2. GEOMETRIC EFFECTS

3. ON EUV EMISSIONS IN M. S. Tillack, K. L. University of California San Diego

4. SPHERICAL & PLANAR Sequoia and Y. Tao Center for Energy Research

5. TARGETS Laser Plasma and Laser-Matter Interactions Laboratory

6. Laser-produced Sn plasma is a leading light source option for next-generation semiconductor lithography. Conversion efficiency (CE) is a critical parameter for an attractive and economic system. Measurements show ~2x reduction in CE using microsphere targets instead of slabs. We have modeled and measured the expansion dynamics and emissions in planar and spherical Sn targets to help explain these observations. The results all can be explained by differences in the electron density. OVERVIEW

7. wafer mask EUV source laser ( Koay et al, SPIE 5751, 2005) (www.intel.com/technology/silicon/lithography.htm) Next-generation semiconductor lithography based on laser-produced plasma

8. Experimental Arrangement Nd:YAG 7 ns 500 mJ 1064 nm Nd:YAG 0.15 – 0.5 ns 250 mJ 532 nm Transmission grating spectrometer: EUV wavelengths 2–20 nm Nomarski interferometer: Wollaston prism, and cube polarizer ~ 20  m resolution 450 mm lens

9. h2d: 2d Lagrangian radiation hydrodynamic code. Distributed & maintained by Cascade Applied Sciences. Tabular EOS and opacity. Mesh refinement capability. Model Description Cretin: Multidimensional non-LTE collisional radiative solver. Distributed & maintained by Howard Scott, LLNL. Sn data generated by FAC atomic code. 1d hydro capability.

10. h2d Electron Density Evolution n e during laser pulse, planar targetn e during laser pulse, spherical target h2d shows the same trends as our experimental results –Spherical target has lower density and smaller plasma Laser spatial profile in this case is an 100  m flat top

11. Possible reasons for lower CE Higher losses due to hydro expansion Higher losses due to hydro expansion Mismatch of laser spot with target Mismatch of laser spot with target Different angular distribution of EUV emissions Different angular distribution of EUV emissions Spectral differences (out-of-band emissions) Spectral differences (out-of-band emissions) Opacity effects Opacity effects I slab = 43 nVs I sphere = 26 nVs Emon Signal

12. The UTA of Sn is an efficient source at 13.5 nm Konstantin Koshelev, Troitsk Institute of Spectroscopy Sn +6 Sn +7 Sn +8 Gerry O’Sullivan, University College Dublin Sn Xe Sb Te I Light comes from transitions in Sn +6 to Sn+ 14, between 4p 6 4d n and 4p 5 4d n+1 or 4d n-1 (4f,5p) Out of band light wastes energy and places an unnecessary heat load on optics

13. Experimental Parameters Laser: 5x10 11 W/cm 2 Wavelength1064 nm Pulselength7 ns (fwhm) Spot size70 µm (fwhm) Target: Composition100% Sn Geometrythick slab or 100~150 µm powder 100 µm

14. h2d energy balance in a slab target Little hydro loss in either case More energy transferred to electron heating

15. h2d energy balance in a spherical target More energy lost to radiation Consistent with smaller, less dense plasma

16. The plasma density profile is very different in the two targets Planar targets have larger plasma size (the tails of the laser beam miss the spherical target) Planar targets have higher density in the corona Before pulse During pulse

17. Spherical Target Planar Target Abel inversion of interferograms along the centerline t = 3 ns past peak laser irradiance measurement plane x y

18. Comparison of EUV spectra Narrower spectrum is consistent with lower electron density

20. Interferometry shows significant differences in electron density between planar and spherical targets. h2d results are in general agreement, and further predict that radiation cooling is larger in spheres. The EUV spectrum is narrower in spheres, also suggesting lower density SUMMARY Lower ablation rate may explain the differences, and offer solutions. ( M. Key, Physics of Laser Plasma, Chap. 14, 1991)