Journal of Micro/Nanolithography, MEMS, and MOEMSCharacterizing positive chemically amplified resists via in-situ impedance monitoring
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We present a novel, accurate methodology for investigating the kinetics of photoacid diffusion and catalyzed-deprotection of positive-tone chemically amplified resists during postexposure bake (PEB) by in-situ monitoring the change of resistance and capacitance (RC) of resist film during PEB. Deprotection converts the protecting group to a volatile group, which changes the dielectric constant of the resist. Thus, the deprotection rate can be extracted from the change of capacitance. The photoacid diffusivity is extracted from the resistance change because diffusivity determines the rate of change of the acid distribution. Furthermore, by comparing the RC curves, the dependence of acid diffusivity on the reaction state can be extracted. The experiment is done on Shipley UV5 resist with 248-nm wavelength illumination, an Agilent LCR meter is used to measure RC curves, and resist thickness loss is recorded. The kinetics of non-Fickean acid transportation, deprotection, free-volume generation and absorption/escaping, and resist shrinkage are analyzed. A comprehensive model of these chemical/physical mechanisms is proposed, whose parameters are extracted though a divide-and-conquer strategy with high accuracy. The theoretical RC curves fit the experiment with rms error less than 0.1%. The effects of free volume, polymer relaxation, and deprotection-induced polarity change on acid diffusion are also identified.