Bending of Lamellar Microdomains of Block Copolymers on Nonselective Surfaces

Symmetric diblock copolymers self-assemble into lamellar microdomains which can be used to create line and space patterns on substrates by controlling the orientation of microdomains. Previously, it has been reported that the orientation of lamellar microdomains can be controlled perpendicular to the surface by modifying the substrate surface to become nonselective to each domain. For practical use of the block copolymer based patterns for nanofabrications, of great interest is precise control over the lateral placement of lamellar microdomains. Commonly predefined guiding patterns containing chemical and/or topographical contrast have been used to direct the lateral placement of lamellae on substrates. In the absence of guiding patterns, the lamella microdomains produce fingerprint-like surface patterns on nonselective substrates as reported previously with a variety of model diblock copolymers, where the most well-studied diblock copolymer is poly(styrene-b-methyl methacrylate) (PS-b-PMMA). The fact that this fingerprint-like pattern contains many regions of high curvature of bent lamellae suggests the cost of lamellae bending may be relatively small. Previously, Nealey and co-workers reported that the lamellar microdomains of PS-bPMMA faithfully follow the lithographically predefined surface patterns of chemical contrast. They showed that bent block copolymer lamellae could be obtained when homopolymers were added to the copolymer. More recently, Wilmes et al. examined the extent to which patterns with different curvatures can be created within the same block copolymer thin films. They showed that the lamellae conform to patterns with radii of curvature equal to the equilibrium domain spacing. In this Note, we report our investigation on the bending characteristics of lamellar microdomains resulting from directed assembly using topographic guiding patterns. We designed the topographic guiding patterns as elbows with varying corner angles. By controlling the surface of guiding patterns nonselective to the microdomains, we rendered the orientation of lamellae perpendicular to the surfaces of bottom and sidewalls of the guiding patterns. Detailed experimental observation of the degree of bending and tilting of lamellae at the sidewall surface is reported along with a theoretical investigation using self-consistent-field calculations, which provide additional knowledge regarding the free energy and interfacial characteristics of the experimentally observed morphologies. We also extended our investigation to a block copolymer hybrid system which is a mixture of an organic diblock copolymer, poly(styrene-b-ethylene oxide) (PS-b-PEO), and an organosilicate (OS) resin. The effect of larger orientational correlation length of the hybrid system on the bending property of lamellar microdomains will be discussed.