1. 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

2. 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

3. 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.

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

5. 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)

6. 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

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

8. 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

9. 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