Polymer photochemistry at advanced optical wavelengths

As lithography is extended to 157 nm, the molecular absorptivity becomes hi gh for most organic polymers. Polymer photochemistry depends on photon abso rption, and the higher energy associated with 157 nm light should lead to h igher quantum yields of photoproducts. Polymers representative of those com monly 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 Phi (S) and Phi ( X) of a number of lithographically significant homopolymers and copolymers at both the 157 and 248 nm wavelengths. It was found that polymers containi ng 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 lo ss 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 outga ssed products exceeded one, implying a reaction mechanism that recycled the initially produced radical. Direct polymer photolysis is significant at 15 7 nm and must be considered in resist design given the relatively high abso rbance of most organic molecules at 157 nm. (C) 2000 American Vacuum Societ y. [S0734-211X(00)03506-X].