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An argon plasma in a magnetic cusp [1,2,3] thermalizes fast tin ions from the laser-plasma exhaust and guides their heat onto a large area beam dump. Demonstrations of plasma stability and particle control [2,3] have shown the magnetic field requirement to be modest and easily achievable with a very compact magnetic circuit. The plasma required for an extreme ultraviolet (EUV) source power of 500W has a pressure of 640Pa and is contained in a cusp magnetic well with a containment B field of only 40mT. Superconducting magnets are not required and the conventional magnet power totals only 4kW. A cusp magnetic circuit of mild steel serves as part of the source chamber wall and has very small fringing fields. In 2015 [2] stable containment was demonstrated of an argon plasma at the exhaust power, temperature and density required for this application. Without B-field containment the same degree of tin ion control would require a large gas number density, necessitating the use of hydrogen for low EUV absorption, with its associated disadvantages of dissociation, tin hydride chemistry and re-cycling concerns. Without a guide B-field, the hydrogen throughput to carry away excess plasma heat is very large and the hydrogen flow is not directional, in contrast to the flow of a magnetically-controlled plasma, making scaling of the hydrogen technology very difficult. Substantially raised EUV source efficiency (40% improvement) is achievable via plasma guidance that conforms the shape of the cusp magnetic well to the EUV collector optic, allowing a collection solid angle as high as 7 sterad. This translates into a laser power as low as 30kW for 500W of EUV power. Because the plasma braking mechanism for fast tin ions is dominantly via electron Coulomb collisions [3], non-reactive argon gas can replace hydrogen as the plasma positive ion, and its flow rate can be much lower. In the argon cusp tin LPP source, the exhaust power is guided precisely onto a ring-shaped beam dump that receives argon ions of energy too low to cause back-sputtering of tin. A typical beam dump heat loading for a 500W EUV source will be 100W cm-2 or less. This combination of low-stress components, easy argon recycling and high source efficiency makes the further development of this technology very important for high volume manufacturing.
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