Characterization of the surface morphology and grain growth near the surface of a block copolymer thin film with cylindrical microdomains oriented perpendicular to the surface

A surface morphology characterization was conducted via atomic force microscope (AFM) observations using thin film specimens of a polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene triblock copolymer with perpendicularly oriented polystyrene cylinders. We sought to determine whether the cylinders were oriented parallel or perpendicular to the film specimen at the near-surface region. Furthermore, the grain size was successfully evaluated using the AFM observations with an image analysis method using Fourier transform (FT)/inverse FT techniques for the grains at the near-surface region of the thin film specimen. The grain size in the interior of the specimen was also evaluated using small-angle X-ray scattering measurements. From the comparison of the results, it was found that the grains were slightly larger (~53% larger) on the surface than in the interior of the specimen. However, this difference might have been due to differences in the evaluation methods. Nevertheless, both results for the grain growth on the surface and in the interior of the specimen exhibited power-law behavior with the same exponent (~0.45) with <55 min of thermal annealing, in which the cylinders gradually oriented perpendicular to the film surface while simultaneously improving the hexagonal lattice regularity. By contrast, with more than 55 min of thermal annealing, an abrupt increase in the grain size was noticeable both on the surface and in the interior of the sample. Therefore, it can be concluded that the grains could not grow quickly during the structural changes in the interior of the specimen (progressive perpendicular orientation of the cylinders and improvement in the hexagonal lattice regularity). After completing the orientation and achieving lattice regularity, the grains were finally able to grow quickly.