Characterization of fluoropolymer resist for 157-nm lithography

Fluoropolymers are key materials for the single-layer resists used in 157-nm lithography. We have been studying fluoropolymers to determine their potential for use as the base resin. We developed a polymer that has high transmittance and high dry-etching resistance by attaching the cyclohexylcyclohexyloxymethyl (CCOM) blocking group to a monocyclic fluorinated polymer. The dry-etching rate of the 32 % blocked polymer was 1.5 times that of a KrF resist and its absorption coefficient at a 157-nm exposure wavelength was 1.1 /mum. The polymer with various photoacid generators (PAGs) was compared with the patterning profile and we found that the shape of the resist pattern depended on the DeltaH, which was the change in enthalpy before and after the PAG generated acid. DeltaH is related to acidity and triphenylsulfonium nonaflate has one of the highest acidity in the PAGs. The polymer with triphenylsulfonium nonaflate resolved a 55-nm line and space pattern. We also investigated the relation between the dissolution characteristics, transmittance and molecular weight of the PAG. We found that as the PAG's molecular weight was low, maximum dissolution rate (Rmax) tended to become high and transmittance tended to become high. For onium salts, in particular, as the anion's molecular weight of the PAG was low, transmittance tended to become high, and as the cation's molecular weight of the PAG was low, Rmax tended to become high. These are guidelines in choosing a PAG.