Image Formation in Electron Projection and E-beam Direct Write Lithography Resists

Abstract

We have demonstrated that current DUV resist platforms can be optimized to develop new EPL resists and that these new resists have a greater degree of freedom in their design than their optical counterparts. Free from the constraints of optical lithography, we have been able to use high levels of PAG that would be too strongly absorbing at DUV, and produce resists that are 3 to 10 times more sensitive than UV6. Monte Carlo simulations of these phenolic resists indicate that the 100KeV electrons will travel straight through the film, leaving straight sidewalls, as verified by SEM cross sections of EPL-optimized resists. Recent work indicates that a new set of formulations will enable even better sensitivity than initial EPL resist requirements. The image formation process for an e-beam exposure on a positive chemically amplified resist has been simulated using discrete models from exposure through development. A ‘percolation network formation for diffusion-reaction development’ model for chemical y amplified resists was used to compare simulation results with experiment for 45nm contact hole arrays, showing excellent agreement. Ionization statistics show that most of the secondaries generated in the resist have energies well below 100eV, peaking at around 10eV. At these low energies impact ionization or plasmon generation are not possible, which means other types of interactions between secondary electrons and PAG come into play for photo-acid generation. This is consistent with a "scavenging" interaction described in the literature. Photoacid diffusion, solubility range and threshold have been identified as contributors to sidewall roughness, though further study is warranted.