Redistribution-Induced Transverse Orientation in Cylinder-Forming Block Copolymer Films

Block Copolymer (BCP) directed self-assembly is a promising pathway for fabricating highly ordered nanoscale patterns. We show that for cylinder-forming polystyrene-b-polydimethylsiloxane (PS-b-PDMS) BCPs (11 kg/mol-b-5 kg/mol, <i>f_DMS</i>=32.9%) self-assembled on substrates with trenches and mesas of 500 nm width<i>,</i> the PDMS cylinders of period =19 nm can orient in plane, transverse to the trench edge. The transverse orientation is often attributed to capillary flow, which involves polymer redistribution from the mesas to the trenches during annealing^1,2. To better understand the redistribution process, we use atomic force microscopy to characterize the surface topography of the film after spin coating and solvent annealing with acetone for trenches of various depths. Assuming conservation of the BCP volume, we determine the BCP film thickness on the mesas and in the trenches as a function of trench depth and annealing time. We find that the ratio of film thickness on the mesas to that in the trenches for the as-spun film decreases as trench depth increases, being approximately 1:3 for a trench depth of 50 nm (3 L₀) and approximately 1:9 for a trench depth of 100 nm (6 L₀). This ratio also decreases with longer annealing time as more polymer flows from the mesas into the trenches, with the time for the BCP to flow off the mesa ranging from 5 minutes to less than 2 minutes.<br/>The self-assembled BCP structure is characterized for a range of film thicknesses and trench geometries. Transverse cylinders are only observed in the case of 16 nm (L₀) deep trenches with an annealing time sufficient to allow redistribution of BCP from the mesas to the trenches. On further increasing the annealing time, the cylinders gradually align parallel to the trenches. In the case of deeper trenches, the transverse morphology does not occur regardless of annealing time. Therefore, we believe sufficient polymer on the mesas is critical for the transverse morphology, and a trench depth matching a monolayer thickness facilitates the appropriate polymer distribution. Further, the transverse orientation is metastable and eventually transitions to the parallel orientation. This work illustrates how the BCP self-assembled structure correlates with the redistribution process, where the initial and final state are determined by trench depth and annealing time. The ability to control cylinder orientation provides greater flexibility and capabilities in designing and fabricating nanoscale patterns through BCP directed self-assembly.<br/>[1] Liu, R.; Huang, H.; Sun, Z.; Alexander-Katz, A.; Ross, C. A. <i>ACS Nano</i> <b>2021</b>, 15, 16266-16276.<br/>[2] Jung, Y. S.; Ross, C. A. <i>Nano Lett.</i> <b>2007</b>, 7, 2046-2050.