Novel strategies for novolak resin fractionation : Consequences for advanced photoresist applications

The importance of well-characterized novolak resins in the manufacture of high performance photoresists is paramount. Two novel fractionation methods for novolaks synthesized by solution condensation are reported in this paper. Solid resin solutions in methanol and ethyl lactate were fractionated by ultracentrifugation and liquid-liquid centrifugation (using water and hexane as non-solvents). The techniques resulted in high yield, low-polydispersity (high M w ) resins that exhibited superior dissolution rates and lithographic performance when compared to resins obtained by conventional isolation techniques. The effects of solvent/non-solvent ratio, temperature, and centrifuge rpm on resist performance were evaluated. It was found that at sufficiently high rpm and at 25°C, the M w and the polydispersity of the fractionated resin increases and the dissolution rate decreases as the solvent/non-solvent ratio decreases. Slow dissolution rates and improved lithographic performance characteristics in experimental i-line formulations were obtained for resin solutions fractionated by liquid-liquid centrifugation using a 1/2 ratio of ethyl lactate (and/or methanol) to hexane. Scaling laws between dissolution rate and polymer molecular weight were derived and the data seem to support reptation-type diffusion. The use of appropriate process manipulation in order to precisely control resin properties and to design novel resins is discussed.