The Dissolution Behavior of Tetrafluoroethylene-based Fluoropolymers for 157-nm Resist Materials

Main-chain-fluorinated base-resins, using the copolymer of tetrafluoroethylene and functional (hexafluoroisopropanol (HFA) group) norbornene, were synthesized. Partial protection of the hydroxyl group as an ethoxymethyl group was achieved by two methods: by copolymerization (Method A) or by polymer reaction (Method B). The partial protection by copolymerization was conducted by copolymerizing TFE with a mixture of protected and unprotected monomers (Method A, copolymerization). Partial protection was also carried out by reacting the hydroxyl group of the polymer, which is composed of TFE and unprotected monomers with ethoxymethyl chloride in the presence of an amine (Method B). In the polymer reaction, only the exo position of the norbornene unit was protected. Their fundamental properties, such as transparency at 157 nm and solubility in a standard alkaline developer, were characterized and studied. High transparency, i.e., absorbance better than 0.4 μm-1, was achieved with both methods. However, the polymer prepared by the polymer reaction (Method B) was deprotected more quickly; it also had a higher dissolution rate and stronger development contrast than the polymer prepared by copolymerization (Method A). Positive-working resists based on these fluororesins were developed and 55 nm dense lines could be delineated by exposure at 157 nm wavelength with alternating phase shift mask on a 0.9 NA 157 nm exposure tool.