Performance properties of near-monodisperse novolak resins

Novolak resins are the most complex and important polymers used in photoresists for microlithography. We have developed methods for fractionation of novolak resins based on supercritical fluid technology to afford polymers with molecular weights distributions (MW/MN) approaching 1.0. Properties (glass transition, dissolution, and lithographic) of monodisperse novolak resins demonstrate extremely exaggerated molecular weight effects. I-line resists formulated from these ultra-fractionated resins show extraordinarily diverse lithographic properties, ranging from low-to-high contrast, the absence or presence of microgrooving and residue, and photospeed and thermal properties that are highly controllable. Ultimately, a prototype resist formulation based on mixtures of fractions combining high resolution (0.35 microns) and fast photospeed (125 mJ/cm2) was demonstrated. New rules for the influence of novolak molecular weight on photoresist performance are presented.