Fluoropolymer resists: Fundamentals and lithographic evaluation

Abstract

Hexafluoroisopropanol (HFA) extensively utilized in the design of 157 nm resists provides attractive but complex dissolution characteristics. Poly(norbornene hexafluoroalcohol) (PNBHFA), which is equivalent to polyhydroxystyrene for 248 nm lithography, exhibits linear dissolution kinetics in 0.26 N tetramethylammonium hydroxide (TMAH) aqueous solution while weaker developers could induce swelling, resulting in multi-stage dissolution. We have found that the dissolution rate of PNBHFA in 0.26 N TMAH (3,000-8,000 A/sec) is not controlled by polymer molecular weights but affected more significantly by the endo/exo isomer ratio of NBHFA employed in polymerization. PNBHFA with a higher exo concentration dissolves more slowly and exhibits less hydrogen-bonding. The H-bonding interaction is not affected by molecular weight. The HFA OH in the exo isomer interacts more strongly with an acid generator and ester. Dissolution of exo-enriched PNBHFA is more strongly inhibited by addition of an acid generator. Lithographic performance of our PNBHFA blend resist is affected by the endo/exo ratio of NBHFA. Copolymers of two similar TFMAs with NBHFA and vinyl ether bearing norbornane hexafluoroalcohol (VENBHFA) exhibit very different surface segregation when blended with PNBHFA. The hydrophilicity of our TBTFMA copolymer system has been vastly improved by incorporating a polar comonomer carrying two HFA groups, which has decreased the 157 nm absorption to 0.9-1.1/μm. The transparency can be further increased by using such co- and terpolymers as a dissolution inhibitor or contrast booster of the most transparent vendor fluoropolymers.