Polymer photochemistry at advanced optical wavelengths

Abstract

As lithography is extended to 157 nm, the molecular absorptivity becomes high for most organic polymers. Polymer photochemistry depends on photon absorption, and the higher energy associated with 157 nm light should lead to higher quantum yields of photoproducts. Polymers representative of those commonly employed in 193 or 248 nm resists were selected for this study. A gel permeation chromatography based method was developed to determine quantum yields for chain scission and crosslinking on thin polymers films coated on silicon wafers. This method was applied to determine the ΦS and ΦX of a number of lithographically significant homopolymers and copolymers at both the 157 and 248 nm wavelengths. It was found that polymers containing hydroxystyrene only undergo crosslinking while acrylate and methacrylate polymer only undergo chain scission. The film loss of 157 nm exposed poly-t-butyl acrylate and polymethyl methacrylate was found to be very high and attributed primarily to side chain cleavage of the esters, while no film loss of polyhydroxystyrene was detected. The analysis of outgassing materials showed that ester elimination in poly-t-butyl acrylate was responsible for all outgassed products and that the sum of the quantum yields of all outgassed products exceeded one, implying a reaction mechanism that recycled the initially produced radical. Direct polymer photolysis is significant at 157 nm and must be considered in resist design given the relatively high absorbance of most organic molecules at 157 nm.