Determination of coupled acid catalysis-diffusion processes in a positive-tone chemically amplified photoresist

Acid diffusion during postexposure baking is viewed to be a limiting factor in the extension of lithography using chemically amplified resists to form ation of nanoscale features. Quantification of thermally activated reaction -diffusion kinetics in these materials is therefore an important step in un derstanding the extendability of this class of resist systems. Previous inv estigations have addressed this issue, however there is poor agreement amon g them, and too few data exist in the literature to allow the systematics o f the effect of polymer, photoacid generator, added base or other resist co mponents on the diffusion process to be understood. We describe in this art icle a combined experimental and modeling protocol that is designed to eluc idate the chemistry and physics of the reaction-diffusion process. Because it is physically based, not phenomenological, it provides a means of develo ping a set of predictive, mutually comparable data. that will allow new ins ights to be developed into the nanoscale behavior of chemically amplified r esist materials. We apply the protocol to a p-t-butyloxycarbonyloxystyrene/ bis(t-butylphenyl)iodonium perfluorobutanesulfonate positive-tone photoresi st system. The resulting kinetics measurements show that diffusion is envir onment sensitive and describable with two limiting diffusion coefficients. The Arrhenius parameters for the coefficients in p-t-butyloxycarbonyloxysty rene are D-0=1.9x10(8) cm(2)/s and E-a = 36.5 kcal/mol; those for diffusion in the deprotected polymer product p-hydroxystyrene are D-0 = 9x10(8) cm(2 )/s and E-a = 22.1 kcal/mol. The coefficients are much smaller than previou sly reported, resulting in a very slow diffusion rate. The model indicates that the considerable image spreading observed during the postexposure bake process is attributable primarily to the efficiency of the catalytic chemi stry. Our results suggest that numerical models currently used for predicti on of imaging in chemically amplified resists may require refinement in ord er to be useful for feature sizes below 100 nm and for new classes of resis t systems. (C) 2000 American Vacuum Society. [S0734-211X(00)01004-0].