Abstract
The industries of microelectronics constantly strives to increase the speed of microprocessors and the storage density of hard disk drives. Historically, the number of transistors on a computer chip has approximately doubled every 18 months, a trend known as “Moore’s law”. Photolithography, the traditional patterning methodology used to fabricate these devices, has become prohibitively expensive. Alternative patterning technologies that enable high-resolution and high-throughput at lower cost must be developed for the semiconductor manufacturers are to continue their historical pace of “smaller, faster and cheaper devices”. Block copolymers (BCPs) offer an attractive alternative patterning technology since they can self-assemble on length scales from a few to hundreds of nanometers, and can self-assemble into various morphologies. With the use of BCP thin films as the etching mask, the self-assembled nanopatterns can be transferred into the underlying materials, which is referred as BCP lithography. For BCP self-assembly, to acquire a variety of nanostructures would require to synthesize a series of BCPs with different volume fractions. In this study, a simple method to create a variety of nanostructures via the self-assembly of a single-composition silicon-containing block copolymer (BCP) is developed. Herein, we demonstrate that, by using selective solvents for the self-assembly of a silicon-containing block copolymer, polystyrene-block-polydimethyl -siloxane (PS-PDMS), the phase behavior of intrinsic BCP can be enriched due to the strong segregation of the PS-PDMS enabling the diversity of the phase behavior of PS-PDMS/solvent mixtures and the clear-cut phase transitions during solvent evaporation. By taking advantage of the clear-cut order-order phase transitions of the PS-PDMS in solution and the vitrification of the PS domains upon solvent evaporation, a variety of metastable phases can be acquired by simply tuning the selectivity of solvent for casting; most interestingly, the final morphologies from the casting are independent of the corresponding evaporation rate. For practical uses, the self-assembled BCP samples may encounter further thermal treatment during the manufacturing process; it may give rise to the phase transformation from the cast morphologies. To examine the stabilities of those phases from casting, the forming cylinder and gyroid phases are investigated by time-resolved SAXS and electron tomography experiments. Phase transformation occurs during thermal treatment, indicating that the cast morphologies are metastable phases at which the formation of intrinsic lamellar phase through order-order transition (OOT) can be clearly clarified in reciprocal space and real space. Those results offer new insights into the phase behaviors of the silicon-containing BCPs for practical applications, in particular for BCP lithography. For the applications in lithography, nanostructured thin films with oriented periodic arrays over large areas are desirable. Most engineering applications demand thin films in which the orientation of the structures, such as lamellae or cylinders, is perpendicular to the substrate. However, for the PS-PDMS BCP systems, the low surface energy PDMS block will prefer to wet the air free surface (air/polymer interface) due to the favorable enthalpic interactions which will result in the parallel oriented nanostructures. Herein, we suggest a new concept to give entropic-driven orientation using BCPs with star architectures to balance the interfacial interactions for minimum Gibbs free energy state. Star-block copolymers of PS-PDMS are used as an exemplary case to demonstrate the effect of architecture on the controlled orientation. As demonstrated experimentally and theoretically, induced perpendicular orientation of BCP nanostructures for cylinder- and lamella-forming PS-PDMS star-block copolymers can be achieved by increasing the arm number of the star-block copolymer to suppress the effect of interfacial interactions. Those results offer new opportunities for the applications of BCPs in the thin-film state by exploiting complex block architecture. Thin films of block copolymers are widely seen as enablers for nano-fabrication of planar devices (2D devices). However, the inherently three-dimensional structure of block copolymer microdomains could enable them to make 3D devices and complex patterns. On the basis of the systematic studies with respect to the self-assembly of the silicon-containing BCPs, we aim to demonstrate the appealing applications by exploiting the strongly segregated PS-PDMS for 3D nanopatterning. By taking advantage of solvent annealing techniques, PS-PDMS thin films with different morphologies, can be acquired from a single-composition sample. Consequently, by taking advantage of high etching resistance of the silicon-containing block, various topographic SiOx can be fabricated after reactive ion etching treatment. The forming topographic SiOx patterns can be further used as a topographical substrate to give multi-layer nanopatterned thin films using a layer-by-layer sequential process via directed self-assembly (DSA). By combining top-down lithography and bottom-up self-assembly, this approach suggests a feasibility to fabricate three-dimensional nanopatterning for various applications.