E. Bryan Coughlin, PhD, University of Massachusetts (Amherst)
Block copolymer (BCP) materials that self-assemble into nanoscale patterns with lengths of less than 100 nm on the silicon substrates have always attracted interests from both academy and industry. The well-defined microdomains of BCP systems can be utilized as scaffolds to achieve secondary patterns of interest. Consequently, there have been numerous efforts focused on lateral and orientational control of three-dimensional BCP nanostructures, and the incorporation of functional materials with a broad range of properties within the BCP microdomains, such as metallic compound, semiconductor quantum dots (QDs) and functionalized nanoparticles.
Recently, impressive work has been done to demonstrate controlled, directional self-assembly of cylindrical phase BCP systems on the nanoscale. The PS-b-PEO BCP system gains exceptional interests among the well-studied BCP systems. As a result of the distinct character of the component segments, the amphiphilic BCP organizes into well-defined nanostructured patterns, including a lamellar phase, a hexagonal phase of rod-like micelles, a cubic phase of spherical micelles, as well as a bicontinuous cubic phase, as the volume ratio of the two blocks changes. Since many applications require near-perfect lateral order in BCP thin films, external forces have been used to promote long-range order, such as thermal annealing, solvent annealing, electro-magnetic fields, topographically and/or chemically patterned substrates. Solvent annealing method was first reported by Kim et al. to induce the ordering and orientation of BCP microdomains. Later, similar effect was observed in a PS-b-PEO thin film and the origin was attributed to a copolymer/solvent concentration gradient normal to the film surface. However, since two blocks are covalently bonded in a BCP molecule, limited methods can be applied to further modifications. As a result, massive efforts have been made for the selective removal of one domain in BCP templates, such as chemical etching. UV degradation and ozonolysis. Recently, synthetic chemists have found that o-nitrobenzyl (ONB) ester group can be used as an efficiently UV-cleavable linker between the two blocks and it is a promising method to prepare nanoporous thin films.
In parallel with the studies on nanoporous templates fabricated by BCPs, line patterns with good lateral order also draw extensive attention of researchers since the line templates will have many potential application such as nanolithographical masks multifinger devices (field effect transistor), and photonic band gap materials. To that end, dedicated efforts have been made to achieve line patterns in nanometer scale with good long-range lateral order and transfer them to secondary patterns, which were obtained either relying on etch contrast between two blocks or reconstruction of the master template where the minor block was swelled and pulled to the surface by its selective solvent. There are few reports on completely removal of the minor cylindrical microdomains to create a “clean” and straightforward mask template for secondary line pattern fabrication, as it holds the promise for creating patterns with high fidelity and uniformity. We have recently reported the first example of highly ordered silica and Au line patterns from photocleavable PS-hν-PEO templates. A versatile method to manipulate well-ordered line patterns from photocleavable PS-hν-PEO with an o-nitrobenzyl (ONB) junction was presented. Highly ordered PEO cylindrical microdomains oriented parallel to the silicon substrates were achieved by solvent assisted direct self-assembly, followed by the selective cleavage photocleavable junctions by UV irradiation, and extraction of the minor PEO phase. We demonstrated the accessibility of transferring polymeric template to inorganic (silica) or metal (Ag) lines, as evidenced by SFM and SEM. The line patterns from photocleavable BCPs provides higher quality master templates, comparing with reconstruction method, which made it a good candidate for further applications. This approach opens up opportunities in future block copolymer design and application as opposed to conventional reconstruction method.