The design of viable imageable photoresist materials for Extreme Ultra-Violet, EUV (13.5 nm), radiation requires the understanding of the energy conduits that form to deliver chemically relevant resonant energies within a material, Starting with absorption of a 92 eV ionizing radiation that, upon absorption, initiates a spawning of photoelectrons and secondary electrons that distribute energy across the population of generated secondary electrons with each subsequent generation’s electrons increasing in number and lower in average energy until they attain 0 eV. The loss of energy is the result of both elastic and inelastic scattering. At energies above 30 eV absorption is localized to atoms on the molecules in the resist matrix and the electrons that are spawned continue to react locally until then around 30-20 eV, the deep-valence region, they begin to delocalize and with continued ionization until 10 eV, that below this interacting through the molecular orbitals, the secondary electrons begin to react as virtual photons until below 3 eV where they attain energies that resonate with vibrational energies to form stable free radicals and ions that if not trapped by a molecule with a resonant low lying unoccupied molecular orbital (LUMO) may be captured by a hole in the material or substrate or they may get trapped in the material system as they are, without reacting.
The resist designer applies knowledge concerning the interaction of high energy particles and photons with gases and biological systems to the lower energies used for lithography truncating at below 3 eV in the condensed matter of the polymeric resist matrix. This truncation occurs because the role of (quantum) resonance of a molecular bond with a “virtual photon” is typically between 3 and 5 eV. However, because of the presence of excess electron, stable anions, and radicals (as) reactants this truncation is erroneous; and chemistries may occur at energies approaching 0 eV. These chemistries result from vibrational resonances of the transient electron with a low energy unoccupied molecular orbitals (LUMO) of the matrix molecules. This behavior is quantum in nature and is not classical.
This paper discusses the radiolysis processes as follows:
1. the local nonmolecular processes from 92 eV photon absorption to 30 eV,
2. then the ionization in the deep valence below 30 eV to 10 eV,
3. followed by the region of virtual photons from10 eV to 3 eV,
4. and then examine below 3 eV to 0 eV to finish.
Describing the processes in the higher energy regions is difficult due to the short attosecond to femtosecond reaction time-resolution to monitor them but the paper proposes methods to accomplish. Below 30 eV the plasmonic charging work function pose problems for some methods like Total Electron Yield measurements of low energy electrons but time resolved and various analytical methods using EUV exposure will prove useful.
The paper concludes by looking at the relationship to line-edge-roughness, stochastic print failures, and research opportunities that will take us to the edge of EUV lithographic chemistry.