POSITIVE DEEP UV RESIST BASED ON POLY(α-ACETOXYSTYRENE)

Abstract

α-Acetoxystyrene (ACOST) undergoes radical polymerization in contrast to α-methylstyrene. However, the polymerization is a slow equilibrium process with a ceiling temperature (T_c) of 47°C at 1mol/L. Poly(α-acetoxystyrene) (PACOST) possesses a tertiary benzylic carbon adjacent to an ester oxygen in its backbone and is converted upon heating to ca. 220°C to poly(phenylacetylene) and acetic acid. The temperature of deesterification involving the polymer main chain can be lowered by generating a strong acid in the film. However, the acid-catalyzed deesterification of PACOST is accompanied by simultaneous depolymerization. Due to the solubility alteration through the structural modification and the molecular weight reduction by depolymerization, PACOST containing triphenylsulfonium hexafluoroantimonate functions as a sensitive positive resist incorporating "chemical amplification." PACOST is very transparent in the deep UV region and the resist containing 4.8wt% of P ₃SSbF₆ has an optical density (OD) of 0.3/μm at 248nm. The resist has a sensitivity of ca. 5mJ/cm² and a contrast (γ) of 4.3 when postbaked at 130°C for 5min and developed with xylenes. Owing to its high glass transition temperature (T_g>200°C), the positive images do not show any thermal flow at 180°C and owing to the aromatic nature of the matrix polymer, the resist is as stable as novolac-based resist systems in CF₄ plasma. Unfortunately, however, the resist film tends to suffer from solvent-induced cracking during development due to the stiff polymer chain.