The hidden energy tail of low energy electrons in EUV lithography

Electronic processes in extreme ultraviolet lithography are key to understand chemical reactions that lead to exposure of photoresist because of recent evidence that even very low kinetic energy electrons (E_k → 0 eV) might be very efficient in dissociating the molecules in photoresists. However, an assessment of the distribution of electrons involved in the cascade at kinetic energy approaching zero eV is not trivial in solid state. In this work, we use electron yield spectroscopy from thin photoresist films based on poly(hydroxystyrene) with and without photoacid generator (PAG). The results show that the addition of PAG boosts the electron yield by a factor 2 with respect to the polymer only photoresist. We then elaborate an analytical form of the transfer function that describes the physics of the photoemission by accounting for the energy distribution inside the photoresist material during exposure to extreme ultraviolet light. By fitting the model function to the spectroscopic data, we obtain an estimation for the distribution of electrons which lie inside the material at energy too low to be measured by an external detector, in other words the hidden tail of low energy electrons. For a quantitative comparison, we also use another approach based on MonteCarlo simulation of electronic scattering effects to calculate numerically the magnitude of the electron cascade. Using Chariot simulator, a statistically significant number of electronic trajectories (6 x10⁷) was calculated and the energy distribution of electrons are compared both outside and inside the photoresist film.