  Next generation EUV sources for photolithography use light produced by
laser-produced plasmas (LPP) from ablated tin droplets. A major challenge for
extending the lifetime of these devices is mitigating damage caused by
deposition of tin debris on the sensitive collection mirror. Especially
difficult to stop are high energy (up to 10 keV) highly charged tin ions
created in the plasma. Existing solutions include the use of stopping gas,
electric fields, and magnetic fields. One common configuration consists of a
magnetic field perpendicular to the EUV emission direction, but such a system
can result in ion populations that are trapped rather than removed. We
investigate a previously unconsidered mitigation geometry consisting of a
magnetic null by performing full-orbit integration of the ion trajectories in
an EUV system with realistic dimensions, and optimize the coil locations for
the null configuration. The magnetic null prevents a fraction of ions from
hitting the mirror comparable to that of the perpendicular field, but does not
trap any ions due to the chaotic nature of ion trajectories that pass close to
the null. This technology can potentially improve LPP-based EUV
photolithography system efficiency and lifetime, and may allow for a different,
more efficient formulation of buffer gas.
