Photoacid generator–polymer interaction on the quantum yield of chemically amplified resists for extreme ultraviolet lithography

Abstract

The transmissivity of thin photoresist films and its variation during exposure are key parameters in photolithographic processing, but their measurement is far from straightforward at extreme ultraviolet (EUV) wavelength. In this work, we analyze thin films of chemically amplified resists, specifically designed for EUV lithography, synthesized with two different backbone polymers and two different photoacid generators with concentrations ranging from 0 to 140% baseline. The static absorption coefficient α and the variation of transmissivity upon exposure to EUV light (i.e. the Dill parameter C) are measured experimentally with our established methodology. The Dill parameter C, or bleaching, is interpreted in terms of outgassing and it is correlated with the rate of photoacid decomposition to extract the exposure kinetics and the quantum yield. In addition, the dose to clear of each formulation is measured to determine the lithographic sensitivity. It was found that not only the photoacid molecule but also its interaction with the polymer backbone affects bleaching and thus the quantum yield. These experimental observables (α, the Dill parameter C and dose to clear) allow us to determine the amount of clearing volume of the photoresist per unit photoacid. The clearing volume is then discussed as a microscopic figure of merit for the deprotection radius in chemically amplified resists and in light of the pursuit of ultimate resolution in EUV lithography.