Study and control of the interfacial mass transfer of resist components in 193-nm immersion lithography

The interfacial mass transfer issues of resist components in ArF immersion lithography were investigated both for topcoat resist system and for non-topcoat resist system. PAGs and photoacids are known for the major components that leach out from the resist to water and topcoat has been simply expected to shut off this leaching. In this study, topcoat layer was acutally verified significantly to reduce the PAG leaching, but PAG (or photoacid) diffusion from the resist to topcoat was found as another issue. This resulted in T-top formation in the imaging due to the loss of photoacid nearby the resist surface caused by this diffusion. A material approach to reduce this interfacial diffusion was proposed through a thermodynamic consideration of the materials system. The free energy change for PAG-polymer solubilization is the key and the Solubility Parameter value was considered to represent a good measure of this free energy change. The experimental results well matched this analysis. On the other hand, a perfect suppression of the leaching without using topcoat would be more desirable if it is feasible. The influence of structural parameter of the resist component was investgiated. The hydrophobicity of PAG was found to well correlate inversely to the leaching amount of the PAG into water, and thereby a technical guideline was obtained in designing resist materials to make them resistant to the leaching. By optimizing each resist component on this bases, we actually demonstrated a resist that showed the PAG leaching level reduced down to 10^-13[mol/cm²], which we believe far below to what we are today with conventional dry resists. A resist sample was thus developed targeting the non-topcoat approach and was examined for its imaging performance with a realistic immersion lithography tool without using any topcoat. The resist actually showed good imaging results fully with the benefits that we expected with the immersion exposure.