(236f) Image Formation in Thin Films of Polymeric Resists

Next-generation nanopatterning requires imaging materials that achieve sub-20 nm resolution in ultrathin films. Current manufacturing practices are based on projection lithography with chemically-amplified (CA) resists; however, such ``top-down’’ lithographic processes are approaching their intrinsic resolution limits, so alternative techniques like ``bottom-up’’ block copolymer (BCP) self-assembly are increasingly attractive.  The physicochemical parameters that control imaging in top-down and bottom-up lithography are very different, but we find that interactions at the free surface and substrate interfaces will control the three-dimensional image in both systems. To examine BCP self-assembly, we cast thin films of lamellar poly(styrene-b-methyl methacrylate) (PS-PMMA) copolymers on epitaxial templates. The three-dimensional domain shape is measured with variable-incident-angle small-angle X-ray scattering (SAXS). We find significant deformations in domain shape near the free surface and substrate interfaces. This behavior is attributed to the different surface tensions of PS and PMMA, as well as different affinities of PS and PMMA for the underlying epitaxial template.  To study imaging in CA resists, we consider a model resist system of poly(4-hydroxystyrene-co-tertbutylacrylate) loaded with triphenylsulfonium photoacid generator (acid catalyst). Thin films of CA resist are nanopatterned with electron-beam lithography, baked to generate the chemical image, and the reaction products are measured with SAXS. We find that image resolution varies with depth into the film, where the reaction front is significantly broader near the film surface. Such behavior is consistent with a surface excess of acid catalyst, depth-dependent catalyst diffusion rates, or both of these factors. These studies demonstrate that the types of interactions at each interface can dominate a nanopatterning process based on polymer thin films.