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This chapter presents a summary of plasma-surface interactions in electrode and condenser-optic materials in plasma pinch sources for EUV light generation, with special emphasis on DPPs. In DPP EUV devices, electrodes at the source are exposed to short (10–20-ns) high-intensity plasmas, leading to a variety of erosion mechanisms. Erosion of the electrodes is dictated by the dynamics of the plasma pinch for configurations such as the dense plasma focus (DPF), Z pinch, and capillary. The transient discharge deposits 1–2 J∕cm2 per pulse on electrode surfaces. Large heat flux is deposited at corners and edges, leading to enhanced erosion. Understanding of how particular materials respond to these conditions is part of the rigorous design of DPP electrode systems. Erosion mechanisms can include physical sputtering, current-induced macroscopic erosion, melt formation, and droplet and particulate ejection. Erosion at the surface is also governed by the dynamics of how a plasma can generate a vapor cloud, leading to a self-shielding effect, which results in ultimate protection of the surface bombarded. Determining which will dominate—either microscopic erosion mechanisms such as physical sputtering, or macroscopic mechanisms such as melt formation and droplet ejection—remains an open question in DPP electrode design. This is because such mechanisms are inherently dependent on the pinch dynamics and operation of the source.
In addition to plasma-surface interactions in electrodes, for condenser optics, especially collector optics, erosion is due to fast ions and neutrals born in the plasma pinch, leading mainly to physical sputtering and other bombardment-induced mechanisms. If the surface is composed of more than one species, which is mostly the case, then radiation-induced and thermally activated effects govern the behavior of the surface and govern lifetime levels of the exposed material. Exposure includes debris from electrodes, high-energy ions and neutrals, highly charged ions (HCIs), background impurities, photon radiation [13.5-nm and out-of-band (OOB)], and redeposited eroded mirror material. Figure 35.1 presents an overview of DPP plasma-surface interaction modeling that can be complemented by materials testing. It shows the transient plasma region (region 1) to the left and the quiescent expanding plasma region (region 3) to the right where the collector optics is located. Region 2 comprises a number of debris mitigation schemes that can also be modeled and experimentally tested. A number of modeling simulators can couple regions 1–3 in a self-consistent manner. This can be useful in designing mirrors that are compatible with the debris mitigation schemes selected as well as the EUV fuel used at the source. This is especially important if alternative EUV radiator fuels are selected, such as Sn or Li. Designs at the source with alternative radiator fuels can be studied and their effect on mirrors assessed.
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