Study of energy delivery and mean free path of low energy electrons in EUV resists

The relative importance of secondary electrons in delivering energy in photoresist films was assessed by performing large area exposures and by quantifying the inelastic mean free path of electrons in a leading chemically amplified positive tone EUV resist. A low energy electron microscope was used to directly pattern large (~15μm x 20μm) features with 15-80 eV electrons followed by analyzing the resulting dissolution rate contrast curve data. In the 40 to 80 eV regime the energy delivery was found to scale roughly proportionally with electron energy. In 15 to 30 eV regime however, this energy scaling did not explain the resist thickness loss data. The dose required to lower the resist thickness down to 20 nm was found to be 2-5X larger for 15 eV electrons than for 20, 25 and 30 eV electrons. Using scattering models from the literature including phonon scattering and optical data deduced electron energy loss spectroscopy and optical reflectometry, the inelastic mean free path values at energies between 10 eV and 92 eV range between about 2.8 and 0.6 nm respectively.