Measurement of the Critical Ionization Level for Resist Polymer Dissolution Using a Polypeptoid Platform

Reactive dissolution of a photoresist polymer in aqueous base to form a processable image is an important step in building patterns on semiconductor chips. The critical ionization (CI) theory states that a specific minimum fraction, the CI level, of the ionizable moieties in the exposed polymer is required for the chain to become soluble. This fraction’s value strongly influences pattern quality but has only been estimated in prior studies. Here, the CI level of a specific polymer platform has been measured to be 20% using a family of polypeptoids. This platform has controlled chain length, ionizable monomer content, and chain end groups. Using reaction-diffusion chemical kinetics modeling, we show that polypeptoids dissolution above the CI level is consistent with theory. At the CI level, qualitatively different gelation-dissolution behavior occurs compared to levels above it. The methodology used in this work is broadly applicable to understanding fundamental aspects of reactive dissolution.