Angular ion emission characteristics of a laser triggered tin vacuum arc as light source for extreme ultraviolet lithography

The angular resolved emission of tin ions from a laser triggered vacuum arc to be used as light source for extreme ultraviolet lithography is presented. Ion energies of more than 200 keV for emission angles up to 50° with respect to the optical axis are observed. The angular emission characteristic is strongly anisotropic with a pronounced peak for fast ions into a cone with an opening angle of roughly 10° at an angle of 35° with respect to the optical axis. These ions also exhibit a distinct energy distribution function compared to the more isotropic emitted bulk of ions, which can be referred to different mechanisms of production. Looking at the discharge current parameters, the production of the directed fast ions can be connected with a peaked increase in the impedance, which gives hint to a plasma instability as origin of those ions. The emission of isotropic emitted ions is in agreement with a model of plasma expansion into vacuum. The emission characteristic is also strongly dependent on the parame...