Orientation of nanodomains of star-shaped (PMMA-b-PS)₆ in thin films with different molecular weights

To apply block copolymer (BCP) to nanolithography, vertically oriented nanodomains in thin film are needed. When the chain architecture of BCPs was changed from linear to star shape, vertically oriented cylinders and lamellae were obtained by using only thermal annealing without any special treatment. This is because the vertical orientation is entropically favored over parallel oriented one.
Here, we study the effect of the molecular weight of (PMMA-b-PS)₆ star shaped block copolymer on the orientation of nanostructres on a silicon oxide changed. When the molecular weight is small, but still larger than the critical molecular weight corresponding to the order-disorder transition (ODT), parallel orientation is observed. This is because the entropic loss arising from the formation of vertical oriented nanostructures is not large; thus the orientation of nanostructures is greatly affected by surface treatment of a substrate. On the other hand, at large molecular weights, the entropic penalty for parallel orientation becomes huge, resulting in vertical orientation.