Mechanical properties of polymeric nanostructures fabricated through directed self-assembly of symmetric diblock and triblock copolymers

Grating arrays of polystyrenic nanostructures were fabricated by directed assembly of lamellae-forming poly(styrene-b-methyl methacrylate) diblock and poly(methyl methacrylate-b-styrene-b-methyl methacrylate) triblock copolymer films on chemical patterns and subsequent removal of polyacrylic regions by soft x-ray blanket exposure and fluid development. The collapse of gratings induced by capillary forces in a fluid rinse was observed when the aspect ratio of gratings was above a critical value or the critical aspect ratio of collapse (CARC). In stark contrast to the performance of traditional polymer photoresists, the CARC of gratings fabricated from block copolymers decreased monotonically with increasing L_S. For a given pattern period (L_S), the CARC of polystyrenic gratings fabricated from diblock copolymers was larger than that of gratings fabricated from an analogous triblock copolymer. The apparent elastic moduli of gratings that were calculated from CARC data using an elastic cantilever beam bending model decreased monotonically with increasing L_S/L₀ ratio, where L₀ is the natural domain period of the block copolymer. This result is discussed in terms of possible L_S-dependent changes in chain conformation, line width roughness, and domain shape of nanostructures fabricated from self-assembled block copolymer films.