Molecular design for stabilization of chemical amplification resist toward airborne contamination

Abstract

This paper describes the first logical approach to the design of chemical amplification resists that are stable toward airborne contamination. This molecular design is based on the observation that uptake of N-methylpyrrolidone (NMP) by thin polymer films is primarily governed by glass transition temperatures (Tg) of the polymers. To prove the validity of our Tg theory, we lowered the Tg of the IBM tBOC resin (Tgg = 130°C), which is based on poly(p-t-butoxycarbonyloxystyrene) (pPBOCST), by simply employing itsm;ta-isomer (Tg = 85°C). Although the para- and meta-isomers contain a very small but the same amount (0.6 and 0.55 wt%) of casting solvent after prebake at 100°C for 5 min, their NMP uptakes have been reported to be vastly different (931 and 99 ng/wafer after 1 hr in airstream containing 15 ppb NMP, respectively). Refractive index measurements by the wave guide technique of the polymer films baked at 100°C have clearly indicated that the meta-isomer film is denser than the para-isomer film, suggesting slower diffusion ofNMP into the meta-film. Resists were formulated with 4.75 wt% oftriphenylsulfonium hexafluoroantimonate, exposed on a PE500 in the UV2 mode, allowed to stand in contact with 50", 100 ppm NMP for 5 min, postbaked, and developed with anisole in the negative mode. The para-isomer film was almost completely wiped off the 5 wafer, indicating a tremendous poisoning effect of NMP whereas the meta-isomer resist printed 1 urn features uniformly. The meta-isomer with a T g below the prebake temperature is annealed well and therefore much less sensitive to NMP contamination. This concept has led to the design of environmentally very robust chemical amplification resists that provide positive images upon development with aqueous base.