Generating Organized Designs Within Block Copolymer Thin Films Through Precise Application of Controlled Thickness Gradients

Self-assembly is the spontaneous formation of intricate micro/nanostructures by nanomaterials such as liquid crystals, peptides, and DNA driven by secondary forces among their molecular components. Block copolymers (BCPs) in thin films exemplify this process, allowing for precise control of diverse polymer properties at scales smaller than 100 nm. The nanopatterns generated by BCPs self-assembly hold promise for various applications, including bio/optical sensors, MEMS/lithography, and semiconductor devices.<br/><br/>However, practical applications of nanopatterns created by BCPs require exact control over the alignment of nanodomains, which is accomplished using diverse directed self-assembly (DSA) methods. These techniques, such as graphoepitaxy and chemoepitaxy, utilize top-down approaches to control the alignment and positioning of BCP nanopatterns. Despite advancements in manipulating BCPs structures, achieving non-defected and highly oriented nanopatterns often involves expensive, multi-step procedures and sophisticated equipment. A new method has been suggested to address this issue, focusing on the automatic alignment of lamellar structures in BCPs thin films with controlled thickness variations.<br/><br/>This method exploits the influence of geometric alignment, promoting extended alignment of lamellar structure along a gradient in film thickness. However, ensuring uniform thickness across the film remains crucial for subsequent nanofabrication processes. To address these challenges, we have effectively showcased the generation of meticulously structured nanopatterns using BCPs with a consistent thickness across a wide area. This accomplishment was achieved by implementing a straightforward two-stage thermal annealing method to introduce a controlled thickness gradient in the BCPs thin films. This method employed an economical and uncomplicated thermal imprinting technique to establish a micro-scale thickness gradient, contrasting the expensive demands of traditional photolithography methods.<br/><br/>The process of thermal annealing in two stages was meticulously planned. First, annealing at a lower temperature of 160°C facilitated the initial alignment of BCPs nanodomains along a gradient in film thickness, leveraging the geometric anchoring effect. Subsequently, a second annealing step at 250°C was applied to enhance lamellar nanopattern alignment further and ensure uniformity in film thickness. During this high-temperature annealing, the BCP thin films, exhibiting curvatures up to 350 nm, underwent spontaneous alignment and flattening via thermal reflow. The effectiveness of these processes was confirmed through detailed analysis using scanning electron microscopy (SEM) and grazing incident small-angle x-ray scattering (GISAXS). The second annealing step induced the flattening of the BCPs thin films, potentially creating shear stress in the direction of the thickness gradient. This effect contributed to enlarging the grain size of the lamellar nanodomains of BCPs initially aligned by the thickness gradient, thus establishing a highly ordered nanostructure across the thin film.<br/><br/>This novel approach to guiding BCPs self-assembly enables the formation of well-oriented vertical lamellar nanopatterns with consistent thickness, circumventing the need for intricate and expensive nanopatterning techniques that typically involve advanced micro/nanofabrication processes. Apart from creating the initial master mold, this method offers a straightforward and efficient strategy to achieve precise nanopatterning, presenting a cost-effective and accessible alternative for nanolithography.