(624b) Probing Ion Diffusion in Chemically Amplified Resists through Experiments and Atomistic Simulations

Quantitative reaction-diffusion models are critical to the design of high-resolution lithographic processes based on chemically amplified resists (CARs). CARs consist of a glassy polymer resin and an acid-anion catalyst. When heated at moderate temperature, the catalyst diffuses through the polymer and drives a deprotection reaction that changes the polymer’s polarity. It is well known that reaction kinetics is controlled by slow diffusion of the acid-anion pair, but it is very difficult to quantify the diffusivity through direct measurements, let alone examine the role of polymer-ion or ion-ion interactions on mobility. We present a concerted experimental and computational effort to probe catalyst diffusion in a model resin of poly(4-hydroxystyrene-co-tert-butyl acrylate-co-styrene), or P(HOSt-tBA-St), which is converted in the presence of an acid catalyst to poly(4-hydroxystyrene-co-acrylic acid-co-styrene), or P(HOSt-AA-St). We employed an inert catalyst analogue (sodium triflate) to examine long-time dynamics in both terpolymers over a broad temperature range. Depth profiling experiments using time-of-flight secondary ion mass spectrometry offer a direct measure of sodium triflate diffusion at temperatures near and below the glass transition (T_g), revealing the trends in diffusivity as a function of concentration and polymer composition. Multi-microsecond molecular dynamics simulations provide insight into ion-ion association, polymer-ion interactions, and transport mechanisms that influence diffusivities at temperatures well above the Tg. The unified picture of sodium triflate dynamics emerging from this multi-faceted approach reveals that while ion-ion association is present and decreases as ion concentration and temperature are lowered, the extent of association does not have an appreciable effect on ion diffusivities in either simulations or experiments. Polymer composition has a subtle effect on ion diffusivity at high temperatures, with slightly enhanced diffusivities in P(HOSt-tBA-St) relative to P(HOSt-AA-St). This effect is diminished at temperatures near and below T_g, where ion dynamics are primarily governed by interactions with HOSt repeat units. To test whether the diffusivities in our model inert system are transferable to a reactive CAR (acid triflate catalyst), these values were used to predict reaction kinetics using a mesoscopic model of fast deprotection coupled to Fickian diffusion, and outcomes were compared with experimental reaction kinetics. The initial reaction kinetics was significantly accelerated relative to predictions, indicating that transient states during reaction may locally enhance the catalyst mobility. The long-time reaction kinetics was consistent with the timescale of ion pair diffusion in the inert system. This study highlights the potential of atomistic modeling coupled with targeted experiments for interrogating the physical and chemical processes that control pattern formation in next-generation lithographic materials.