Debris studies for the tin-based droplet laser-plasma EUV source

Abstract

We are developing a mass-limited, laser plasma target concept that utilizes excited state transitions in tin ions as the source of 13.5 nm radiation, offering in-band conversion efficiencies greater than 1%. The ultimate objective of this EUV source strategy is the utilization of a target that is completely ionized by the laser. To determine the viability of this source for EUVL, we are making extensive measurements of the debris emanating from the target. Here we report on some of these measurements. Also under investigation are various methods of debris mitigation. We have previously shown the effectiveness of electrostatic fields for repelling ions from mass-limited targets, demonstrating improvements in multilayer mirror lifetimes in excess of an order of magnitude, positioning water droplet targets within reach of the EUVL roadmap requirements. Our investigation of debris utilizes various diagnostic techniques including ion collection, ion sputtering and witness-plate capture of particulate debris, and extensive post-mortem microscopic materials analysis.