The reaction-diffusion mechanism of photoacids is dependent upon many variables including the reaction rate, size of the acid counterion species and temperature. Recent experiments have demonstrated the acid diffusion can be controlled by a combination of lower processing temperature with low activation energy resists as well as increasing the size of the acid. In an attempt to understand the fundamentals of the deprotection reaction process, we have studied the deprotection kinetics of model 193-nm photoresists with Fourier transform infrared (FTIR) spectroscopy as a function of exposure dose, post-exposure bake (PEB) temperature, and copolymer content. Under all conditions, the deprotection kinetics initially follow a 1st-order reaction rate law, but then breaks down as the deprotection level increases. This behavior can be understood by a limiting acid diffusion coefficient in the presence of high levels of deprotection. To understand the significance of deprotection extent and copolymer content, we studied the homopolymer of methyladamantyl methacrylate and copolymers containing (50 and 59) mole % alpha-gamma-butyrolactone methacrylate (?-GBLMA). The copolymer content decreases the reaction rate systematically with increasing levels of ?-GBLMA in the copolymer photoresists. Therefore reaction-rate kinetics models need to consider the initial copolymer composition for accurate prediction of the deprotection kinetics. Interestingly, the methylene adamantyl deprotection products could also be monitored using FTIR and leave the thin films at elevated temperatures. These measurements are the initial basis for understanding the influence of copolymer composition on the reaction kinetics and the outgasing of the products in these model resists.
Citation: Abstracts SPIE
Pub Type: Journals
copolymer, line edge roughness, photoacid, photoresist, reaction diffusion, spectroscopy