Polymer nanostructures in sub-micron lithographically defined channels: film-thickness effects on structural alignment of a small feature size polystyrene–polyisoprene–polystyrene block copolymer

Abstract

The use of phase separation in block copolymer systems to generate regular nanopatterns at surfaces may be an alternative to advanced photolithography. Here, substrates with photolithographically defined rectangular channels (of depth 60 nm and widths 166–433 nm) are used to direct nanoscale phase separation of a polystyrene-block-polyisoprene-block-polystyrene (PS–PI–PS) block copolymer into aligned periodic superstructures. This nanoscale phase separation results in the rapid formation of parallel and narrow polystyrene (PS) cylinders orientated in a 2D hexagonal arrangement within a polyisoprene (PI) matrix for the polymer composition used here. The PS–PI–PS system is shown to be extremely amenable to simple processing methods allowing precise and homogeneous coating of the substrates. Effects of polymer film thickness are investigated in depth, given that polymer film thickness plays an essential role in the orientation/architecture of the structure formed. It was observed that control of film thickness can determine the orientation of cylinders (parallel or vertical) with respect to the substrate surface. In films where the PS cylinders are aligned parallel to the substrate surface careful control of the processing parameters facilitates the fabrication of regular multi-layer systems (i.e. layers of cylinders) within the channel. The directional effect imposed by the channels is not limited to polymer nanostructures within the channels as long-range order and alignment can be observed at film thicknesses that extend above the channels and onto the outer surface of the substrate. However, as thickness increases, this ‘directing’ effect conferring alignment is lost.