A laser-produced plasma extreme ultraviolet (EUV) source by use of liquid microjet target

Transcription

1 A laser-produced plasma extreme ultraviolet (EUV) source by use of liquid microjet target Takeshi Higashiguchi Keita Kawasaki, Naoto Dojyo, Masaya Hamada, Wataru Sasaki, and Shoichi Kubodera Department of Electrical & Electronic Engineering and Photon Science Center, University of Miyazaki, Miyazaki, Japan Work supported by MEXT (Ministry of Education, Culture, Science and Technology, Japan) under contract subject Leading project for EUV lithography source development - Gakuen Kibanadai, Miyazaki, Miyazaki , JAPAN, TEL/FAX:

2 Background: EUV light source for the next generation lithography Engineering Test Stand Bill Replogle, Sandia National Laboratories International Technology Roadmap for Semiconductors (ITRS) Proc. SPIE 4688, 3 (22) D. Attwood, Soft X-Rays and Extreme Ultraviolet Radiation (Cambridge University Press, 2).

3 Joint requirements for EUV source: 3.5 nm (2%BW), 5 W, 7-7 khz Joint Requirements for EUV Source SOURCE CHARACTERISTICS REQUIREMENTS Wavelength 3.5 (nm) EUV Power (in-band) 5 (W) (after intermediate focus) Repetition Frequency > 7 - khz * Integrated Energy Stability ±.3% (3σ over 5 pulses) Source Cleanliness 3, hours (after intermediate focus) Etendue of Source Output 3.3 mm 2 sr (max)* Maximum Solid Angle to Illuminator.3.2 (sr) (depending on particular optical scheme)* Spectral Purity 3-4 nm (DUV/VUV) TBD 7% (design dependent)* > 4 nm (Vis/IR) TBD * Not agreed among participants Kazuya Ota, Source Workshop, Santa Clara, CA, USA (23).

4 Emission intensity (arb. units).5.5 EUV emission spectra from possible plasma materials Sn Xe.5 Li Wavelength (nm) EUV CE (%) 3 rd International EUVL Symposium Date : November - 4, 24 Venue : Sheraton Grande Ocean Resort Miyazaki, JAPAN Committee : Conference chairperson : Prof. Y. Horiike Program committee chairperson : Dr. S. Okazaki Deadline of abstract submission : July st, URL : info@euva.or.jp 2 3 Laser intensity (W/cm 2 ) The Optimal Source Path to HVM D. Myers, B. Llene, I. Fomenkov, B. Hansson, and B. Bolliger (CYMER)

5 Schematic diagram of experimental setup using planar Li target Wavelength: 64 nm Laser energy : <.4 J Pulse width: ns (FWHM) Spot size: 3 μmφ EUV energy meter (Mo/Si mirror) τ delay ω 2ω 5 mj 6 mj ns 8 ns Dual laser pulses Lens f = 5 cm Spectrometer (2 lines/mm) XRD Faraday Cup Li planar target Pumping outlet Focal spot CCD

6 EUV spectrum of a laser-produced lithium plasma Laser intensity: 7 x W/cm 2 Laser energy : 5 mj Pulse width: ns (FWHM) Spot size: 3 μmφ Intensity (arb. units) Wavelength (nm)

7 Laser intensity dependence of EUV CE using a single laser pulse EUV CE (%).5.5 Li Sn Laser intensity (x W/cm 2 ) Intensity (arb. units) Angular distribution cos.5 θ Angle (degrees) T. Higashiguchi et al., (submitted).

8 Estimation of an electron temperature using bound-free transitions in Li 2+ ions Electron temperature (ev) Laser intensity (x W/cm 2 ) Emission due to bound-free transitions Intensity (arb. units) T e ~ 25 ev Wavelength (nm) Y. B. Zel'dovich and Y. P. Raizer, Physics of shock waves and high-temperature hydrodynamic phenomena (Dover Publication, Inc., New York, Mineola, 966).

9 Pulse separation time dependence of EUV CE utilizing dual laser pulses Pre-pulse: 532 nm, 6 mj, 8 ns (< 2 x W/cm 2 ) Main pulse: 64 nm, 5 mj, ns, 3 μm (7 x W/cm 2 ) Optimum delay separation time: 2-5 ns 3 EUV CE (%) 2 T. Higashiguchi et al., (submitted) Pulse separation time (ns)

10 Ratio of electron temperature as a function of pulse separation time Pre-pulse: 532 nm, 6 mj, 8 ns (< 2 x W/cm 2 ) Main pulse: 64 nm, 5 mj, ns, 3 μm (7 x W/cm 2 ) Ratio of electron temperature Pulse separation time (ns) CR model Ion population Li + Li n e = x 2 cm -3 Li 2+ Li Electron temperature (ev) T. Higashiguchi et al., (submitted). [] D. Colomband and G. F. Tonon, J. Appl. Phys. 44, pp (973). [2] M. J. Bernstein and G. G. Comisar, J. Appl. Phys. 4, pp. 729 (97).

11 Objective: Enhancement of the EUV CE Enhancement of the EUV CE by use of dual laser pulses Plasma hydrodynamics of a Li plasma should be regulated. Optimum Li plasma parameters could thus be realized.

