Uncovering Hidden Structure in Polymer Films with Soft X-ray Reflectivity

Studies on lamellar forming block copolymers (BCPs) organized parallel to a substrate are important for understanding assembly in confined conditions and the impact of surface effects. These have typically been conducted using either hard X-rays or neutron reflectivity where the contrast mechanism makes it difficult to distinguish depth dependent structural variations. Using soft X-rays the optical constants of a material can be controlled by varying the energy near an atomic absorption edge. As you approach the edge, the real and imaginary components can change dramatically as a function of functional group type, concentration and orientation. Additionally, unlike hard X-rays or neutrons, the absorption is no longer negligible and shifting energies can significantly change the depth profile of the electromagnetic field, providing additional control over sensitivity as a function of film depth. This technique is applied to two BCP multilayers with different affinities for a surface. Both systems show variations in interface width as a function of substrate proximity, where a strong surface affinity resulted in an increase in the interface width and a weak affinity resulted in a decrease. This type of surface induced behavior has important implications for BCP lithography.