New strategies for high resolution photoresists

Abstract

2-[4-(2-hydroxyhexafluoro isopropyl)cyclohexane] hexafluoroisopropyl acrylate (AF) was prepared as a key monomer for 157 nm photoresist platforms. In order to balance transparency with other desirable traits such as etch resistance, new strategies for high resolution photoresists were developed and are discussed. (1) The α-trifluoromethyl group was introduced on the polymer backbone and the resulting polymer showed unexpectedly high transparency at 157 nm (A = 1.6 μm-1) despite a carbonyl group in all the monomers. A new 157 nm transparent monomer that will provide a solubility switch necessary for base solubility was designed. (2) p-t-Butoxy-tetrafluorostyrene was copolymerized with AF. The high acidity of p-hydroxy-tetrafluorostyrene allowed us to increase the content of the monomer in the system to produce a novel polymer that is highly transparent at 157 nm (A = 2.1 μm-1). The polymer showed good 248 nm lithographic performances. (3) Introduction of fluorine on the polymer backbone was carried out. Due to a lower electron density of the vinyl groups with fluorine, these vinyl monomers can be copolymerized with electron-rich vinyl monomers such as norbornene using radical initiators. 2-Methyl-2-adamantyl trifluoroacrylate (MAdTFA) was synthesized. An alternating copolymer with a norbornene derivative was found to have high Tg and gave clear images using 248 nm imaging.