Image Formation In Thin Films of Chemically-Amplified Photoresists

Next-generation nanofabrication requires economical lithographic processes that achieve sub-22 nm resolution. Current manufacturing practices are based on projection lithography with chemically-amplified (CA) photoresists. CA resists are comprised of a polymer resin (reactant) blended with a photoacid generator (catalyst); a coupled reaction-diffusion mechanism drives image formation, where resolution is controlled by slow diffusion of the acid catalyst. Extending the lifetime of CA resists requires a comprehensive understanding of the physical and chemical variables that control image formation in ultrathin films. Challenges associated with thin films include surface segregation of acid catalyst, adsorption of airborne contaminants, and changes in polymer mobility due to thin film confinement. Currently, there are no methods that offer spatially resolved feedback for the reaction-diffusion mechanism. In this talk, we demonstrate that image formation in CA resists can be accurately measured through the film thickness with small-angle X-ray diffraction. Proof-of-concept experiments are based on poly(4-hydroxystyrene-co-tertbutylacrylate) resin loaded with 6.5 wt% triphenylsulfonium sulfonate photoacid generator. The CA resist is cast in 80 nm films on silicon/silicon nitride substrates, and the spatial distribution of acid catalyst is generated with electron-beam patterning.  Image formation is completed at temperatures in the range of 130-140⁰C, which are near the glass transition temperature of the reactant. Preliminary results demonstrate that spatial extent-of-reaction can vary through the film thickness, and measurements of the thin film glass transition suggest that enhanced dynamics at the free surface are responsible for this behavior.