The acid-catalyzed deprotection of glassy polymer resins is an important process in semiconductor lithography. Studies have shown that the reaction kinetics in these materials is controlled by slow diffusion of the acid−anion catalyst, but trends deviate from models of a first-order reaction coupled to a composition-invariant Fickian diffusivity. We present a concerted experimental and computational effort to examine catalyst diffusion in a model resin of poly(4-hydroxystyrene-co-tert-butyl acrylate-co-styrene), or P-(HOSt-tBA-St), which is deprotected 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 to examine long-time ion dynamics in both terpolymers with atomistic molecular dynamics simulations and compared the calculated Fickian diffusivities with direct measurements of ion diffusion fronts using time-of-flight secondary ion mass spectrometry. Our results demonstrate that ion diffusivities in P(HOSt-tBA-St) and in P(HOSt-AA-St) are similar near and below the glass transition, consistent with a diffusion process that is dominated by interactions with the polar HOSt units. We then compared the bulk reaction kinetics measured by Fourier-transform infrared spectroscopy with reaction kinetics obtained using mesoscopic reaction−diffusion models. We found that initial reaction kinetics is significantly accelerated compared to predictions based on the long-time ion diffusivities of our 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.
Polymeric chemically amplified resists (CARs) are critical
materials
for high-throughput lithographic processes. A photoactivated acid-anion
catalyst changes the polymer’s solubility via a deprotection
reaction, which enables pattern development through selective dissolution.
To capture observed reaction kinetics, reaction-diffusion models employ
a catalyst diffusivity that is accelerated by reaction. However, the
microscopic origin and factors contributing to this phenomena remain
unclear. Herein, we employ detailed atomistic molecular dynamics simulations
to examine the impact of protecting group removal and material relaxation
on catalyst mobility. We report data on polymer density, catalyst
dispersion, excess free volume, and segmental dynamics with increasing
time/extent of deprotection. We then propose simple kinetic Monte
Carlo algorithms that can describe both molecular dynamics simulations
of deprotection reactions and experimental data.
Fundamental understanding of the physical processes controlling deprotection in chemical amplified resists (CARs) is critical to improve their utility for high-resolution lithography. We employ a combined experimental and computational method to examine the impacts of excess free volume generation, reaction byproducts, catalyst clustering, and catalyst counter-anion chemistry/size on deprotection rates in a model terpolymer CAR. These studies suggest that catalyst diffusion can be enhanced by a combination of excess free volume and reaction byproducts, and that counter-anion chemistry/size plays a key role in local reaction rates, which stems from differences in the rotational mobility of the catalyst.
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