Ion Diffusion in Chemically Amplified Resists

Abstract

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.

Characterization of polymeric surfaces and interfaces using time‐of‐flight secondary ion mass spectrometry

“…There are characterization needs in polymer science where ToF‐SIMS could be very valuable. As an example, studies have shown that ToF‐SIMS depth profiling can probe penetrant diffusion in glassy polymers 102 as well as interdiffusion of polymers and small molecules 152 . The key advantage of using ToF‐SIMS for diffusion studies is the intrinsic chemical contrast, which is particularly important when probing materials with complex chemical motifs.…”

“…Depth profiling can be used to probe diffusion of a small molecule penetrant in a thin polymer film. As an example, Bottoms et al examined diffusion of sodium triflate in a polymeric photoresist, 102 where sodium triflate was employed as an inert analogue of a common catalyst (acid triflate). They prepared bilayers with a base “feeder layer” containing sodium triflate and a top “diffusion layer” that initially contained no ions.…”

“…The optimized diffusion model is shown by the red line. Adapted with permission from Reference [102]…”

Accelerated Diffusion Following Deprotection in Chemically Amplified Resists

“…Diffusion of small penetrants in inert polymer melts and glasses has been a subject of numerous studies, with atomistic and molecular simulations expanding our fundamental understanding of these processes. − It is now well established that a subdiffusive regime can be present for displacements over length scales of a few nanometers. This was attributed to (1) the movement of penetrants being controlled by the size, shape, and connectivity of an infrequently present transient volume between polymer chains, as well as the size of the penetrant, , and (2) the strength of enthalpic interactions between the penetrant and polymer, impacting short-range penetrant dynamics. , With recent advancements in modeling and computational power, molecular simulations have been increasingly employed to study ion diffusion in polymers, in the presence of strong ionic interactions. − In addition, recent molecular simulations have shown that intermolecular interactions are an important consideration for catalyst diffusion in inert CAR systems. ,− All of these prior studies provide a solid fundamental framework for molecular diffusion in polymeric systems without reactions.…”

“…Prior literature has shown that catalyst diffusion during reaction has an apparent time dependence . Others have hypothesized that this time dependence is associated with changes in polymer composition, ,, catalyst–polymer, or catalyst–catalyst interactions, − or a short-lived increase in free volume from the generation and (assumed) fast escape of volatile reaction byproducts. − In our most recent study, we examined the first two hypotheses using a model CAR system based on a terpolymer resin and inert catalyst analogue (ion pair) with atomistic simulations and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) experiments. The findings showed that in the absence of reaction, ion pair diffusion is polymer composition and catalyst concentration invariant.…”