Chemical Patterns from Surface Grafted Resists for Directed Assembly of Block Copolymers

We demonstrate a direct e-beam patternable one-component block copolymer (BCP) resist to fabricate a chemical pattern for the directed assembly of a symmetric block copolymer. The resist consists of a low molecular weight poly(styrene-block-methyl methacrylate) with a hydroxyl group at the PMMA chain end (PS-b-PMMA-OH), which anchors the chains to the surface. This short-tethered PMMA block provided sufficient sensitivity to allow scission by e-beam. The length of the untethered PS block was fine-tuned to impart the required contrast between the patterned and the unpatterned region for 1:1 assembly of an overlying BCP blend. Two BCP resists with a PS fraction of 0.25 (16SM) and 0.34 (18SM), with a total molecular weight less than 20K, were synthesized, and the assembly of a ternary BCP blend was studied. 16SM- and 18SM-anchored substrates showed nonpreferential and PS preferential surfaces, respectively. Both 18SM and 16SM could be patterned by e-beam to fabricate a 1:1 chemical pattern with a line pitch of 70 nm for the assembly of a BCP ternary blend. 18SM gave fewer defects than 16SM due to an increased contrast in interfacial energies between adjacent stripes in the chemical surface pattern. Two additional PS-b-PMMA-OH polymers with a molecular weight of 39K (F PS = 0.76) and 69K (F PS = 0.83) were synthesized to study the effect of PS cross-linking upon exposure to e-beam. As the PS fraction increases, the line pattern becomes blurred and ultimately ineffective in guiding the BCP assembly. The blurring is attributed to cross-linking of adjacent PS chains.