High-performance block copolymers with large Flory-Huggins interaction parameters (χ) have become essential for achieving sub-10nm lithographic patterning. These materials enable directed self-assembly (DSA) with exceptional feature resolution, driven by their ability to form well-ordered microdomains at extremely small length scales. The development of high-χ block copolymers requires precise molecular design, advanced synthetic methods, and a deep understanding of segregation strength to meet the demands of next-generation nanofabrication.
The molecular design of high-χ block copolymers focuses on maximizing the thermodynamic incompatibility between blocks while maintaining processability. Polystyrene-block-poly(dimethylsiloxane) (PS-b-PDMS) has been a benchmark material due to its high χ value, which facilitates sub-10nm feature formation. The strong immiscibility arises from the significant differences in cohesive energy density between the siloxane and styrene segments. Further improvements involve incorporating polar or non-polar groups to enhance χ without compromising thermal stability or etch selectivity. For instance, block copolymers with fluorinated or silicon-containing segments exhibit increased χ values, enabling smaller domain sizes. The introduction of rigid or bulky side chains can also amplify segregation strength by reducing conformational entropy, promoting faster phase separation.
Synthetic approaches for high-χ block copolymers must balance precision and scalability. Living anionic polymerization remains a gold standard for producing low-dispersity block copolymers, particularly for styrenic and diene-based systems. However, advanced techniques like ring-opening metathesis polymerization (ROMP) and controlled radical polymerization (RAFT, ATRP) have expanded the range of accessible monomers, including those with high χ-inducing functionalities. Silicon-containing blocks, such as polyhedral oligomeric silsesquioxane (POSS)-based polymers, are synthesized via these methods to achieve high etch contrast and thermal stability. Metal-containing block copolymers, incorporating elements like iron or platinum, are synthesized through coordination polymerization or post-polymerization modification, offering additional functionality for niche applications.
Enhancing segregation strength is critical for achieving sub-10nm patterning. The product χN, where N is the degree of polymerization, dictates the degree of microphase separation. High-χ systems allow for lower N values while maintaining strong segregation, enabling smaller domain sizes. Strategies to increase χ include incorporating monomers with disparate solubility parameters or introducing specific interactions such as hydrogen bonding or ionic interactions. For example, block copolymers with zwitterionic segments exhibit enhanced χ due to Coulombic interactions. Another approach involves using sequence-controlled polymerization to create gradient or tapered blocks, which can fine-tune interfacial tension and improve ordering kinetics.
Emerging materials in this field include silicon-containing and metal-containing block copolymers, which offer unique advantages. Silicon-rich blocks, such as polysilanes or polycarbosilanes, provide high etch resistance and compatibility with semiconductor processing. These materials can form sub-5nm features when paired with organic blocks, making them suitable for extreme ultraviolet (EUV) lithography. Metal-containing block copolymers, such as those with ferrocene or metallocene segments, introduce magnetic or electronic properties while maintaining high χ values. These systems are particularly promising for applications requiring both nanoscale patterning and functional material properties.
The self-assembly kinetics of high-χ block copolymers must be carefully controlled to avoid defects in sub-10nm patterns. Rapid phase separation is often desired to minimize processing time, but excessive segregation can lead to kinetic trapping. Thermal annealing remains a common method, though solvent vapor annealing offers better control over domain orientation and ordering. The use of additives, such as small molecules or homopolymers, can further modulate interfacial interactions and reduce line-edge roughness. For instance, selective solvents or plasticizers can swell one block preferentially, enhancing mobility without disrupting the overall morphology.
Challenges persist in the integration of high-χ block copolymers into manufacturing workflows. Achieving defect densities below acceptable thresholds requires optimization of both material properties and processing conditions. The high χ values that enable small feature sizes can also lead to increased sensitivity to surface energetics, necessitating precise substrate modification. Additionally, the etch selectivity between blocks must be sufficient to transfer patterns into underlying layers without distortion. Advances in block copolymer design, such as incorporating cleavable junctions or crosslinkable segments, are being explored to address these issues.
Future directions in high-χ block copolymer research include the development of bio-derived or sustainable alternatives that maintain performance while reducing environmental impact. Another area of interest is the integration of dynamic or stimuli-responsive blocks, enabling reconfigurable nanostructures for adaptive applications. Computational modeling and machine learning are increasingly employed to predict χ values and optimize monomer combinations, accelerating the discovery of new materials.
In summary, high-χ block copolymers represent a critical enabler of sub-10nm patterning, with molecular design, synthetic innovation, and enhanced segregation strength being key focus areas. Silicon-containing and metal-containing systems offer additional functionality, pushing the boundaries of nanofabrication. Continued advancements in this field will rely on interdisciplinary collaboration to overcome remaining challenges and unlock new applications in nanotechnology.