Debris characterization and mitigation from microscopic laser-plasma tin-doped droplet EUV sources

The EUVL collector mirror reflectivity degradation can be measured as erosion of the mirror surface caused by the high energy ion emissions. Characterizing the ion emission permits the analysis of the mechanisms of erosion and provides the capability to reduce the high energy ion emission which directly reduces the erosion rate. The degradation can also be measured as deposition of particulate debris on the mirror surface. The debris particles have sizes of only a few nanometers. We have demonstrated that the use of electrostatic repeller fields mitigates large fraction of the particle transfer. Our microscopic tin-doped droplet target is a mass-limited target and is designed to limit the flux of uncharged particulate matter emanating from the target, with the eventual objective of only generating charged material. The latter then may be inhibited from degrading EUV optics with the use of electrostatic repeller fields and other mitigation schemes. We present tin-doped droplet target ion emission characteristics in terms of ion energy distribution obtained using our ion spectrometer. Extensive studies on particle generation by controlling plasma conditions and the repeller field effect on individual ion species and particles is also described.