Photopatternable Imaging Layers for Controlling Block Copolymer Microdomain Orientation

Abstract

Block copolymer (BCP) lithography has emerged as a promising strategy to create highly regular and dense dot or line arrays at the sub-50 nm length scale. [1][2][3][4][5][6] Block copolymer lithography refers to the use of self-assembled domain structures, typically spheres, cylinders, and lamellas in thin-film form as a template for the addition and subtraction nanofabrication processes. [5] Mainly two pattern geometries have been studied extensively: dense arrays of dots and dense arrays of lines and spaces. The former can be generated from sphereforming BCP or from cylinder-forming BCP with domains oriented perpendicular to the substrate, and the latter from cylinder-forming BCP with domains oriented parallel to the substrate or lamella forming BCP with domains oriented vertically to the substrate. These periodic arrays based on self-assembly in BCP thin films have been examined for fabrication of magnetic storage media, quantum dot arrays, photonic crystals, and nanowire transistors. [4,[7][8][9][10] Vertically oriented domain structures have distinct advantages in terms of pattern transfer to the underlying substrate and fabrication of high-aspect ratio features.[11] In the case of perpendicular cylinder or perpendicular lamellae, the orientation of the BCP microdomains can be controlled by solvent evaporation, electric fields, directional crystallization, [12][13][14][15] physical constraints (topography), or chemical patterns. [16][17][18][19] Perhaps the most common means to induce perpendicular domain orientation, however, is to chemically modify and control the interaction between the BCP and the substrate. [20,21] For example, the substrate can be chemically modified by using self-assembled monolayers (SAMs) or random copolymer brush. SAMs of alkylthiols (on gold) or alkylsiloxanes (on Si/SiO 2 ) with polar or non-polar terminal groups have been utilized to modify the substrate. [22][23][24] Exposure to different doses of X-ray in the presence of oxygen can further alter the polarity and hence the wetting behavior of the SAM resulting in a symmetric, neutral or asymmetric BCP morphology. [25] Mansky and coworkers demonstrated that the interfacial energies between the PS and PMMA blocks of P(S-b-MMA) and the substrate can be carefully balanced by grafting an end-hydroxy functionalized P(S-r-MMA) polymer on the SiO x /Si substrate. [20,26,27] Covalent grafting of P(S-r-MMA)having f St (where f st is the styrene fraction) of 0.58 resulted in the neutral wetting behavior and the vertical orientation of lamellar or cylindrical domains. In this original report covalent grafting is usually achieved through the dehydration reaction between the end-hydroxy functionalized P(S-r-MMA) and the native oxide layer of silicon substrate. Ryu et. al recently reported an elegant chemistry to create substrate independent neutral surface through the self thermal crosslinking reaction of benzocyclobutene (BCB)-containing random copolymers at temperatures above 200°C.[28] The high temperature required for efficient cross-lin...

Can ionic liquid additives be used to extend the scope of poly(styrene)-block-poly(methyl methacrylate) for directed self-assembly?

“…56 On the other hand, if the substrate has nonpreferential interactions with both blocks, then the domains orient perpendicular to the substrate. [57][58][59][60][61][62][63] A concern when blending an additive, such as an IL, into the BCP is that the surface energetics may change drastically, necessitating the identification of new compositions for "neutral" underlayers.…”

Microdomain Orientation of PS-b-PMMA by Controlled Interfacial Interactions

“…28,29 Potentially the most robust of these strategies is surface modification via a neutral or nonpreferential wetting of the substrate, enabling A-b-B block copolymer to promote microdomain orientation perpendicular to the film surface by the balanced interfacial interaction, which is precisely tunable by the relative composition of A-r-B random copolymer on the substrate. 25,[30][31][32] By varying the composition of the random copolymers, surface properties ranging from PS to PMMA characteristics were observed from the dewetting behaviors of PS and PMMA, resulting that the interfacial energies on the substrate were balanced at an approximate styrene fraction of 0.58 in the P(S-r-MMA). This corresponds to the surface neutrality obtained from the dewetting experiments by homopolymers such as PS and PMMA, which has been believed to allow both lamellar and cylindrical microdomains of PS-b-PMMA to orient normal to the film surface.…”

Self-assembly and structure-property behaviour of block copolymer/ionic liquid composites

“…59 Hence, ionic liquid-block copolymer blends offer exciting prospects for the field of nanofabrication, where different morphologies can be used as templates for the synthesis of porous materials/membranes or for the generation of nanoscale patterns using block copolymer lithography. 59,62 In particular, the ability to achieve sub-10 nm domain sizes by simply blending PS-b-PMMA with an ionic liquid provides an alternative to the synthesis of block copolymers with high χ parameters. 11,63 Although the domain sizes achieved in these materials are still larger than obtained in a number of more recently reported novel block copolymers, 62 Unfortunately, a comprehensive experimental phase diagram has not been reported for neat PS-b-PMMA, however, the phase behaviour of neat PS-b-PI has been extensively studied and can serve as a good proxy.…”

“…59,62 In particular, the ability to achieve sub-10 nm domain sizes by simply blending PS-b-PMMA with an ionic liquid provides an alternative to the synthesis of block copolymers with high χ parameters. 11,63 Although the domain sizes achieved in these materials are still larger than obtained in a number of more recently reported novel block copolymers, 62 Unfortunately, a comprehensive experimental phase diagram has not been reported for neat PS-b-PMMA, however, the phase behaviour of neat PS-b-PI has been extensively studied and can serve as a good proxy. The experimental phase diagram for neat PS-b-PI is slightly asymmetric around ƒ' PS = 0.5.…”

“…60 Snedden et al also reported success with this method by synthesising cross-linked polymer-ionic liquid composite materials from a huge array of monomers and ionic liquids, many of which possessing unique properties from materials reported previously. 62 Some of the highest ionic conductivities ever achieved in polymer electrolytes were reported a few years later with this same method. 63 Polymer gels obtained by polymerising methyl methacrylate (MMA) and a small amount of cross-linker in the ionic liquid…”