Mark Tillack, S. S. Harilal, and Yezheng Tao Debris mitigation for solid target LPP – Recent results at UCSD – Mark Tillack, S. S. Harilal, and Yezheng Tao EUV Source Workshop San Diego, CA 10 November 2005 Financial support provided by General Atomics and the University of California through the UC Discovery Grant program

Target density is a key parameter for conversion efficiency, out-of-band emission and debris mitigation We studied LPP debris and light emission from Sn-doped foams – 100 mg/cc RF foam – 0.1-1% solid density Sn Targets provided by Reny Paguio, General Atomics e.g., 0.5%Sn = Sn1.8O17.2C27H54 Diagnostic capabilities: – EUV TGS – E-mon – Faraday cup – visible spectroscopy – 2-ns iCCD – interferometry

The minimum number of Sn atoms required to generate 10-100 mJ is small The total number of atoms to produce 100 mJ in-band is only ~1012 for a 10-ns pulse and average transition rate ~1012 s-1 <1 nm for a 500-µm spot Data courtesy of Joe MacFarlane, Prizm Comp. Sci. Konstantin Koshelev, Troitsk Institute of Spectroscopy The optical depth of Sn is only ~7 nm at full density. Beyond that, light is reabsorbed.

Lower density reduces out-of-band emission, without degrading CE Peak CE occurs at a higher laser intensity with low-density targets

Magnetic diversion appears more effective on doped foams B=0.6 T B=0.6 T solid Sn 25 wt% Sn 135 mg/cc total B only slows ions from solid Sn • B has a large effect on signal • Not clear yet how much is CH and how much is Sn (Al plume confinement)

Gas stopping is marginal with solid Sn Ion yield at 12˚, 15 cm Faraday cup vs. SRIM estimate Conversion efficiency at 45˚, 78 cm E-mon vs. attenuation calculation calculated • Only hydrogen stops ions at acceptable pressure • Collisionality in plumes affects the range

Foam targets appear to offer little advantage for gas mitigation Faraday cup signal Time (s) Time (s) Faraday cup signal

A combination of techniques may succeed Faraday cup measurements at 12˚, 15 cm from fully-dense target 100% dense photo (or electron impact) ionization peak occurs when gas is present We found evidence of a synergistic effect between a magnetic field and a background gas

Summary We have found several techniques to be partially effective at mitigating debris: B, E, gas, low density. Synergistic (multiple) effects offer some hope. Optimal control of density and opacity is essential. The tolerable amount of debris is low; we need a better quantification of the requirements vs. energy More work is needed: H2, higher sensitivity, B+gas+foam