**3. EUV radiation-driven plasmas**

As discussed earlier, Figure 1 shows the general configuration of a DPP system for EUV 13.5-nm light generation. Another "coupling" effect of the DMZ in the source system (e.g. from the electrode materials of the source through the DMZ to the collector mirror) is the fact that the intense EUV and UV radiation generated from the 13.5-nm radiators (e.g. Xe or Sn) can induce a secondary low-temperature plasma at the surface of the collector mirror by ionizing the protective gas used for debris mitigation such as argon or helium [Van der Velden et al, 2006, Van der Velden & Lorenz, 2008]. The characteristic plasma in this region is found to be of low temperature (e.g. 5-10 eV) and moderate densities (e.g. ~ 1016 cm-3). The photoionization process can lead to fast electrons that induce a voltage difference the order of 70 V. In addition, due to the sheath region at the plasma-material interface between the plasma and the mirror the ionized gas particles (e.g. Ar+ or He+) can be accelerated up to about 50-60 eV. This energy in the case of Ar ions is relatively low and in the so-called sputter threshold regime for bombardment on candidate collector mirror material candidates. In addition, carbon contamination could also be accompanied by this plasma exposure. These candidate materials are typically thin (~20-60 nm) single layers of Ru, Rh or Pd, all of which reflect 13.5-nm light very efficiently. Only few studies have been conducted to elucidate how these low-energy ions may induce changes that can degrade the optical properties of the 13.5-nm collector mirrors. Van der Velden and Allain studied this effect in detail in the *in-situ* experimental facility known as IMPACT to determine the sputter threshold levels at similar energies [Allain et al, 2007]. In the work by van der Velden et al. the threshold sputtering of ruthenium mirror surface films were found to be in close agreement with theoretical models by Sigmund and Bohdansky. The sputter yields varied between 0.01-0.05 atoms/ion for energies about 50-100 eV and models were found to be within 10-15% of these values.

propagation. These analyses along with erosion material modeling (discussed in Section 4) are mainly used to dictate materials selection for electrode materials in EUV DPP

One particularly important "coupling" effect between the debris mitigation zone region and the collector optics region is the use of inert mitigation gases (e.g. Ar or He) that in turn are ionized by the expanding radiation field and thus generate low-temperature plasma near the collector mirror surface. This phenomenon is briefly discussed in Section 3. Each candidate radiator (e.g. Li, Sn or Xe or any combination) will result in a variety of irradiation-induced mechanisms at the collector mirror surface. For example, if one optimizes the EUV 13.5-nm light source for Li radiators, the energy, flux and mass distributions will be different compared to Sn. Both of these in turn are also different from the standpoint of contamination given that both are metallic impurities and Xe is an inert gas. The former will lead to deposition of material on the mirror surface. In the case of Xe, thermal deposition would be absent however the energetic Xe implantation on the mirror surface could lead to inert gas damage such as surface blistering and gas bubble production for large doses. Debris mitigation systems would have to be designed according to the

As discussed earlier, Figure 1 shows the general configuration of a DPP system for EUV 13.5-nm light generation. Another "coupling" effect of the DMZ in the source system (e.g. from the electrode materials of the source through the DMZ to the collector mirror) is the fact that the intense EUV and UV radiation generated from the 13.5-nm radiators (e.g. Xe or Sn) can induce a secondary low-temperature plasma at the surface of the collector mirror by ionizing the protective gas used for debris mitigation such as argon or helium [Van der Velden et al, 2006, Van der Velden & Lorenz, 2008]. The characteristic plasma in this region is found to be of low temperature (e.g. 5-10 eV) and moderate densities (e.g. ~ 1016 cm-3). The photoionization process can lead to fast electrons that induce a voltage difference the order of 70 V. In addition, due to the sheath region at the plasma-material interface between the plasma and the mirror the ionized gas particles (e.g. Ar+ or He+) can be accelerated up to about 50-60 eV. This energy in the case of Ar ions is relatively low and in the so-called sputter threshold regime for bombardment on candidate collector mirror material candidates. In addition, carbon contamination could also be accompanied by this plasma exposure. These candidate materials are typically thin (~20-60 nm) single layers of Ru, Rh or Pd, all of which reflect 13.5-nm light very efficiently. Only few studies have been conducted to elucidate how these low-energy ions may induce changes that can degrade the optical properties of the 13.5-nm collector mirrors. Van der Velden and Allain studied this effect in detail in the *in-situ* experimental facility known as IMPACT to determine the sputter threshold levels at similar energies [Allain et al, 2007]. In the work by van der Velden et al. the threshold sputtering of ruthenium mirror surface films were found to be in close agreement with theoretical models by Sigmund and Bohdansky. The sputter yields varied between 0.01-0.05 atoms/ion for energies about 50-100 eV and models were found to be

**2.3 EUV radiators, debris generation and debris mitigation systems** 

sources.

radiator used.

**3. EUV radiation-driven plasmas** 

within 10-15% of these values.
