Mean Free Path of Electrons in Organic Photoresists for Extreme Ultraviolet Lithography in the Kinetic Energy Range 20-450 eV

Abstract

Theblur caused by the nonzero mean free path of electrons in photoresistsexposed by extreme ultraviolet lithography has detrimental consequenceson patterning resolution, but its effect is difficult to quantifyexperimentally. So far, most mean free path calculations use the dielectricformalism, which is an approximation valid in the optical limit andfails at low kinetic energy. In this work, we used a modified substrate-overlayertechnique that exploited the attenuation of the Si 2p core level originatingspecifically from the native silicon dioxide to evaluate the attenuationof electrons traveling through 2 and 4 nm of photoresist overlayersto provide a close estimation of the inelastic mean free path relevantfor photoresist lithography patterning and for electron microscopy.The photoemission spectra were collected in the proximity of the Si2p edge (binding energy & SIM;101 eV) using synchrotron light ofenergy PLANCK CONSTANT OVER TWO PI & omega; ranging between 120 and 550 eV. The photoresistfilms were prototypical chemically amplified resists based on organiccopolymer of poly hydroxystyrene and tertbutyl methacrylate with andwithout triphenyl sulfonium perfluoro-1-butanesufonate photoacid generatorand trioctylamine quencher. The inelastic mean free path of electrons,in the range that is relevant for photoresist exposure in extremeultraviolet lithography (20-92 eV), was found to be between1 and 2 nm. At higher kinetic energy, the mean free path increased,consistently with the well-known behavior. The presence of the photoacidgenerator and quencher did not change the mean free path, within experimentalerror. Our results are discussed and compared with the existing literatureon organic molecules measured via dielectric formalism and electrontransmission experiments.