Figure 8 shows a schematic diagram of the reasons for the occurrence of defects. Please note that this is only a hypothesis. Suppose we have an AB block copolymer in an ideal state, which is microphase-separated in an ideal state, and a defect-free lamellar structure is formed. In reality, however, all block copolymers are distributed and contain impurities that have been killed during growth. If this is used for DSA, for example, there may be cases where immature B chains dissolve in the A chains. In the case of annealing, there is a possibility that defects may occur from this. On the other hand, if it is dry-etched, the B chain may remain on the substrate. Although this idea is still hypothetical at this point, we believe that this may be one of the causes of the occurrence of defects.
Figure 9 shows the results of Kosaka et al. When block copolymers with the same molecular weight and composition but different PDI were coated on the substrate and annealed, those with PDI = 1.13 did not phase separate well. To further verify this result, we synthesized block copolymers with PDI = 1.5, 1.1, 1.05, and 1.01 with equal molecular weight and composition, and investigated the correlation between the number of defects and PDI in detail. If it is proven that the number of defects decreases with decreasing PDI, it is concluded that PDI greater than 1.01 should not be used.
Although GPC is an excellent method for determining the relative molecular weight or PDI of polymers, it does not accurately represent the molecular weight of block copolymers. This is because GPC is only a relative molecular weight based on standard PSt. For example, the results obtained vary greatly depending on the model of the equipment and the type, length, and number of separation columns. We have been focusing on this issue for a long time and have verified the results using six GPCs of different instrument manufacturers and models, and have confirmed that there is an error of ±1000 for Mw and ± 0.1 for PDI.
Figure 10 shows the molecular weight of PSt measured by TOF-MS, one of the absolute molecular weight measurement methods, although the data is now 25 years old. PSt with Mw (GPC) = 10,000 and PDI (GPC) = 1.02, synthesized by anionic polymerization, was measured by TOF-MS, giving an Mw of 10,060 and a PDI (TOF-MS) of 1.007.
Figure 11 shows the TOF-MS results of the ring polymer shown in Figure 5. The values obtained by the membrane osmometry (Mn = 18, 300, Mw (GPC) = 18, 000, and PDI (GPC) = 1.01) were obtained as Mw = 18, 630 and PDI = 1.01 by TOF-MS, although these values were also measured 25 years ago. The reason why the PDI (GPC) values did not differ from PDI (TOF-MS) compared to the linear polymer is not well understood.