Thermally induced orientational flipping of cylindrical phase diblock copolymers

Cylinder forming PS-b-PMMA block copolymer (BCP) thin films were deposited on pre-patterned surfaces with densely packed and nanometer wide trenches defined by conventional top down approaches and arranged to form a periodic grating. The films were subsequently annealed at 250 degrees C for times ranging from 30 s to 900 s in a rapid thermal processing device and their morphology evolution within the topographically defined structures was systematically investigated. Irrespective of the surface neutralization, an irreversible orientational flipping of the BCP microdomains inside the trenches was observed and attributed to de-swelling of the polymeric film as a consequence of a progressive desorption of the solvent retained inside the film. Systematic investigation of the annealing process indicates that the flipping is strongly dependent on the geometric parameters of the trenches and on the relative position of the analysed trench with respect to the periodic topographic structure.