Fluorinated Quaternary Ammonium Salts as Dissolution Aids for Polar Polymers in Environmentally Benign Supercritical Carbon Dioxide

Environmentally benign supercritical carbon dioxide (scCO2) has been utilized as an “ecologically responsible” solvent for a wide variety of applications. However, nonpolar scCO2 is generally a very poor solvent for polar polymers which limits its use as a processing solvent in high resolution photolithography. In this paper, we report the synthesis of fluorinated scCO2 compatible additives and demonstrate their utility to help dissolve polar polymers in scCO2. We have designed and synthesized a series of quaternary ammonium salts (QAS) as scCO2 additives. The ability of these salts to assist in the dissolution of model acidic and protected polar polymers has been compared. For our studies we have chosen three different polar reference polymers. Addition of millimolar quantities of the QAS to the scCO2 solvent helped to dissolve the protected model polymer under moderate supercritical conditions. To explicitly investigate the interactions between the QAS additive and the various functionalities on the polymers we have carried out computational simulations using a fully atomistic force field adapted from optimized potentials for liquid simulations (OPLS). These studies combining both theory and experiment showed that the architecture of the cation and the basic nature and size of the anion are crucial in designing more efficient additives. Finally, on the basis of these findings we report the lithographic evaluation of a standard commercially available polymer photoresist using an appropriate scCO2 compatible QAS additive after development in scCO2. Well-resolved, negative-tone patterns as small as 100 nm were obtained by e-beam exposure, indicating the excellent potential of using scCO2 as the processing solvent in the presence of the QAS additives to form high-resolution structures.