12 Experimental setup using ultrashort, high- intensity laser pulses Ti:Sapphire laser pulse Adjustable pulse width Compressor Spectrometer EUV energy meter 25 mj τ pulse fs- ps 7 ps LPP Adjustable separation time Michelson interferometer HM M M2 3 fs x 5 W/cm mj 6 x 3 W/cm mj τ delay = -8 ps Lens f = 5 mm Li-based microjet target (8 μmφ, 44%) Pumping outlet

13 EUV spectra from a Li-contained plasma at different laser intensities Li concentration (by mass): 44.4% Jet diameter: 8 μmφ Laser energy: 25 mj I L = 2.8 x 5 W/cm 2 I L =.8 x 2 W/cm Intensity (arb. units) Cl 9+ O 5+ (3d-2p) Li 2+ (Ly-β) O 5+ (4d-2p) O 5+ (4p-2s) O 5+ (3d-2s) Li 2+ (Ly-α) Wavelength (nm) Intensity (arb. units) Cl 9+ O 5+ (4p-2s) O 5+ (3d-2s) O 5+ (4d-2p) O 5+ (3d-2p) Li 2+ (Ly-α) Wavelength (nm)

14 Subpicosecond pulse width dependence of the EUV CE Li concentration (by mass): 44.4% Jet diameter: 8 μmφ Laser energy: 25 mj 3.5 nm EUV CE (a.u.) nm (2%BW) Pulse width (ps) APL 86, 2352 (25)

15 Optimum pulse width explained by the resonant absorption process Resonant absorption is dominant in a steepened density profile. Absorption thickness 3 3 Lcr = Lopt 3 8k cos θ L c opt S τ pulse c S 2 x 7 cm/s, λ = 2π/k =.8 μm, L opt 82 nm for θ = τ pulse 4 fs [] C. Garban-Labaune, E. Fabre, C. E. Max, R. Fabbro, F. Amiranoff, J. Virmont, M. Weinfeld, A. Michard, Phys. Rev. Lett. 48, 8 (982). [2] V. L. Ginzburg, Propagation of Electromagnetic Waves in Plasmas (Pergamon, New York, 97). [3] E. Parra, I. Alexeev, J. Fan, K. Y. Kim, S. J. McNaught, and H. M. Milchberg, Phys. Rev. E 62, R593 (2).

16 Enhancement of the EUV CE by use of dual subpicosecond laser pulses Soft x-ray CE (%) Exp. 3.5 nm (2%BW, 2π sr) Cal. (hydro-code) Pulse separation time (ps) A one-fluid two-temperature Lagrangian code well reproduced the experiments. - Plasma hydrodynamic - Atomic code - Plasma emission APL 86, 2352 (25)

17 Optimum delay time explained by the plasma expansion (simpler estimate) n e = cm -3 n crit =.7 x 2 cm -3 r Jet = 4 μm (2r Jet = 8 μm) v exp c s 2 x 7 cm/s τ exp 4 ps

18 Experimental setup using dual nanosecond laser pulses Intensity (a.u.) 2 pre-pulse (532 nm) EUV spectrum Wavelength (nm) EUV energy meter (Mo/Si mirror) ω 2ω A 2ω prepulse was focused loosely enough into the jet target to produce a plasma but not to produce EUV emission. τ delay Lens f = 3 mm ω 2ω 7 mj mj ns 8 ns Dual laser pulses ( Hz) Spectrometer (2 lines/mm) I main = 3 x W/cm 2 (75 μm spot) Li-based microjet target (7 μmφ, 45%) Focal spot CCD Energy meter Pumping outlet

19 Enhancement of the EUV CE by use of dual nanosecond laser pulses.5 EUV CE (%) % APB 8, 49 (25). 3.5 nm (2%BW, 2π sr) Pulse separation time (ns) A plasma expansion time to its critical density was responsible for the optimal pulse separation time. τ crit n n e crit / 3 r v Jet Exp 8 ns n e = 23 cm -3 n cric = 2 cm L = μm 2r Jet = 7 μm v Exp = 5 x 5 cm/s@ 2 cm -3

20 Optimum plasma conditions obtained for the Lyman-α emission at 3.5 nm Intensity (arb. units) Li 2+ (Ly-β) O 5+ (4p-2s) O 5+ (4d-2p) Li 2+ (Ly-α) w/o pre-pulse w/ pre-pulse O 5+ (3d-2s) Wavelength (nm) Optimum dual laser parameters produced a plasma where Li 3.5 nm emission became dominant. APB 8, 49 (25). Ion current (arb. units) CR model Ion velocity (x 7 cm/s) Population Population Li Electron temperature (ev) Li 2+ (3.5 nm) population: T e 8 ev O 5+ (3. nm) population: T e 25 ev Li + w/o pre-pulse w/ pre-pulse (Δτ = ns) Li 2+ Li 3+ O O+ O 2+ O 3+ O 4+ O 5+ O 6+

21 Summary Optimum dual laser parameters for the EUV CE enhancement Subpicosecond pulse width dependence revealed the optimal prepulse width of 4 fs, which corresponded to resonance absorption time of a short scale-length plasma. Dual laser pulse irradiation increased the in-band EUV CE from.8% to.2% for subpicosecond laser pulses, and from.5% to.5% for nanosecond laser pulses. Pulse separation time dependence indicated the optimum time separation of 5 ps and ns for subpicosecond and nanosecond laser pulses, respectively, corresponding to the plasma density decrease to its critical density due to the plasma expansion. The difference of the optimum delay time is mainly due to the difference of the plasma expansion velocities determined by different laser intensities.