Tuning domain size of block copolymers for directed self assembly using polymer blending: molecular dynamics simulation studies

A 10% batch-to-batch variation in molecular weight for a low polydispersity block copolymer (BCP) used for DSA lithography could result in more than a 6% change in critical dimension (CD). Therefore, there is a strong motivation and need to develop and understand methods for fine-tuning the domain size of DSA BCPs to meet the CD and pitch specifications that will be required for practical implementation of DSA processes. This study investigates two methods of fine-tuning the domain size of a specific batch of BCP through blending of the BCP with another polymer, either 1. a set of similar homopolymers (HPs) or 2. a similar BCP of different molecular weight. Each method was investigated and compared using a coarse grained molecular dynamics simulation. For BCP-HP blends, the domain size increases as the amount of HP increases because the HPs tend to slightly swell the BCP domains. A design heuristic was developed for guiding the determination of how much HP to add to obtain a desired pitch. For blends of different molecular weight BCPs, two different scaling regimes were identified; one regime is majority large chains and the other regime is majority small chains. Based on the simulation results, the domain scaling can be mapped across the full range of blends by simply measuring three points: the pure small chain domain size, the pure large chain domain size, and a 50/50 blend of the small and large chains. Comparing the two different blending methodologies, BCP blending with other BCPs is a more versatile approach because it can be used to either increase or decrease the domain size of a base BCP while HP blending can only increase the domain size. HP blending is also potentially problematic because the HPs tends to aggregate in the middle of each block’s half domain which can have a significant effect on CD uniformity due to the strong effect of local variations in the concentration of the HP and local variations in HP conformation. The use of BCP blends with other BCPs should be more favorable from a CD uniformity perspective because this approach is much less susceptible to initial local fluctuations than HP blends because all BCPs go to the A-B interface. The coarse grained molecular dynamics simulation is well suited for comparisons such as this and can produce design rules that are needed for experimental implementation of polymer blending with BCPs to tune domain size.