Particles

“…For thin films, the introduction of a neutral layer, typically a random copolymer, between the BCP and substrate and/or air can induce a perpendicular orientation of lamellae and cylinders. [12] Our new approach to controlling the interaction between block copolymers and the surrounding medium is based on the concept of a neutral layer derived from a mixture of two surfactants. In this design, each surfactant would be preferential for one specific domain.…”

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

“…[ 1,2 ] Block copolymer self-assembly therefore offers a cost-effective, high throughput pattern generation process with applications in nanolithography, [3][4][5][6][7][8][9][10][11][12] photonic crystals [ 13 ] and nano-scale arrays. [ 14 , 15 ] Several different block copolymer systems have been used for pattern generation, but the incorporation of Si into one of the blocks is particularly useful because it provides a high etch selectivity between the blocks which is convenient for transfer of the self-assembled pattern into other materials.…”