Thickness and Microdomain Orientation of Asymmetric PS-b-PMMA Block Copolymer Films Inside Periodic Gratings

The ordering process of asymmetric PS-b-PMMA block copolymers (BCPs) is investigated on flat SiO₂ surfaces and on topographically patterned substrates. The topographic patterns consist of periodic gratings of 10 trenches defined by conventional top-down approaches and subsequently neutralized using a P(S-r-MMA) random copolymer (RCP). When the ordering process is accomplished on a flat surface at a temperature ranging between 180 and 230 °C, cylindrical microdomains perpendicularly oriented with respect to the substrate are observed irrespective of annealing temperature. In contrast, when the ordering process occurs on topographically patterned substrates, different phenomena have to be considered. The simultaneous effect of the flow around the gratings and the BCP flux from the zone located between adjacent trenches (mesa) into the inner part of the trenches results in significant thickness variations of the confined BCP film. Therefore, the amount of BCP inside the trenches depends on the width of the mesa region, which acts as a BCP reservoir. Moreover, within each trench group, the BCP thickness progressively decreases from the external to the central trenches composing the periodic grating. The thickness variation of the BCP film within the trenches strongly affects the ordering process, ultimately leading to different orientations of the microdomains within the trenches. In particular, when the annealing temperature is 190 °C a precise confinement of the BCP within the trenches featuring a perpendicular cylinder morphology is observed. At higher temperatures, mixed or parallel orientations of the microdomains are obtained depending on the width of the trenches in the periodic grating.