Origin of Debris in EUV Sources and Its Mitigation

Particle emission is an unavoidable consequence of using a plasma to generate photons. These particles will interfere with the collection mirrors for the EUV radiation. Low-energy atoms from sputtered material or condensable fuels such as Sn or Li can deposit on the mirrors. Higher-energy ions and charge-exchanged neutral atoms can sputter, implant, and roughen the mirrors. Both of these effects reduce the reflectivity. This chapter describes the sources of the energetic and condensable particles, analyzes mitigation schemes aimed at reducing their flux to the mirrors, and offers two new ideas to mitigate their effect on the mirror's surface. While this chapter deals most directly with DPPs, many aspects are relevant for LPP systems as well. Consider the plasma source shown in Fig. 36.1. Though a dense plasmas focus (DPF) is drawn, this section applies equally to any DPP, such as a Z pinch or vacuum arc. A dense pinch plasma is produced close to the inner electrode of the source. During the pinch, energetic fuel ions are created in the dense hot plasma and then move outward in all directions as the plasma expands.