Photo-Cross-Linking of Poly(4-hydroxystyrene) via Electrophilic Aromatic Substitution: Use of Polyfunctional Benzylic Alcohols in the Design of Chemically Amplified Resist Materials with Tunable Sensitivities

The use of polyfunctional benzylic alcohols as latent cationic cross-linkers for polymers such as poly(4-hydroxystyrene) has been explored. Photogeneration of acid within thin films of the phenolic resin containing the latent cross-linker results in their thermal cross-linking via electrophilic aromatic substitution in a self-perpetuating process. This cationic photo-cross-linking reaction has been used to design resist materials that show remarkably high sensitivities in the three common modes of lithography: deep-UV (0.2 mJ/cm²), E-beam (0.1 μC/cm²), and X-ray (<10 mJ/cm²). A comparative study involving numerous crosslinkers with related structures demonstrates a direct correlation between the resist sensitivity, or the initial rate of cross-linking, and the stability of the carbocationic intermediate that is formed in the rate-determining step. In some cases, the sensitivities of these chemically amplified materials are so high that they have surpassed the capabilities of many existing exposure tools. To make the resists more versatile, tuning of sensitivity can be achieved through the addition of an electron-rich species to the resist to partially capture the acid photogenerated within the polymer thin film.