Laser heating of noble gas droplet sprays: EUV source efficiency considerations S.J. McNaught, J. Fan, E. Parra and H.M. Milchberg Institute for Physical Science and Technology University of Maryland College Park, MD 20742 http://lasermatter.umd.edu
Abstract Our recent experimental studies of intense laser pulse interactions with noble gas droplets have shown that for fixed pulse energy, pulse duration is an important parameter for EUV source efficiency [1]. In general, one must first identify the desired emission feature. This feature will usually be associated with a particular ion state that should be present in high relative concentration for the full duration of the laser pulse. We have found that if this spectral feature is associated with an excitation process, the optimal laser pulse duration is set mainly by droplet size. In that case, it is desirable that the laser heating occur when the droplet is at or above the critical electron density. If the spectral feature is associated with recombination, this condition is relaxed, and the most EUV efficient laser pulse duration is generally longer. For that case, it is determined by achieving at least several recombination cycles. [1] S.J. McNaught, J. Fan, E. Parra, and H.M. Milchberg, Appl. Phys. Lett. 79, 4100 (2001)
Experimental Set-up Sample EUV spectra Nozzle Signal (AU) Filter transmission 10 20 30 40 Wavelength (nm) Signal (AU) 10 15 20 25 30 35 40 Wavelength (nm) To EUV grazing incidence monochromator (λ = 2-44 nm) Amplification circuit Laser Nd:YAG (λ = 1064 nm) t = 100 ps @ 10 Hz Cluster, Droplet Beam X-ray diode Silicon photodiode with 150 nm Al coating
Pump Probe Experimental Setup LASER Nd:YAG (1064 nm) Pulsewidth : 100 ps Energy: 200 mj/pulse @ 10 Hz Peak intensity: 5 10 14 W/cm 2 Probe delay line (τ delay = 0 14 ns) X-ray detector (>1.5 kev) Side scatter CCD Focal spot CCD Nd:YAG Laser Probe 200 mj TFP Pump 200 mj TFP f = 150 mm 15 cm Droplet jet Paraboloidal condenser τ delay Grazing incidence spectrometer (λ = 2-44 nm)
Sample EUV Spectra Kr 9+ Signal (AU) Kr 7+ Kr 6+ Kr 5+ KRYPTON 10 15 20 25 30 35 40 Wavelength (nm) Ar 7+ Ar 7+ Ar 6+ Ar 9+ Ar 5+ Signal (AU) Ar 8+ Ar 6+ Ar 4+ Ar 4+ ARGON 10 20 30 40 Wavelength (nm)
Droplet Size Distributions Argon droplets (600 psi, 138 K) Droplet size histogram d mean = 4.8 µm σ = 0.9 µm Small argon droplets 0 2 4 6 8 10 Droplet diameter (µm) 3 mm Krypton droplets (200 psi, 143 K) 300 µm Droplet size histogram d mean = 10.2 µm σ = 2.1 µm Large krypton droplets 0 2 4 6 8 10 12 14 16 18 20 Droplet diameter (µm)
Argon Droplet Plasma Emission Droplet plasma EUV emission (Argon droplets, d mean = 4.8 µm) EUV emission at 26.0 nm 2p 6 4f 2p 6 3d in Ar 7+ ions Recombination from Ar 8+ 120 E i (Ar 7+ ) = 143 ev 100 Ar 8+ emission lines observed Signal (a.u.) 80 60 40 τ decay Ar 9+ EUV (pump) Ar 9+ EUV (probe) Ar 7+ EUV (probe) Ar 9+ decay time = 190±60 ps Ar 7+ decay time = 2.1±0.3 ns EUV emission at 16.6 nm 2s2p 6 2s 2 p 5 in Ar 9+ ions 20 Electron collisional excitation 0 0 2 4 6 8 10 12 14 E i (Ar 9+ ) = 479 ev Delay (ns) Ar 10+ lines not observed
Krypton Droplet Plasma Emission EUV emission from probe (Krypton droplets, λ = 10.0 nm) Signal (a.u.) 160 140 120 100 80 60 40 20 0 10-µm droplets 7-µm droplets Decay time = 2.5±0.4 ns Decay time = 1.7±0.2 ns 0 2 4 6 8 10 12 14 Delay (ns) EUV emission at 10.0 nm 3p 6 3d 8 4p 3p 6 3d 9 in Kr 9+ ions Recombination from Kr 10+ E i (Kr 9+ ) = 275 ev Poor indicator of laser coupling
Droplet Plasma X-Ray Emission X-ray emission from probe (Argon, E > 1.5 kev, d mean = 4.8 µm) X-ray emission from probe (Krypton, E > 1.5 kev) 100 80 160 140 120 10-µm droplets 7-µm droplets Signal (a.u.) 60 40 Decay time = 160±30 ps Signal (a.u.) 100 80 60 Decay time = 295±50 ps 20 40 20 Decay time = 210±20 ps 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Delay (ns) Delay (ns)
Green Probe Scattering Images Argon droplets (4.8 µm diameter) 0 ns 125 ps Green scattering signal (integrated in center channel) 100 250 ps 500 ps 80 Signal (a.u.) 60 40 20 Decay time = 265±60 ps 3 ns 10 ns 0 0 2 4 6 8 10 12 14 Delay (ns)
Disassembly Time Scales Droplet disassembly time Laser-droplet plasma disassembly τ 1/ 3 d N 0 2 = e crit cs Ncrit Initial electron density: N e0 2 10 23 cm -3 Plasma critical density: N crit 10 21 cm -3 Plasma sound speed: c s 10 7 cm/s Disassembly time (ps) 400 350 300 250 200 150 100 50 0 Ar Ar Kr 0 2 4 6 8 10 12 14 Kr t = τ crit (c s = 0.9 x 10 7 cm/s) 16.6-nm EUV (Ar 9+ ) x-rays (>1.5 kev) Droplet diameter (µm)
The Laser Droplet Interaction t = 0 time N e = N e0 N e = N crit N e < N crit N e << N crit Case #1: Short probe delay Pump Probe X-rays or excitation emission Recombination emission Case #2: Long probe delay Pump Probe
Conclusions Characterization of the nozzle in the droplet flow regimes is important for the proper interpretation of EUV / x-ray emission data. Laser coupling efficiency is highly dependent on the target size and the duration of the laser pulse. Optimum pulse width for best radiation generation efficiency is dependent on the type of emission: (1) EUV recombination emission t crit < t laser < t recomb (2) EUV excitation or x-ray emission t laser t crit
Preliminary Xenon Results: Size histograms Pressure (psi) 10000 1000 100 10 1 0.1 P T phase space of gas jet Frequency 1.0 0.8 0.6 0.4 0.2 0.0 500 psi, 240 K <d> = 5.4 ± 1.3 µm 100 psi, 190 K <d> = 7.4 ± 1.8 µm 100 150 200 250 300 Temperature (K) 2 4 6 8 10 12 14 FWHM Diameter (µm)
Preliminary Xenon Results: EUV spectra 2.0 Signal (AU) 1.5 1.0 0.5 spectra at 100 PSI, 190 K 0.0 10 12 14 16 18 Wavelength (nm)