Fundamental aspects of norbornene-maleic anhydride co- and terpolymers for 193 nm lithography: Polymerization chemistry and polymer properties

Abstract

Radical co- and terpolymerizations of norbomene (NB) derivatives and maleic anhydride (MA) were in situ monitored in dioxane-d8 at elevated temperatures by 1H nuclear magnetic resonance (NMR) spectroscopy. The 5-substitution on norbornene reduces the polymerization rate. The endo and exo isomers of t-butyl norbornene-5-carboxylate are incorporated in the polymer at the same rate. In terpolymerization involving methacrylic monomers (t-butyl methacrylate, methacrylic acid), the third homopolymerizable monomer is much more rapidly incorporated in the polymer, with the ratio of NB to MA in the polymer significantly deviating from 1/1. This observation indicates that the commonly believed chain transfer (CT) mechanism is unfounded. In contrast, acrylic acid and t-butyl acrylate are more uniformly incorporated in the terpolymer. Another evidence against the CT polymerization was obtained by applying the mercury method to analyze the monomer reactivities using gas chromatography. UV absorption of MA-NB polymers at 193 nm tends to be fairly high, which is a compounded effect of modest intrinsic absorption of the anhydride structure itself and a significant contribution from the polymer end group. The anhydride ring can undergo hydrolysis during development when the matrix is polar enough to allow aqueous base penetration, which could enhance the development contrast but the anhydride hydrolysis could lower the storage stability.