Towards molecular-scale kinetic Monte Carlo simulation of pattern formation in photoresist materials for EUV nanolithography

Modelling the pattern formation process in photoresist materials for extreme ultraviolet (EUV) lithography in a stochastic and mechanistic manner, with molecular-scale resolution, should enable predicting the effect of variations of material parameters and process conditions, leading to insights into the ultimate resolution limits. In this work, we present the results of the first steps toward that goal. We describe the physics of the development with time of cascades of electrons and holes, created by the stochastic absorption of 92 eV photons, using a kinetic Monte Carlo model with molecular resolution. The thin film material is modelled assuming a cubic array of lattice sites, at a distance that is consistent with the molecular density of the photoresist material that is considered. The simulation of the cascading process is based on the experimental optical energy loss function, extended to include also excitations with momentum transfer. The method allows for including the Coulomb interactions between charges. In contrast to earlier work, within which the high-energy electrons move ballistically until scattering takes place, the trajectories are in our model formed by stochastically determined interconnected molecular sites. In future extensions of the model, this approach will facilitate including in a natural way a transition from delocalized electron transport at high energies to hopping transport of localized electrons at low energies. The simulations are used to study the sensitivity of the average number of degradations per absorbed photon and the average electron blur length on the rates of elastic scattering and of molecular degradation, and on the energy that is lost upon a molecular degradation process.