# EUV-Induced Plasma: A Peculiar Phenomenon of a Modern Lithographic Technology
## 3 Bulk Materials Facing EUV Photon Induced Plasma
### Ionic Particle Fluxes  
Particle densities and velocities towards nearby walls determine particle fluxes. Atomic hydrogen radicals have no charge and therefore plasma-induced electromagnetic fields do not influence their trajectories meaning that their flux is being determined by diffusion. Ionic species, on the other hand, are charged and are therefore accelerated by the electric fields created due to the potential difference between the bulk plasma and the surface (see previous sections). The distribution of the impact energy of these ions is determined by the electron energy distribution function (EEDF) of the EUV-induced plasma to a large extent. In EUVL, the EEDF might be significantly altered by secondary electron emission from EUV irradiated surfaces, and therefore this secondary electron emission also affects the energies and fluxes of ions towards relevant surfaces [10; 21; 61; 79]. Metals emit electrons when they are illuminated with electromagnetic radiation with photon energies above the work function W. Of these electrons, only a small part is emitted as primary electrons straight from the surface and have energy \(E_{prim}~{}=~{}hv-W\). However, the absorption length for EUV photons in metals is much larger than the electron mean free path in the same material. Primary electrons that are not created on the surface, but deeper in the material, undergo many scattering events before they reach the surface. This also leads to the emission of low energy (1-2 eV) secondary-electrons from the surface [80]. This effect results in a significantly lower average electron energy than one would expect solely based on the difference between the photon energy and the material's work function. The research on the impact of secondary electron emission on EUV-induced plasma properties is limited to numerical work. Early simulations [10] on EUV-induced plasmas in 1 Pa argon show that secondary electrons lower the mean electron energy by approximately 10 eV between 50-200 ns after the EUV pulse. After 400 ns following the EUV pulse, there is no difference in charged particles density distributions, plasma potential, and average ion impact energy between simulations with and without the inclusion of secondary electrons. Recent simulations [21] in 5 Pa hydrogen yield similar observations: after 600 ns after EUV-irradiation, the influence of secondary electrons is negligible. However, the impact in the first 400 ns was larger with a 2/3 reduction in the potential when compared to the situation without the inclusion of secondary electrons. The reason for the different impact of secondary electrons on EUV-induced plasmas in argon and in hydrogen is the fact that the plasma is much denser in argon due to the larger photoionization cross section. Therefore, the relative contribution of the secondary electrons is much smaller. Experimental work on the particle fluxes in EUV-induced plasmas has been scarce due to the limited availability of high power EUV sources. A decade ago, initial attempts to experimentally characterize the EUV-induced plasma in argon with Langmuir probes were unsuccessful [33]. The photo-electric currents that obscure the measurement of the I-V curve were the main reason for this. Later attempts showed that Langmuir probe measurements are feasible, starting from 1 \(\upmu\)s after the EUV pulse [65; 81]. Photo-electric currents still hinder probe measurements during the EUV pulse. Recently, temporally resolved retarding field energy analyzer (RFEA) measurements of ion fluxes in hydrogen plasma were published [82]. The investigated pressure range was 10-100 Pa. Similar to previous work [33], the measurements were troubled by EUV-irradiation during the first 1.5 \(\upmu\)s. The measured ion energies peaked at bias potential, which is evidence for a collisionless space-charge region (often named the sheath region) that separates the plasma bulk from the plasma-facing surfaces. The thickness \(s\) of this sheath was estimated while using the Child-Langmuir law [60].