ACS Nano VOL. 5, NO. 6, 4617–4623 (2011)

Self-organized block copolymer (BCP) systems have been shown to have a host of applications, most notably in the fabrication of inorganic structures used in electronic, optoelectronic, and magnetic devices.  Absolute and precise control of pattern orientation (i.e., to the surface plane) is central to their possible use and requires a profound understanding of phase behaviour and structure evolution during post-annealing of the BCP films.

The focus of this research is the demonstration of pattern orientation coupled to pattern alignment being achieved for cylinder forming poly-(styrene-b-ethylene oxide) (PS-b-PEO) films. The research highlights unexpected cyclic transitions with anneal time of the polymer structure between perpendicular and parallel arrangements of microphase separated cylinders in these types of  films using in situ time-resolved light scattering data combined with ex-situ time evolution AFM experiments. This is the first time such observations have been reported.

This work is another step forward to understanding the structure evolution and also controlling the alignment of block copolymer nanocylinders independently of thickness and external fields. The research demonstrates continuous control of pattern structure and alignment through solvent exposure and in particular, how orientation of the pattern can be determined independently of the initial film thickness, surface segregation effects, and “solvent fields”.

Figure: Cyclic transition in thick (177 nm) PS-b-PEO films exposed to toluene for different annealing times. A transformation similar to thin film happens but at higher transition rate. A parallel structure is formed after 40 min (c) as opposed to 60 min in Figure (1c). The images are 2 _ 2 μm².