Abstract
The fabrication of nanostructured thin films from the self-assembly of degradable block copolymers (BCPs) has attracted extensive attention in the past decades, and a variety of appealing applications in different research areas have been suggested by using the nanostructured thin films. To create useful BCP thin films for practical uses, controlled ordering of self-assembled nanostructures is essential. Directed self-assembly (DSA), which combines the top-down with bottom-up methods, is of great interest in micro electro mechanical systems (MEMS) due to the ability to control orientation and improve the lateral ordering of the nanostructured BCP thin films. In this study, we aim to examine a specific block copolymer system, star-block polystyrene-block-polydimethylsiloxane (PS-PDMS) copolymers with cylinder- and lamellae-forming phases as nanostructured thin films with perpendicular orientation and controlled lateral ordering by DSA using topographically nanopatterned substrates fabricating from a specific lithographic approach. Our previous studies found that entropic effect can be used to control the orientation of BCP thin films. Specifically, the architecture of star-block copolymers can be used to regulate the entropic contribution to the self-assembled nanostructures. As demonstrated, for star-block copolymers with the same volume fractions of PS and PDMS, perpendicularly oriented BCP nanostructures could be induced via an entropic effect regulated by the number of arms. Here, we aim to investigate the nucleation and growth mechanisms of the self-assembled star-block PS-PDMS directed by the fabricated nanopatterns through thermal annealing. In particular, we are interested in the ordering process of the star-block copolymers through the type of secondary (i.e., heterogeneous) nucleation for microphase separation. Accordingly, the nucleation mechanism for the microphase-separated star-block PS-PDMS will be systematically studies by using topographic nanopatterns with different trench dimensions and different geometric textures. As a result, the controlled orientation and also the lateral ordering of self-assembled block copolymers through the nucleation and growth can be examined to provide the fundamental understanding of the DSA approach. By taking advantage of the high etching contrast of silicon-containing PDMS as compared to PS, the PDMS block will be oxidized into SiOx through the treatment of reactive ion etching (RIE) while the PS block can be simultaneously degenerated. As a result, the well-ordered perpendicular SiOx cylinders or lamellae can be obtained through such a graphoepitaxy approach followed by dry etching (RIE) process.