Free Volume Enhances Catalyst Diffusion in Reactive Polymer Resins*

Quantitative reaction-diffusion models are critical to the design of high-resolution lithographic processes based on acid-catalyzed deprotection of glassy polymer resins. Prior works show that catalyst diffusion is enhanced by the deprotection reaction; however, the underlying mechanisms remain unclear. Literature suggests that the generation of excess free volume due to the fast removal of reaction byproducts could be central to the apparent reaction-diffusion rates. We examine the validity of this mechanism in a model polymer using experiments and atomistic simulations. Extent of reaction was examined through infrared absorbance spectroscopy, and densification of reacting films was probed through spectroscopic ellipsometry. Comparison of these data indicates that most reaction occurs prior to full relaxation of the polymer. Atomistic simulations follow the effects of deprotection and subsequent volume relaxation on catalyst mobility. We establish that catalyst diffusion is enhanced in recently reacted regions and slowly relaxes to that observed in the absence of reaction. Estimated relaxation rates of excess free volume are similar to those of catalyst mobility, suggesting that excess free volume generation could impact deprotection kinetics.